Printing


Introduction

This is the first unit in a series that covers board assembly processes in more detail than was possible in the brief description given in Fabrication and assembly process outline. You should however remember to relate your reading here to that process sequence. So, if you can’t immediately remember the process order, now is the time to do some revision!

It is also by far the longest unit! This reflects the fact that:

Typical volume production printer

Typical volume production printer

In his paper Where quality is lost on SMT Boards, Mangin calculated the first-pass yield for a populated board, based on typical values for component counts and process defect rates, and came up with an expected yield of only 42%! More significant still was the breakdown between defect causes (Figure 1), which showed that almost two-thirds of defects were attributable to the paste printing process. Even with a paste print defect rate halved (50ppm), the loss in board yield would be 21%, with paste problems remaining the most significant cause.

Figure 1: Distribution in percent of process-related defects on a populated board

Figure 1: Distribution in percent of process-related defects on a populated board

Is this theoretical analysis valid? A survey of US manufacturers carried out in 1993-94 showed that solder paste problems indeed accounted for between half and three-quarters of defects. This has been the driving force behind many subsequent improvements in machines and material.


Solder paste printing principles

Printing is an application process by which all lands on a board are provided with solder paste or glue in one stroke. Unlike most text or graphics applications, the paste printing process needs to apply significant quantities of ink, and the two techniques which can be used are screen printing and stencil printing, which was developed from the earlier screen printing process. In both cases, the printing principle is the same, as shown in Figure 2.

Figure 2: Principle of the stencil printing process

Figure 2: Principle of the stencil printing process

Solder paste is pressed by a squeegee onto the board through defined openings (usually called ‘apertures’) in a stencil or screen. Important steps in the process are the moving of the paste ahead of the squeegee, the flowing of the paste into the openings, the levelling of the solder paste deposit, and the release of the paste onto the board.


Screen printing

In screen printing, a screen is employed as an image carrier. This consists of a rigid frame on which is stretched a mesh (or ‘gauze’) made from fine polyester or stainless steel wires. The mesh acts as a support for a stencil of the required image, which is produced in a photosensitive emulsion applied to the mesh. Emulsion is normally applied to both sides of the mesh, and ‘built up’ to a defined thickness on the underside (the side in contact with the board). The mesh support allows total freedom of stencil design, which can include areas which would otherwise be unsupported – think of a children’s lettering stencil and the webs which need to be left in place in letters such as A and O.

In use, the screen is fixed just above the board (Figure 3), and the mesh of the screen pushed down into contact by a flexible squeegee as it moves across the screen.

Figure 3: Screen printing process

Figure 3: Screen printing process

The squeegee blade presses the paste into the open apertures of the image, and the surplus is removed by the squeegee as it passes across each aperture. The screen then peels away from the printed surface behind the squeegee, leaving the paste that was previously in the mesh aperture deposited on the board beneath (Figure 4). The paste flows slightly immediately after printing to reduce the ‘mesh marks’ left in the print, a process known as ‘levelling’.

Figure 4: Screen printing process

Figure 4: Screen printing process

The process is sometimes referred to as ‘off-contact’ printing, since the screen only contacts the printed surface at the point where the squeegee passes over it. A typical value for this ‘snap off distance’ is 0.5mm for each 100mm of frame width.

The total thickness of the emulsion mask and the mesh determines the thickness of the paste deposited. Emulsion masks were originally limited to around 50µm thickness, because PVA-based photo-emulsions changed colour after UV exposure, causing an exponential increase in exposure time with emulsion thickness. However, modified emulsions, with faster cross-linking and without the colour change property, allow thicker masks (up to 1mm) to be made. As a rough rule-of-thumb, the print thickness will be approximately two-thirds of the overall mesh-plus-emulsion thickness: using 80 mesh stainless steel screens, with emulsion coatings 25–300µm thick, solder deposits in the range 200–500µm can be produced.

Typical solder cream particles are 45–75µm in diameter. This particle size affects the paste rheology, and also sets the size of the screen mesh (Table 1).

Table 1: Typical screen parameters
Mesh density
(per inch)
Wire diameter
(µm)
Mesh opening
(µm)
Emulsion
thickness (µm)
40
140
495
330
80
100
224
300
120
63
140
150
165
50
100
115

A minimum ratio of 1:3 between particle size and open dimension of the mesh is normally necessary, using the largest mesh aperture possible in order to reduce the risk of screen clogging.

The mesh support also sets an upper limit to the viscosity of the paste used, and therefore to its metal content. Viscous pastes:

Industrial printers need to use screens and emulsion stencils, because unsupported stencils are unable to produce the complex patterns needed. However, because solder paste patterns are less complex, and have a lower open area, it proved possible to dispense with the support mesh and use a direct stencilling approach.

Stencils are the method of choice for almost all solder paste printing, but you will still encounter screen printing in board fabrication, for tasks such as screening solder mask and legend. Here the patterns are more complex than with solder paste, covering a larger percentage of the board area, and the materials used have finer particles and are less viscous.


Stencil printing

In contrast to screen printing, stencil printing is an ‘on-contact’ (or ‘in-contact’) process. The stencil is a metal mask which rests directly in contact with the surface of the board. In stencil printing, the board is moved into contact with the stencil before the squeegee starts to move (Figure 5). When the squeegee has completed its stroke, the board and stencil are then separated vertically, which releases the paste from the stencil, producing well-defined edges to the print. It is usual, but not essential, for the stencil to remain fixed and the board to be raised for printing and lowered at a controlled speed afterwards.

Compared with screen printing, stencil printing gives:

Figure 5: Stencil printing (‘on contact’ printing)

Figure 5: Stencil printing (‘on contact’ printing)

The density of electronic components continues to increase, and the lead pitch on integrated circuit components to decrease, and ‘fine line’ printing (where the width of printed lines is <300µm) has become a routine requirement for SM designs. This is generally also linked to a requirement for pastes with a higher metal loading. Consequently, stencil printing has become the industry standard method of solder paste application. However, it is still common to hear the equipment referred to as a ‘screen printer’!

The process depends on the interaction of several factors:

If one of these factors is incorrect, printing quality will be poor, and therefore the printer itself is only one of the decisive factors in the whole process. The process window (the region where process values can vary but still produce good results) can be enlarged by careful choice of materials and design.

Stencils

Stencils for solder paste printing are normally bought in from specialised manufacturers. The starting point is the PCB layout CAD data, which is used to define the size and position of openings, allowing for the intended size differences between lands and stencil openings. The stencil manufacturer then has to include other corrections for the tolerances of the processes being used.

A complete stencil consists of a square or rectangular rigid frame to which the stencil is attached. The frame design is specific to the printer to be used, and is most commonly fabricated from square aluminium tube for the optimum combination of rigidity and lightness, although cast aluminium frames are used for small printers.

Loading the stencil

Loading the stencil

Most frequently, a polyester mesh is laid on the frame with its filaments at 45° to the frame, tensioned and then glued to the frame, the metal stencil is glued on this mesh, and the mesh is cut away from the central printing area (Figure 6). The mesh is often filled with emulsion to make it impermeable to solder paste. Metal stencils are, however, sometimes directly glued to the frame. Tensioning is also required in this case to ensure a taut, flat surface.

Figure 6: Features of a completed stencil

Figure 6: Features of a completed stencil

The stencil frame evidently takes more room to store than would a separate stencil foil. A number of removable foil systems have been developed so that stencils can be stored separately from frames.

DEK Micromount stencil in frame

DEK Micromount stencil in frame

In the original implementation of this idea, the stencils were stretched in only one direction, corresponding to the print stroke direction of the printer. The advent of vector printing, which attempts to improve solder release by using a squeegee at an angle to the print, has led to the development of methods for tensioning the stencil in both axes.

The stencil foil itself may be made by one of three manufacturing techniques: etching, laser cutting and electroforming.

Etched foils

Chemically etched stencils are usually produced either from brass (which may subsequently be nickel plated) or from an etchable grade of stainless steel. Brass will yield sufficiently to allow printing on surfaces which are less than perfectly flat. Stainless steel is more expensive, but has improved durability. In the US stencils made from molybdenum are used, but these are not popular in Europe.

The process is shown schematically in Figure 7. A resist is applied to the stencil material and the apertures are photographically defined. The resist is then ‘developed’ to remove unexposed areas, and the apertures thus opened up are chemically etched.

The artwork dimensions are not the same as those of the openings to be made because the etchant ‘undercuts’ the photoresist, so there is an inevitable degree of over-etching. The artwork is therefore modified by the stencil manufacturer (a process known as ‘wobbling’) to reduce the aperture sizes to compensate for this. The wobbling process, in combination with etching, produces the rounded corners which are characteristic of etched stencil apertures.

Figure 7: Manufacturing steps for chemically etched stencils

Figure 7: Manufacturing steps for chemically etched stencils

Chemically etched stencil

Chemically etched stencil

Etching is carried out from both sides of the foil in an attempt to produce near-vertical side walls. This double sided process can create a ‘waist’ within the aperture, although this can be reduced by electro-polishing.

Etching is the least expensive manufacturing method, but the practical lower limit for aperture dimension is the thickness of the material (150µm). Etching is also the least accurate method in terms of aperture positioning, and it is difficult to produce a quality stencil with openings for components with pitches smaller than 0.5mm. Nevertheless, except for fine-pitch applications, the majority of stencils are produced in this way.

Laser cut foils

The laser cutting process produces stencils directly from the PCB CAD data, with no intermediate steps, such as photoplotting (Figure 8). The aperture data for the stencil is modified to allow for the width of the laser beam, and fed directly to the laser cutting machine. Size and positioning are therefore very accurate, and laser cutting can be used for component lead pitches down to 0.3mm. The limitation then is due to the aspect ratio of the hole: where the aperture is smaller than about 1.5x the stencil thickness, paste release is impaired.

Figure 8: Manufacturing method for a laser cut stencil

Figure 8: Manufacturing method for a laser cut stencil

Laser cut and electropolished stencil

Laser cut and electropolished stencil

The laser cutting operation is carried out from the bottom side of the stencil (board-side during printing), to ensure that the slight taper introduced by the cutting process opens out towards the board. This is claimed to enhance solder paste release during printing. However, the square corners typical of laser cut apertures are believed by some users to make cleaning more difficult.

The stencil material is almost always stainless steel, and the grades used can be substantially more robust than for etching, where a fine grain is mandatory. By contrast, laser cutting takes no account of grain boundaries, and operates well even for annealed materials.

Another advantage of the technology which is used by some assemblers is that the stencil can be cut from measurements of an actual board, rather than the artwork from which it was generated. This compensates for any inaccuracies in board manufacture.

Laser cut stencils are produced using CAD data, so size and positioning are very accurate and modification is easy. However, since laser cutting is a serial process, with apertures formed one at a time, the price of a laser cut stencil is high, particularly for a densely-packed board.

Electroformed foils

Electroformed stencils are made of nickel, by an ‘additive’ electrochemical process, in contrast to the ‘subtractive’ process of etching and laser cutting. Photoresist is applied to a metal base plate and exposed through a photoplot of the aperture pattern (Figure 9).

Figure 9: Manufacturing steps for an electroformed stencil

Figure 9: Manufacturing steps for an electroformed stencil

Electroformed stencil

Electroformed stencil

After processing, a resist pattern is left only where apertures are required. A plating process builds up nickel to the required thickness around the resist areas. The resist is removed and the electroformed stencil separated from the metal base.

The advantage of this type of stencil is the extreme smoothness of the aperture walls which results in easier flow of the paste into the aperture during printing, and possibly lower adhesion of the paste to the walls during release. A slight tapering of the stencil walls is also present.

A side effect of the manufacturing process is that electroformed stencils provide a small gasket around each aperture which helps reduce paste bleed onto the underside of the stencil. One user reported reducing the need to clean the underside of the stencil from once every five prints to once every thirty.

The cost of an electroformed stencil is between that of an etched and a laser cut stencil, and its accuracy is similar to etching.

Stencil thickness

The most commonly specified stencils are etched from stainless steel 150µm (0.006in) thick. Other standard thicknesses are 200µm (0.008in) and 125µm (0.005in), the latter for fine-pitch applications. Molybdenum is a candidate for very fine pitch slots needing 100µm (0.004in) thick stencils, but the resulting paste volume is only rarely sufficient to create satisfactory joints over the whole.

Etching, often in combination with laser cutting, is used to produce stepped stencils, partially etching areas around apertures where a thinner than normal deposit is required. This construction enables solder paste deposits of different thickness to be produced simultaneously for different types of components – for example, a reduced thickness may be required at fine-pitch component locations, in order to improve print definition, or a thicker deposit needed for components such as through-hole connectors. When a number of different thicknesses are produced on a single foil, this is sometimes referred to as a ‘multi-level’ stencil (Figure 10).

Figure 10: Schematic cross-section of a multi-level stencil

Figure 10: Schematic cross-section of a multi-level stencil

Although stepped stencils are more expensive, the attraction is that the same stencil can be used for printing paste volumes suitable for both large surface mount components and fine-pitch leads. Despite concerns that such a stepped stencil would only be ‘cleared’ by a compliant squeegee, squeegees of both hard rubber and metal have been proven in practice, provided that the stencil profile is correct, the areas of reduced thickness are not too small, and an allowance is made for a transitional area (2–3 mm width), as squeegees do not cope well with a sharp transition.

Stencils are generally satisfactory for printing solder paste for chip components down to the smallest passive component sizes and for integrated circuits with lead spacing down to 0.4mm. Below this level, the deposit usually becomes too thick and excess paste is a major problem, arising largely from the impracticality of reducing stencil thickness below 100µm.

Apertures

Laser-cut stencils are usually more consistent than etched equivalents in their control of aperture dimensions. This is partly because suppliers have been slow to recognise the importance of using cross-rolled material to ensure regular grain shape in X and Y directions, or else have difficulty in etching these tougher materials. There is always a slight taper in etched apertures, depending on the metal thickness and whether etching is from one or both faces: the latter gives better results and is customary except for stepped stencils.

The apertures in a stencil are, ideally, designed to be slightly smaller than the corresponding pads, with the aim of getting a bleed-resistant seal between the pad surface and the underside of the stencil. In practice, for the smallest pads, the need to have a sufficient paste area makes this difficult to achieve.

Wall finish

There has been much debate as to what is the best finish for aperture walls to aid paste release, ensuring that all the paste is released from each aperture. This has become more difficult with reducing pad sizes. Competing claims for promoting paste release are made for:

Internal wall of an electroformed stencil

Internal wall of an electroformed stencil

One school of thought advocates a degree of roughness on aperture walls as an aid to reliable paste transfer, using the analogy of wet sand in a child’s seaside bucket: ‘If the bucket walls are smooth, it can be difficult to get the sand out; if the walls were corrugated, the sand would be more easily ejected.’

As laser cutting produces a comparatively rough wall finish, it is not surprising that manufacturers of laser cut stencils favour this less than perfectly smooth wall approach! Alpha Sigma Technology stencils are subjected to a proprietary side-wall preparation process, in which small indentations are provided to retain small amounts of the paste vehicle and so ‘lubricate’ the paste’s passage through the aperture. This is analogous to high performance car engines, where the cylinder walls have minute indentations to retain lubricating oil.

In the final analysis, there is no substitute for evaluating different stencil types for the specific application, particularly for fine-pitch work.

Self Assessment Questions

Without looking at Figure 6, try drawing a diagram showing the construction of a typical stencil for solder paste printing, naming the materials used for the components other than the stencil foil itself.

show solution

Self Assessment Questions

Use a diagram to describe how etched stencils are made, explaining the approach used to produce near-vertical side walls.

show solution

Self Assessment Questions

What two other methods are used for manufacturing stencils for solder paste printing? Draw up a table comparing all three methods, explaining briefly:.

show solution

Life

The useful life of a stencil varies considerably, depending on complexity, stencil size and thickness and the distance between apertures. Based on his experience as a contract assembler, Boswell reports that stencils last between 5,000 and 50,000 prints, but much longer lives have been reported by OEMs. In all kinds of company, the life of a stencil is frequently cut short by damage during handling.

The squeegee choice

Screen printing uses relatively soft polyurethane rubber squeegees, as these:

Note that the presence of mesh prevents flexible squeegees entering the apertures and scooping out paste.

In stencil printing, either polyurethane rubber (‘polymer’) or metal squeegees may be used.

Polymer squeegees

In stencil printing, a soft squeegee blade will accommodate uneven boards, but its flexibility will also deflect into the apertures and scoop out the paste which has just been printed. A soft rubber squeegee is also prone to wear, and its lack of a sharp, square edge will give relatively poor print definition.

Polymer squeegee

Polymer squeegee

Hard, polyurethane rubber with limited flexibility gives improved print definition and longer life, but needs high pressure to clean the stencil surface of paste. If machine settings are incorrect, a hard squeegee may ‘coin’ the stencil surface, leaving an impression of the board outline in the stencil foil.

There are a number of types of polymer squeegee including both trailing and diamond-shaped. Usually two squeegees will be fitted back to back as shown in Figure 11, in order to make it possible to print on both forward and return strokes, with the squeegee assembly ‘hopping over’ the pile of paste. This is preferable to having to return the paste roll manually to its start position.

Squeegees lifted to show the paste roll

Squeegees lifted to show the paste roll

Figure 11: Trailing squeegees fitted back to back

Figure 11: Trailing squeegees fitted back to back

All rubber squeegees will deform to some extent, resulting in what is known as ‘scoop’ or ‘scavenging’. This effect is illustrated in Figure 12. The result is a reduced volume of solder paste, leading to ‘insufficients’ and ‘opens’.

Figure 12: Schematic view of print scavenging

Figure 12: Schematic view of print scavenging

Metal squeegees

By contrast, the transverse stiffness of a metal squeegee prevents the blade from bending into a solder pad opening because the squeegee is supported on both sides of the pad. It rides along the stencil surface and shears the solder paste in the plane of the top surface of the stencil, but without dipping into and scavenging solder paste from the apertures.

Metal squeegee with paste-retaining side-bar

Metal squeegee with paste-retainingside-bar

Provided that both board and stencil are flat and accurately parallel to each other and to the plane of motion of the squeegee, metal squeegees not only give good definition, but can also operate at lower pressure than hard rubber squeegees. This lower pressure reduces bleeding where solder is forced out at the bottom of the aperture, past the seal between stencil and pad, leading to solder bridging after reflow.

Experiments by Intergraph reported that, compared against a polyurethane squeegee, the solder pads produced by a metal squeegee:

The metal squeegee was also reported to leave a more consistent paste deposit whether the pads were parallel or perpendicular to the squeegee motion, and to be more forgiving of variations in squeegee pressure, solder paste, and board to substrate alignment. 18% fewer printing defects have been cited by one high volume manufacturing operation.

These effects are just as important in non-fine-pitch printing applications, where large apertures make polymer squeegee deflection even more of a problem, although manufacturing tolerances are wider.

Unexpectedly, experience has shown that metal squeegees are compatible with stepped stencils, with excellent printing results reported when 3mm is allowed for each 50µm of step-down.

The higher friction between a bare metal squeegee and stencil can result in stencil wear. Early non-durable finishes provided good performance only for a short working life, but highly adherent, wear-resistant polymer coatings have now been developed by companies such as Transition Automation. In these, the lubricated edge is metallurgically bonded to the squeegee base material rather than plated, so that the lubrication lasts as long as the squeegee. This allows the squeegee to slide smoothly on the stencil surface with less friction than a polymer squeegee. Reduced friction leads to reduced registration shifts which in turn improves output quality. Decreased stencil movement during printing, particularly during acceleration, extends the life of the stencil. The lubricated edge also serves to prevent stencil scratching.

Metal squeegees are more durable than polyurethane rubber blades. High volume manufacturing operations may change polymer squeegees as often as once a day, whereas metal squeegees may last for over 100,000 prints. More significantly, the printing quality of a polymer squeegee reduces because the topology of the edge changes as the squeegee is abraded by friction with the stencil and corroded by contact with solder paste. These effects are not visible on the surface of a metal squeegee, even after many print cycles.

So, while metal squeegees can cost 5–15 times as much as polymer units, they are often justified on the basis of higher throughput, lower sensitivity to printing variables, and higher output quality. However, operators must be careful when handling metal squeegees, as they ‘do not bounce when dropped’, and for that reason it has been commented that ‘the life of a metal squeegee can be less than one print’!

Metal squeegees work well with metal stencils. They are, however, not recommended for traditional screen printing, since they can damage both the polymer mask and the underlying mesh.

Self Assessment Questions

Describe the advantages and disadvantages of soft rubber, hard rubber, and metal squeegees for stencil printing solder paste.

What type of squeegee would you choose for an assembly containing many fine-pitch parts on a small board?

show solution

The printing process

The printing process was illustrated in Figure 2. As we saw, it starts with the application of paste to the stencil, and then involves four main activities:

  1. ‘Rolling’ the solder paste on the top surface of the stencil ahead of the squeegee.
  2. The paste flowing into the stencil apertures, filling them.
  3. The squeegee cleaning the top surface of the stencil, defining the thickness of the paste deposit.
  4. Removing the board from the stencil, when the effects of gravity and surface tension combine to release the solder paste from the walls of the aperture, leaving the paste on the board.

Applying solder paste to the stencil

Applying solder paste to the stencil

The first three of these activities take place while the squeegee moves across the stencil, apertures being progressively filled and the paste sheared flat until the whole stencil pattern has been filled.

To understand the printing process, we need to think of the forces acting on the squeegee, stencil and paste. The paste acts as a fluid, transmitting hydrodynamic pressure applied to it and being forced through the apertures by pressure from the squeegee.

The forces acting on the paste and stencil are shown in Figure 13 and Figure 14. As the squeegee is moved across the stencil, the rolling resistance of the paste exerts an upward pressure on the squeegee. The magnitude of this lift force depends on the viscosity of the paste, the speed of the squeegee, and the angle of the blade. The pressure applied to the squeegee must overcome this lifting force in order to fill the apertures and clean the top surface of the stencil.

Figure 13: Forces acting on the squeegee during printing

Figure 13: Forces acting on the squeegee during printing

Figure 14: Triangle of forces

Figure 14: Triangle of forces

Solder paste levelling

The paste is ‘levelled’ by the squeegee cleaning the top surface of the stencil, and its cleanliness is an indicator of stencil printing quality. The major influences are squeegee pressure and speed.

Note, however, that there is a ‘process window’ for squeegee pressure, with bleeding also occurring if the pressure is insufficient to create a seal between board and stencil, and that a similar effect is produced by stencil damage.

Exercise

Using a diagram to show the forces acting on solder paste during printing, explain how you would expect the squeegee pressure required for a good print to be affected by the speed and angle of the squeegee.

Compare your explanation with that given in the next two sections.

Solder paste ‘rolling’ and filling

To get the aperture full of paste requires both a sufficient flow rate and a sufficient fill time. Apertures which are not completely filled will not release paste onto the board, usually resulting in clogged stencils, and defective solder joints.

The flow of the paste depends on the pressure gradient, the paste viscosity, and the dimensions of each aperture. The smaller the opening, the higher the pressure required to press the paste through it. The hydrodynamic pressure depends on two things.

A typical squeegee speed is in the range 10–100 mm.s−1, with slower speeds needed for small apertures. When fine-pitch parts are to be assembled, a typical speed has been 25 mm.s−1 – the time taken by the print stroke is often a factor limiting the capacity of a SM assembly line. For this reason, pastes with improved printability are being developed, which allow higher speeds, and give increased machine throughput: the latest pastes claim to be printable at speeds as high as 200 mm.s−1.

As well as being affected by squeegee pressure and speed, levelling is influenced by the squeegee angle. A smaller angle will increase the lifting force, and increased squeegee pressure is needed to overcome this.

The downward force on the squeegee is typically in the range 2–8 N.cm−1 of squeegee length, the value depending on the printing conditions, and in particular the squeegee speed.

Other influences on the printing behaviour are the squeegee hardness and the stencil surface finish – a slightly rough surface is considered useful to promote paste rolling during printing.

Solder paste release

For on-contact (stencil) printing, paste release is determined by the separation speed of the board from the stencil. The adhesion of the paste on the board has to provide the shearing force to overcome the adhesion of the paste to the stencil walls. This hydrodynamic shearing force depends on the separation speed. Board release speed is typically a few mm.s−1. As with filling, it is the release of fine-pitch apertures which is the limiting factor in the printing process.

Self Assessment Questions

Draw a table showing the main machine parameters which affect each of the four activities in the stencil printing process (rolling, filling, levelling, and separation). Which of these are inter-related?

show solution

For printing through fine-pitch apertures, to reduce scooping and improve solder release, it is advantageous to print along the length of the aperture rather than across its width. The only possible compromise for a four-sided package is to position it at an angle to the direction of squeegee movement, but to require layouts to be designed with the larger ICs at such an angle is not acceptable.

Vector printing, in which the travel of the squeegee is at an angle to the normal X-Y board (and stencil) directions, allows the user to adopt variable positioning (generally 45°) without altering the board layout. This can be achieved either by rotating the board and stencil under a (fixed) squeegee assembly (MPM), or by swivelling the squeegee above the (fixed) board and stencil (DEK). In the first machine, there are alignment implications; in both, the useable stencil area is reduced for a given size of stencil frame.

Adjustable-angle squeegee head

Adjustable-angle squeegee head

Printer operation

The printing cycle

As well as the stencil printing process described above, a printer must be able to carry out several other operations to complete the print cycle, which comprises:

  1. Transport and mounting.
  2. Alignment (stencil to board).
  3. The printing process.
  4. Inspection of solder paste deposits (optional).
  5. Cleaning of the underside of the stencil (after some cycles).

The complete operation ‘cycle time’ is important in determining the throughput and capacity of a surface mount line. To avoid inspection adding to the cycle time, it is often carried out as an off-line operation, or using a separate work-station.

The description given below is of a printer integrated into a production line, but the comments made about board mounting, alignment and cleaning requirements apply generally. Note that there are many variations in the way mounting, alignment and cleaning are implemented, and that stand-alone printers will also handle boards manually rather than automatically.

Typical semi-automatic printer

Typical semi-automatic printer

Board mounting

The board is normally transported into and out of the working area by conveyor belts. Once in the working area, the board is stopped in the desired position either mechanically or by using an optical sensor. The board then has to be both clamped rigidly in position to prevent lateral movement, and supported to resist the downward forces during the print stroke, which would otherwise lead to warping and solder bleed under the stencil.

Fixturing can take several forms, including:

Pin board support

Pin board support

The solution chosen will depend on the application. Providing adequate support during second-side printing can be problematic, particularly when the first side assembly is densely packed with components, or the board is thin and/or flexible. A common approach is to use a dedicated tooling plate which is machined to accommodate the components. This gives better support than a bed-of-nails fixture, especially around the board periphery.

‘Nest’ for second-side printing

‘Nest’ for second-side printing

Clamping may be mechanical, with edge clamps which are thin enough not to impede squeegee travel (and which are correspondingly sharp!). Alternatively, the board may be held against the tooling plate with vacuum assistance, which has the apparent advantage that it will hold warped boards flat (although this task is adequately carried out by the downward pressure of stencil and squeegee).

Alignment

Accuracy of alignment is critical. The aim is to position the board accurately and repeatably to align the solder lands on the board pattern with the stencil apertures. This requires three adjustments (X, Y, q): as the range of angular adjustment is small, alignment is often implemented using X1, X2, and Y adjusters as shown in Figure 15.

Figure 15: Principle of a stencil alignment mechanism

Figure 15: Principle of a stencil alignment mechanism

Manual adjustment has now largely been replaced by vision systems using a CCD camera to image fiducial marks on the board. A number of different shapes of fiducial have been used in the past, although the industry tends now to use a solid filled circle between 1mm and 3mm in diameter.

Fiducial camera in action on a semi-automatic machine

Fiducial camera in action on a semi-automatic machine

In many systems, the camera is inserted between the stencil and the PCB and looks both down to the board and up to the stencil, either using pneumatic shutters to determine the direction in which the camera points or viewing both targets simultaneously (Figure 16). In operation, the camera traverses diagonally across the board to take in both fiducials, and returns to a rest position outside the range of vertical motion of the board fixturing.

Figure 16: Operation of vision alignment system

Figure 16: Operation of vision alignment system

The sequence of operations is as follows:

  1. board moves to board stop.
  2. board is clamped.
  3. camera moves to fiducial 1 and locates its position.
  4. camera moves to fiducial 2 and locates its position.
  5. camera moves to rest position.
  6. stencil is aligned.
  7. board support moves up.
  8. squeegee operation commences.
  9. board support moves down.
  10. board is unclamped and moves out of machine.

Other printer manufacturers do not site the camera between stencil and board, and therefore require a precision camera movement.

Fiducials may be referenced either to light background (the stencil) or dark background (the board). Modern software systems work on the basis of fiducial scoring against target/accept scores. In the DEK system, a stored outline would yield a score of 999 for a perfect match. Practical values are 950 on a board and 900 on a stencil, with a target of 700 and an accept score of 500.

The accuracy of alignment depends on:


Alignment compromises

Differences in temperature and humidity during production can lead to small differences in dimensions, particularly on large boards, so there is no guarantee that the stencil will exactly match the board even though these have been generated from the same CAD artwork.

or this reason, the best any alignment system can do is to provide perfect alignment between stencil and the board at one point, and to minimise the errors at all others.

A vision alignment system looks at two fiducials, and can correctly align the fiducials to the line joining them and average the error along this (Figure 17).

Figure 17: Error averaged between fiducials

Figure 17: Error averaged between fiducials

The one point of perfect match is somewhere between the two fiducials. Often this point is midway between them, but some boards may have non-central areas which are particularly difficult to print. In such cases, it is possible to ‘weight’ the fiducial correction, so that the optimum alignment occurs at the desired board location. In all cases, however, exact alignment in X,Y and q, is only achieved at one place, and the inaccuracy increases with distance from this point.

Self Assessment Questions

A colleague complains that the PCBs you have printed are misaligned, because only the pads in the centre of the board have been accurately printed, and suggests that the vision alignment system is faulty. Explain why this is not likely to be the case, and what the probable cause is.

show solution

Temperature control

If possible, temperature should be kept constant within close limits: 21°C ±2°C is a typical specification. This is primarily in order to avoid any effect on the process resulting from changes with temperature in the viscosity of the solder paste, but also helps keep alignment consistent by maintaining constant stencil dimensions.

Stencil cleaning

Stencil cleaning is a low-technology part of the printing process but is nevertheless vital. Cleaning is required both at the end of a run when the stencil is removed from the printer, and it is necessary to remove all accumulations of solder paste from the apertures and the stencil surfaces before the deposits harden, and also in-process.

The underside of the stencil gradually acquires solder paste through:

The rate at which this happens will depend on print parameters and stencil condition.

The underside of the stencil should therefore be cleaned periodically. This is particularly important for fine-pitch applications, because a small degree of contamination of the board by solder paste degrades the print through smearing. If not cleaned off, the resulting print smearing causes an increased likelihood of solder shorts and solder balling.

In both cases, cleaning may be by hand, or automatic. Programmable in-process stencil cleaning may be built-in to an automated stencil printer, while separate automatic spray-cleaning tanks may be used for stencils after a print run.

Automatic stencil cleaners are designed to allow unassisted cleaning of the underside of the stencil at user-programmable intervals (typically every few prints). Typical systems for cleaning the stencil underside use a lint-free wipe, running between supply and take-up rolls, so that an unused area of the paper is pressed against the underside of the stencil. Contaminants and paste removed from the stencil are trapped on the roll of material.

Close-up of under-stencil cleaning mechanism

Close-up of under-stencil cleaning mechanism

On some machines, this operation can be run dry or wet, using a cleaning fluid ‘sprinkled’ onto the absorbent paper by a solvent bar wetting system. Machines may be programmed for different combinations of wet and dry wiping, using wet cleaning to loosen dried solder paste residues.

Depending on the manufacturer, the cleaning process may also be assisted by vacuum, which is claimed to help remove solder paste from stencil openings and improve the clearance of partially-clogged apertures. The vacuum system operates in conjunction with twin blades on the under-stencil wiper.

Self Assessment Questions

Why does solder paste gradually accumulate on the underside of the stencil, and what problems are likely to occur if this is not regularly cleaned?

show solution

Paste conditioning

Paste needs to be stored correctly and allowed to reach operating temperature before the container is exposed to (potentially moist) air. Preparation for use should also include a degree of ‘paste conditioning’ to ensure that:

With a conventional stencil printer, the third of these is generally carried out by ‘kneading’ the paste, using the squeegees to work it backwards and forwards over a blank section of the stencil. This operation can be programmed on the printer and linked with automatic paste replenishment.

However, straightforward mixing to achieve homogeneity before application to the stencil is only possible when paste is supplied in pots, and even then it is easy for over-zealous stirring to incorporate bubbles of air into the paste. With cartridges, because direct working of the paste is not viable, any homogenisation necessary must be built into the dispense routine. For an automated system, one approach is to incorporate a circuitous paste flow route within the dispenser.

Automated paste dispenser

Automated paste dispenser

Assessing the results

Print quality

Visual assessment of paste deposits offers a quick and helpful guide to whether the process is under control. Best results are obtained with relatively low magnification (×4 – ×10) using a magnifier or projection microscope, as these allow the whole area to be scanned relatively quickly.

What are you looking for? A working definition of an acceptable print is one that has good definition and registration without any defects (such as slumping, scavenging, bridging and peaking):

Figure 18: Illustration of printing defects

Figure 18: Illustration of printing defects

More detailed standards are available, such as those originally developed by Martin Marietta, covering topics such as:

Be aware that, because they have not yet been formalised by IPC or similar international regulatory bodies, workmanship standards may differ slightly between assembly houses.

Paste measurement

The measurement of the paste deposit is crucial to quality control. There are two significant aspects:

The methods of evaluating a print vary between fully automated inspection, both for coverage and paste height, and occasional operator visual checks. There is a trend towards building in automatic checks at the end of the print cycle, either within the printer or as a separate feature. As always, there are variations in practice between different manufacturers and users, depending to some extent on the required cycle time and the sophistication and speed of the optical arrangements. This is an area where significant progress has been made in recent years.

Optical inspection for coverage has in the past relied on there being a visual difference between a pasted and bare pad: this is very easy when printing onto a nickel-gold finish, but very much more difficult when printing onto solder surfaces.

The so called ‘Z-check’ for paste height can be carried out easily with a light-section microscope or laser equivalent. As shown in Figure 19, the height of the print, and some information on the topography of the surface, can be gained using oblique illumination through a slit, and viewing from above.

Figure 19: Method of operation of a light-section microscope

Figure 19: Method of operation of a light-section microscope

Both visual measurement systems are ‘non-contact’, eliminating any need to touch the solder paste or any surface on the board substrate, and are suitable for measuring wet solder paste as soon as it is printed. However, both approaches ignore the real aim, which is to deposit a known volume of solder paste, and are particularly susceptible to error due to thickness variations from effects such as scooping.

There are two general methods available for measuring solder volume.

  1. The direct optical approach, where a scanning laser sectioning equipment is used to generate a three-dimensional digital image of the solder deposit, from which the volume can be computed.
  2. An indirect method, which measures the weight of the solder deposit, and compares this against a perfect print. Because the weight of solder paste on an average board is only of the order of a few grams or tens of grams, it is necessary to weigh the board both before and after printing.

While this second method is of limited utility, particularly in detecting stencil blockages and other localised defects, the approach has been used to monitor the amount of paste which is actually used for making the product, giving a guide to the scrap levels. These are surprisingly high in many companies, despite the fact that solder pastes may cost £50–£140 per kg. Figures of under 50% utilisation are not uncommon.

Self Assessment Questions

Think about the process of printing, and identify the ways in which solder paste might be consumed but not turned into good product.

show solution


Pressure printing systems

Limitations of conventional printing

Conventional stencil printing techniques have fundamental limitations as regards paste handling:

The result is that, over the last decade, there have been many attempts to produce a viable sealed-paste print system. These attempts have accelerated since 1997/8, as there has been substantial focus on such methods as solutions to the challenges of high production rate printing.

The most fundamental constraint for fast printing is that the hydrostatic pressure in the paste is determined by the squeegee speed, but there are other considerations:

Approaches to pressure printing

Sealed head, pressure printing systems take subtly different approaches to the design challenges of:

However, there are a number of features common to all systems:

DEK ProFlow DirEKt imaging

The first of these heads to be launched was the DEK ‘ProFlow DirEKt Imaging’ head shown schematically in Figure 20.

Figure 20: The DEK ‘ProFlow DirEKt Imaging’ head

Figure 20: The DEK ‘ProFlow DirEKt Imaging’ head

The paste is supplied in a cassette, conceptually like a printer toner cartridge.

Multicore paste cassette system for pressure printing

Multicore paste cassette system forpressure printing

The Multicore Direct Imaging System Cassette shown is that company’s implementation of what is an open standard, which anyone can adopt without licence payment. The cassette holds 1.25kg of paste (substantially more than a standard cartridge) and is emptied through the holes on its top surface, which are sealed with removable tape when supplied. The collapsible plastic pouch which forms the body is flexible, which allows for a degree of manual kneading of the paste if desired.

The cassette base-plate is an integral part of the paste conditioning system, the holes in it working in conjunction with those on the transfer head to create a meander path for paste conditioning.

During the print stroke, the paste is pressurised by a piston which acts directly on the top surface of the plastic pouch of the paste cassette, keeping the conditioning chamber constantly supplied. At the end of the print stroke, pressure is removed from the system.

Whilst travelling across the stencil, the trailing wiper foil of the paste retention system ‘scoops’ the paste from the stencil surface, keeping the stencil surface clean and also inducing a rolling motion within the conditioning chamber, to help maintain the paste in optimum condition for printing.

MPM Rheopump

The MPM system uses standard paste cartridges. As the pump head moves across the stencil, pneumatic pressure and friction between paste and stencil combine to cause the paste to roll inside the paste chamber, as shown in Figure 21. The circular shape of the chamber enhances paste rolling and eliminates ‘dead spots’ in the head.

Figure 21: Principle of the MPM Rheopump print head

Figure 21: Principle of the MPM Rheopump print head

Paste is ‘pumped’ into the stencil aperture onto the circuit board pad, then sheared from the main body of paste by the trailing edge blade. The blades on this head are mounted at 45° and made of specially coated metal, using Permalex technology from Transition Automation.

The MPM head has closed-loop feedback from the paste chamber to control the pressure applied to the paste feed. Capacitative sensors prompt the operator to supply additional paste when the cartridge level is low.

As with DEK, considerable effort has been put into the design and materials of the ‘side dams’, which fill the spaces between the two blades. The challenge is to eliminate paste wastage and build up of paste at the sides. MPM side dams are now made of a composite polymer material with improved wear characteristics, and the side dam pocket has been relieved to conform better to the blades. At the same time the blade and backup bar have been repositioned to protect blades from being damaged and provide better blade support at their interface with the side dam.

A key enhancement in the MPM system has been the development of the Variable Volume Actuator, designed to stop paste bleeding out of the pump at the end of the print stroke. The sequence of operation is:

MPM Rheopump head with Variable Volume Actuator

MPM Rheopump head with Variable Volume Actuator

You may notice that this is comparable to the technique used on some Archimedean dispense heads, where a small reverse action at the end of the stroke is used to stop droplet formation at the tip and possible ‘dribbling’.

Some pressure printing practicalities

As with dispensing, care has to be taken in material selection – not all pastes benefit from an enclosed chamber! In fact, certain paste formulations will even harden over time, this cold welding of the particles possibly being caused by a combination of a chemical reaction and the paste being pressurized. Also, depending on the head design, the paste may roll faster in the chamber than with squeegee blades, causing some paste formulations to shear thin faster than normal. On the positive side, however, costly agents added to extend paste life on the stencil may no longer be necessary.

Priming is the equivalent of initially charging the stencil before first use. Typically this is carried out away from the printer, with a removable sole plate in place, and with the head inverted to ensure that any trapped air is evacuated. When the print orifice is completely full, the head is primed and can be inverted and positioned on the stencil. Then, as with squeegees, sealed heads may take two or three passes initially for optimum print settings to be achieved.

All the designs allow for removal and storage, so that the paste can be removed from the printer, resealed and stored under optimum conditions during intermittent printing.

Cleaning the head can present some difficulties, generally requiring mechanical paste removal followed by immersion in cleaning fluid. The ease of cleaning varies between designs: for example, the MPM head has a push rod which extrudes paste from the chamber.

With the traditional squeegee, different sizes of board are accommodated by selecting a squeegee and applying the appropriate extruded length and volume of paste from the cartridge so that the paste roll is around 25-50mm wider than the board. Where necessary, polymer squeegees can even be cut to length. With sealed paste systems, this becomes impracticable, so heads are supplied in different widths.

As with squeegees, a skim of paste on the surface may be left on any unsupported areas and there can be more side overlap between head and pattern, especially with fixed sizes of head. In addition, the paste contact area is much greater, with the result that there is a higher total downward force on the board. For these reasons, the board generally needs to be supported better and over a wider area than with squeegee printing. Where a rail system is used for holding the board, tooling has to support the print head at each end of its stroke.

DEK experience suggests that there are two differences between sealed paste systems and conventional squeegee printing.

Results from pressure printing

Tests have suggested that pressure printing can be carried out more repeatably at the higher print speed range of the paste. However, like printing with squeegee blades, the speed attainable is directly dependent upon the type of paste being used. Higher viscosity pastes may require lower print speeds and possibly higher print pressure.

Typically, improved filling pressure means that fine pitch parts can be printed at higher speed, giving higher throughput, although the rate remains material dependent. Faster cycling is also aided by:

Of the other advantages claimed by sealed paste systems over conventional squeegee printing, maintained solder paste quality and reduced paste wastage is the most substantial and immediately quantifiable benefit. Because the paste doesn’t come into contact with air, there is no drying out and crusting of paste on the stencil, and little waste during changeover, shift-change, clean-up and down-time. Although the actual performance will depend on batch sizes and printer idle time, scrap figures in the range 0.5% to 2% have been reported, as against as much as 30-50% for conventional squeegee printing.

Not only is there a direct cost saving, but the costs of disposing of hazardous waste are cut, and operator exposure to solder materials and solvents minimised. A valid point has also been made that the reduced amount of superfluous paste leads to a cleaner, safer printer and generally results in a reduced requirement for maintenance.

Other advantages include:

MPM results at 0.3mm pitch

MPM results at 0.3mm pitch

Self Assessment Questions

What advantages would you expect to get if your assembly house announced that they had just purchased a pressure-printing system? And what changes might you as a designer need to make?.

show solution