Flow in practice


Introduction

In this section we are specifically reviewing the methods of depositing solder paste and glues, but you might wish to extend your research into the latter topic, because the issues are related to the way that polymers are used for encapsulation purposes.

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Solder paste

For solder paste, the flow behaviour directly affects the way in which the paste is deposited, and therefore:

In paste, the solder particles are homogeneously distributed in an organic carrier phase (or ‘matrix') with or without the aid of dispersion agents. There are many coating materials, such as non-drip paints, which exhibit shear thinning and thixotropy, and which contain an organic carrier phase and dispersed filler particles.

However, in these the carrier usually has a yield value high enough to counteract the gravitational force on the particles and keep them suspended, whereas in solder pastes, there are a number of factors which make attaining the desired paste rheology more of a challenge.


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Applying paste

There are four methods in general use for applying solder paste to a board:

  1. pin transfer

  2. dispensing

  3. screen printing

  4. stencil printing

Each method needs different paste flow characteristics and correspondingly different formulations. Table 1 gives a set of representative specifications for metal content and viscosity which is frequently seen in the literature. Remember, however, that these figures are very much simplified and that, for example, satisfactory pastes for stencil printing can be formulated with a wide range of metal contents.

Table 1: Paste metal content and viscosity for different application methods
nominal metal content (%) range of viscosity (Pa.s) application method
90 800–1,000 stencilling
88 600–800 screen printing
86 400–600 screen printing/dispensing
81 300–400 dispensing
75 200–300 pin transfer

Pin transfer

Systems have been developed for solder dot placement using multiple pin transfer. The pin array is first immersed to a controlled depth in a level trough of solder paste and then touched down on the pads of the circuit board. The size and design of the pins and the properties of the solder paste determine the quantity of paste transferred.

The paste selected for this placement method must be sufficiently tacky to stick to pins during transfer, but also be fluid enough to rapidly refill the gaps in the trough left by the pins. The trough can be made to move slightly between each pin pick-up, in order to reduce the latter problem.

Dispensing

The solder pastes formulated for syringe dispensing have different thixotropic agents, to ensure that flow occurs only when pressure is applied and that the deposited dot has a sharp cut-off with no stringing of the paste from point to point. The solder particle size needs to be more tightly controlled and of a finer size distribution than is normally necessary for screen printing, in order to avoid clogging of the syringe tube.

Most automatic dispensing machines use syringes pre-loaded by the paste manufacturer. Syringes are usually made of rigid polyethylene or polypropylene which will not bow or bulge, so that, at normal working pressures, the piston is both air-tight and paste-tight. Great care must be taken to exclude any trapped air which would otherwise cause ‘misses’ in dispensing and continued dispensing of paste after pressure has been released.

Printing

The use of a screen printer, predominantly with a stencil to define the pattern, is the usual method employed, and this will be covered in detail in Board assembly.

The complexity of the printing process is demonstrated by the number of forces involved in paste transfer:

So solder paste needs careful formulation to ensure printability, because during printing a series of motions occurs:


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Slump

After solder paste has been printed or dispensed, it tends to slump and spread, because it is a fluid. This spreading may cause solder bridging between the pads and inadequate solder joint stand-off height. The degree of ‘cold slump’ depends on:

Some cold slump may also be seen with dried pastes which are left for an extended period, in the form of ‘crumbling’ at the edges, caused by loss of solvent.

A second type of slump – ‘hot slump’ – is also generally recognised. This occurs during the preheat part of the reflow cycle, where the flux vehicle is more mobile and may be less able to keep the heavy solder particles in suspension, so the paste tends to spread or slump due to the effect of gravity.

Self Assessment Questions

What processes are available for applying solder paste to an assembly, and how do these differ in the requirements they make on the flow characteristics of the paste?

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Adhesives, fluxes, solder mask and legend

Pin transfer

This process, also referred to as ‘dip and dab’ or ‘stamping’, is conceptually very simple – a tool is dipped first in the adhesive or flux to be applied and then onto the work-piece. Whilst the first few applications may be slightly short of material, the process soon reaches equilibrium. All that is needed is a constantly renewed surface of known adhesive thickness into which to dip the tool, and consistent timing for the application part of the process. On typical machines, the even layer of material is produced using a doctor blade held over a rotating pot – the blade retains surplus fluid in the well behind it.

Though apparently crude, the process has been used extensively for applications where only a small amount of adhesive is needed. It is particularly useful when glue dots of substantially different sizes are to be applied simultaneously, and in its automated form is most commonly found on die placement equipment. Reduced to its absolute essentials, the ‘dip and dab’ method will be found useful when trying to apply very small amounts of resin prior to component placement during manual assembly.

The key to pin transfer is to have a material of the right viscosity. Pin transfer, usually automated, has also been widely used for applying flux to small assemblies, such as flipchips and Chip Scale Packages. The success met with has been somewhat variable, with the thickness deposited being variable.

Stamping processes are also used to transfer ink for component coding. A typical process uses a steel blade to wipe a thin layer of ink into etched cavities, from which it is removed using a silicon tampon, which is then pressed onto the surface to be marked. Clearly in this case the material needs to have an appropriate viscosity and surface tension which will allow it to end up in the right place! Typical marking inks contain either adhesives or surface etches to aid this.

Printing

Printing, using either screen or stencil, is perfectly possible for polymeric compounds. The only provisos are that compounds used have sufficient working life, that they contain no solvents which might adversely affect the screen or stencil, that the particles of filler used are of appropriate small size, and that there exists some means of cleaning which is compatible with screen or stencil.

Printing is becoming increasingly used for applying chip attach adhesive to assemblies which are to be wave soldered. However the process is without problems. Ideally one wants a ‘pile’ of resin which can be deformed during device placement to ensure that both chip and board are in contact with the glue, but stencil printing typically leaves a flat top surface. To overcome this, a number of proprietary techniques are employed such as printing with the stencil slightly off-contact and using special stencils. This is an area where printing competes with dispensing, and making the correct choice is something that requires good liaison between designer and assembler.

Printing through patterned screens is also the traditional method by which printed circuits were made, by printing etch resist. Nowadays in board fabrication the main uses of this sort of printing are in depositing legend and peelable solder mask. The process parameters are different in these two tasks: the first requires a thin, well-defined print; the second needs a very thick deposit.

In each case, the materials are designed to exhibit shear thinning and thixotropy, so that they will flow well during the print stroke, yet retain their shape once the screen is removed. Particularly in the case of peelable solder mask, cosmetic reasons suggest that the material be chosen so that it will ‘level’ after printing, so that it will form a slightly flatter surface without mesh marks, but not of course spread too far over unwanted areas.

Printing through blank screens, to give an overall coat of medium, is increasingly being used for solder mask deposition on account of its cost-effectiveness. Carried out with vertical board and vertical screens, and some means of confining the medium in contact with the screen, it is possible to print simultaneously on both sides of the board. However, this creates problems in handling boards until the solder mask has been cured.

Printing is also used for patterning polymer conductors and resistors, both for making thick film circuits and for embedding components within conventional boards. Yet other materials are deposited on glass as part of an LCD display.

Self Assessment Questions

Identify as many as you can of the fluid materials other than solder paste that are used in fabricating and assembling boards, and where the way that they flow has an impact on the processes used to apply them. What are the main processes used for each?

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