Alternative techniques and off-board assembly


Introduction

So far you have studied most of the processes involved in making a board, from solder paste printing through to wave soldering. However, we haven’t yet looked at more specialised solder assemblies, nor at the important area of ‘off-board assembly’, the combining together of boards by mechanical means, including wire attachment.

We shall first be considering the common manufacturing challenge of a double-sided design that uses mostly surface mount components, but has just a few through-hole parts such as connectors. Unless it is possible to design the board underside to be suitable for overall wave soldering, the traditional options for soldering through-hole parts on such a board have been hand soldering, selective wave soldering, or single-point automatic soldering. Having outlined the issues for all of these, and realised that they are relatively costly, we will be examining alternative ways of automating the process of applying solder just to selected areas.

We will then be looking at ways of making the soldering process more efficient and cost-effective, before concluding with a brief review of the more mechanical methods of attachment. Note that this last item is not intended to be more than an introduction to what is in practice a complex area, especially for the production engineer.


Hand assembly

Components mounted after machine soldering and cleaning, because they are unable to withstand soldering heat, or because their constructions are not sealed against ingress of flux or solvent, are often referred to as ‘non-wets’, and the operation is sometimes called ‘back-loading’. Examples of such parts are batteries, connectors, electrolytic capacitors, power devices, quartz crystals, relays and switches. These are usually, though not always, of through-hole construction. For these, hand soldering will be required.

Hand-soldering station

Hand-soldering station

Modern component leads are normally plated, which allows easy soldering without pre-tinning. However, if components are stored for too long a period, the plating can become sufficiently oxidised to prevent good soldering. There are standard methods of test, using sophisticated equipment, but a simple test is to hold the soldering iron bit to one side of the wire and the solder to the opposite side. If the component lead is clean enough, the solder will melt and flow along the wire as the iron and solder are moved simultaneously along the lead. If the lead is contaminated or oxidised, the solder will blob and not flow, and not tin the lead correctly.

The tip temperature is checked by using an electronic thermometer, a digital instrument based on measuring the voltage from the thermocouple junction, in the same way as for reflow oven profiling. Whilst many soldering irons have a means of adjusting the tip temperature, it should not be assumed that the temperature achieved is always that indicated. There may be systematic errors, which can be corrected by calibration, but a more frequent cause of temperature variation is the thermal lag between the heater and the iron tip. If the work needs heat, then the tip temperature is depressed until the sensor has detected the loss of temperature and the heater has been able to compensate. Thermal delays between heater, sensor, and tip mean that in a practical soldering situation the temperature first dips and then over-recovers, reaching stability only after some time. It is thus particularly important to select a tip size that is adequate for the task undertaken.

When hand soldering, use the correct gauge of solder to apply sufficient solder to the joint in between 3 and 7 seconds. If too fine a gauge is used, the dwell time will be too long, and too heavy a gauge of solder may produce flooded joints, or ones which are not properly wetted.

The soldering sequence is to pre-tin the iron tip, apply the soldering iron to the joint, and then apply the solder to the other side of the joint. Keep the iron on the joint until the heat melts and flows the solder round the joint. With heavier gauges of wire, it may be necessary to run the iron around the joint to improve the flow, although at no time should the solder come in direct contact with the tip. The solder is removed from the joint first, followed by the iron, once the solder has completely flowed.

For axial components the body of the component should be flat to the board and the wires should not be bent too sharply to avoid fracture. Components such as metal can transistors and diodes, where the lead goes through a glass seal, are prone to damage when the lead is cut, as this action creates a destructive shock wave.

Normally all component leads are cut to length before soldering, as this prevents there being a bare end to the lead. Care has to be taken during pre-forming and cutting to allow a sufficient length of wire protruding from the through-hole. Normally 3 mm is regarded as a maximum, but there is more of a problem with leads which are too short – if the wire is not clearly visible through the joint, then how do we know that it has been soldered? All standards require the joint to be visible.

How do you know whether these pins have been wetted,
or just covered with solder?

How do you know whether these pins have been wetted, or just covered with solder?

Care is also needed to clinch the component wire to the right angle and in the right direction relative to other joints. Figure 1 indicates how incorrect clinching can have an effect on the incidence of bridging defects after wave soldering, and the potential for bridging also exists with hand soldering.

Figure 1: Importance of correct clinch angle and bend direction on adjacent leads

Figure 1: Importance of correct clinch angle and bend direction on adjacent leads

Where the lead is flimsy or not supported, for example in the case of a flying lead, then it is particularly important to make sure that the joint is made mechanically before solder is applied. Otherwise, it is possible for the lead to move during the process of joint solidification, producing what is called a ‘disturbed’ joint, which may be electrically intermittent or entirely open circuit.



Selective machine soldering

There are a number of designs when one wants to use a machine soldering process, but needs to confine it to just a section of the board. For example:

The first two of these requirements are normally met by temporary solder masks or physical barriers, both of which mask off areas of tracking and pads from the solder wave. For the third, custom pallets are commonly used, but there are an increasing number of selective soldering solutions.

Temporary solder mask

There are three approaches to providing a temporary solder mask:

Removing peelable solder mask from boards

Removing peelable solder mask from boards

Peelable masks should peel off easily from the surface and the holes, but there is often a problem in achieving complete removal: a dotted line of residual mask may be seen around the perimeter of the peeled area; material may be left in via holes. The nature of such adhesive residue is critical: it must either not be detrimental to circuit function, or else be dissolved completely in a cleaning process. Inspectors are rightly suspicious of residues in plated holes: even if non-conductive, they may trap flux and contamination during subsequent assembly. The greatest problems are seen with liquid masks, as these penetrate the holes and vias to give a mechanical key.

Unfortunately, some resins can become very difficult to remove once they have been subjected to reflow temperatures, so it is important to make sure that the mask material has been designed to cope with the double soldering process. Improved results have also been reported from buying boards with the peelable mask a little under-polymerised, so that it reached the right physical properties only after assembly.

An alternative is to apply a selective mask post reflow/pre wave. This can be done by hand, or in volume by using selective coating equipment (dispensing or spray).

Peelable masks vary widely in their ionic properties and flux absorption characteristics, but should always be removed either before or during the cleaning operation. Not only may the mask itself degrade, but flux tends to become trapped underneath. Whether the resulting contamination originates from the mask or the flux, corrosion during life may be the outcome.

Quote

A point I would like to make is that people don’t pay much attention to materials that are not part of the final assembly. Why worry about the latex mask? I’m just going to throw it away anyway. Well, they can have detrimental effects. The same thing applies to other ‘temporary’ materials like water soluble solder mask, water soluble temporary spacers, and water soluble tapes. Just keep in mind that every material has an effect, and every material has to go somewhere.

Doug Pauls on TechNet, 23 August 2000

Physical barriers

There are many mechanical ways of restricting solder flow. Where only small areas of the board are to be screened off, three solutions are:

However, the custom pallet may well be the preferred way to assemble the increasingly common designs where only a few areas are to be wave soldered. The pallet screens most of the underside, with apertures for the solder areas, and internal relief to allow flush fitting on top of SM components. The windows need careful design, with chamfered edges to aid solder access.

Selective soldering equipment

There are a number of ways of automating selective soldering, the simplest of which (at least in concept) is to use a robotically-controlled soldering iron, where solder and iron are applied sequentially to the joints to be attached. This is most suitable for applications where only a few joints are needed. Where more joints are to be made, some alternatives are illustrated below:

Pin transfer

An array of transfer pins with a concave top surface, mounted on a carrier plate, is submerged in a solder bath. When raised, each pin holds a solder ball, which is applied to the joint from below, so that all joints are made simultaneously in one cycle. The transfer pin remains in contact with the solder bath, and this ensures adequate heat transfer to the joint. Flux is applied by transfer pin or spray. This process is relatively fast, but needs custom jigging, so is best suited for high volume applications.

Figure 2: Pin transfer method for selective soldering

Figure 2: Pin transfer method for selective soldering

after Streckfuss

Mini-wave

This is a more flexible process, based on a CNC controlled transport system which moves the board sequentially over preheat, spray fluxing and solder stations. Flux is sprayed selectively, but the whole board is preheated, to prevent warping. A nozzle directs a fountain of molten solder onto the joint area, and the overflow falls back into the bath along the nozzle sides. Given a programmable nozzle and conveyers, product changes are easy to make, requiring only a new program.

‘Solder fountain’ used for selective soldering

‘Solder fountain’ used for selective soldering

Figure 3: Mini-wave method for selective soldering

Figure 3: Mini-wave method for selective soldering

after Streckfuss

Stencil dip

This process is a variant of the custom pallet system, but requires just two stencils, one for flux spraying and the other for soldering. The soldering stencil is made in heat resistant material and completely covers the circuit board, with apertures corresponding to the solder joint locations on the board. The assembly is lowered onto the soldering stencil, which is then lowered onto a soldering area created in a solder bath by a pump and solder nozzle. Simultaneously, flux is sprayed on the next board. This system can be made highly flexible, provided stencils can be changed quickly.

Figure 4: Stencil dip method for selective soldering

Figure 4: Stencil dip method for selective soldering

after Streckfuss

Although wiper blades can be used to skim oxide from the solder surface, most of these processes benefit from the use of a local nitrogen atmosphere to minimise dross formation.

Self Assessment Questions

Review the material so far and list the methods described in it which can be used for applying solder to just part of an assembly. For each method, describe an application (that is, a type of assembly) for which the method would be particularly appropriate.

show solution



Intrusive reflow

The reaction of process engineers with a background in reflow soldering to the description of the many methods of applying liquid solder will probably be to throw up their hands in horror! Certainly all these techniques are expensive in equipment or jigs, or costly in labour. There is also some concern that they will lead to reduced yield and increased rework.

The alternative of using solder paste and reflowing all the components is attractive, and cost reduction and process simplification are major driving forces behind what is called ‘intrusive reflow’ or (more descriptively) ‘pin-in-paste’ or ‘Pin-In-Hole Reflow’ (PIHR). The process sequence for this is shown in Figure 5.

Figure 5: The intrusive reflow process route

Figure 5: The intrusive reflow process route

The proponents of PIHR would claim that it reduces the number of manufacturing operations and the cost of assembly by replacing hand or wave soldering operations and the associated rework. It also involves just a single flux, which eliminates any problems of compatibility and cross-contamination.

The key prerequisite for PIHR is that components must be able to withstand the reflow conditions. Particularly as we move towards lead-free solders, whose higher melting points require increased zone temperatures, this can be a consideration. In fact some of the reluctance amongst assemblers to adopt PIHR is that connector housings made of materials that will withstand reflow conditions have to be made of more expensive base resin, and are consequently more expensive. Electrolytic capacitors also present a challenge.

Overall PIHR produces an effective solder joint. However, the solder volume is limited to that deposited as paste, and is consequently generally leaner than the normal wave-soldered through-hole pin (Figure 6). Visual standards may need to be reconsidered and staff retrained. An example of the results of the intrusive reflow process can be seen in the associated video. VideoClip1 Intrusive reflow of axial components.

Views from both sides of a Pin-In-Hole-Reflowed connector

Views from both sides of a Pin-In-Hole-Reflowed connector

Figure 6: Different profiles of wave-soldered and PIHR joints

Figure 6: Different profiles of wave-soldered and PIHR joints

The main issue within intrusive reflow is getting enough solder volume. It is possible to calculate the amount needed from the dimensions of the aperture, making allowance for the fact that only 50% by volume of the paste converts to liquid solder, the balance being flux – though only 10% by weight, flux is substantially less dense than solder.

We can deposit more paste by allowing paste to penetrate into the hole during the printing process. The degree of penetration depends on the printing speed, the width and angle of the blade, and the squeegee pressure. With the correct settings penetration of 45–85% can be achieved, but the settings are important to ensure that there is not excessive bleed onto the underside of the board, which will cause contamination to spread from board to board (Figure 7).

Figure 7: Penetration of paste into through holes

Figure 7: Penetration of paste into through holes

Given that just pasting into the hole will generally not give enough solder, we have to consider how else to create a greater solder volume. The simplest way is to expand the deposit beyond the periphery of the copper track, relying on the fact that, when melting, solder will withdraw from the solder mask and form part of a single joint. [Note that, if solder paste is to be printed on solder mask, then the correct material for the application must be selected] This pull-back is shown in the linked video clip, which also indicates that there is little margin between satisfactory joints and ones where bridging occurs. VideoClip2: Intrusive reflow of connectors, showing potential for bridging.

An alternative for increasing the deposit, shown in Figure 8, is to expand both diameter and length, and this is frequently done on connectors and for other applications where leads are closely spaced in one direction. Where yet more solder is required, but there are two rows of pins, then the ‘teardrop’ design is appropriate. Other assemblers prefer to use square deposits, seeking to maximise the area of the solder paste print.

Square paste deposits

Square paste deposits

Figure 8: Options for different aperture designs

Figure 8: Options for different aperture designs

Of course there are some components, such as connectors with multiple rows of pins, where none of these approaches can be used to give sufficient solder paste to form satisfactory joints. The alternatives are then to dispense additional solder paste where required, as a separate operation, or to use a multilevel stencil as shown in Figure 9. The multilevel stencil, typically produced by electroforming, is particularly useful where there are both through-hole and fine pitch components, because the technique allows the fine-pitch areas to be created in a thinner foil, with PIHR components provided with a thicker deposit.

Multi-level stencil

Multi-level stencil

Figure 9: Schematic cross-section of a multi-level stencil

Figure 9: Schematic cross-section of a multi-level stencil

Another factor to be kept in mind is the pin projection. As suggested in Figure 10, the pin projection should be kept between 1.0–1.5mm, so that excessive paste is not displaced by the lead, to be lost to the final fillet.

Figure 10: The impact of lead projection on the paste deposit

Figure 10: The impact of lead projection on the paste deposit

The problem with visual standards has already been noted, but there is also an additional problem at the test stage that the insertion process can lead to some build-up of solder on the pin tip and to flux residue on the pins, which means that it is recommended that pins should not be used as test points.

Hole paste displacement by inserted leads

Hole paste displacement by inserted leads

Self Assessment Questions

Your latest design has only a small number of through-hole components: a connector, a relay, a quartz crystal and a toroidal transformer. Critically discuss the assembly options available to you.

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Some design considerations for reflow

Through-hole components present a challenge for reflow, but then so do certain types of part that are intended for surface mounting. In this short section we are considering just two of these, the particular challenges produced by small components, and by screening cans, bus-bars, and similar components requiring a continuous solder fillet.

Small components

With small components, great care has to be taken with pad design in order to avoid problems with solder balling underneath the component. Sometimes this will produce an immediate short circuit; more seriously, there can be longer term reliability issues. The problem starts (Figure 11) when a component is pressed onto two piles of paste, with the result that some solder paste is displaced underneath the body and does not subsequently reflow. This can be seen clearly using X-ray techniques, but is perhaps not as visible by standard inspection, there being insufficient solder to form the traditional ‘solder bead’.

X-ray image showing the formation of solder balls

X-ray image showing the formation of solder balls

Figure 11: Impact of aperture shape on solder balling

Figure 11: Impact of aperture shape on solder balling

The key solution is to modify the aperture for small components in some of the ways shown in Figure 12. All of these aim to make enough solder available on the outside of the joint, whilst minimising the free solder paste that might otherwise create solder balls. Procedures of this type are not always necessary with 0603 chip components, but, as we move towards small sizes, assemblers will often have preferences for modified pads. This is another area where you should consult with your partners.

Figure 12: Footprint modifications to reduce solder balling

Figure 12: Footprint modifications to reduce solder balling

Parts with continuous fillets

The challenge with mounting screen cans comes when we try to reflow them rather than hand solder – creating a continuous seam with a soldering iron is something that is well established as a hand process, and can even be seen on your tin of corned beef!

With reflow, however, it is not possible to create a continuous band of solder paste, since the mask to do this would have an unsupported central section that would fall out. The normal way of treating this is to interrupt the continuous deposit with ‘webs’ in the stencil, sufficient in number to ensure that the central section stays firmly attached and can itself produce correct prints of paste for the components inside the screening can. However, this interrupted pattern usually results in the connection between can and metallisation not being continuous. One way of overcoming the problem, which is in fact a general way of creating arbitrary shapes is to create a modern equivalent of a screen. You will recall that traditional silk screens have a stencil mask made of polymer supported by a continuous mesh. In this case, the ‘mesh’ is provided by electroforming an equivalent pattern, but only within selected stencil apertures.

Figure 13: Stencil design for RFI enclosures

Figure 13: Stencil design for RFI enclosures

A second difficulty with any vertical metallised surface is keeping the solder where it needs to be. Problems such as the migration of solder paste up the side walls can create both poor fillets and voids and blow holes (Figure 14).

To overcome the problem Tecan has developed and patented a ‘reflow Plimsoll line’, where a transition in the plating on the can, seen as a coloured line, creates a barrier to solder, so that the fillet is created only below this line. This is a modern equivalent of keeping solder in position by marking a piece to be hand-soldered with a soft graphite pencil. For more information on this technique visit the Tecan website at http://www.tecan.co.uk/.

Can soldering showing paste migration (LH), compared with optimised stencil and can (RH)

Can soldering showing paste migration (LH), compared with optimised stencil and can (RH)

Figure 14: ‘Plimsoll line’ reduces the impact of paste migration

Figure 14: ‘Plimsoll line’ reduces the impact of paste migration

Simultaneous double-sided reflow

So far we have considered ways of automating a range of soldering processes, and the move towards using a double reflow process, applying additional solder paste where necessary. However, this is still a procedure that involves two heat cycles, and the operating cost of reflow equipment is high, both in terms of energy used and time taken. Something similar can be said of the reflow plus wave process – again a double process, with cost and yield implications.

One simplification proposed was to reflow both sides of the board simultaneously, but the clear concern here is how to retain components on the underside. Whilst sufficient surface tension is available in molten solder to prevent underside components dropping off on second-side reflow, this assumes that the solder joint has already been made; solder paste has little tack during the pre-heat phase.

The solution to simultaneous double-sided reflow is to use an adhesive to retain the component during the solder cycle, in the same way as we would for wave soldering. But of course we need to cure the adhesive in order to give it some strength, and this has to be done after the components have been assembled, and by implication when solder paste has also been applied to the board. So how do we cure the adhesive whilst not changing the properties of the solder paste?

The solution to the problem of providing at least a temporary cure by means other than heat was (at least apocryphally) generated in the early hours at the bar of the SMART Brighton conference in 1998! It involves using materials intended for chip bonding, where initial strength is given by exposure to ultraviolet light, and the final cure carried out in the pre-heat stage of reflow. Note that the glue deposits have to be positioned towards the outside of the package, so that at least some of the material may be initially cured by UV.

Choice of materials is critical for this application. It is not just a question of using ‘superglue’, as these adhesives dispense poorly and are difficult to control! There are competing considerations within the formulation, in order to allow the material to cure fast with a moderate ultraviolet exposure level, yet also cure by heat alone, so that material in the shadowed area becomes fully cured during reflow. There is the complication, for example, that some of the resin is protected from oxygen by the component, whereas other sections of the material have free access to the atmosphere – thin films take longer to become tack-free because the reaction is inhibited in the presence of oxygen.

One of the difficulties in any experimentation is devising a reproducible and standardised test, in this case to ensure that there is sufficient adhesion. The work carried out by Loctite evaluated the wet strength using the sliding impact test for the acceptance of SMT adhesives defined by Siemens SN-59651: there is a similar Bellcore test TR-NWT-000078.

Exercise

We have concentrated on the materials aspect of SDSR, but not considered how to apply both glue and paste to the same side of the board, in preparation for device placement. How do you think this might be done?

show solution

SDSR is a viable process, particularly when the components on the underside do not include large parts, because the materials and processes are fairly well understood. It also has the demonstrable benefit that the solder joints on the underside are only reflowed once, so that both sides of the board have a minimum of intermetallic growth. However, it does create complexities of its own, and the need for additional processes, so is not yet universally accepted.



Area arrays

So far in this module we have only lightly touched on the topic of area arrays. You will recall from the section on components that these offer substantial advantages over quad flat packs, allowing more connections within a given area and at the same time placing these connections further apart. In fact, so much progress has been made with the original 1.27 mm pitch style that this is almost a ‘standard’ component, requiring no special facilities other than a rework station and some X-ray capability in order to assure quality and deal with any problems.

Given these advantages, it is not surprising that the BGA format has been continuously extended both in the number of leads and in reducing the volume of the package. Not only has the ‘tape BGA’ package enabled the BGA to become thinner than the original OMPAC, but the grid spacing of the balls has been successively reduced, together with the ball diameter. The resultant package is now often little bigger than the die inside, and the ball size and associated process and reliability issues is little different from the ‘flip chip’, where the solder bumps are attached directly to the silicon chip itself. Detailed consideration of the flip chip will be given in Technology Awareness, but if you are interested in a preliminary view of this technology (and have time at this stage in your studies), there is a very useful resource at http://extra.ivf.se/ngl/B-Flip-Chip/ChapterB.htm.

Elsewhere in this module we make the point that the reliability of a joint depends on the stress applied to it. In the case of two materials of different CTE, joined together by pillars of solder, the stress in the solder will be a function of:

This last is particularly important – you may recall IBM’s purpose in developing the column BGA was to reduce the stress caused by mismatch between the ceramic body of the part and a laminate substrate.

So how do we deal with components with small balls, where both connected area and stand-off have been reduced? The answer lies in applying an adhesive between the surfaces, which spread the stresses caused by differential expansion, and reduces the stress applied to individual solder balls. This process is referred to as ‘under-filling’, and is generally carried out after the devices have been tested electrically, in order to make the rework task possible.

A range of resins are available for this purpose: all need to be very pure, and exhibit good adhesion, with compatible CTE. Another desirable property is that they should not be too rigid, so that they can comply with the joints as they expand and contract.

Whether or not under-filling is needed by any particular application will depend primarily on the physical characteristics of components and board and on the expected environment of the end-product. If the requirement involves continued temperature excursions during life, then under-filling is recommended. Under-filling is also advantageous if it is anticipated that assembly will be subjected to mechanical stresses during life, as these have a comparable effect to the strains imposed by thermal mismatch. Mobile phones are an example of such a product.

But how do we get the adhesive between components and board, given the small spacing? A few of the materials used are applied before solder assembly, and combine the functions of flux and underfill. However, this method often produces voids in the interface, which can make local stresses worse. Probably the preferred option is to apply the resin after soldering, making use of the fact that capillary attraction will draw fluids into the gap.VideoClip3: Underfilling operation on transparent test flip-chip, showing capillary action. This requires adhesives of suitable viscosity, containing small filler particles. Unfortunately, resins with suitable CTE and thermal conductivity require substantial amounts of filler, and much development work has been needed to produce underfills where the filler particles are not trapped at the edges, leaving the material at the centre of the chip relatively unfilled and with a higher CTE.

Given the right material, and considerable advances have been made over recent years, a package can be underfilled in under 5 seconds. Material is ‘piped’ along one edge (or two adjacent edges), and allowed to be pulled underneath the package before subsequent applications are made. Depending on how far one is prepared to allow the resin extend beyond the package boundary, three or four applications of resin may be needed, with a final ‘piping’ of resin around the periphery to create a proper fillet.

Self Assessment Questions

Explain to your Production Manager why s/he might need to invest in a machine for underfilling µBGA packages.

Show solution


Mechanical attachment

In this section on mechanical attachment, we are considering only forms of mechanical attachment where an electrical connection is intended. This might be a simple loop in the bare end of a wire placed underneath a washer and secured by a bolt, or considerably more sophisticated, as in the case of a press-fit connector.

However, the basic principles are the same – in order to get an electrical connection, there must be intimate contact between the mating parts, so that the electrical connection is not compromised by the build-up of oxides or other surface films. Typically this involves what is often referred to as making the connection ‘gas-tight’, where deformation and pressure combine to create an interface protected against the environment.

There are many types of joint, and correspondingly a number of ways of ensuring that this metal-to-metal connection is secure. As an example of what might be involved, consider the stages involved in making a good connection to your car battery – this involves cleaning the surfaces, applying as much pressure as possible, and finally coating the surfaces to protect them from environmental attack. Not every electronic connection is quite as gross as this, but the general principles are worth bearing in mind.

Also be aware that with all mechanical connections the integrity of the metal-to-metal contact, which often is almost a diffusion weld between the two metal surfaces, is compromised if the bond is broken and then an attempt made to remake it.

In the section that follows, we are considering three types of gas-tight connector. In crimping, the internal barrel of a metal part is pressed against a bare wire with sufficient force to create a mechanical link and break through the oxide layers1; the insulation displacement connector (IDC) ‘cuts through’ both insulation and oxide; the press-fit connector uses interference between the inside of the hole and a pin in order to create local points of bare contact.

Microsection of satisfactory crimp termination

Microsection of satisfactory crimp termination

Crimping

In a crimped joint, a stripped wire is inserted in a metal sleeve, which is then distorted by pressure so as to make an internal contact between wire and sleeve body. The sleeve may be part of a connector pin or some other style or terminal, but the same general requirements apply to the crimped joint.

There are many types of crimp tool (Figure 16) available, the mechanical form depending on the force needed to distort the crimp barrel – this can be quite substantial for heavier cables. In all cases a crucial requirement is that the tool chosen is correct for the type and size of crimp tag being used, and this is selected according to wire size. Only by getting a correct match of wire, tag, and crimped tool will a satisfactory gas-tight and reliable joint be created.

Figure 15: Basic crimping tool

Figure 15: Basic crimping tool

The sequence of making the crimp is to strip the insulation from the end of the wire using wire strippers, and taking care not to nick any of the wire. As with the crimp tool, correct selection of the stripping head is important. The aim is to leave a multi-strand wire in a neat bundle which may be inserted easily into the sleeve. Some slight twisting action may be needed to ensure that no whiskers are left, as this is a frequent cause of short circuit between crimped joints. The crimp barrel on the terminal or connector is placed over the wire and the jaws centrally positioned over the barrel, with the jaws square and central. In a hand tool, the crimp is normally made by squeezing the handle and the tool released by squeezing the handle still further, after which the handle can be opened and the wire and crimp removed.

Wire insulation stripping

Wire insulation stripping

Crimping of termination

Crimping of termination

Crimp tool and insulated crimp after crimping operation

Crimp tool and insulated crimp after crimping operation

All crimped joints need to be checked visually, and it is common practice to sample check tags to ensure they are correctly crimped. Just looking at the form of the crimp will give a good indication of whether or not it is correct. The crimp should be central to the barrel and square, with no whiskering.

Test crimps can be checked for insulation damage by pulling the wire from its tag and checking the insulation is only slightly deformed. If there is excessive damage to the insulation then the crimp is too tight and must be readjusted.

The conductor must be positioned correctly in the connector, and many connectors have an inspection window in the crimped area through which bare wire should be visible.

After crimping the connector should be free from fractures, rough edges and flash. Where conductors protrude through the connector ferrule, this projection should be a minimum, to bring it level with the connector insulation, and the gap between insulator and ferrule should be as specified.

Further information on generally-accepted standards for crimped joints is given in Figure 16.

Figure 16: Some crimping recommendations
The conductor must be positioned correctly in the connector. Many connectors have an inspection window in the crimped area. If bare wire is not visible through this window, the joint is suspect. Figure 16: Some crimping recommendations
The connector must be free from fractures, rough edges and flashes. The conductors must protrude through the connector ferrule for a minimum length to bring it level with the connector insulation. Figure 16: Some crimping recommendations
The crimp must be in the correct position on the connector to give maximum strength to the joint and to avoid damage. Figure 16: Some crimping recommendations
The insulation gap must be as specified. Figure 16: Some crimping recommendations
Many crimped connectors are also crimped to the insulation. The insulation must be held firmly. Figure 16: Some crimping recommendations
There must be no loose strands of wire. These are a common cause of short circuits. Figure 16: Some crimping recommendations

IDC

Insulation-displacement connectors were developed to provide a fast reliable means of terminating multiway cables. Like crimping, the connection is made (Figure 18) by pressure contact between terminal and conductor to break down intervening oxide (as well as ‘displacing’ the insulation around the conductor), using controlled methods of pressing together connector and cable.

Figure 17: Comparing an IDC connection (LH) with a crimped connection (RH)

Figure 17: Comparing an IDC connection (LH) with a crimped connection (RH)

Sometimes special jigs and presses are needed; in other cases the connectors are self-jigging (Figure 18).

Figure 18: Assembly of a typical IDC connector

Figure 18: Assembly of a typical IDC connector

Press-fit connectors

The rationale for using press-fit connectors, rather than solder, is four-fold:

The original (1960s) press-fit connector was a ‘force-fit’ type, a ‘crunch connection’ made by inserting a square pin in a round hole. There were inevitable problems with component tolerancing and board finish, with reliability on temperature cycling and after exposure to (corrosive) atmosphere, and with removal and reinsertion because of distortion of the through-hole.

Many attempts have been made to find a practical design which will provide a gas-tight connection, applying continued pressure between pin and board metallisation, and which will survive both the board stress-relieving itself over time and the variable stresses of temperature cycling.

Views from both sides of a press-fit connector

Views from both sides of a press-fit connector

Exercise

Carry out a web search on press-fit connectors and look for the different design approaches used to try and provide a reliable joint with acceptable insertion force and minimal board damage.

Examine the principle of operation for each, and the way in which the pins are distorted on insertion so as to maintain contact pressure throughout life, whilst making it possible to slide the pin in and out of the hole.

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Requirements for success in mounting press-fit connectors are:

Box build

Mechanical processes are at the heart of what is usually referred to as ‘box build’, that is the assembly of printed circuit boards with other elements to create a completed product. More and more this is the province of the contract assembler, who may even package units with manuals in individual cardboard boxes, and ship them to warehouses ready for sale to the public. The rationale for this is that it allows the assembler’s client to concentrate on design and marketing, without getting involved in any aspect of manufacturing.

Box build involves a variety of mechanical processes, securing parts with screws, by welding, by riveting or by clipping. We will be making the point in Design for eXcellence that the challenges here are to make products that can be assembled in one way only, as quickly and efficiently as possible, and subsequently disassembled readily into their constituent parts for recycling.

Torque-limited screwdriver used for mechanical assembly

Torque-limited screwdriver used for mechanical assembly

Riveting used to attach mechanical component to assembly

Riveting used to attach mechanical component to assembly

The main requirement for the box build stage is that the designer should:

What is involved in the box build task will very much depend on the intended volume of manufacture: procedures that are acceptable in the ‘craft’ conditions of a prototype shop may be totally unacceptable in a volume-manufacturing environment. Consideration there has to be given to simplifying the process stages, and making as much use as possible of mechanical aids. Rather than use individual fasteners, the product may be clipped together, welded together, or stuck together, using a variety of techniques.

Don’t forget in your design to allow for the effects of the physical environment. Will continued temperature cycling, or shock, or vibration, cause parts to become loose, so that connections go open-circuit or the whole assembly falls apart? There are also safety aspects to consider, such as clearances for mains voltages, and ensuring that children’s fingers can’t access dangerous parts! And, of course, you will take into account the nature of the final application, which may suggest rounding corners and including other safety features.

As you can see, box build is a minefield for the layout designer, and one where you would do well to take advice from the manufacturing professional.

Self Assessment Questions

What are the important factors to take into account when making electrical connections by mechanical methods?

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Manufacturing standards

So far in this module we have contented ourselves with indicating what good products look like in terms of components and solder joints, and given some examples of defects, but have not explicitly addressed the question of manufacturing standards.

Most companies will have their own preferences, but over the years there have been a number of attempts to create common standards, as this reduces costs and the potential for error. Many of these specifications started life with military establishments, but there has been a move to reduce defence expenditure by passing the responsibility to the manufacturer. As a result, the main manufacturing standards are those produced by IPC, or variants on these.

IPC-A-610 (The Acceptability of Electronic Assemblies) describes the standards for solder assemblies, in the same way as IPC-A-600 (Acceptability of Printed Boards) described acceptable standards for the boards themselves. More recently, this has been supplemented by IPC/WHMA-A-620, which describes standards for cable and wire harness assemblies.

With all these specifications, there is an emphasis on what the product looks like and can therefore be inspected for compliance to, rather than on the inherent quality or otherwise of the processes that made the product. Mere compliance with the manufacturing standards is insufficient, and any audit of a company will need to look also at the procedures for managing quality, in particular controlling individual processes.