Design for Product Build

Unit 6: Board assembly

If you are the sort of student who starts by printing the Unit text, you will be surprised that this page is relatively short, though the list of Unit contents is extensive. This disparity is caused by the fact that, as with Units 1 and 2, the text covers the basic principles, and there is a considerable amount of supplementary information about the processes and the way they are operated and controlled. We suggest that you read the main text first, and then dip into the additional material, not forgetting to tackle the SAQs in the linked documents. Throughout your studies, remember that our concentration is on how the processes fit together, how they are controlled, and the many influences on the final outcome.

Other supplements for this unit are Unit 1, which covered solder materials, and in particular solder paste, and Unit 2, where we considered the printing process and how a solder joint is made.

By the way, this Unit focuses on board assembly: the important area of ‘off-board assembly’, the combining together of boards by mechanical means, including wire attachment will be the focus of Unit 9.

Unit contents

The manufacturing sequence

We have already seen in Fabrication and assembly process outline that there are a number of different possible manufacturing sequences, depending primarily on the mix of surface mount and through-hole technology employed by the components specified. Figure 1 shows a typical range of process routes, for a range of assemblies where the top side is populated with surface mount components, using reflowed solder paste.

Figure 1: Process routes for main mixed technologies

Process routes for main mixed technologies

The same reflow process may be used for the under-surface, but, if there are any through-hole components, the underside also needs to accommodate the solder joints to their leads. We will see later that there are alternatives, but traditionally these joints have been made by immersion in molten solder, to survive which any surface mount parts on the underside need first to be attached mechanically. This combined process is shown as the “white route” in Figure 1, representing what has long been the most usual mixed-technology process sequence.

But bear in mind that there will always be variations, depending on the application and requirements. For example, one major company used to apply both adhesive and solder paste to SMDs on the underside, reflowing the paste and simultaneously curing the adhesive, before inserting through-hole parts and wave soldering. Although the wave-soldering process removed most of the solder from the paste applied to the underside, the fact that the solder paste had previously wetted the component terminations and board significantly improved the quality of the solder joints. In this case, the yield improvement paid for the extra process steps.

Figure 1 is very much a simplification, in that it indicates none of the control and monitoring elements that combine to create a quality process. As you read about individual processes, make a note of the ways in which the quality of materials influences the outcome and the ways in which the process is controlled both during the operation and afterwards, and reflect on the requirements for training everyone involved, to ensure that every process step is correctly carried out.

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Solder paste printing

Although solder paste printing always appears as the first process, we should not forget that the bare board has to be received, inspected and stored, hopefully in a protected and dry environment. Boards that are old, so that their solderability is impaired, or have been allowed to absorb atmospheric moisture, so that there is a danger of delamination during reflow soldering, may need a degree of pre-conditioning by baking before use. Depending on the form of automation used on the line, they may also need to be stacked in the board loader or into cassettes.

The paste deposition process to which the board is presented will use a screen printer, but typically nowadays this will be configured as a stencil printer, with solder paste deposited through a stencil of appropriate thickness and pattern definition. A typical stencil printer will use a laser-cut or electroformed stencil 125µm or 150µm thick, and a metal squeegee or (increasingly) a pressure printing head.

Recommended reading

More details on printing basics in our paper Screen and stencil printing.

 

Tuning the printing process for each application involves proper stencil design, appropriate choice of materials and correct setting on the printer, both as regards the internal printer settings and the registration of the print to the board. It cannot be stressed too much that paste deposition is a critical process, because we will only achieve correctly-soldered joint if we have the right amount of solder in the right place. As the first process in the assembly line, it is critical to the high-yield manufacture of printed circuit assemblies, and failures occurring at this point are difficult to correct later on, requiring time and skill, and adding to the total cost.

Figure 2: Typical volume production printer

Typical volume production printer

In an early 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 3), 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 3: 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, and this was the driving force behind many subsequent improvements in machines and materials. Whilst Mangin’s work has not been repeated directly, there is some evidence that the situation has improved as a result of the attention given. For example, if you visit http://www.ppm-monitoring.com/, you will see that the quoted results for screen printing are lower than for other parts of the process. However, if you look in more detail at the monitoring procedures, you will find that the results are based on only relatively few samples, and reflect the visual standard of the pasted board, rather than the resulting solder joint. That is not to take away from the value of this ongoing exercise, but serves as a reminder that the results you get reflect the question you asked! Certainly the observation that AOI is frequently used after paste deposition for critical applications such as column BGAs suggests that problems such as blocked apertures can still potentially cause defects.

Recommended reading

In the linked paper on Screen and stencil printing, our focus was on the general principles of the process, and not the practicalities. These are covered in our paper Solder paste printing, which covers some practical issues associated with printer operation and assessing the quality of the results. The information here on board support, alignment and under-stencil cleaning will offer some insight into why real printers are relatively complex and expensive, although the process itself is inherently simple, and one that may be carried out on a hand printer.

 

In the same way that real printers are more complex than the basic process suggests, real pastes are matched to the application and equipment, as you may remember from your earlier studies in Unit 1. Specifically, the flow characteristics of the paste have to be appropriate for the equipment, and the on-stencil life sufficient for the mode of operation. As we pointed out in Unit 1 under Practical solder paste issues, solder pastes vary considerably between suppliers, so it is not uncommon for process engineers to be very specific in their likes and dislikes.

Whilst appearing as smooth creams, real pastes are also slightly abrasive, especially with lead-free materials where the metal balls lack the lubricating quality added by the lead . In consequence, higher rates of abrasion are reported, so that stencil life may once again become an issue as it was in the early days of solder paste printing.

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Placement

Placement of a surface mount component is conceptually very straightforward, but is now an increasing challenge because of the wider range of components, from very small devices to large, fragile components requiring precision mounting. The aim is to select the correct component, and position it within close tolerance on the pasted board, applying sufficient pressure to ensure that intimate contact is made between component termination and paste, so that the part is kept temporarily in place by the “tack” in the paste and a good joint can be created during reflow.

With the move to lead-free pastes, with their inherent higher surface tension but lower wettability than lead-tin eutectic solder, precision of placement has become more critical, and there is little margin for error, especially with the very smallest chip components. For example, it has been reported that the incidence of tombstoning with 0201 and 0402 chip components is greatly increased if they are not exactly centred during placement.

Whether the task is carried out by hand, or (as is more usual) by machine, the process is one of supplying components in bulk to the placement station, picking up single components, and then placing them accurately. It involves identifying both the correct part and the correct location and orientation, it being important in the case of components such as active devices and electrolytic capacitors to ensure that the parts are mounted as intended and not at 90° or 180° to the design, which plays havoc with electrical performance! And so does assembling the wrong component . . . much attention is given by assembly houses to ensuring that parts are correct, with frequent checking, often with two operators involved, particularly when new reels of components are being mounted on the placement machine and at first-off inspection. Because the consequences of an error are very significant, and can affect whole batches, it is not surprising that placement equipment is increasingly fitted with some means of verifying part function, especially for smaller components that bear no direct part marking.

Recommended reading

Our material on component placement is split between two papers: that on SM component placement gives information on the ways in which components are fed to the pick-up point and vision systems are used to align component and board. These aspects are common to all types of placement equipment. However, as you will realise if you have visited an assembly house, practical placement equipment can take many different forms. These reflect the different design approaches taken by different machine manufacturers, as well as the requirements of different types of assembly house.

 

Traditionally, in volume manufacture, there was a distinction between “chip shooters” used for the very fast assembly of small components, and slower precision placement machines used for more challenging applications. These machines were typically set up for long runs, and the key metric was the achievable placement rate. Note that this was not the quoted placement rate for the machine, where manufacturers would give the theoretical result under the most favourable conditions, but a measurement based on the real situation, where placement parameters can be optimised, but it is not possible to mount all the reels in the position nearest to the pick-up head, nor to place consecutive components so close that the movement can be accommodated within a single machine cycle, nor to eliminate totally the time taken by reel changeover. Fortunately, quoting unachievable placement rates has been accepted by the industry as counter-productive, and a more realistic metric for placement rates has been devised by IPC.

In today’s changed market, large volume assembly is a less frequent challenge than is kitting a line with placement equipment that will allow small batches of varying products to be assembled cost-effectively and with minimal change-over time. The result is a greater emphasis on presenting components grouped together in a transfer trolley rather than mounting one reel at a time, and increasingly on using intelligent feeders that can be mounted in an arbitrary position, with the placement machine detecting both component identity and location. At the same time, single machines are typically used to mount all sizes of component, although larger components will still be placed at a lower rate, to ensure that over-fast movement between pick-up and placement does not result in unwanted movement with respect to the placement nozzle and consequent misplacement.

Supplementary information

If you have time, it is worth looking at the specifications for typical machines, and seeing how each machine seeks an effective compromise between placement rate, placement precision, and the ability to handle a variety of component types. Some comments on the major designs are to be found in our second paper on Practical placement systems.

 

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Reflow soldering

Recommended reading

The reflow process is comprehensively described in our paper Reflow soldering, which we suggest you read before proceeding further, so that you are aware of both principles and equipment associated with this key process. What the paper perhaps does not sufficiently emphasise is the need for the reflow process to be matched both to the paste and the application, in order to ensure that all joints have a thermal experience that is as close as possible to each other and to the optimum process for the solder paste.

 

The early stages of the reflow process may need adjustment in order to ensure that the flux is fully active by the time that the solder starts to melt, assisting the wetting of solder to component terminations and board, and allowing a joint to be created. And the central portion of the profile has to transfer sufficient thermal energy to the board to ensure that melting and wetting takes place. At the same time, the temperature experience of the components and the board must lie within their capacity to survive without deterioration, from the points of view of maximum temperature, time at maximum temperature and rate of temperature change. Components such as ceramic capacitors are sensitive to rate of change, whereas electrolytic capacitors and components with plastic mouldings, such as connectors, are sensitive to absolute temperature and the duration of exposure.

At the same time as the process must be adjusted to meet the requirements of flux and components, the overall process sequence will determine whether a second reflow process may be required, in which case the paste must be specified (and validated) to ensure that the flux remains sufficiently active during the second soldering process.

You may have noticed the emphasis on ensuring that the temperature experience should be as nearly the same as possible for all joints, but of course in a practical application this is not possible because of the different thermal masses associated with each joint. The purpose of profiling individual assemblies, rather than setting the reflow oven only once for all types, is to minimise the spread in temperature across the assembly. As one expects intuitively, larger components are slower to heat up than smaller ones, and the hardest assemblies to heat are those that are densely populated with components of high thermal mass. For some of these assemblies, a convection reflow system struggles to give a satisfactory temperature range (usually referred to as “Delta T”), and condensation soldering (the new name for vapour phase soldering) may be preferred as an alternative to running a slow and hot profile.

As with any assembly stage, many defects are attributable to factors within the stage. However, since reflow is the last process of an in-line single-sided SMT assembly line (except for any cleaning activity), many faults found at this point are either caused by (or at least influenced by) problems that originated at an earlier process, or are principally due to the materials, components or board used, or to aspects of the design. The total range of faults and causes is therefore not just those produced by incorrect implementation of the reflow process.

Faults that can be seen after reflow can be divided into five broad categories, some of which are illustrated below:

1 This classification is that of the principal reflow process cause for the defect. For example, whilst tombstoning is often an indicator of a placement problem, it is considered here as potentially being a ‘wetting defect’.

Figure 4: Solder balling simulation with solder paste reflowed on a ceramic substrate

Solder balling simulation with solder paste reflowed on a ceramic substrate

Figure 5: Solder short

Solder short

Figure 6: Non-reflowed paste

Non-reflowed paste

Many of the problems that become apparent after reflow soldering are due to the effects of surface tension, which results in the solder not being confined to the intended joint, but migrating down holes and up leads. However, the final effect in our paper Surface tension effects is the generally helpful movement of components that takes place as they “float” on the solder surface during reflow. The surface tension acts to reduce the surface area of the joints, which generally also centres the components, and is frequently relied upon to overcome minor placement problems. However, as lead-free materials wet less well than their tin-lead eutectic predecessors, the reflow process is becoming less forgiving.

The extent to which these problems are defects requiring rework will depend on the severity of the problem, the position of the circuit and the application. One problem that will always need rectification, and is sufficiently common to warrant having its own topic page is Tombstoning, a type of problem that has many possible causes and is a reminder that:

Supplementary information

Weeding out defects is not the only reason that we carry out a visual inspection. In fact, it is arguably more important to use feedback as a means of detecting processes that are going out of specification, or potential problems. Read our paper on Other solder-related failure causes for an insight into some of the potential problems that can be screened for at this stage.

 

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Insertion

Reflow soldering was developed for mounting SM devices, and the expectation in the 1980s was that component insertion would soon be consigned to history. In practice, many products still make at least some use of through-hole parts, although typically these are in the minority. The result is that, although automatic insertion machinery is seen less frequently than in the past, there is often a requirement for an assembly house to have at least some ability to mount through-hole parts.

With the use of through-hole parts being restricted to a minority usage on each board, and insertion often being quite a complex manoeuvre, particularly if accompanied by cropping and forming, it is still very common for through-hole parts to be assembled by hand, with some machine assistance for preforming. The most beneficial aids for this process involve helping the operator to identify the correct component and the appropriate location and orientation. In consequence, projection systems are still fairly common. More details about this process in our paper Component insertion, which discusses the issues of component presentation, although detailed consideration of equipment practice is more suited to a mechanical engineering course.

Exercise

Spend a short time examining the through-hole components on any sample mixed technology board. Ask yourself:

 

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Through-hole soldering

Once the through-hole components are in place, we need to solder the terminations to the lands, making a connection that extends from one side of the board to the other. This requirement is common to most specifications, and arises from the fact that defects in the through-hole metallisation will prevent solder being pulled through the hole and wetting the pad on the opposite side. The requirement is thus a process indicator rather than a requirement based on the strength of the connection, which is significantly (perhaps as much as ten times) over-designed.

Having an interference fit is incompatible with insertion, so there needs to be clearance between the lead and the internal diameter of the plated hole, and this necessarily requires a significant amount of solder. Whilst, as we shall see later, this can be provided by modifying the paste process, the traditional way of soldering through-hole components has been to apply molten solder to the joint, either by hand or using a machine to create all the required solder joints in one pass. Although machine soldering is more efficient, there is still a place for hand soldering, especially for adding sensitive components at the end of the assembly sequence and for carrying out repair and rework operations.

Hand soldering

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.

Figure 7: Hand-soldering station

Hand-soldering station

The tip temperature is checked by using an electronic thermometer to measure the voltage from a 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 preferred 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.

Component retention

During the hand soldering process the component leads must not move relative to the hole, especially during the solidification stage, or the joint will be “disturbed”, that is it will be of dull appearance, high resistivity and suspect reliability. So we need to hold components in place.

In some cases components will be held against the board during soldering, using a compliant material such as a foam, but for other applications the lead is either formed to provide some spring resistance to movement, or else “clinched” to retain the part.

For axial components the body of the component will normally be flat to the board, so the leads must be bent, but not so sharply as to induce 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.

Figure 8: 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?

Particularly where there is little clearance between boards, it is sometimes necessary to cut the wires quite short, and a good way of achieving pin witness and the shortest possible lead is to cut the leads after soldering, using a machine such as the Blundell Cropmatic.

Where individual components are inserted and then clinched, for example when an automatic insertion machine is used, care is needed to ensure that the clinch is both at the right angle and in the right direction relative to other joints. Figure 9 indicates how incorrect clinching can have an effect on the incidence of bridging defects, particularly with wave soldering, although the potential for bridging also exists with hand soldering.

Figure 9: 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.

Machine soldering

Recommended reading

Machine soldering involves first applying flux to the surfaces to be joined and then exposing them to a clean molten solder surface, as with hand soldering, but subjecting all joints on a board to these conditions during a single machine pass. The equipment used for this, where the underside of the board is swept by a wave of clean solder, was originally referred to as “flow soldering”, but is nowadays generally referred to by the more descriptive form “wave soldering”. How this is achieved is discussed in our paper on Wave Soldering. This is an extended discussion, covering the background to the process, issues of holding components in place, the basics of fluxing, pre-heat and solder wave, and concluding with some aspects of practical machines and how their performance is maintained.

 

Supplementary information

Whilst today’s wave soldering machines are significantly more sophisticated than earlier models, and offer improved process control, it is interesting to review one of the original Fry’s machines and see how the same problems existed 40 years ago, and how these were tackled. Details at this link.

 

Wave soldering is an extremely complex process, with much variability, so it is not surprising that in many cases a significant amount of rework needs to be carried out in order to get a satisfactory result. Much depends on the design, on the solderability of the materials, and on the setting and maintenance of the wave soldering machine, especially with older models. In consequence, it was formerly the case that an inspection stage was always carried out immediately after wave soldering, inspection being accompanied by immediate rectification of any faults. Such “view and touch-up” constituted a significant additional cost, and accounts for the preference for reflow soldering shown by many process engineers.

Supplementary information

Fortunately, most of the issues can be avoided by correct design and taking appropriate precautions during soldering. Our paper on Wave Soldering Problems illustrates typical problems, and how these may be overcome, and is required reading for anyone who designs or specifies assemblies for wave soldering.

 

Combining soldering processes

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 the common manufacturing challenge of a double-sided design that uses mostly surface mount components, but has only 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.

Hand soldering is both slow and variable in quality, and the machines developed to simulate the hand-soldering process are still relatively slow and unreliable, requiring significant process control effort. So some kind of selective wave soldering is the most viable option. The simplest way is to use conventional wave soldering equipment, but transport the board over the wave in a “template”, a carrier that exposes only part of the board under-surface to the wave. Such custom templates add to the tooling costs, and require careful design to allow good access of solder to the joint. They also need to be cleaned at least occasionally during use, in order to remove the build-up of flux residues.

Recommended reading

For a discussion on some other possible approaches, read our paper on Selective machine soldering.

 

Self Assessment Question

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

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Simplifying the sequence

We have seen that wave soldering is a process that has yield concerns, and can add cost, particularly if there are surface-mounted components on the underside that cannot be wave soldered, so require through-hole component soldering as an additional process. One way in which the overall process can be made more efficient and cost-effective is to use reflow soldering to mount through-hole components as well as SM parts. This is a process referred to as “pin-in-paste” or “intrusive reflow”, and implementing this requires both additional solder paste and some acceptance of joints that are visually different, generally leaner, but with at least the potential for solder spikes on the end of the leads, which have been pushed through the solder paste.

Of course, there are component materials problems when moving from wave soldering to reflow, in that the whole body is exposed to reflow temperatures, whereas the board in wave soldering acts as a screen against soldering heat. In order to simplify the process by using pin-in-paste, one may need to purchase components with a higher temperature withstand capability, and potentially a higher price.

Recommended reading

More about this process in our paper on Intrusive reflow.

 

Self Assessment Question

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.

show solution

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 without 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 is 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.

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Other assembly processes

Cleaning

Whatever flux is used during soldering, there will inevitably be some residue. Early flux residues were either harmful, or believed to be so, and it was common practice to clean boards after solder assembly. Even when low-residue and no-clean fluxes were developed, and where there was no reliability requirement for the residues to be removed after soldering, many customers still preferred the look of the visually-clean circuit, and continued to specify a cleaning process.

Recommended reading

Cleaning is not only a process that adds cost without necessarily adding value, but it is fraught with problems about materials, processes and the definition of what constitutes an adequately clean board. Materials and processes have undergone a significant change since the signing of the Montreal Protocol in 1987 and the consequent removal from the market of some highly-effective cleaning materials. If you have not already done so as part of your study of Unit 2, read our paper To clean, or not to clean to gain an appreciation of the issues and processes involved.

 

Cleaning covers a very wide range of methods, all of which seek to bring the “soil” into intimate contact with a material that will disperse it more widely, and remove the bulk of the soil from the part being cleaned. Bringing the dispersal agent and the soil together may be done by condensing vapour onto the surface or by immersion, and the process may be assisted both chemically with surface-active materials to improve the wetability of the surface, and by mechanical agitation, produced during boiling, by spraying, or by using ultrasonics.

The choice of cleaning method is intimately related to the choice of flux and the solderability of components, as well as to customer preferences. In many respects an effective approach is to use a water-soluble flux of relatively high activity, that gives a good result but needs to be removed, and has been formulated to allow cleaning to take place with a mixture of water and surfactants. This approach optimises flux effectiveness and gives a clean result, but there are always concerns about process controls and the effect of moisture on components and board. These complications, added to the cost of the additional processing, mean that most assembly houses will prefer to use a no-clean process where this is compatible with the customer requirement.

Whilst cleanliness may be apparent on the surface, many users are highly sensitive to the possibility of ionic contamination remaining on the assembly, with possible adverse implications for reliability in the presence of moisture. Although it is hard to correlate tests of cleanliness with survival under humid conditions, several tests have been devised in an attempt to detect the effect of any residues on the conductivity of a solution used for an extended wash cycle.

Supplementary information

More details at this link, together with a critique of this monitoring process, which works well for ionic soils, but not for other foreign materials.

 

Non-wets

The presence of a cleaning process may also influence whether or not it is necessary to carry out additional assembly work after the main cleaning has been performed. 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.

Conformal coating

Another reason for cleaning is because the board is intended to be conformally coated. This is a process that aims to delay the deleterious effects of moisture, few enclosures being sufficiently hermetic to protect a board against the affects of hot, humid air. Conformal coating is an approach that finds favour for severe applications such as automotive and military. Its aim is to provide a thin overall covering to the assembly surface that will improve insulation resistance and high-voltage breakdown, and protect against electromigration.

Materials such as acrylics and silicones are applied by dipping, painting or spraying, all trying to produce an even, thin coating without any imperfections, to reduce both material use and stress in the assembly. These coatings need to be applied to a clean surface, rather than sealing in contaminants that may themselves cause damage. As Hamlet puts it, “It will but skin and film the ulcerous place, whiles rank corruption, mining all within, infects unseen.”

Recommended reading

A wide variety of materials is available, and several different application methods. Before you tackle the next activity, read the Concoat paper Conformal coating: Selection and application (PDF file; 282Kb)

 

Activity

What are the issues relating to the use of conformal coating that will impact on your design, as viewed from a process, manufacturing and servicing perspective?

Compare your answer with this one.

 

The majority of assemblies can be manufactured using the processes that already have been described, but other processes are occasionally employed. For example, additional support may be given to large components by applying adhesive, either a material that can be dispensed or a hot-melt type. And some assemblies may use mechanical processes, involving screws or rivets to secure components such as connectors, that are more akin to those we will consider in Unit 9.

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Rectification processes

In-line rectification

Any consideration of a set of manufacturing processes would be both incomplete and misleading if we failed to acknowledge that not every process works correctly first time. Rectification processes are inevitable, both those carried out in-line, as part of the processing, and the procedures of repair and rework that follow formal test and inspection stages.

Activity

Looking at each of the processes in turn, what in-line rectification procedures might be used? And what are the dangers associated with each?

When you have made you own assessment, look at our comments.

 

Repair and rework

Recommended reading

Whether the input is a failed part from the field, a reject found at final inspection, or a defective joint or component weeded out after a soldering process, the procedure required is the same, and is discussed in our paper on Repair and rework. This is recommended reading, because it deals not just with the practicalities, but also the reasons why we should do as little rework as possible, and control what we do very carefully.

 

If you have ever tried to carry out any rework on assembly, you will know that some design features make the activity more difficult than it need be, usually because of problems over access. More about Design for Repair in Unit 8.

Manufacturing standards

All rectification processes need to be explicitly specified and monitored, trying to use feedback to improve the original process. This also applies to the more formal test and inspection procedures that are applied during board assembly, and which form the subject of our next Unit. But what is a good product, and what a reject? Although we have already given some examples of defects, we have not yet 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.

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Organising the line

Whilst we have considered all the individual processes used in assembling a board, we have not explicitly dealt with the way that a manufacturing line is organised, and the ways in which work-in-progress is fed from one stage to the next. Companies making small batches with considerable variety will normally choose independent items of capital equipment, handling boards on trays or cassettes, but of course always with due regard to the anti-static precautions that we considered in Unit 3.

However, many larger items of equipment are primarily designed for in-line application, so will contain integral board transport mechanisms with input and output conveyors, even if they are not coupled together into a single line. Common practice is to join together printing, placement and reflow equipment, with the conveyors between the machines not moving continuously, but used for intermediate work-in-progress (WIP) storage and access for inspection.

In cases where continuous operation is desired, the aim is to reduce WIP to a minimum and to harmonise the machine cycle time (usually referred to in the electronics industry as the “takt” time) of individual process stages. This means attention to “line balancing”, to ensure that the takt time of each machine is compatible. This may need the simplification of some tasks, for example carrying out less than an exhaustive inspection of the paste deposit, or the splitting of other tasks between different stations. Examples of the latter occur both in automated and manual placement, where more than a single machine or operator is needed to give the required “beat rate”, and the task is carefully partitioned between the operating stations, to maximise the placement rate by simplifying the task and “spreading the load”.

Care has to be taken when setting up a conveyor system, choosing mechanisms that are appropriate and flexible. For example in the usual case where a board is supported only on its edges to prevent damage to parts on the underside, changeover between board types will require an adjustment to the spacing between the support edges along the whole length of the conveyor system. Most practical board transport systems will therefore automate this aspect, usually keeping the board transport edge at the front of the machine static, and moving its partner at the rear.

When integrated in a machine, the board transport in and out of the machine is supplemented by some means of fixing the position of the board and supporting it during the operation. Overall, the board transport module performs the following functions:

The feeding and removal of boards to and from the working area is mostly carried out on conveyor belts. The Surface Mount Equipment Manufacturers Association recommend that support edges of 4.75 mm are provided on both sides of the board (Figure 10). In order to provide satisfactory support of the board on the conveyor, no components may be located on these edges.

Figure 10: Board transport

Figure 8: Board transport

Typical speeds for run-in and run-out are 300–400 mm/s. The board is stopped at the required position by means of a mechanical stop or an optical sensor, the latter having the advantage that the PCB does not receive any impact as the result of colliding with a mechanical stop. The board can be positioned (Figure 11) in the working area in three ways, the third of which is now almost universally adopted:

Figure 11: Board positioning options

Figure 9: Board positioning options

Whilst edge transport is sufficient to support the board during transfer, additional support will often be required during processing. For example at printing, to ensure that an effective seal is made between stencil and board, and at placement, to ensure that the board does not bend too much due to the placement forces.

When components are placed on the first side, the board under-surface is flat, and a platen may be used; for second-side printing and placement, the underside is no longer flat, and alternatives are to use support pins, as shown above, or a nest, as shown in our paper on Solder paste printing.

As we saw in Reflow soldering, board support of a different type may also be needed to prevent board sag, and this is commonly carried out using a taut guide wire, although individual boards can be manually fitted with edge stiffeners. In every case, once there are components on the underside of the board, the conveyor has to contact the board only by the edges, and there must be clearance beneath the conveyor to accommodate any tall components.

To think about

When we consider the inputs and outputs for a practical production line, we are not just concerned with bare boards and components in and good parts out, but a considerable amount of waste is generated. Some of the waste associated with printing is classed as “hazardous waste” on account of its metal content: materials contaminated with solder paste need separate treatment, as does dross removed from the wave soldering machine. In addition, other items are consumed that do not form part of the end-product. Reflect on what these might be before reading our paper on Assembly waste.

 

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What influences the process?

We hope that by the time you have read this far, including taking a careful look at the supplementary materials, you will have realised that board assembly is not just a single process route, but rather a group of processes from which the manufacturing engineer selects the most appropriate approach for the application.

The choice made will be influenced by a number of features of the design, especially the format of the available components, and by the environmental specification required by the end-user, in particular the temperature range and the humidity requirement. Often when the choice is made it is easy to overlook the need to ensure that the materials and processes chosen are compatible with each other and with the intended use.

Cost is a primary influence on the process, and in particular on the choice of component and board supplier, as against the specification of these parts. But even here a wise manufacturing engineer will prefer to pay more for materials and components in order to maximise yield, as well as tuning the processes to give best performance with the parts available.

Whilst cost is usually a main consideration, encouraging the simplification of processes and the use of automation as far as possible, other influences on the process include customer preferences, for example for post-soldering cleaning, and assembler preferences, based on the equipment and operator skills available.

Do not underestimate the influence that the preferences of the assembly house may have on the process route as well as the process detail, or the variety of responses there may be from different assembly houses, based both on their capability and their history! Given the nature of the EMS market, it is perhaps not surprising that in recent years many major multi-site players have deliberately set company-wide standards for equipment and processes – the so-called “global platform” – to facilitate moving assembly projects between manufacturing sites in order to ensure security of supply and the lowest possible price. Of course, whether they have actually achieved a level playing field, and whether it is possible to transfer production between plants with little or no impact on the end-customer, is a moot point . . .

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