Lead-free Implementation

Unit 9: Design implications

Contents

Working towards Assignment 3

By this time you should have submitted Assignment 2, or at least be refining the completed draft. Before you put the material totally behind you, we recommend you read Assignment 3, from which you will see that a brief summary of the situation (from both Assignments 1 and 2) is required as an introduction, in order to make the Assignment 3 report self-standing. [Of course, you may wish to review you recommendations in the light of tutor feedback on Assignment 2!] But a second reason for asking for a “lead-in” to the report is to ensure that you think carefully about keeping your recommendations for implementation consistent with the technology choices you have made previously – the whole report has to “hang together”.

This unit’s focus is on the “front end” of the company, and the work needed by Marketing and Design/Development. This will affect the resources required and the control to be exercised, but will also impact on all AMS’s customers in terms of the evaluation needed in order to minimise technical and commercial risk. This is a holistic concept, and you should not expect to find all the information you need on this aspect within Unit 9. For example, there are reliability issues (Unit 8) and component issues (Unit 6) that will impact on the company’s relationship with its customers.

Introduction

So far in the module we have concentrated on the materials and processes required for lead-free manufacture; in this final third, we want to be able to pull together all the aspects of what the company needs to do to be lead-free.

Whether your company is an OEM or a contract manufacturer, the crucial decisions are made early on by Marketing and Design, and it is the potential consequences of the technical decisions made at this time that are the focus of this short Unit.

Environmental issues in general, and lead-free considerations in particular, need to be firmly on the agenda both in the initial marketing discussion and in the preliminary work that results in an outline design. This should be a team effort involving those who can reflect the opinions and needs of the final user, those with a focus on commercial aspects of the design, and those engineers who are involved in meeting the environmental requirements of the new product. In some cases, the task will simply be lead-free conversion, but for totally new products there is an opportunity to look more widely at issues such as:

Such an approach can be informal, but increasingly there are drives to integrate an formal environmental review into the design and development process. For an insight into how environmentally-aware design can be carried out and organised see PD ISO/TR14062:2002. This document is accessible through British Standards Online (http://bsonline.techindex.co.uk) using your Athens log-in. The standard aims to integrate environmental aspects into product design and development, balancing them against factors such as product cost and performance. It stresses the need to think environmentally from the outset, and has a useful list of publications. As you might expect from your study in Unit 1, ISO 14062 takes a “Life Cycle Analysis” approach.

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What's different about lead-free?

Whatever the choice of materials, any company needs designs that are compatible with the process available, so as to give the best possible yield. So we need good Design for Manufacture in its broadest sense, whether Design for Fabrication, Design for Assembly or Design for Test. There are many guides as to how this should be attempted, but our question in this module is “What is different about a good lead-free design, as distinct from a product made from lead-containing solder?”

Whilst the answer has been summed up accurately as “Not a lot!”, there are still some subtle differences. Which is why another common observation is that designs that create problems for the assembler when lead-containing solder is used will cause at least as many problems with lead-free, and often more.

WWW Research


Before reading further, try a preliminary search to establish whether you can find any material on design aspects of lead-free soldering. We tried lead-free +solder +"pad design". Some of the resources you explore will be useful later on.

 

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Design actions

Apart from the obvious changes in the choices of materials, boards and components, the reasons for more subtle changes between using eutectic tin-lead and lead-free solders lie in the different characteristics of the solder, in particular its higher melting point, its higher surface tension, and the differences in the extent and speed with which it will wet surfaces. Some of these differences have an effect on manufacturing practice; others have implications for the designer.

Wave soldering

Surface tension and wetting rate changes affect both wave soldering and reflow soldering, but are most evident with wave soldering, as lead-free solders are substantially less fluid than eutectic tin-lead solder. The first requirement is that, within the relatively short contact time available, the solder should wet all the component terminations and pad areas without leaving any solder ‘skips’ or shadowing, problems that are particularly acute when soldering bottom-side SMT components. Secondly, as the assembly leaves the trailing edge of the wave, the solder should drain away from each joint sufficiently to permit ‘pin witness’ and leave a set of solder joints without bridges (Figure 1) or spikes (Figure 2).

Figure 1: Solder bridging between pins

Solder bridging between pins

Figure 2: Solder spikes

Solder spikes

 

Exercise

Reflect from your experience on how these problems might be overcome, or look on the web for ideas.

Then look at our comments on solder skips and bridging, which suggest both machine methods and design approaches to help overcome these common problems.

Remember that the problems with lead-free are just the same as with eutectic tin-lead, but the impaired flow characteristics of lead-free solders make it less prudent to ignore good practice.

 

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Reflow soldering

For reflow soldering, the major reported problem has been in tombstoning. As explained at this link, the primary cause of tombstoning is a difference in the wetting forces on the two ends of the component, a result of thermal and design factors, and of varying degrees of wetting. Much work has been done on this, as you will find, from the following exercise.

WWW Research


Browse the web for information on preventing tombstoning. Is there much useful information? You might like to bear in mind that difficulties with tombstones become much more severe with small parts . . . we found that the information collected by Dan Teerstegge on 0201 Sized Components was particularly helpful.

Draw your own conclusions as to what needs to be done and whether going lead-free is likely to make an impact.

Now look at our answer.

 

If you have to assemble 0201 components, there are many issues to consider other than just the use of lead-free solder. For example, tombstoning problems will become worse if the paste print is not consistent.

Supplementary Information

To get satisfactory paste release, Boardman and Houston’s paper (Designing an 0201 assembly process for high yield and reliability, Advanced Microelectronics, January/February 2004) recommends that the ‘area ratio’ (the ratio of the area of the open aperture divided by the area of the aperture walls) should be greater than 0.65 for an electropolished laser-cut stencil, and 0.6 for an electroformed stencil. [The difference lies in the improved surface finish of the electroformed stencils, which results in smaller variations in print volume.]

The area ratios used by Flextronics (Investigating 0201 printing issues and stencil design, Mei Wang et al, Circuits Assembly, May 2003) were slightly lower, although electroformed stencils were again preferred for smaller apertures. However, the Flextronics team also found that the solder deposit could be increased by using paste made with finer solder powder and by defining the pads with solder mask.

A good general resource on assembling small components is Bob Willis’s 0201 Printed board design and assembly issues in the Circuit Assembly “Process Defect Clinic”, May 2002.

 

But tombstoning is not the only aspect of soldering that will be affected by the changes in solder characteristics . . .

Exercise


Make a list of the ways in which changes in wetting and surface tension might influence the outcome of the soldering operation.

Now look at our short paper on surface tension effects. All of these effects can be expected to be subtly different where the solder used is lead-free, rather than eutectic tin-lead.

 

In Unit 8 we considered the issue of incomplete pad coverage. Whilst this has a greater effect on the appearance of the joint than on its reliability, designers may prefer to increase the stencil apertures, where this is possible. Kevin Pigeon comments in Considerations for printing lead-free solder pastes (Surface Mount Technology Magazine, July 2004): “Many manufacturers currently use reduced aperture-to-pad ratios to prevent bridging and solder beading. Due to differences in the solderability characteristics of lead-free circuit board finishes and the inability of lead-free solders to spread as well as tin-lead, reduced stencil apertures may need to be opened back up to a 1:1 aperture-to-pad ratio. This ratio should not result in bridging because density differences between lead-free and tin-lead solder pastes results in less slump with lead-free pastes.” [Note that access to this paper requires (free) registration with SMT web site]

A final design issue is the concern with reflow that voids in the solder have been reported, particularly with area arrays, and the consequences for reliability can be serious if the voids are large. For BGAs, a connection with pad finish has been reported (Investigating voids, Keith Bryant, Circuits Assembly, June 2004), with ENIG producing a large number of small voids at the pad-to-ball interface (potentially leading to tearing defects in the joint), and voids in OSP joints tending to be much larger and close to the joint open edge.

Note that, apart from the need to choose the right materials and reflow profile in order to reduce voiding, it has also been found that in-pad vias contribute to voiding1 as well as increasing the occurrence of tombstones.

  1. Changing to Pb-free profoundly impacts the manufacturing production process, Brooks, Day and Goudarzi

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Component movement

The different wetting, mobility and surface tension of lead-free solders also have an affect on the final position of the soldered components. For example, with Ball Grid Arrays, assemblers have made use of the fact that a considerable amount of alignment of the package onto the pads takes place once ball and paste have flowed to become a single molten mass. Movement can be seen clearly in this Seho video clip. Reduced flow (from a lead-free solder) equates to less movement during self-positioning, and thus imposes tighter requirements on placement accuracy.

Further reading

Whether you are trying to reduce tombstoning on a small chip component or deal with the lack of realignment on a BGA, a key is to get the best possible alignment between stencil and board. Look at this Cookson article on Improving PWB to stencil alignment for some suggestions and ways in which this can be optimised.

 

Higher temperatures

The higher temperatures of lead-free soldering mean that the designer must specify components that will survive the process, as was discussed in Unit 7. At the same time, the increased temperature of lead-free processing can make a difference to the most unexpected aspect of the circuit: look at the Polyonics web site to see how the lead-free process can have an adverse impact on the legibility and appearance of such a common-or-garden thing as a label. [Bear in mind that labeling becomes even more important in the lead-free situation, as we need to know the materials of which board assemblies are made in order to be able to rework them effectively]

Two other higher-temperature aspects that influence design are:

Figure 3: Specify sufficient copper plating

Specify sufficient copper plating

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Board support

The higher melting point, and higher temperatures at which wave soldering and reflow soldering are carried out also increases the likelihood of the board sagging in the machine, with the adverse consequences for reliability that have been talked about in earlier units. We strongly recommend providing some means of supporting the centre of the board, particularly when larger or thinner boards are used. A typical implementation in a wave soldering machine is shown in Figure 4.

Figure 4: Centre board support for wave soldering

Centre board support for wave soldering Centre board support for wave soldering

To allow the board to be supported, the designer should leave a sufficiently wide strip free of components, as shown in Figure 5. A typical width for this ‘no-go’ strip is 3–4mm, and it should be placed more or less in the centre of the board. Fortunately, where circuits are small, and handled as a multiple-up panel, it is often possible to locate the no-go strip in the waste material between circuits.

Figure 5: Different ways of indicating a board support no-go area

Different ways of indicating a board support no-go area Different ways of indicating a board support no-go area

The designer should also make the position of the component-free strip clear to the assembler; Figure 6 shows a top-side position marker for the board support area to assist in aligning a wave soldering machine.

Figure 6: Board support top-side position marker for wave soldering machine alignment

Board support top-side position marker for wave soldering machine alignment

As well as making allowance for board support, the designer also has to remember to leave a margin for clamping at the periphery of the board; an allowance of 4mm all round is suggested (Figure 7).

Figure 7: Requirement for edge clearance

Requirement for edge clearance

Where boards are carried on conveyor belts, the Surface Mount Equipment Manufacturers Association recommend that support edges of 4.75 mm are provided on opposite sides of the board (Figure 8). In order to provide satisfactory support of the board on the conveyor, no components may be located on these edges.

Figure 8: Board transport

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Review and preview

The higher melting point of lead-free solder, and its different wetting characteristics, will typically result in the process having smaller margins than eutectic tin-lead, thus needing tighter control. Experience suggests that there will probably be some knock-on effect in terms of reduced yield and increased cost. As a result, assemblers will almost certainly become more critical of designs and more dependent on designers taking DfM aspects into account. In other words, lead-free will place even more emphasis on the need for good design practice. [This linked paper on pad design contains some thoughts on this aspect, but is not intended to be definitive or complete]

This unit has been relatively short, and comes at the end of a group of units looking at the practical implications of the move to lead-free. So, before we move on to think about how the transition can be tackled in departments of the company other than Design, perhaps it is time for you to review the story so far, and perhaps even draw up a fresh list of the issues that need to be tackled by the management team as a whole. To aid in this we have included some links to a number of short video clips in which Bob Willis explains, for his “Lead-Free Casebook”, the practical issues that affect each part of the manufacturing process.

We have supplemented this by references to some articles that we feel are useful in bringing together the topics about which you need to be fully aware before tackling the final part of this module. Note that there is no hidden agenda in the order in which these resources are listed!

Supplementary Information

Bob Willis video clips on:

Topic
Link(s)

Legislation

video

transcript

Components

video

transcript

PCB

video

transcript

Paste printing

video

transcript

Placement

video

transcript

Reflow

video

transcript

Wave

video

transcript

Hand soldering

video

transcript

Inspection

video

transcript

Rework

video

transcript

Additional resources

Lead Free Production, Under the Microscope: How the Introduction of Lead Free Processes will Change the Appearance & Inspection Criteria for PCBs by Bob Willis. Available on registration at the Vision Engineering web site.

Lead-free electronics assembly: How will this unfold? by Prismark Partners

Lead-free, a report by Promosol

The HDP User Group Implementation Guidelines

 

As you skim the material, be aware not only of the effect of lead-free on processes, materials and designs, but also think about some of the issues we’ll be looking at in the next units:

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