Lead-free implementation

Unit 6: Implications for assembly

Contents

Working towards Assignment 2

This unit contains much of the detailed information that you will need to tackle Assignment 2.

We assume that you will have selected a process and materials (or perhaps a combination of different processes and materials), and Part 1 of the task is about whether the equipment is lead-free capable. The additional scenario for Assignment 2 gives valuable insight into how the factory is laid out and equipped. You will be able to draw valid conclusions as to which equipment is likely to be lead-free process compliant, and which may need upgrade or replacement. At the same time, there is information about how the process is controlled. Consider this scenario in parallel with the sections in the unit that give an overview of process changes and detail of the soldering processes.

In Part 2 of the Assignment you become involved in procuring components that will work with your chosen process to provide a reliable lead-free assembly. There is a great deal of information in the section on components, so you would be well advised to look really carefully at Task 2 in parallel with your study of the components section of this unit.

Overview of process changes

In this Unit we are trying to give an overview of the changes to the process, plus some links that will indicate the depth to which members of your company’s team will have to go when considering the implications for each of the key items of equipment. So, what are the changes?

As with the units so far, many of the “solutions” contain a substantial amount of information, to check that you are going along the right lines. But from here on, particularly where the issues are clear-cut, we won't insult you with trivial solutions!

Research Topic

What impact will the transition to lead-free make on the processes and equipment used for assembly?

As well as consulting the resources indicated below, we would encourage you to create and maintain a detailed list of the issues that need to be considered, as this will help you formulate your answers to Assignment 2.

Resource Link(s) Notes

Review SAMC’s June 2003 presentation on the impact of lead-free on the process. The speaker notes are almost a transcript of the presentation.

Check Martin Allison’s summary slide.

Read Bob Willis’s article Lead-Free Hands On Experience in the July 2003 issue of Circuits Assembly.

Skim Altera’s Reflow Soldering Guidelines for Lead-Free Packages.

Read Section 2 of the ESPEC document Promoting the commercial adoption of lead-free solder and evaluating its reliability by Hirokazu Tanaka et al. We will return to this in Unit 8.

Browse Solectron’s presentation on Lead-free production. Slides 6–10 and 28–32 are concerned with manufacturing, but other parts of this presentation will be referred to in later Units.

Interview with
Phil Hamilton (1999)

video
transcript

Most of the points Phil makes are still spot-on, despite the date of the interview.


Review our summary of the issues

Last revised 3 July 2004

Our summary makes the comment that the change at printing is not significant. There are those who would take issue with this! The fact is that moving to a different solder paste, lead-free or not, equates to buying a material that will have different rheological properties, will print differently, will slump to a greater or less extent, have a shorter or longer life on the stencil, and so on. Not major issues, but things that have to be taken into consideration.

You might also have worked out for yourself that the particles in lead-free solder paste are harder, and have the potential to cause more damage to stencil and squeegee blades, in particular the former. This has indeed been reported. Also, given that more active fluxes are likely to be required, this may have implications for the storage and handling of the paste.

The most significant issue relating to printing is probably that of segregating lead-containing and lead-free materials, bearing in mind that lead contamination of lead-free material may impact on the reliability of the assembly, as well as making it non-compliant should the percentage of lead exceed 0.1%. Hygiene, housekeeping and similar issues become very important, particularly as there is no visible difference between solder pastes . . . Was this egg from a battery hen or free-range? Difficult to tell until you cook it!

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Soldering processes

Having skipped over placement, where the only difficulty may be with the vision systems and lead-free balls on BGAs, we arrive at soldering processes. And here there are substantial changes, and not just to the materials. Unless the application allows the use of a zinc-containing solder, or the sensitivity of components dictates one of the quaternary alloys, most soldering operations will need to be carried out with an alloy that melts at 217°C or more. Unfortunately, one can’t just “scale up” the settings from lead-containing solder by making everything 35°C hotter, for example, reflowing at 255°C, because this causes damage to components and boards, as well as putting a strain on the equipment.

Reflow processes

The critical question with reflow is by how much the temperature will need to be increased, compared with normal conditions for tin-lead solders. The indication is that the critical feature is the integral of the time over liquidus temperature, and that equally successful results can be achieved either by using a short spike or a more extended exposure at a lower temperature. In Figure 1 these are referred to as ‘angle’ and ‘hat’ profiles1.

1 Whilst the ‘hat’ profile might seem to be the kinder to components, in fact many parts will withstand brief exposure but not an extended time at lower temperature. JEITA make the interesting comment in their 2002 Roadmap that component manufacturers may take a different view of the requirements from component users – caveat emptor!

Figure 1: Some alternative profiles for lead-free reflow soldering

Some alternative profiles for lead-free reflow soldering

Conventional wisdom a couple of years ago was that a peak temperature in the region of 240–250°C would be necessary for SAC. However, encouraged by reports by commentators such as Suraski that SAC reflowed just as well at a 230°C spike, considerable work has been ongoing to keep the temperature to a minimum. However, success with such a low value of over-temperature is critically dependent on equipment, firing atmosphere and flux.

A small margin inevitably reduces the process window in terms of setting and control, as shown in this slide. More critically still, it is necessary to pay close attention to the variation in temperature across the assembly, usually referred to as the ΔT.

Supplementary Information


For a view on what the process conditions should be, look at the presentation that Sue Knight and Dave West of Sanmina-SCI gave under the title Lead-free: What are the real process constraints?

Keeping the temperature range as small as possible means giving an even greater attention to profiling, aiming to set conditions so that all components on the assembly undergo the same temperature experience.

Supplementary Information

Reflow is intimately associated with the choice of paste. A good review of the issues is Peter Biocca’s paper Flux chemistries and thermal profiling: avoiding soldering defects in SMT assembly on the Loctite-Henkel web site. Other papers from this author will help you plan your experimental approach for paste evaluation.

The need to keep ΔT to a minimum is one of the factors that generally dictates that the lead-free reflow soldering process be carried out at a slower conveyor speed and with more convection than with eutectic tin-lead. But these are not the only equipment-related aspects, and we invite you to explore the factors involved in selecting reflow equipment.

Research Topic

What are the factors involved in selecting reflow equipment?

Resource Link(s) Notes

For the Soltec view, go to the Step 1 page, and explore the Reflow Soldering Equipment Selection link.

Look at the Heller equipment presentation.

Search for "reflow equipment" +lead-free in order to access the view of other manufacturers such as Electrovert and Ersa.

Interview with
Phil Zarrow (2001)

 

Read Fred Dimock’s article Effect of high-temperature requirements for lead-free solder in the July 2004 issue of Circuits Assembly.

show solution

One equipment aspect which has been revisited is the need to carry out reflow in a nitrogen atmosphere. Certainly, when interviewed in 2003, Chris Hunt of NPL believed this to be the case (video and transcript at these links).

However, this is a moving target because of the way in which paste manufacturers have risen to the challenge of providing a paste that will reflow adequately without the cost burden of providing nitrogen. Unfortunately, providing an inert atmosphere demands both changes to the equipment (not always possible as a retrofit) and a continuing high cost of gas, both measured in €s and the energy cost to the environment. Inerting is more popular for wave soldering, for reasons that will become clear if you follow this link.

The need to keep the temperature at a minimum, coupled to the advantage given by soldering in an inert environment, has had the affect of resurrecting the technique of vapour phase soldering (or condensation soldering) that was developed during the 1970s, but has more recently withered away because of the improvements in convection ovens coupled to the high cost of the materials used for vapour phase. It appears, however, that this technique is on the verge of a comeback fuelled by three factors:

There is some information on condensation soldering at this link, but we would encourage you to look at these links to presentations by EPM-IBL and Asscom, two vapour phase solder machine manufacturers. EPM-IBL have also provided a real time video of equipment in action, described at this link. Note that the pre-heating carried out is now very much more controlled than previously, so that tombstoning and other bad experiences from the early days are less likely to appear.

Finally, read this Seho paper, which comes to the conclusion that convection ovens are still the most appropriate method for volume manufacture, smaller scale operations might benefit from using vapour phase. Much of this of course has to do with the mechanisation aspects of the different equipment, and the batch nature of most condensation soldering.

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Wave soldering

Wave soldering with lead-free materials has been somewhat more problematic than for reflow, and has been further complicated by the parallel drive to carry out the process with VOC-free fluxes. This is an aspect to which we will return later. The indication is that the process is feasible with high yield, although the process window is relatively narrow. We invite you to explore the issues relating to wave soldering equipment in this next exercise!

Research Topic

What are the factors involved in selecting wave soldering equipment?

Resource Link(s) Notes

For the Soltec view, go to the Step 1 page, and explore the link “Wave Soldering Equipment Selection"

interview with
Jim Morris (2003)

 

Look at the Seho equipment presentation.

Search for "wave soldering equipment" +lead-free in order to access the view of other manufacturers such as Electrovert, Heller and Invicta.

show solution

Last revised 3 July 2004

Equipment factors apart, many of which will suggest the use of a new pot, if not a totally new machine, there are a number of process issues associated with wave soldering. For example, there is the problem of maintaining the composition of the solder. We have already pointed out that adding small percentages of copper increases the liquidus point, so that the solder used to top up the bath may need to be copper-free. There is an increasing tendency to recommend the use of on-site monitoring, rather than relying on laboratory analysis.

Supplementary Information

The paper by Minoru Ueshima and Osamu Munekata of Senju, The Quality of Flow Soldering on the contamination of Cu for Sn-3Ag-0.5Cu solder, which appears on the EFSOT web site, suggests a 0.5% limit for copper contamination on the basis of soldering results and joint structure and performance.

 

The assembler also has to decide which material to use, and here there is a genuine choice between SAC and Sn0.7Cu, trading off cost against performance, and bearing in mind the compatibility of the solder with that introduced by other soldering activities, such as HASL board finishing and rework.

By the way, don’t be tempted to think that, even if your equipment is lead-free compliant, you will be able just to empty out the pot and refill with the new type. People who have hands-on experience of wave-soldering equipment know well that there are many crevices and corners in a machine in which solder and contamination can become trapped! The alternative to simply buying a new pot is to drain the pot, fill it with pure tin and run it for a period and then discard the contaminated tin. When we say “discard” of course we mean “recycle” because there is a market for the lead-containing material as a solder constituent for less sensitive applications. You need to do a deal with your solder supplier . . .

As with any new material, there are always some wrinkles to be observed when setting up the process. A good example of this is the way in which copper will collect at the bottom of a bath when it is cool, creating “fingers” of copper into the bulk of the solder. These are not dispersed under normal re-melting, but need the pot to be run 10–20°C higher than normal for an hour or so, before reducing the temperature to normal and using on production circuits.

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Other types of soldering

Soldering is not just a machine process, but hand soldering is used, particularly for rework. Read Simon Hawkins’ presentation for an overview of the process and what needs to change for lead-free. BGAs present a particular rework challenge, where control is very important, as emphasised in Paul Wood’s presentation.

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Process problems

In Unit 5 we mentioned the common process problem of tombstoning in the case of lead-free exacerbated by the relatively slow wetting that takes place. Other problems include fillet lifting and leaching.

Trials with bismuth-containing solders and double-sided through-hole joints exhibited ‘fillet lifting’, which was first described by Vincent and Humpston in 1994. The problem appears to be caused by low melting-point liquid collecting on the land as the joint solidifies. This liquid comes either from the presence of bismuth in the solder and/or lead dissolving from the component finish. The fillet pulls away from the land as the joint cools and the solder contracts, as shown in Figure 1 and these slides.

Figure 1: Mechanism of fillet lifting

Mechanism of fillet lifting

The problem occurs to a greater or lesser extent with most lead-free solders, depending on the conditions and whether any lead contamination is present in the solder, and debate continues as to whether or not there is any adverse reliability implication.

WWW Research

Read this article and then browse for "lead-free solder" +"fillet lifting" and make up your own mind whether fillet lifting will be a serious issue for your company’s products.

With all soldering tasks there is an issue about solderability, and particularly maintaining parts in a solderable state after storage. But the problems are generic, rather than related specifically to lead-free. More about solderability, solderability testing and changes with solderability with time at this link.

Whilst one can overcome poor solderability by increasing the soldering temperature, care has to be taken not to overdo this! Not only can damage to components occur, but high-temperature increases the rate of leaching of copper from the surface of both lead-frames and pads. It is reported that wave soldering at 265°C can remove 10µm of copper – only 10% of the lead-frame thickness, but one-third of a typical copper pad.

Supplementary Information

Indium Corporation and Motorola carried out extensive work on paste development, and ran across some other problems in the course of transferring prototypes to manufacture. The work has been widely reported, but there is a good summary of the issues in this presentation by Steve Dowds.

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Components

Almost every observer of the lead-free scene agrees that the principal problem in going lead-free is the supply of components – getting information about components, getting components with the right specification, and managing the transition between a non-compliant part and a compliant component, even though there may not necessarily be any visible difference between the two. In fact, with some manufacturers, you may not be aware from the accompanying paperwork whether or not the component is lead-free or lead-free compatible, or full of ‘nasties’! As part of Assignment 2, we shall be asking you to look at a real circuit which was converted to lead-free at the beginning of 2004.

Supplementary Information


A general resource for all issues relating to components is this presentation by Angus Westwater of Rohm.

Lead-free terminations

Availability of components with lead-free terminations was a major problem, although lead incompatibility is only crucially important where bismuth-containing solders are to be used, and with BGAs.

For packages with lead-frames, nickel-palladium with a gold flash has historically been the most common substitute for solder plating. This has 1–3 µm plating of nickel, followed by a 0.02–0.15 µm layer of palladium, and a 3–10 nm gold flash. The palladium protects the nickel from oxidation; the nickel keeps the base metal in the lead frame from diffusing into the palladium; the gold improves wettability. Texas Instruments pioneered the use of this material in the late 1980s, but its slow uptake has been due to supply and price issues, most of the world supply coming from Russian and South Africa. There is also competition from palladium use in catalytic converters for vehicles.

A simpler option is a pure tin finish, a return to the dominant finish in the 1960s. However, there are concerns about the potential for the growth of tin ‘whiskers’, which grow from the surface of the stressed metal. There is more about this in Unit 8. Whisker growth can be reduced by adding lead (not very helpful!), and alternatives being investigated are high-tin alloys with bismuth, silver and copper. This is an area where solutions are still being sought.

A number of area packages such as BGAs use solder ball terminations, and it seems likely that these will migrate to SAC. However, this will not work for some complex devices, such as CBGA parts, which use a high-melting solder to define a sufficiently high stand-off from the substrate in order to enhance reliability.

WWW Research

Take soundings from some representative manufacturers as to the approach they are taking to make their components lead-free.

Suggestions:

 

Supplementary Information


If you have had an X-ray in hospital, and observed the use made of lead shielding, you will know how good a barrier to X-rays lead is. The question that one inevitably asks is whether X-ray inspection will do an adequate job with lead-free in critical applications such as inspecting BGA joints. Here is the answer from David Bernard of Dage.

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High-temperature withstand

The higher time at temperature will affect component choice and specification, and was originally predicted to result in the demise of the SM aluminium electrolytic capacitor, although versions being developed can now withstand 250°C.

The electrolytic capacitor failure mechanism involves breakdown of the internal gel electrolyte, and is fairly specific to that part. However, there are a number of other components where high temperature produces distortion. Any plastic moulding is vulnerable, depending on the material of which it is made and the stresses that are embedded in the construction during moulding. Connectors are a good example of components that have been known to “melt” under reflow conditions. It’s not just the simple parts – you might have noticed a previous reference to the very small dimensional changes that can result in the failure of a relay to operate correctly.

Problems have also been reported with LEDs damaged by a combination of heat and mechanical stress, where the epoxy softens, allowing inner parts of the assembly to move if any residual stress remains as a result of the assembly operation.

Supplementary Information

The amount of work needed to requalify components for the higher lead-free temperatures is substantial, as indicated in the presentation by Robert Hilty on Lead Free Component Qualification at the TURI site. The information on resistance to soldering heat is near the end of the paper.

To think about . . .

Does anyone care about components surviving? Thankfully, more and more people are becoming concerned. This report from Angus Westwater (video and transcript) dates from 1999 . . . there was interest and collaborative work then, but how much progress has really been made since?

 

Polymer mouldings are not the only instance of a generic constructional feature with lead-free implications. Another such is those components that contain solder. These can present a challenge about their lead content – are they covered by the exemptions to the Directive? – but they also need to be able to survive lead-free assembly.

Tin and lead are a unique combination, in the sense that varying the percentage of tin produces useable solder alloys whose melting temperatures range from 183°C to over 300°C. This has led to their use in component manufacture and similar processes that require a differential between melting points.

One example where solder is used inside a component is for die attach in power products, where the bonding material generally needs to have both high thermal and electrical conductivity. The most common solders for this application are either high-lead (such as Pb5Sn, Pb5Sn2.5Ag, Pb2.5Ag2Sn) or high-tin (Sn25Ag10Sb, and Sn8.5Sb). Whilst the two lead-free alloys in this list are used extensively, they have solidus temperatures of only 228°C and 236°C respectively. This creates a problem when parts are to be assembled with lead-free solders, and are specified for reflow at 245°C for extended periods!

There are as yet no straightforward inexpensive solutions: a Japanese group reported work on an AlZnMgGe alloy, but their results were not encouraging; known alternatives include high-Au alloys such as Au20Sn and Au3Si, but these are considerably more expensive than high-Pb alloys, and are arguably too rigid for some applications.

The search continues at NCMS among other places, for a solder that will have a high melting point yet be free of both lead and precious metals. Encouragingly, although no details have been published, HOTSOL, a “High Operating Temperature Solder with Zero Lead”, has been the subject of a UK Smart Award and it is currently being evaluated in a pan-European project. Watch http://www.tcore.co.uk/Research/Solders.htm for details.

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Moisture sensitivity

Plastic moulded packages also have lead-free issues, although direct distortion due to the application of heat is not normally a problem. However, moisture absorbed by the packages does give rise to major problems.

Handwerker expressed concern over plastic-moulded packages that higher reflow temperatures “will have a severe, negative impact on component performance and, therefore, on the component ratings”. Certainly it has been reported that absorbed moisture, leading to popcorning, is a worse problem at the higher temperatures needed by lead-free systems.

Moisture sensitivity of devices has gradually increased in importance in recent years, as the result of integrated circuits becoming smaller and thinner, with the silicon “nearer the outside”. As a result, for several years there has been a pair of moisture sensitivity specifications that set out a number of levels of sensitivity, proposing both safe times for exposure on the shop floor in an uncontrolled humidity environment, and a means of restoring the part to its initial manufactured and packed status by gentle but thorough drying.

The consensus of opinion is that the higher temperatures of lead-free soldering make parts typically two levels more sensitive, and the prediction is that dealing with MSD (moisture sensitive devices) will become as big an issue as ESD was a few years back.

To think about . . .

Take a look at the actual specifications relating to moisture sensitivity:

J-STD-020B Moisture/Reflow Sensitivity Classification for Nonhermetic Solid State Surface Mount Devices

J-STD-033A Handling, Packing, Shipping and Use of Moisture/Reflow Sensitive Surface Mount Devices

Are the two slides in this presentation an adequate and current summary? What will the practical impact be for a typical contract manufacturer?

 

 

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A step-wise transition

Changing to lead-free compliant components from lead-containing parts involves potentially three steps, though they are not always achievable in this order:

Step 1 Components are selected to withstand lead-free processing – the higher temperatures of the lead-free process take a number of components beyond their safe operating limit.

Step 2 These components are provided with lead-free terminations – where current component terminations contain lead, there may be compatibility problems between these and some lead-free solders

Step 3 All materials in the components are free of lead. [For “lead”, one might also read “bromine” or any of the other materials that are under threat because of the RoHS Directive.]

For most purposes the combination of the first two steps is what will be needed for RoHS compliance. However, as a user, you need to be aware of any components that might still contain lead, even though it may be in a form that is permitted, for example, as a high-lead solder.

For a transitional period, many companies will elect to buy components with lead-free terminations, even though these are not intended to be soldered with tin lead solder. This enables the supply line to be “flushed”, so that there will be no problems in applying a lead-free solder to components that are subsequently found to have lead-containing terminations. However, it is worth keeping in mind that, in order to move to lead-free soldering, the components themselves will have to have the right terminations and be capable of the higher temperatures of lead-free reflow.

 

Supplementary information

We recommend that you make a practice of examining every new type of component and assuring yourself that you understand how it is made. It is only in this way that you will be able to understand the compliance issues facing the manufacturer of the part.

If you need generic information on the construction of components, look at the material at these links.

Note that components may be able to withstand a high temperature for a short time, but less able to survive longer exposure, albeit at a slightly lower temperature. The specification of components is an area where extreme care is needed.

 

 

Supplementary information


How do you find out whether your parts are lead-free compliant, lead-free compatible, have lead-free terminations or are just plain leaded? This is a task that Dionics have tackled. Skim their presentation to learn something about the issues. More about this problem in later Units.

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Marking

The move to lead-free must also be communicated clearly within the whole of the supply chain. This is probably one of the critical areas for control, and something that we will be visiting again in a later Unit. The user is highly reliant on the manufacturer making a clear declaration as to whether a component has lead-free terminations, and whether it will withstand lead-free soldering. Then, of course, there are details of what temperature the components will withstand, bearing in mind that higher specification components may command a premium price or be less available.

Much of the marking of lead-free compliance will need to be on the packaging of the components supplied to the assembly house, rather than on the component body; fortunately more room is available for the assemblies that these components are used on because the marking requirements are more extensive. This is because, whilst a simple “lead-free components + lead-free solder” statement may suffice to demonstrate compliance, a repairer will need to know which of the many materials has been used for the assembly, and an end user in the automotive industry may need to be told of the (permitted) lead content of specific parts. Before you move on, you might like to explore some of the issues about marking and proposals that are currently being discussed.

WWW Research


Marking is a major issue and fills a whole page on the Soldertec web site. How far has the collaborative work gone? Is there yet a totally workable standard that everyone can sign up to? What conclusions can you draw?

 

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Other implications for lead-free assembly

The major impact here is in the control of different parts, particularly where both leaded and lead-free parts are in use. A decision has to be made as to whether to create a new set of part numbers, or take the risk that some of lead-containing parts may exist as a residue within a larger lead-free stock.

It is not just those who convert to lead-free who will be affected. Kay Nimmo of Soldertec commented that changes to component supply will create issues of obsolescence with standard components, because suppliers will make only the lead-free option. This will affect everyone in the automotive and aerospace sectors, for example, and not just those using lead-free.

In theory, it would be attractive to be able to offer both tin-lead and lead-free products, as this would allow a smooth transition. However, in practice, using multiple alloys adds to complexity and cost:

Different processes mean different algorithms for inspection and replacement equipment, different reflow profiles (and perhaps different equipment) and different wave soldering procedures and equipment – at the very least the pot and the flux have to be changed. Inspection criteria are also different, whether inspection is automated or manual.

Running two lines, one for leaded and the other for lead-free, also creates capacity issues, with one line perhaps being under-utilised, whilst the other is overloaded. At the same time, many operators (probably all personnel) have to be trained in both processes, both processes have to be maintained, and engineering changes have to be controlled across both processes.

To support two processes and two lines, two part numbers are needed for the same item. This means they have to be stocked separately, inevitably with increased inventory costs. Also, it is likely that splitting the volume between two different component types will increase unit cost.

In short, running multiple alloys is unlikely to be attractive and most commentators will suggest that companies run with just lead-free if this is at all possible.

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