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Lead-free Implementation

Lead-free implementation

Unit 3: Alternatives to soldering

Contents

Working towards Assignment 2

By this stage you should have completed Assignment 1, and be thinking about Assignment 2. Whilst Part 2 of the assignment relates primarily to Unit 6, Part 1 asks you to select processes, or combinations of processes to meet the needs of all AMS’s customer products. As you read this unit, try and draw on your knowledge of AMS’s products to see whether alternatives to soldering might be appropriate for some applications.

Introduction

So far in the module we have looked at the pressures to go lead-free; now we look at the implications of this move. There are broadly three areas to consider in our response, finding alternatives to lead solder, making appropriate adjustments to board materials and finishes, and making sure that components are both lead-free and will survive the changed process conditions.

However, putting the issues that way makes the assumption that lead-containing solder has simply to be replaced by a lead-free equivalent. Whilst that is appropriate for many applications, it is not the only solution. That is why, before we turn our attention to alternative solders in Unit 4, we are taking a short excursion to look at some alternative construction methods.

Few of these are new, but they are all still used in electronic manufacture. Also, as should be clear by the end of this unit, many of these alternative constructions are themselves affected by moves to lead-free elsewhere. This is just one example of the holistic approach that is needed when considering electronic interconnection.

Alternatives to solder

Solder is not the only medium of interconnection, although it has won primacy of place over the last 100 years. At this point, we invite you to think about all the other ways in which electrical connections can be made.

Research Topics

Reflect on your own experience of a wide variety of electronic assemblies, and of all the ways in which connections are made other than by soldering. Then try a web search for alternatives to solder. Here you will have to think laterally in devising ways of generating ideas and at the same time filtering out the chaff! Some ideas for search terms:

Hint: It’s also worth quickly researching the history of electronics. Browse for terms like "point-to-point construction".

 

show solution

Last revised 22 June 2004.

Whilst keeping in mind the wider range of connections whose appropriateness will depend on the application, we are next going to focus our attention on ways in which polymers can be used to remove the need for any solder, and then look at some of the mechanical methods of making electrical connection.

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Polymer replacements for solder

In the solution to the last search we deliberately said little about ways in which conductive polymers were employed in electronics or the different configurations used – this is your next research task.

Research Topics

Search the web using two search terms:

If you can search a single-site, also try "conductive adhesive" at www.chipscalereview.com, which gives around 25 articles.

Remember that this is a “skim and dip in” activity, not a command to dig deep!

What clues do you get from your search about the different types of conductive polymer and how they are used?

Last revised 23 June 2004.

 

Most polymers are good insulators, with resistivities in the range 1010 Ω.cm for PVC to 1018 Ω.cm for PTFE (Figure 1), and this insulating property is a major reason why these materials are widely used throughout the electrical and electronic industries in applications such as insulators, dielectrics, and resists. [More information on conduction, resistivity and dielectric properties can be found at this link]

Figure 1: Comparative volume resistivity of common electronic materials

Figure 1: Comparative volume

During the 1970s, research began on the development of a new class of polymer materials that exhibited intrinsic conductivity without the need to incorporate metal fillers, and these are beginning to be used for specialist applications1.

1 Key material names to look out for are polypyrrole and polyaniline, but expect most of the papers you find to require substantial knowledge of materials science.

 

However, adding metallic fillers to conventional polymers both makes them conduct electricity and enhances their thermal conductivity. The conductivity achieved depends on the type of metal, the particle size and shape of the filler, and the percentage loading of filler. Silver is a commonly-used filler, especially for die attach, but there have been attempts to use cheaper materials, such as carbon and copper. As with any task involving the compounding of polymers, other fillers will be added to the resin in order to enhance the flow characteristics and mechanical properties. For example, carbon fibres have been added to create a more robust joint.

There are three forms in which polymers are used in a conductive role:

The first two technologies were devised primarily for board to board connection, and some more detail is given at this link.

The third technology, using a substantially uncured polymer loaded with metal powder, is by far the most common, having a 30-year history of use for die attach in the semiconductor and hybrid microelectronics industries. These filled resins conduct equally in all directions, as distinct from anisotropic2 materials, that conduct only through their thickness. These were developed during the 1980s for making contact to very thin conductive coatings used for displays. Read our short summary of the two types of conductive adhesive at this link.

2 There may be differences in all kinds of characteristics depending on whether one is measuring through the thickness of a material or across its surface. For example: An epoxy-impregnated mat of woven glass fibre may expand much more in thickness than in the direction of the glass fibres, because these constrain its expansion. Some materials with structured fillers may conduct electricity quite well in the thickness direction ('Z-axis') yet be almost totally non-conductive from grain to grain across the surface of the film. These differences with axis are referred to as ‘anisotropy’, as distinct to the materials being 'isotropic', that is the same in all directions (from the Greek).

It is the isotropic conductive adhesive that has attracted much attention as a potential solder substitute mostly because of increased environmental concern about lead, but also in response to the drive toward materials which are more fatigue-resistant than solder.

Inevitably the document we have written constitutes only a simplistic view of the issues relating to the electrical and mechanical performance of ICAs. Some of these issues are discussed in Ken Gilleo’s presentation. As befits one of the gurus in this sector of the industry, Ken is very bullish about ICAs, and they are indeed very well worth considering if you have a severe weight problem, as they are far lighter than conventional solders.

Supplementary Information

Good sources of reference material on the topic of conductive adhesives are:

Epotek http://www.epotek.com/default.htm

Emerson and Cuming http://www.emersoncuming.com/ToolKit/TechPapers/

SMTA http://www.smta.org
(membership comes free with SMART Group membership)

If you need more information about this topic, contact Martin Tarr.

Last revised 22 June 2004.

 

As well as finding application as a direct solder substitute for assembly, you may well have discovered during your browsing other uses for conductive adhesives. Examples of this are for flip-chip bumps, as a replacement for solder, and in board fabrication, where conductive materials have been used as a substitute for conventional plated metallisation, particularly in low-volume applications.

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Mechanical attachment

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

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

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

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

Exercise

In the light of the preceding paragraphs, reflect on the types of mechanical connection used in products with which you are familiar, and try and identify what are the important factors that you need to take into account when making electrical connections by mechanical methods.

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When it comes to using mechanical methods to make connections, there are three main types of ‘gas-tight’ connection that are briefly described in the associated links:

Hopefully you will have opened the press-fit connector document and carried out the task in it, so will have been duly impressed by the ingenuity of engineers in providing innovative solutions to the problem of making a metal-to-metal contact. Another ‘gas-tight’ connection is used in the wire-wrap joint, a technique that was much in vogue for backplanes for large computer systems and is still used for prototypes and other applications where modifications are expected. Again this is an alternative to conventional solder technology, so we would encourage you to spend a short while on the next piece of web research.

Web Research

Search the web to find information on wire wrapping.

If you looked for "wire wrapping" or "wire wrap", you will now know that the term is more common amongst workers with precious stones than in electronics! You could try a filter – let us know if you find something more effective than –jewelry.

You should think laterally, perhaps focusing on the uses of wire-wrapping, or try a composite search term such as "wire wrap" +standards.

 

now read our comments on this technology

 

Last revised 23 June 2004.

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Lead-free and mechanical connections

So a number of different technologies are available, all of which have a place, depending on the application. Some of these even present an opportunity to move away from solder. However, assuming that we will probably have to stay with a solder technique, but that there will be some mechanical connections, will there be any interaction?

Exercise

What impact will going lead-free have on mechanical connections?

We suggest taking a look at the excellent collection of papers at the Tyco Electronics lead-free web site and skim-reading at least those by George Chou and Robert Hilty Effects of Lead-Free Surface Finishes on Press-Fit Connections and Eric Verhelst and Tom Ocket Lead-free manufacturing: Effects on press-fit connections.

Look for hints as to the effects that lead-free soldering has on other aspects of the process. This will be useful background when you come to look at the implications of lead-free for components in Unit 6.

show our comments

Notice from our solution that we are once more making the point that lead-free is not something to be implemented in isolation, but has wide implications for the whole assembly

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Reviewing the options

Of course, when selecting alternative technologies, one has to be careful not to end up in an even worse situation! We have to take into account such issues as:

Where mechanical connection methods are inappopriate (and this depends very much on the application) the performance limitations and practical difficulties associated with adhesives suggest that the most likely outcome is the further development of lead-free solders. Which leads us neatly to looking at those materials in Unit 4!

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