Harsh environment applications for lead-free solder systems was the subject of a talk by Steve Brown of Cookson Electronics

Citation

Ling, J. (2004), "Harsh environment applications for lead-free solder systems was the subject of a talk by Steve Brown of Cookson Electronics", Soldering & Surface Mount Technology, Vol. 16 No. 2. https://doi.org/10.1108/ssmt.2004.21916bab.002

Harsh environment applications for lead-free solder systems was the subject of a talk by Steve Brown of Cookson Electronics

Harsh environment applications for lead-free solder systems was the subject of a talk by Steve Brown of Cookson Electronics

These environments are in the automotive, aerospace, military medical and industrial fields, and involve temperature cycling, a corrosive environment which may include vacuum, vibration or both. Steve covered the range of temperature experienced in the automotive environment, and the implications for military use. Lead-free must meet performance requirements; no one needs tin whiskers in space!

SAC is the alloy of choice in most applications, but there are variations on the theme involving silver, bismuth, indium alloys and gold alloys for high-end applications. Steve took us on a comprehensive sweep through the various aspects of SAC, and its variations, costs, effect on tombstoning, the wider pasty range. There is, he said, no unique "best solution" for every set of circumstances, tin-silver-copper is a good all-rounder, but every company must have room for something else. SAC is a primary choice, but its behaviour leaves a bit to be desired. He listed some proposals for the prevention of interconnect failures. They are as follows:

1. 1.

   change package design;

2. 2.

   compliant joints;

3. 3.

   underfills;

4. 4.

   total encapsulation; and

5. 5.

   board design.

Why do tin whiskers cause so much concern? Well, they cause short circuits, they can grow in ambient temperatures, there is no accepted accelerated test, and remember that products may be stored for many years before use. Nowadays automotive electronics are safety critical, tin whiskers are caused by compressive stress on the tin. Compression is caused by high stress coatings, deformation, intermetallic coating and thermal mismatch. Stress can relieved by reflow, the use of a matt tin finish, a Ni/Ag flash or a pre-bake at 150°C.

In terms of intermetallic management, he said that the use of a conformal coating can mask a tin whisker, but there is still a problem with harsh environments. On a brighter note, he concluded by saying that the situation is better now than it was even 1 year back. Another 12 months and who knows.

The acknowledged expert on product marking Kay Nimmo of Soldertec tackled the vexed question of lead-free and RoHS marking and labelling systems. This is indeed a complex issue, finding out what type of marking systems people would prefer. However the results of the survey were now in and available from the Web site: www.tintechnology.biz/soldertec/soldertec.aspx?page_id=1637&SelectedMenu_ID=73

Kay said that WEEE labelling will be put on all equipment after 13 August 2005. There were several options – CE marking, a crossed out dustbin, and smart chips – are all possibilities. Nothing has been requested officially from RoHS yet, it is still under discussion.

Whilst there is no legal requirement for marking for lead-free, how do you demonstrate compliance? It may well be an industry requirement – but this requirement can vary within the supply chain, it depends on the viewpoint of manufacturers, distributor, assembler, recycler, etc. There is no consumer marking.

Kay went on to explain about the difficulty in establishing an acceptable international labelling standard. There were various proposed JEDEC labels, and she suggested that a look at their Web site www.lead-free.org would be invaluable.

Mike Fenner of Indium Corporation looked at the practical aspects of lead-free soldering. He discussed some of the difficulties, and what you have to do to convert over to lead-free. Lead free is market driven, and you need to accept that this represents a fundamental change in the process of manufacturing your products.

Whilst there are some 200 alloys which have low melting points, the choice is pretty well made with SAC which has a melting point of 217°C compared to183°C for Sn/Pb. It takes longer to get to 217°C and it takes more energy to keep it there. There is an increase in DT. Suitable PCB finishes include HASL, OSP, Organic Ag, ENIG, Pd/Ni, and immersion tin. You will need to consider poorer wetting, harder surfaces, longer times, higher temperatures, using the same equipment. Various options include a change of PCB finish, a change of components, and a change in solder paste.

His suggestion was to go away and practice quietly in a corner somewhere, and you need to do it NOW! There are many variables to consider, and you may well want to draw up a wish list for a lead-free solder paste, but you have to expect the unexpected. There will be new inspection criteria, as SAC alloys do not wet as well as the old ones, they are more cohesive. Mechanical properties are different, they are stronger but they are less compliant, and failure models are different.

Mary Gregory from A & D Automation encouraged the view towards vapour phase soldering with lead-free. She looked back to the early machines that were crude and difficult to operate. But nowadays VPS as a means to transfer heat is too good to ignore; it provides efficient heat transfer using perflurorcarbon liquid, which is healthy stuff.

Nitrogen is no longer needed. An inert vapour atmosphere in vapour phase soldering gives the best possible solder reflow conditions, where the DT is as close to zero as it is possible to get. The IOR pre-heat, variable heating levels, and Soft Temperature Rise allows you to control the temperature rise at 6°/s and there is an economical reflow alternative for high volume SMD manufacturers when they are considering their alternatives for lead-free solder reflow.

New VPS equipment can produce a board every 20s. For lead-free no modifications are needed, all you do is change the liquid to one with a boiling point of 230°C. Is the vapour expensive? No, said Mary, it costs about 1.5p per min for the vapour.

Helmut Leicht, Managing Director of the manufacturers IBL, covered some of the problems associated with lead-free which included temperatures exceeding 230°C and wetting problems. Whilst traditional convection reflow equipment is already installed, and the process knowledge is well founded, if equipment has to be replaced then the problems associated with overheating and bad wetting will be eliminated by the use of an inert gas.

Vapour Phase has some disadvantages, in that no high volumes are available and VPS is not a mature process in the field. However, these can easily be outweighed by the facts that the equipment can be used immediately, there is absolutely no risk of unsoldered joints, no risk of overheating, and there is the best possible wetting in a 100 per cent inert gas atmosphere in a sealed container.

VPS joints typically show finer structure and less oxidation, and wetting is also better.

Densely populated boards solder superbly with VPS but not with convection ovens. With VPS you get excellent heat transfer through layers with less risk of "popcorning" of different materials with the lower temperature of reflow at vapour phase, and good temperature pressure in the best thermal environment.

Inline machines are now available, and Herr Leicht was keen to emphasise that VPS is worthy of serious consideration when considering equipment replenishment with lead-free soldering demands.

That another 50 people wished to attend and had to be turned away indicates the fast-growing awareness of the vital importance of the move to lead-free soldering. The concept of lead-free has been around for a very long time, but it has been left to the SMART Group, amongst a few others, to bring the need for "action this day" to be the call, rather than "we'll have a look at it next week".

The facts and figures given in the paper from Martin Allison of Senju accurately indicates the position of the UK. It makes worrying reading. It is not that the industry will not move to lead-free, for it knows that it has to. But it seems to be leaving it all rather late, and other countries who have positioned themselves to meet the demands of to-day will benefit whilst the UK pursues the "jam tomorrow" philosophy.

Plate 1 shows the speakers from the seminars.

Plate 1 All eight speakers from the 6th Annual Lead-free seminar