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Emerald Group Publishing Limited
Copyright © 2004, Emerald Group Publishing Limited
The SMART Group lead-free assembly and soldering workshop
The SMART Group lead-free assembly and soldering workshop
Keywords: SMART Group, Speedprint technologies
Held at the premises of Speedprint Technologies Ltd, Upton, Poole, Dorset on Thursday, 27 November
Bob Willis, Technical Director of the SMART Group, welcomed over 40 delegates to their meeting held at the premises of Blakell Europlacer Ltd. The day consisted of a presentation on the subject of lead-free soldering, followed after lunch by a practical "hands-on" look at the application of, and subsequent processing with, lead-free solders on the Blakell screen printing, component placing and reflow soldering equipment arranged in their imposing showroom.
Attending such a SMART Group meeting evidences the wealth of knowledge and experience in the field of lead-free. It was therefore not surprising to see that many delegates came from companies who have to face up to the changes necessary before July 2006. In Bob Willis they have the perfect mentor. He opened the meeting by referring the delegates to the SMART group/DTI programme on reliability testing, in which you can benchmark your assembly process. A unique concept, and one which should be taken advantage of.
Getting down to the lead-free heart of the matter, Bob Willis covered several areas. He reminded delegates that the NPL Interactive Cookbook on lead-free is now in its 3rd edition, and apart from three new CD issued by Soldertec on hand, wave and reflow soldering, it is the only lead-free training manual in existence. He looked at some key points in legislation, essentially with effect from 1 July 2006 assemblies and components imported into, and manufactured in, Europe have to be lead-free. Here, the amount of lead contained has to be less than 0.1 percent by weight. So companies need to implement a plan for change as soon as possible. A visit to the Web site: www.dti-gov.uk sustainability would be useful for the latest feedback on issues and support on latest information.
Concerning PCBs themselves, generally there would be no change in the base materials, but the automotive industry are considering higher Tg materials. All solder finishes work with lead-free, and whilst there has been some talk about delamination and barrel cracking these areas do not give rise to concern.
Looking at solder finishes, in the range of gold, solder levelled, silver, tin and OSP, electroless Ni/ Au was the most popular, followed by HASL, silver and OSP. OSP wets well, and the HASL finish has some lead-free options, for which a visit to the SMART Group Web site is recommended.
Temperatures for lead-free would typically be as follows.
Wave solder – 260-265°,
Convection oven – 240-245°;
Vapour phase – 230-240°.
Concerning the IEC/IPC component requirements, the change to lead-free will move the goalposts higher up, so it is worth looking at the JEDEC Web site which shows draft charts on times and temperatures.
Bob then looked at some practical examples of component failure though incompatibility. Hand soldering is not difficult, but there can be tip corrosion and tip damage, so look at what your soldering iron suppliers are offering for lead-free. With regard to wave soldering, Bob has worked with Electrovert, using different flux technologies, and found that VOC materials are more stable in elevated temperatures. Alcohol fluxes tend to be exhausted at higher temperatures, so talk to your suppliers.
Concerning solder pots, bear in mind that the cost of solder will be going up to 2 or 3 times the current price. The dross is therefore expensive, but lead-free does not make more dross.
Corrosion seems to be the main problem, all manufacturers have reported it, but using a proprietary coating over the stainless steel seems to work. When changing over to lead-free, it is easier to swap pots.
Defects such as fillet lifting during wave soldering were discussed. There was no evidence to suggest that this causes a reliability problem. Pad lifting was caused by expansion, and this is one area where a higher Tg may be of benefit. PCB design change can have an impact on fillet lifting. Concerning adhesive dispensing, Bon said that there were not many problems associated with placement.
With wave soldering, try to get rid of shorts, also with some alloys you get a duller appearance. Change alloy, more expensive yes, but the performance can be the same as what you are doing now. Bob recommended centre board supports for wave soldering on single-sided boards. He has also done some tests to prove that tin erodes surfaces, he has used various alloys here.
With print and reflow process, no problems, printing is simple with lead-free, as is the reflow process, although there are some concerns on metallurgy between two surfaces that need to be understood, also spread during reflow. Lead-free does not wet as well as conventional, is a fact of life, some people have been modifying the apertures to cope, successfully.
The use of vapour phase will increase, by about 10 per cent or so. There are five main suppliers in the field, results of tests showed that the process soldered well, there was some tombstoning but overall it is an excellent process.
Mixed technology wave soldering – if you get a small amount of lead into the interface the temperature drops, but otherwise OK. Double- sided reflow soldering – works well, but again during the secondary reflow, the boards can flex, so think about board support. Simultaneous sided reflow soldering – no problems.
Inspection – there are no inspection standards for lead-free, and it might be as well for IPC to leave the basic criteria alone. What we should have are reference joints for various materials. Bob circulated some SMART Group inspection posters showing different lead-free joints for discussion amongst the delegates. The use of X-ray is effective, too. Yield information on lead-free does not exist, but Bob reminded the assembly that the SMART Group has already started a project on PPM monitoring to compare tin/lead and lead-free processes.
Rework and repair – demonstrations by use of video showed that lead-free works fine, and the removing and replacing of components could be achieved without difficulty. However, said Bob, with lead-free we might start to see a move back to contact methods. You have to think about temperature demand to reflow the solder, but it works fine. Hot air pencils work well for removal of small components, even the very smallest 0201 and 01005 chips.
Finally, Bob talked through the lead-free line, which he ran the Hanover Fair earlier this year, the process line was demonstrated, and the parameters for the production line were discussed. The line used tin/silver/copper paste alloy, and Bob talked about the experience in detail, summarising that all the data were on the SMART Group Web site.
A few questions came at the end; use of airknives? Bob has done only one lead-free experiment, he found that you could remove shorts on some, but not all, boards, you cannot always get the knife close enough to give best performance. Tried soldering under nitrogen? Not yet. Who will "police" the use of lead-free? Your competitors!
Competitors? What competitors? When it comes to lead-free, it seems that the SMART Group has no competitors, either. A thoroughly enjoyable, informative and useful morning, with an equally valuable practical session in the afternoon.
John LingAssociate Editor,Soldering & Surface Mount TechnologyNovember 2003