SMART Group. "lead-free update 2001"
SMART Group
The SMART Group "lead-free update 2001"
Keywords: SMART Group, Lead-free solder
The opening papers of this SMART Group 3rd annual seminar (Plate 1) addressed two problems frequently raised with lead-free soldering – what about the boards and components?
Plate 1 Speakers at the SMART Group lead free seminar. Left to right: Simon Hawkins,Metcal; Richard Camp, AVX; Gerry Andrews, Marconi; Kay Nimmo, ITRI; Nick Jolly,DTI; Martin Wickham, SMC/NPL; Dr Chris Hunt, NPL; Atso Fortsen, Teleste Access Systems, Finland; Neil Carpenter, Emerson & Cumming, Benelux
Gerry Andrews gave a production perspective showing that, for Marconi,Organic Solder Preservative (OSP) finish is their choice. Andrews' confident overview of all the alternatives and the work he and his team have conducted was covered in fine detail. This was a three-year performance review highlighting the company's search for a finish that will meet all the technology changes envisaged.
As well as being lead-free, any finish selected must also be (existing) lead compatible. The evaluation looked at visual inspection, X-ray, micro section,etc. and all results were qualified with photographic examples.
To summarise, lead-free finishes are readily available, they are fully compatible with lead solders, some show lower cost and higher quality yields. Immersion tin is a good "work horse" finish, but can be problematical. Silver gives a much better performance however, Marconi query its use for press fit.
OSP is their selected future number 1. The reasons are strong and well argued– lower cost, environmentally very friendly, no metals are used or thrown away, no storage intermetallic formed, no potential threat of migration or whiskering and very good press fit performance. Finally OSP has good field solderability.
What about components? Richard Camp from AVX started by updating on legislation; the EU's 4th draft recommendation is for the move to lead-free on 1 January 2008.
The USA has no implementation date; Japan remains the catalyst by recommending lead-free for its industry by the end of this year.
What is lead-free? The maximum lead traces allowed, in several medical standards; US EPA and ITRI Soldertec recommend a maximum of 0.2 percent. AVX's preliminary limit is 0.1 percent. Richard showed results of studies of the solder alloys under evaluation, ranking them separately for the wave and reflow process. The finish preferred is Sn.
A pure tin coating is the ideal termination finish, considering the compatibility with different solder alloys/cost/mass production experience. Pure tin coating is fully compatible with the current SnPb. The pure tin termination finish is already in mass production for components with barrel plating MLCC and Tantulum TA microchip.
AVX have no previous experience with formed terminations (J lead) with tin finish.
The potential issues appear to be: Visual assessment of the solder fillet;whisker formation; solderability after multiple reflow and pure tin plating on copper terminations.
For ceramic capacitors, all AVX plated terminations are currently lead-free(and have been for four years). Some surface mount chips do contain lead in the glass or ceramic matrix, but this is sealed and not open to the atmosphere, so is therefore exempt from current lead-free legislation.
The AVX BME (nickel electrode) product is 100 percent lead-free;approximately 35 percent of total production is currently manufactured using this technology. They estimate that 50 percent of production will be lead-free in one year and 100 percent in three to five years.
For a practical view, Atso Forsten, Manager, Design and Production Systems,Teleste Access Systems, Finland, was invited to share his experiences of a development and validation study into the lead-free wave soldering process. This project was shared between Teleste, Multicore (materials) and Ersa (equipment manufacturer).
Atso has extensive experience of soldering from his time at Nokia. Among the considerations with this project were; suitability of flow process (fluid properties), working temperature and joint acceptability, manufacturing and cost of alloy, compatibility with solder machine construction, long-term stability in a real process, alloy stability against impurities, metals dissolution, joint metallurgy against various substrates, etc. After some initial investigations SnAgCu replaced SnCu, with the testing, particularly in alloy and drossing behaviour. Excellent photographs and graphs enhanced this presentation.
After the alloy selection was made, further considerations included; flux type and how it is applied, optimising preheat parameters, review solder pot construction and temperature, also examined conveyor speed/angle versus nozzle geometry.
An early problem encountered was persistent oxide layer on top of the wave;this was solved by phosphorus doping (patent pending), also a new flux formula eliminated higher surface tension that was causing missed and shorted joints.
After much additional work, the selected process has been running for 24 months, with the following success; new nozzle development has reduced defect rates, new solder pot materials have proven stable against SnAgCu alloy, P doping of alloy has improved flow characteristics, fluxes have been developed to cope with oxide behaviour, drossing is under control at ~ 15kg/week,accumulation of impurities is under control, process maintenance rules and instructions have been developed.
The conclusion is that, after extensive trials, wave soldering using SnAgCu has proven a viable technology. The cooperation between Teleste, Multicore and Ersa certainly speeded up the project, Atso is of the firm belief that "the future will be lead-free".
NPL, The National Physical Laboratory, has many projects under way that are lead-free driven. One is the benchmarking and providing a code of conduct for profiling oven/wave soldering equipment, important because of the higher temperatures necessary for lead-free soldering.
Dr Chris Hunt was the first of two speakers reporting the extensive ongoing work on lead free that NPL is pursuing. First, Dr Hunt wanted to explore if there was a problem with lead-free joint reliability and how useful is modelling to answer this question.
The project parameters were outlined, with the experimental matrix comparing with leaded assemblies, a lead free transition and then lead free assemblies.
The reliability testing summary showed that the life time reliability of solder joints is material and component dependent (lead free PBGA can be more reliable than a lead free chip resistor), PCB finish has only a minor influence on solder joint reliability and for the majority of components there are no significant differences with lead free.
The use of modelling is fast but it does not model crack growth; it approximates secondary creep and needs more materials data/computing power. The simulation results were graphically presented.
Modelling can successfully predict aspects of thermal fatigue, although the future challenge is to include crack growth. Modelling will increase as computing power increases and costs change.
Martin Wickham, also from NPL, opened with the conclusion that nitrogen offers significant benefits in the processability of lead-free soldering. He showed the results of the work, partnered by BOC, detailing solderability measurements for lead free alloys, component termination process yield and tin whisker, linked to the effects of using nitrogen. A recent collaborative study of Automatic Optical Inspection (AOI) has been extended to include lead-free assemblies. A second meeting has recently been held, at NPL, with many UK materials and cleaning equipment suppliers, who will cooperate in a "cleaning after lead-free soldering study".
Kay Nimmo from Soldertec at ITRI, has frequently updated SMART meetings on what is happening in Japan. Her most recent visits were in December and January. Kay reported on the recent progress, both with production and promotion of lead-free products.
To set the tone, the reasons for our concern were mentioned, the solubility of lead and components, particularly acid rain and acid landfill. The Japan Electronic Association, JEITA Roadmap 2000, shows that the first adoption of lead-free in Japan in mass produced goods was in 1999. The general use of Pb-free soldering in new products is targeted for 2002. Full use of Pb-free solders in all new products is to take place by 2003.
The Japanese government supports and agrees the JEITA lead free activity;however, their concern is that trade barriers will not be erected and they are keen that there is standardisation on lead-free issues within global industry groups.
Soldertec recommends alloy Sn + [3.4-4.1]% Ag + [0.45-0.9]% Cu and has done so since October 1999.
For wave soldering, the control of dross is improved with the use of phosphorus and controlled atmosphere. The subject of fillet lifting was mentioned and the importance of regularly monitoring the solder bath.
A Japanese equipment company, Mr Flip, has developed a flat linear induction pump, as part of a dual wave system. Improvements in operation include better hole filling, less wave fluctuation, smoother surface, and less solder capacity is needed.
The preferred profile for reflow soldering was shown and how the Japanese had improved this – particularly to improve joint strength.
Much promotion of products in Japan is listed as either lead free or using halogen free materials; in fact, Panasonic has a one-minute slot on TV,promoting their lead-free products. They have also announced plans to introduce lead-free wave soldering to all their manufacturing facilities in Europe.
Kay also showed examples of lead-free products; Hitachi is building PC motherboards and a refrigerator main control panel. NEC mobile equipment,Panasonic – a cell phone, Nissan a keyless car entry system, Sony a Camcorder plus two models of TV and Fujitsu a high end server.
The most famous product is the Panasonic lead-free Mini Disc and a video tape player; for both of these, over one million have been built. In each case Kay linked each of these products with the type of alloy used.
So, Japanese manufacturing sites are on target to go lead-free. Their overseas sites are also implementing this process. Some significant equipment developments are taking place, i.e. new wave shapes and temperature control equipment. The marketing of lead-free products is supported by press and TV adverts.
It was relevant to follow the presentation with the announcement of the SMART/DTI mission to Japan (the team departed two days after this seminar). Bob Willis, Technical Director of SMART, thanked Kay Nimmo for all her recent presentations regarding the activities in Japan.
The alternative to lead-free solder was presented by Neil Carpenter from Emerson & Cuming: conductive surface mount as a solder alternative.
Nick Jolly, from the DTI, joined all the speakers for a lively debate to end the day.
Simon Hawkins, Metcal, and an active SMART Group committee member, chaired the day's proceedings.
A small exhibition was sold out months ago. Included were NPL, ITRI, Concoat,Heraeus, Electrolube, DEK Printing Machines, Loctite-Multicore, Emerson &Cuming, Indium, R.L. Pidduck, Reed Exhibitions, TWI and Earth-Tronics.
A lead-free soldering demonstration was also available during the day.
SMART appreciated those attendees who travelled from Sweden, Norway, Finland,Germany and Benelux.
In a raffle held during the day, £192 was donated to the Indian Earthquake Appeal. First prize was a Wycombe team soccer shirt, won by Jason Cooper of Briton Electronics, whilst Peter Norgate of Planer won the new Lead-Free Cookbook 2, an interactive CD-ROM.
