Lead-free Musings
10/01/2000
By Harold Hyman
Having been involved in various aspects of electronics assembly for more years than I care to admit, I find it interesting to review the challenges that have been presented to assembly engineers. I have also noticed that at most of the technical conferences I have attended, sessions that related to soldering have attracted a great number of attendees.
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I will never forget an expression used by the late Marty Boyle, former vice president of Alpha Metals. He called a solder joint "the magnificent trifle," and that just about sums it up. Regardless of the ingenuity of the component designer and the brilliance of the architecture, a device still needs to be connected to the outside world. This is invariably achieved by soldering - a technology that, in principle, has not changed much over the past 3,000 years. If any one of those hundreds, if not thousands, of "trifles" is not properly made, then the entire assembly fails. That was Marty's nightmare.
So, over the years, the technical community has risen to the occasion by creating new soldering materials and equipment and finding ways to deal with smaller components and greater numbers of I/Os. Devices with "hidden" connections, such as area array components, are aligned, placed and reflowed as a matter of course. Problems that initially seemed insurmountable have been resolved with relative ease. The evolution of solder fluxes, reflow ovens, rework systems, and all the ancillary tools and materials that are commonplace today has seemingly occurred without difficulty. It can be taken for granted that, given a problem, a solution will be forthcoming. Such faith is a remarkable tribute to those involved in electronics assembly technology.
All these musings lead me to the latest revolution in electronics soldering: the introduction of lead-free solders. A combination of national and international legislation, market forces, and ecological responsibility has driven the industry to realize that, like it or not, lead-free soldering is feasible and a reality.
A great deal of work has already been done, mostly in the selection and standardization of suitable solder alloys. By now it is commonly well-accepted that there isn't an easy substitute for near-eutectic tin-lead solders that have a melting point of 183°C. Of all of the alloy compositions that could be used in place of lead-containing solders, the favorites seem to be various combinations of Sn/Ag/Cu or bismuth-containing alloys.
The common denominator of all of these choices is an elevated melting point, which will require a reflow temperature of about 240°C (30°C higher than traditionally used). This, in my opinion, is the new challenge. Components and materials, not to mention equipment, that presently function satisfactorily at traditional reflow temperatures may be able to withstand the rigors of a 30°C increase, but it will be difficult for those that are already marginal to not suffer damage when they face these more arduous conditions. PCB substrates may discolor and/or blister, plastic encapsulated components may delaminate and fluxes may decompose. These problems rapidly become apparent when a rework operation is performed because the overheating effect is at least triplicated.
The board, components and materials are subjected to elevated temperatures when they go through the initial reflow operation and, at this point, they suffer the consequences. If, however, it is deemed necessary to rework a component, then a nearly identical thermal excursion will be twice imposed on the relevant area of the assembly - once during the removal of the defective component and again during the replacement with a new one. If site dressing is required, this will add an extra thermal factor to the damage equation. Such repeated exposures to elevated temperatures take their toll and dramatically demonstrate the limitations of those parts of the assembly that cannot withstand their harmful effects.
How will these problems be resolved? Short term, there have been some excellent examples of early success stories. With considerable extra effort, process engineers have worked within small process windows. Thermal process systems, especially those with automated profiling features, have stepped up and met stringent demands. However, in the long run, we must invest in circuit elements that will not be adversely affected by these harsher operating conditions. New, low-cost materials that are capable of suffering the slings and arrows of lead-free soldering will promote a full-scale, environmentally friendly process. If the past is any indicator, a resolution will undoubtedly be obtained and the practitioners of printed circuit processing will once again rise to the occasion and create an even more magnificent trifle.
HAROLD HYMAN, manager of technical support, can be contacted at SRT, 59 Power Road, Westford, MA 01886; 978-614-4437; Fax: 978-392-0638; E-Mail: [email protected].