Lead-free solder proposals put pressure on electronics manufacturers

Officials of the U.S. Environmental Protection Agency (EPA) want to change how the federal government regulates the use of lead, which has the potential to increase manufacturing costs and compromise reliability of electronic components — particularly those in military and aerospace applications.

By Tom Adams

Officials of the U.S. Environmental Protection Agency (EPA) want to change how the federal government regulates the use of lead, which has the potential to increase manufacturing costs and compromise reliability of electronic components — particularly those in military and aerospace applications.

At issue is the lead-based solder used to manufacture electronic components and printed circuit boards. A draft EPA proposal would reduce the threshold of lead usage at which manufacturers must file detailed reports — at considerable expense — from an annual 25,000 pounds to 10 pounds.

Some experts say these EPA proposals could impose costs on industry great enough to drive small firms out of business.

EPA's assault on lead use is not happening in a vacuum. Lead use has come under fire throughout the world, and is leading to the elimination of lead in products ranging from paint and buckshot to fishing sinkers.

Within the next few years, in fact, leaded solder will be largely removed from microelectronics manufacturing worldwide. In the U.S., "lead-free" is just now becoming a buzzword, yet is causing much uncertainty and confusion.

The transition from leaded solders to lead-free solders is most advanced in Japan. Leaders of Panasonic have already started replacing lead in solder, and plan to have all their consumer electronics products lead-free by 2001. Hitachi is on track for having all of its products lead-free by 2001, and Toshiba officials say they will remove all lead from their cell phones by 2002.

Guiding the switch to lead-free solders in Europe is a proposal from the European Commission called "Waste from Electrical and Electronic Equipment" (WEEE). This proposal calls for consumer-electronics manufacturers to use lead-free solders in all products they sell in Europe by January 1, 2004. The WEEE does not mention products used in military applications.

The U.S. appears to be last in line for the transition to lead-free solder. There is practically no legislation to mandate lead-free solders. An informal survey of several military suppliers showed that most are just beginning to think about the problem.

Hanging over everyone's head, however, is not only the EPA proposal, but also the worldwide trend toward the elimination of lead-based solder.

Worldwide, the percentage of lead going into electronic products falls between 0.5 and 2 percent. Scientific evidence suggests that leaded electronic solders in landfills do not leach into surrounding soil.

Still, some in the electronics industry believe it will probably be impossible to export consumer products containing leaded solders to Europe after January 1, 2004. A study by the U.S. National Institute of Science and Technology estimates that U.S. companies could lose $420 billion in sales from 2003 to 2005 if they stay with leaded solders.

Since military applications do not fall under the same market constraints as consumer applications, military electronics makers might simply keep using leaded solder indefinitely.

Realistically, however, military applications will also become lead-free as designers move toward using commercial off-the-shelf (COTS) components and subsystems. The same manufacturers making consumer products — and the components that go into them — will be supplying COTS items to the military.

If it were not toxic, lead could indefinitely hold its place as the ultimate complement to tin in solders. Lead is inexpensive — 20 cents a pound was one spot price in March — and there is no conceivable shortage of the metal, experts say.

Dozens of potential replacement alloys are available — nearly all of which use tin and one or more other metals — but replacements with the superior flow and adhesion properties of leaded solder are extremely difficult to find.

One of the leading contenders to replace leaded solder is an alloy of tin, copper, and silver. Several such alloys are available, some of which are patented, and their compositions are generally about 96 percent tin, 3 percent copper, and 1 percent silver. Experts believe these alloys can become successful replacements for leaded solders in most applications, although there is not yet a solid history of usage or reliability.

Hotter temperatures

Despite the technical qualities of this group of replacement alloys, these alternative solders have the potential to cause drastic changes in manufacturing processes because they melt at higher temperatures than lead.

Standard tin-lead solder (63 percent tin, 37 percent lead) melts at 183 degrees Celsius. The tin-copper-silver alloys, meanwhile, melt at around 217 C. This relatively large increase in melting point has serious implications for manufacturing, experts point out. Processing temperatures — currently around 225 C for many leaded solder applications — will rise to hotter than 250 C, and sometimes to as much as 260 C in some instances.

Other likely replacements exist as well. Officials of the Institute for Interconnecting and Packaging Electronic Circuits (IPC) suggest a tin-copper alloy as a low-cost alloy for wave soldering, and a tin-silver-bismuth alloy for surface mount applications.

Overall, the switch to lead-free solders will place many demands on suppliers to the military.

First, the temperature profiles of some reflow ovens and some wave-soldering machines can adjust to accommodate the higher processing temperatures. The problem for manufacturers will be in determining just which equipment they can adapt for which product runs. Manufacturers will have to replace some equipment, but no one knows yet which equipment, experts say.

Manufacturers use leaded solder inside components, for example, in the tinning of leads to improve the adhesion of wires. Although noble metals such as gold and palladium are possible substitutes in this application, many existing molding compounds do not adhere well to these metals. This often results in internal delamination.

In addition, it is not clear if replacement solders can achieve the same thorough wetting as leaded solders. One danger is "cold" solder joints, which appear to be bonded but which lift off easily. Experts also are not certain if wetting properties are adequate to ensure complete coverage in through-hole plating, which is still widely used in military applications.

Finally, failure analysts and makers of molding compounds predict that surface-mounted components are likely to suffer internal cracks and delaminations if technicians do not handle temperature requirements well. To counter this threat, at least one molding compound maker is developing new compounds designed to survive the higher processing temperatures.

While new molding compounds may help, the reality is that manufacturers will depart from materials and processes that have long histories of reliability, industry experts say. These manufacturers are taking up new materials and adapting processes in which they have little information about reliability.

The danger of internal cracks and delaminations in components is especially troublesome; if all cracks and delaminations were immediately lethal, it would be possible to make process or material adjustments to avoid the problem. But many cracks and delaminations have no immediate electrical symptoms at all. Instead, they reside in the component until conditions such as humidity, thermal cycling, and mechanical shock cause them to grow and break an interconnect.

As one military failure analyst points out, the difference in reliability between consumer products and military products is profound. No one is very surprised when a two-year-old computer printer fails; it is probably already obsolete anyhow. But military systems must be reliable over periods of five or 10 years or more.

Destructive physical analysis can detect internal cracks and delaminations. Experts also can view cracks and delaminations nondestructively with acoustic micro imaging systems.

Potential defects on PC boards that eventually may have to endure higher solder temperatures are receiving little attention, even though there is considerable talk about the impact of higher processing temperatures on components. Mark Thompson of Prototron Circuits in Redmond, Wash., notes that all major manufacturers of board materials already offer materials that are stable at relatively high temperatures. Although these materials are not widely sold at the moment, their market is likely to increase with the introduction of lead-free solders.

In addition to adjusting to higher processing temperatures, manufacturers and assemblers also face paying closer attention than they do today to the compatibility of various items. Replacement solders other than the tin-copper-silver "standard" have a variety of melting points, some of which are substantially lower than the 217 C melting point of tin-copper-silver. "If we have a low-temperature solder being processed at higher temperatures," one failure analyst observed, "we'll have components falling right off the boards." Some of the compatibility issues are subtler — the whiskering characteristics of various replacement solders, for example.

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