The E.U.'s proposed restrictions on hazardous substances (RoHS) have elicited considerable grumbling from U.S. manufacturers, especially those in the electronics business. The new regulations will cover five classes of material: lead (Pb), cadmium (Cd), mercury (Hg), hexavalent chromium (Cr6+), polybrominated biphenyls (PBB), and polybrominated diphenyl ethers (PBDE). Of these, lead is surely the most ubiquitous—is there any Sensors reader who has never done any soldering? Lead-free solders consisting of tin and another metal such as silver, copper, bismuth, zinc, indium, or nickel have been available for some time, but they can be a real trial to work with and the results are difficult to verify by eyeballing.
Even so, Japan and the E.U. have banned tin-lead alloy solder altogether, and the U.S. has begun to take notice of the estimated 10% of that alloy used in consumer products that wind up in landfills after a woefully short lifetime. For diplomatic, economic, and, of course, environmental reasons good substitutes for conventional solder would be very welcome.
These replacements would need to take the form of conductive adhesives capable of carrying high currents, and lead-free solders with low processing temperatures, high reliability, and good thermomechanical properties, according to Georgia Tech's C.P. Wong. And that's what he and his team are investigating.
One promising material consisting mostly of tin alloyed with silver and copper offers strength, fatigue resistance, plasticity, and reliability, but has a melting point of 217°C, ~30°C higher than conventional solder's 183°C. This elevated temperature would cause manufacturing problems with the inexpensive organic substrates now in use. One solution might be to add metal nanoparticles to the solder.
Another candidate is electrically conductive adhesives consisting of a metal powder filler, typically silver, that conducts electricity inside a polymeric resin. The resin—epoxy, silicon, or polyimide—provides adhesion, mechanical and impact strength, and other desirable mechanical properties. Furthermore, the adhesives are environmentally friendly, require less processing, can be used with low-cost components and substrates, and facilitate size reduction in finished products.
But there are snags here too. The substances suffer from conductivity fatigue, limited current-carrying capability, and poor impact strength, and are therefore used at present only in low-power devices such as driver chips for LCDs.
"After you attach a component to a board with conductive adhesives and then cure it, you must test the connections under conditions of high humidity and heat," Wong says. "When you do that, electrical resistance in the joint increases and conductivity drops. That is a major problem for the industry."
Engineers' and scientists' first surmised that oxidation was the source of the problem. But Wong and researchers at the NSF-sponsored Microsystems Packaging Research Center identified the cause as galvanic corrosion resulting from dissimilarities between metals in the adhesive and the contact. "By understanding this galvanic corrosion, we can develop improved materials that use an inhibitor such as acid to protect the contacts from corrosion, and we can use an oxygen scavenger to grab the oxygen required for corrosion to take place." Wong says. "We can also include a sacrificial material with a lower potential metal that is attacked by the corrosion process first, sparing the conductive materials."
The researchers are working on ways to improve the adhesives' conductivity. One tack is to use self-assembled monolayers, wires <10 Å long, to provide a direct connection through the adhesive. Such monolayers could allow conductive adhesives to seriously compete with conventional solder joints in terms of conductivity and current-carrying capability. They are also looking to improve impact resistance.
The project has received support from the National Science Foundation and the E.P.A.