Concern about the environmental and human health implications of conventional tin-lead alloy solder have caused the European Union and Japan to ban its use altogether. Though lead-based solder alloys haven't yet been outlawed in the United States, electronics manufacturers are becoming increasingly interested in alternatives.
An article in the June 3 issue of the journal Science describes progress made in developing alternative materials, including tin-based solders and electrically-conductive adhesives. While none of these materials is yet as good as the lead-based solder they are designed to replace, the article reports significant progress in developing alternatives that would allow manufacturers to get the lead out of their products.
"Though many challenges remain to be addressed, both lead-free solders and conductive adhesives show much promise as a means of replacing conventional solder materials," said C.P. Wong, a Regents Professor in Georgia Tech's School of Materials Science and Engineering. "But before these alternatives become truly viable, we must develop conductive adhesives that can carry high currents, and lead-free solders that have low processing temperatures, high reliability and good thermal-mechanical properties."
Tin-lead alloys have long been used in interconnects that allow electrical current to flow through electronic components installed on printed wiring boards in a broad range of devices. The lead-based alloy is attractive because of its relatively low melting point, high reliability and good mechanical properties.
Lead-free solders that combine tin with other metals such as silver, copper, bismuth, zinc, indium and nickel are already in use. Among the many possible combinations, an alloy composed mostly of tin – to which silver and copper have been added – has been widely accepted as the most promising lead-free solder. This alloy provides the best combination of strength, fatigue resistance, plasticity and reliability, Wong noted.
However, the melting point of this alloy (217 degrees Celsius) is about 30 degrees hotter than that of the tin-lead alloy with the lowest melting point (183 degrees). Processing at the higher temperature creates potential manufacturing problems.
"When you attach a component to a circuit board in a cell phone or PDA using this alloy, you would subject the components to a higher temperature, which increases unwanted stress and reduces the integrity, reliability and functionality of the equipment," Wong said. "The substrate we are now using must also be reformulated because the low-cost organic materials we now use cannot withstand these temperatures."
The temperature problem, note Wong and co-authors Yi Li and Kyoung-sik Moon, could potentially be addressed by the introduction of metal nanoparticles into the tin-based solder.
Electrically conductive adhesives offer another alternative. They consist of metal powder filler – usually silver – that conducts electricity inside a polymeric resin. The resin, an epoxy, silicon or polyimide, provides mechanical properties such as adhesion, mechanical strength and impact strength.
These electrically conductive adhesives offer numerous advantages over conventional solder technology. They are environmentally friendly, require fewer processing steps and allow a lower processing temperature. These advantages bring lower processing costs, allow the use of lower-cost components and substrates, and facilitate size reduction in devices.
However, electrically conductive adhesives have their own set of disadvantages, including conductivity fatigue, limited current-carrying capability, and poor impact strength. As a result, conductive adhesives are currently used only in low-power devices such as driver chips for liquid crystal displays.
"In certain applications that require high current densities, conductive adhesives still do not measure up to metallic solders," Wong noted. "However, progress is being made at improving the properties of these materials."
"Conductive adhesives have a lot of advantages, but there are a few challenges," Wong noted. "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. When you do that, electrical resistance in the joint increases and conductivity drops. That is a major problem for the industry."
At first, scientists and engineers believed the problem was caused by oxidation. But Wong and colleagues at the National Science Foundation-sponsored Microsystems Packaging Research Center showed that galvanic corrosion, caused by contact between dissimilar metals in the adhesive and contact, was the real culprit.
"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 explained. "We can also include a sacrificial material with a lower potential metal that is attacked by the corrosion process first, sparing the conductive materials."
Researchers are pursuing additional techniques to boost conductivity of the adhesives, including use of self-assembled monolayers – essentially molecular wires less than 10 Angstroms long – that provide a direct connection through the adhesive.
Additional work is exploring ways to improve the impact resistance.
"Recent studies show that with incorporation of these self-assembled monolayers, the electrical conductivity and current-carrying capability of conductive adhesives could compete well with traditional solder joints," Wong added. "This could be a significant advance in improving these materials."
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