Electrically conductive adhesive (ECA) materials offer the electronics industry an alternative to the tin-lead solder now used for connecting display driver chips, memory chips and other devices to circuit boards. But before these materials find broad application in high-end electronic equipment, researchers will have to overcome technical challenges.
Using self-assembled monolayers (SAM)–essentially molecular wires–and a three-part anti-corrosion strategy, researchers at the Georgia Institute of Technology have made significant advances toward solving the challenges. Sponsored by the National Science Foundation, the U.S. Environmental Protection Agency and several electronic interconnect companies, the research is helping ECA materials become more competitive with the metal alloy solders they are designed to replace.
For environmental reasons, manufacturers are moving away from the tin-lead alloys now used when integrating devices into such products as computers, PDAs and cell phones. Japanese manufacturers adopted lead-free electronic interconnection technology in January 2005, and European Union manufacturers are expected to follow suit in June 2006. Though the United States has no official requirement for halting the use of lead, the European and Japanese decisions have spurred new research into alternative materials. Those fall into two categories: alloys that combine tin with such metals as silver, gold, copper, bismuth or antimony, and conductive adhesives that combine flakes of silver, nickel or gold with an organic polymer matrix. Each alternative has its advantages and disadvantages.
Most solders containing two or three metals–such as tin-silver (SnAg) and tin-silver-copper (Sn/Ag/Cu)–have a melting point higher than eutectic tin-lead alloy solder, increasing the thermal stress placed on components being connected. The higher reflow temperatures, up to 260 degrees C versus 230 degrees C, also require more costly circuit board materials and increase energy costs.
Conductive adhesives could simplify electronics manufacturing by eliminating several processing steps, including the need for acid flux and cleaning with detergent and water. Because the materials can be cured at lower temperatures–about 150 degrees C and ultimately even room temperature–they would place less thermal stress on components, require less energy, and use existing circuit board materials.
The challenge appears when connections are tested under conditions of high humidity and heat. Electrical resistance in the joints typically increases, and conductivity drops after operation under such conditions.
At first scientists and engineers believed the problem was caused by oxidation. But researchers at Georgia Tech’s Microsystems Packaging Research Center and School of Materials Science and Engineering showed that galvanic corrosion, caused by contact between dissimilar metals in the adhesive and tin-lead alloys used in device contacts, was the real culprit.
By understanding this galvanic corrosion, they helped develop improved materials that use an inhibitor such as acid to protect the contacts from corrosion, and an oxygen scavenger such as hydroquinone to remove the oxygen required for corrosion to take place. A third strategy is to include a sacrificial material–a metal with lower potential–that is attacked first by the corrosion process, sparing the conductive materials.
Further improvements in conductivity have been made by substituting short-chain dicarboxylic acids for the surfactant stearic acid used to prevent agglomeration of the silver flakes. Replacing or reducing the amount of stearic acid, which acts as an insulator around the silver flakes, improves current flow.
Still, the current density accommodated by conductive adhesives has fallen short of what’s needed to support power-hungry devices like processors. Therefore, researchers in the Georgia Tech laboratory of the author–including Grace Yi Li and Kyoung-sik Moon–developed self-assembled monolayers made up of sulfur-containing conductive materials called thiols. Less than 10 angstroms long, these molecules chemically bind to gold pads in the device and board, providing a direct electrical connection that bypasses the resistance normally found at the interface.
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. But the SAM structures still must be optimized. Testing shows that they begin to decompose above 150 degrees C. So additional research will be needed to develop more stable materials that still can carry the current density required.
C.P. Wong is a regents professor in Georgia Tech’s School of Materials Science and Engineering.