In The News
04/01/2005
Heightening Electrically Conductive Adhesives’ Performance
ATLANTA, GA. - As an alternative to tin-lead solders, electrically conductive adhesives (ECAs) may be used to connect display driver chips, memory chips, and other devices to circuit boards. Before these materials find widespread application in electronic equipment, however, researchers will have to overcome technical challenges, including low current density.
Using self-assembled monolayers-or molecular wires-and a 3-part, anti-corrosion approach, researchers at Georgia Tech have made strides toward solving those problems, with the aid of the National Science Foundation (NSF), the U.S. Environmental Protection Agency, and several electronic interconnect companies as sponsors. At the 229th national meeting of the American Chemical Society, which was held March 13th, the researchers described improvements that could permit ECA materials to conduct electrical current, and also the metal alloy solders they are designed to replace.
 Researcher Grace Yi Li holds samples of electrically conductive adhesive being studied at Georgia Tech’s School of Materials Science and Engineering. |
“In certain applications that require high current densities, conductive adhesives still do not measure up to metallic solders,” says C.P. Wong, a professor in Georgia Tech’s School of Materials Science & Engineering. “By using these self-assembled molecular wires and controlling corrosion at the interface, we can significantly increase the current density.”
Currently, manufacturers are moving away from tin-lead alloys to make connections for integrating devices into computers, PDAs, cell phones, and such. Though the U.S. has no official requirement for discontinuing lead use, European and Japanese decisions have spawned research into alternative materials falling under two categories:
- Alloys that combine tin with metals like silver, gold, copper, bismuth, or antimony; or
- Conductive adhesives that combine flakes of silver, nickel, or gold with an organic polymer matrix.
Most solders containing two or three metals have a melting point higher than eutectic tin-lead alloy solder, increasing thermal stress placed on components. ECAs could, however, facilitate manufacture by doing away with processing steps, such as acid flux and detergent/water cleaning. Because ECAs can cure at lower temperatures, they could produce less thermal stress, require less energy, and use existing board materials. At first, scientists and engineers believed that oxidation caused the problem, but Wong, as well as colleagues at the NSF’s Microsystems Packaging Research Center, showed that galvanic corrosion, caused by contact between dissimilar metals in adhesives and tin-lead alloys used in device contacts, was the real culprit.
More progress was made by substituting short-chain dicarboxylic acids for the surfactant stearic acid used to prevent agglomeration of silver flakes. Replacing or reducing the stearic acid further improved current flow, but the current density accommodated by conductive adhesives fell short of what is required to support devices that need lots of power. Wong and collaborators Grace Yi Li and Kyoung-sik Moon developed self-assembled monolayers (SAMs) comprised of sulfur-containing conductive materials called “thiols.” Less than 10-Å long, they bind to gold pads in the device and board to provide a direct connection that avoids resistance typically found at the interface. The SAM structures are, however, not yet optimized - testing shows that they begin to decompose at 150°C and above.
Synergetix Proposes New Standard for Test Socket Characterization
KANSAS CITY, KAN. - At the BiTS 2005 conference, held March 6-9 in Mesa, Ariz., Synergetix proposed a new industry standard for high-bandwidth electrical characterization of test sockets. Exact electrical characterization is becoming more and more important for comparing and selecting sockets, but there are other choices available to characterize electrical performance at these bandwidths - each with its own limitations.
The new technique, developed by Dr. Eric Bogatin, CTO of Synergetix, and a team of Synergetix experts, is reproducible, relying on standard, off-the-shelf hardware and software tools. Outputs are accurate and verified SPICE models, accurate up to bandwidths >5 GHz, and a behavioral model includes insertion and return loss. Getting measurements with a minimum number of artifacts is difficult. In developing the technique, a number of specific tricks are necessary to get around frequently encountered artifacts. “One of the key features of our technique is the identification of seven specific tricks that must be employed to get accurate, reproducible measurements,” says Bogatin.
Characterizing a socket incorporates identification of each of the return path configurations, and then extracting an electrical model for each pattern using measurements from a network analyzer. Use of a 2-layer board to bus all the return paths together is necessary so that a ground-signal-ground probe can be used to make contact to the signal and its return pins. Two configurations are measured, and the ends of the pins are open and shorted. To measure the signal’s low impedance and return path, the end is shorted by use of two ports of the network analyzer, and the measurement is synonymous with Kelvin measurement. An ideal lossy transmission line models the signal and return pins - fitting the measured data to over 10 GHz.
“The paper I presented at BiTS was very well received by both socket users and socket suppliers,” Bogatin continues. “There is a general belief among the user community that we need standardized test methods for all the important performance metrics of sockets. It is only with standard methods that test engineers will be able to select the socket with the best performance-price tradeoff.”
The first test Synergetix is tackling is electrical characterization. The company believes that the same test should be used by all vendors and users, and is taking the lead to share with the industry a simple, robust, and nonproprietary method.
Intel 2004 PQS Awards
SANTA CLARA, CALIF. - Intel recently honored 26 companies with its Preferred Quality Supplier (PQS) award. Each of these companies provided products and services considered indispensable to Intel’s business in 2004.
The winners are: Advantest; Amkor; Applied Materials; Asyst-Shinko; AZ Electronic Materials; Compugraphics USA; Credence Systems; Daewon Semiconductor Packaging Industrial Co.; Dow Corning; Elec & Eltek Int’l. Holdings; Georg Fischer Piping Systems; Henkel; Hitachi High Technologies; Hon Hai Precision Industry Co.; IBM; ICOS Vision Systems; Kelly Services; KES Systems & Service; Komatsu Electronic Metals Co.; Moses Lake Industries (Tama Chemicals); Munters; Nan Ya Printed Circuit Board; NEC Electronics; Nikko Materials Co.; SPIL; and Tyco Int’l.
-Lee Mather