Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Molecular wires & corrosion control boost performance of conductive adhesives

14.03.2005


Replacing lead-based solder



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 that include low current density.

Using self-assembled monolayers – essentially molecular wires – and a three-part anti-corrosion strategy, researchers at the Georgia Institute of Technology have made significant advances toward solving those problems. At the 229th national meeting of the American Chemical Society on March 13th, the researchers will describe improvements that could allow ECA materials to conduct electrical current as well as the metal alloy solders they are designed to replace.


The research has been sponsored by the National Science Foundation, the U.S. Environmental Protection Agency and several electronic interconnect companies.

"In certain applications that require high current densities, conductive adhesives still do not measure up to metallic solders," noted C.P. Wong, a Regents Professor in Georgia Tech’s School of Materials Science and Engineering. "However, by using these self-assembled molecular wires and controlling corrosion at the interface, we can significantly increase the current density."

For environmental reasons, manufacturers are moving away from the tin-lead alloys now used to make the connections for 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 alternatives fall into two categories: (1) alloys that combine tin with such metals as silver, gold, copper, bismuth or antimony, and (2) conductive adhesives that combine flakes of silver, nickel or gold with an organic polymer matrix.

Each alternative has its own set of 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 Celsius versus 230 degrees, also require more costly circuit board materials and increase energy costs.

Conductive adhesives could simplify electronics manufacture 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 Celsius and potentially even room temperature – they would produce less thermal stress on components, require less energy and use existing circuit board 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 joints 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-supported Packaging Research Center 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. They have since published numerous papers describing strategies for fighting corrosion.

"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 such as hydroquinone to grab the oxygen required for corrosion to take place," he said. "We can also include a sacrificial material with a lower potential metal that is first attacked by corrosion process, sparing the conductive materials."

Further improvements were made by substituting short-chain dicarboxylic acids for the surfactant stearic acid used to prevent agglomeration of the silver flakes. Replacing or reducing the stearic acid – which acts as an insulator around the silver flakes – further improved current flow.

Still, the current density accommodated by conductive adhesives fell short of what’s needed to support power-hungry devices like processors.

To overcome that challenge, Wong and collaborators Grace Yi Li and Kyoung-sik Moon developed self-assembled monolayers (SAM) – essentially molecular wires – made up of sulfur-containing conductive materials known as 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.

"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 said. "This could be a significant advance in improving these materials."

But like most advances, the SAM structures aren’t yet optimized. Testing shows that they begin to decompose at temperatures above 150 degrees Celsius.

"We need additional research in this area to develop more stable materials that are still able to carry the current density required and have the necessary mechanical properties," said Li, a graduate student in Wong’s laboratory.

John Toon | EurekAlert!
Further information:
http://www.edi.gatech.edu

More articles from Power and Electrical Engineering:

nachricht New creepy, crawly search and rescue robot developed at Ben-Gurion U
19.07.2018 | American Associates, Ben-Gurion University of the Negev

nachricht The role of Sodium for the Enhancement of Solar Cells
17.07.2018 | Max-Planck-Institut für Eisenforschung GmbH

All articles from Power and Electrical Engineering >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: First evidence on the source of extragalactic particles

For the first time ever, scientists have determined the cosmic origin of highest-energy neutrinos. A research group led by IceCube scientist Elisa Resconi, spokesperson of the Collaborative Research Center SFB1258 at the Technical University of Munich (TUM), provides an important piece of evidence that the particles detected by the IceCube neutrino telescope at the South Pole originate from a galaxy four billion light-years away from Earth.

To rule out other origins with certainty, the team led by neutrino physicist Elisa Resconi from the Technical University of Munich and multi-wavelength...

Im Focus: Magnetic vortices: Two independent magnetic skyrmion phases discovered in a single material

For the first time a team of researchers have discovered two different phases of magnetic skyrmions in a single material. Physicists of the Technical Universities of Munich and Dresden and the University of Cologne can now better study and understand the properties of these magnetic structures, which are important for both basic research and applications.

Whirlpools are an everyday experience in a bath tub: When the water is drained a circular vortex is formed. Typically, such whirls are rather stable. Similar...

Im Focus: Breaking the bond: To take part or not?

Physicists working with Roland Wester at the University of Innsbruck have investigated if and how chemical reactions can be influenced by targeted vibrational excitation of the reactants. They were able to demonstrate that excitation with a laser beam does not affect the efficiency of a chemical exchange reaction and that the excited molecular group acts only as a spectator in the reaction.

A frequently used reaction in organic chemistry is nucleophilic substitution. It plays, for example, an important role in in the synthesis of new chemical...

Im Focus: New 2D Spectroscopy Methods

Optical spectroscopy allows investigating the energy structure and dynamic properties of complex quantum systems. Researchers from the University of Würzburg present two new approaches of coherent two-dimensional spectroscopy.

"Put an excitation into the system and observe how it evolves." According to physicist Professor Tobias Brixner, this is the credo of optical spectroscopy....

Im Focus: Chemical reactions in the light of ultrashort X-ray pulses from free-electron lasers

Ultra-short, high-intensity X-ray flashes open the door to the foundations of chemical reactions. Free-electron lasers generate these kinds of pulses, but there is a catch: the pulses vary in duration and energy. An international research team has now presented a solution: Using a ring of 16 detectors and a circularly polarized laser beam, they can determine both factors with attosecond accuracy.

Free-electron lasers (FELs) generate extremely short and intense X-ray flashes. Researchers can use these flashes to resolve structures with diameters on the...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

VideoLinks
Industry & Economy
Event News

Leading experts in Diabetes, Metabolism and Biomedical Engineering discuss Precision Medicine

13.07.2018 | Event News

Conference on Laser Polishing – LaP: Fine Tuning for Surfaces

12.07.2018 | Event News

11th European Wood-based Panel Symposium 2018: Meeting point for the wood-based materials industry

03.07.2018 | Event News

 
Latest News

Metal too 'gummy' to cut? Draw on it with a Sharpie or glue stick, science says

19.07.2018 | Materials Sciences

NSF-supported researchers to present new results on hurricanes and other extreme events

19.07.2018 | Earth Sciences

Scientists uncover the role of a protein in production & survival of myelin-forming cells

19.07.2018 | Life Sciences

VideoLinks
Science & Research
Overview of more VideoLinks >>>