Portland, Ore. - Low-temperature, low-profile conductive adhesives are poised to enable ultraflat-panel displays for cell phones, laptop computers and wall-mounted televisions.
Going beyond recommendations to take the lead out of solder, researchers at the Georgia Institute of Technology are working to build a world of electronics free of metal-solder itself.
While the interconnect trade association IPC and other organizations are pursuing a no-lead goal, they "are only looking at low-temperature adhesives as one of many different alternatives," said Ching-Ping Wong, professor of materials science and engineering at Georgia Tech (Atlanta). "We believe that our approach to low-temperature adhesives has much greater potential because we are solving the fundamental problems that make others hesitant to use them."
Other approaches to low-temperature conductive adhesives have been afflicted with self-alignment and low-carrier mobility problems. Wong's group, which included graduate student Grace Yi Li and post-doctoral researcher Kyoung-sik Moon, is working to overcome those obstacles.
Lead-based solder has traditionally been used to affix components to printed-circuit boards, but IPC and other industry organizations have been encouraging a switch from lead to tin alloys, which don't pose the same danger or hazard. In addition, the European Union's Restriction of Hazardous Substances directive will limit the lead in electrical equipment sold in Europe after July 1, 2006.
The IPC road map (lead-free.ipc.org) calls for adding alternative metals as nanoparticles into tin-alloy solder. However, the Georgia Tech researchers propose going straight to electrically conductive adhesives using a metal powder filler, such as silver, that conducts electricity inside a polymer resin. Wong said that these resins-essentially an epoxy of silicon or polyimide-offer greater adhesion, mechanical strength and affect resistance.
Low-temperature conductive polymers also hold an edge over tin alloys, which melt at higher temperatures than lead, lowering the yields of today's lead-free pc boards. Some components are particularly sensitive to heat, so manufacturers are reluctant to switch materials.
"Tin alloys of silver and copper provide the best combination of strength, fatigue resistance, plasticity and reliability, but they still tend to have a higher melting point than lead, thus subjecting the board and CMOS components to higher stress," said Wong. "Gallium arsenide chips are especially sensitive to temperature, making them prime candidates for our low-temperature adhesives."
Wong cited other advantages:
- Tin alloys of silver and copper have a melting point of 217 degrees C, or 34 degrees higher than a tin-lead alloy's melting point of 183 degrees C. But low-temperature adhesives can essentially be applied at room temperature, enabling low-cost, thin and flexible polymers to be used as pc-board materials.
- As an alternative to lead or tin alloys, low-temperature adhesives allow expensive components to be assembled without risking damage from overheating.
- Low-temp adhesives are more expensive but offer a lower profile for circuit boards, adding only a few microns of thickness, perhaps 100 times thinner than metal solder. Some applications, such as cell phone displays, have bitten the-bullet and switched to low-temp adhesives despite their higher cost.
"Besides being lead-free, our adhesives are applied at a lower temperature than lead, plus they provide a lower profile-adding only a few microns to the thickness of a printed-circuit board whereas metal solder adds over 100 microns," said Wong. "This helps make LCD displays for televisions, laptop computers and cell-phones much thinner."
But low-temp adhesives cannot be used in high-current applications today because of the inherently lower carrier mobility of conductive adhesives compared with metal solder. Also, because solder is not sticky, many of the assembly methods used today rely on self-aligning components, whereas adhesives require pick-and-place equipment to be much more accurate. Wong's team at Georgia Tech said it's solving these problems, potentially making low-temperature conductive adhesives applicable for all phases of electronic assembly.