CONDUCTING glues that stick selectively to different materials could bond silicon chips to their connecting wires without the need for solder. This could significantly cut the cost of assembling electronic components and reduce the use of toxic metals such as lead, the main component of solder.
The secret of the glues鈥� selective stickiness lies in the ability of their molecules to alter their shape in response to the nature of the underlying material, says Simon Holder, a chemist at the University of Kent in the UK, who unveiled his smart glues last week at the Sixth International Conference in Materials Chemistry in Sheffield.
Each glue molecule is made up of three blocks 鈥� a styrene molecule flanked by two polymethacrylate groups. In the presence of metals such as gold, silver or copper, the molecule forms a 鈥淯鈥�-shape allowing the styrene block to stick strongly to the metallic surface while the two ends move away.
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Other surfaces cause the molecule to buckle into an arch shape. In this configuration, the ends of the molecules stick only weakly to the surface. This allows water molecules to force their way into the arch and prise the glue molecules away from the surface.
Circuit boards could be sprayed with a smart glue that would stick only to gold electrodes and copper wires. Silicon chips could then be stamped in place and the whole printed circuit board hosed down with water to wash away unwanted glue.
鈥淲e鈥檙e confident that we will then be able to dry out and recover the washed-away glue,鈥� says Holder. 鈥淭his could then be reused, so the glues should be relatively green too. Anything that reduces solder and solvent use has to be good for the environment.鈥�
A conducting adhesive would be an important development for the electronics industry, says Michael Turner of the University of Sheffield in the UK. For example, some companies believe that organic light-emitting diodes (OLEDs) made of plastic will replace existing light-emitting diodes. But OLEDS do not stick very well to the gold electrodes in circuit boards, says Turner. To bond OLEDs to gold, manufacturers have to use several polymer layers, each of which bonds to its neighbours. However, Holder鈥檚 glue could eliminate these extra layers because it bonds to both gold and OLEDs. 鈥淭hese glues sound like a big step forward,鈥� says Turner.
Holder now hopes to improve other properties of the smart glues. At the moment, they act as semiconductors but he is confident of producing a conducting version in the near future by chemically doping the material with iodine.