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Can Conductive Ink Shrink?
Poly-Flex was building circuits using conductive ink, but had problems with component junctions. What was causing this? Mysteries of Science
Transcript
Printed circuit boards are found in nearly every electronic product large and small. Most circuits begin as a rigid board with copper foil on the top and bottom surfaces.
Poly-Flex Circuits was intent on assembling circuits in such a way to reduce wasteful copper etching by printing conductive ink on low cost plastic sheets.
Poly-Flex's process had been perfected and they were producing printed circuits quickly and efficiently. The prototype lab was running even better than expected. The lab then ran a standard temperature and humidity test with a poor outcome.
Careful examination showed that the adhesive was still electrically conductive, but the junction at each electrical component was the problem.
What was causing this and what was the solution for making conductive adhesive junctions stable?
Here's the rest of the story
The lab went to work unraveling the mystery. Testing focused on the interface between the adhesive and the solder finish of the components. Could the solder be melting and fusing to the metal in the adhesive? Was the extra heat somehow preventing oxidation by making the additives more effective? These theories were tested and disregarded. Then, an unusual theory was proposed by a mechanical engineer. The extra heat may have caused the adhesive to shrink. What if the shrinkage was making a better connection?
After a month of additional testing, the lab now had an adhesive that could be hardened at a practical temperature and still pass the tough temperature and high humidity test. The adhesive was purposely made to have a high shrinkage rate at a normal hardening temperature. Shrinkage was the key factor.
One theory was that the epoxy shrinkage pushed the metal adhesive particles so tightly against the components that they either penetrated the oxide layer or no oxide was able to form between them. The best adhesives had the highest shrinkage, but only if they used irregular metal particles that would be more likely to penetrate the oxide layer.
Comments
The statements presented hit the key points. Conductive inks shrink as they cure and for good reason. The shrink pulls the electrically conductive material, metal or carbon, together to form a good continuous electrical connection. The interface, if it has oxide present before the shrink is finished then the conductive particles can move over the interface surface and not make the best connection. As a general statement, conductive inks must have a relatively oxide-free surface to connect. You mentioned heat in your process. Perhaps the interface started fairly oxide free then, with heat, an oxide barrier was created. In which case you might try reflowing in Nitrogen Atmosphere. The nitrogen gas eliminates the oxygen which creates the oxide. Nitrogen reflow equipment does dramatically reduce the formation of oxide when the parts are heated.
Jaye Waas, Renkus-Heinz
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