The Puzzling Z-Axis Adhesive

The Puzzling Z-Axis Adhesive
A new type of z-axis conductive adhesive was mysteriously switching on and off. The cycle was endlessly repeated. What was the cause?
Mysteries of Science


The lab in this mystery was working with IBM on methods for bonding electrical components to a new type of printed circuit material. The initial approach was to use conductive adhesives instead of solder. The lab had good results with metal filled epoxy, but they wanted to try a new type of "Z - Axis" material that only conducted electrical current in the vertical direction.

The adhesive contained tiny conductive metal spheres. When a component was pressed down into the film the little spheres made contact with the metal terminals on the component and to the bonding pads on the circuit board.

The entire IBM circuit was covered with the material. You pressed components into the film while applying heat to soften it and the connection was made. 

The lab built a few dozen assemblies and placed them into systems for actual field testing. But there was an anomaly; these systems seemed to turn off and on by themselves.

The cycle was endlessly repeated, but when the board was pulled out and checked everything was perfect. So what was going on? 

Here's the rest of the story.

The board only misbehaved when it was hooked into the system. While turned on, the system would generate heat. At about 140 degrees F, the Z-axis adhesive connection opened. But at 120 degrees F, the plastic contracted and it made contact again. The result was very repeatable, almost like a thermostat.

The adhesive was acting like a thermal switch. The adhesive was modified so it worked within the temperature range. But the original formulation was used for another application making printable fuses.

When too much current passed, the material heated and opened. The fuse would reset upon cooling. Beyond the obvious lessons of understanding the effect and tracking the cause, there is something more useful here. A defective material for one application can be the solution to a problem somewhere else.


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