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Fireworks on the Plating Line



Fireworks on the Plating Line
A lab made low cost circuits by printing conductive ink on film. Suddenly a cloud of smoke moved over the line. What happen?
Mysteries of Science

Transcript


A factory was making flexible circuitry commonly found in printers, disk drives and other electronic products requiring electrical connections between moving parts.

The factory used large rolls of raw materials wherever possible to make the handling process more efficient.  The factory lab had made a low cost circuit by printing conductive ink on low cost film and coating it with copper. 

This new process had almost no waste and only required conductive ink and plating copper.

The lab created a row of test circuits with improved conductive ink made up of a mixture of carbon black and metal powder.  The roller motors were turned on and the film moved slowly down the line.  Suddenly the film began to distort, shrink and a cloud of smoke moved over the line.

What caused this to happen? 

The rest of the story.

The circuits in the lab ran fine, but there was a very small distance between the point where power was applied, and the copper plating solution.  Current could flow over this short distance without too much electrical resistance. The factory plating line was a completely different situation.

The circuits rested on the top of the 12 inch diameter electrically powered rollers. The copper plating bath was at least 1 foot below the top of the rollers. Electrical current had to flow through the conductive ink over a distance of 12 to 15 inches. According to Ohm's Law, electrical resistance is directly proportional to distance, or length of the conductor.

This meant that the factory run produced 12 to 15 times more resistance than during lab tests. 

Power loss through resistance generates heat. The lab had inadvertently built a resistance heater on the plastic film.

As the film heated and melted, the conductors fell apart and caused sparking. Molten Mylar readily burns so the arcing and sparks were enough to ignite it. The melting and flaming plastic film fell into the plating solution that was mostly water and put out the fire. There was no easy fix to the problem.

The phrase, "Scale-ups are not linear" probably remained burned in the memories of the PhD's that forgot about Ohm's Law.

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