Polymer Thick Film That Was Thinned Out

Polymer Thick Film That Was Thinned Out
A factory is making circuits using polymer thick film. One day the ink was too thin, making it print poorly. What was causing this?
Mysteries of Science


We're in a modern factory with a recently completed building that is making printed circuits by a revolutionary process called Polymer Thick Film. The concept is to print conductors, resistors, insulators, and eventually components, directly onto material. This material can even be a roll of plastic film.

While the idea has been around for many years, this was the first significant investment in the technology and this company was betting everything on this unproven way of making circuits. Most circuits are made by etching away copper foil that has been bonded to a dielectric material, a more costly and much more wasteful process than Polymer Thick Film.

The factory had new equipment and was evaluating inks. After 3 months most of the processes where under control and the first high-volume product was ready for launch.

The circuit conductors were made by printing metallic ink and then dried in a long oven. Then carbon resistor ink was printed so that it contacted the hardened metallic conductive ink.

After drying, the resistance values were measured and the printing process adjusted until the values of the printed resistors were within specification.

The process was under control, and the customer was pleased with the product. More orders were placed.

The carbon ink supplier was a small firm in Minnesota and the circuit company was worried about reliance on only one supplier. The suppliers ink was always perfect, but one day it was too thin making it print poorly.

 There was panic since production only had a 3 day supply. Calls to the ink supplier did not indicate anything had changed, and the ink analysis in the lab did not reveal any issues.

The R&D manager decided to review every step. There was nothing they could pinpoint. Even the plant temperature and humidity were checked. The manager also reviewed packaging and shipping. The packaging was simple; a wrap in a poly bag, seal it, put it in a box, and delivered to UPS. So what was going on?

Here's the rest of the story.

The R&D manager wanted a shipping timeline comparison. The finished ink was poured into plastic containers and then placed in the shipping area so their truck could load it in the morning and drive it to UPS.

"How cold does the shipping area get overnight", the R&D man wanted to know. "It gets quite cold here since the place isn't insulated, but that doesn't matter anymore since we started storing the ink inside", said the ink maker.

"When did you start storing the ink inside?" asked the manager. "We just started doing that on the last shipment", was the reply. Was this a clue?

"Do me a favor", requested the manager, "put some ink in the loading area tonight and ship it tomorrow, just like you did last week". The ink arrived, and sure enough, it worked fine.

Based on this insight, the lab manager placed a container of "thin" ink in the freezer. The next day, the ink was allowed to warm up, and then it was tested. The ink performed perfectly.

The freeze cycle was the key. Apparently, the low temperature caused the plastic binder to thicken. This is just the opposite of many inks that thicken if warmed up for a while, so it was hard to spot.

The solution was to "age" the ink by chilling it. It turned out that the ink thickened naturally after a week or two and then became quite stable. The rapid production ramp-up didn't leave enough time for natural aging so the "chill aging" was needed.

The lesson here is to analyze the entire process and never assume that any step is unimportant.


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