Electronics Assembly Knowledge, Vision & Wisdom
Screen Print Mismatch
Screen Print Mismatch
At a printed circuit shop artwork on the finished sheets did not match up. What was causing this discrepancy.
Mysteries of Science
Dr. Gilleo
Mysteries of Science by Dr. Ken Gilleo

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A factory made printed circuits through a unique "additive", method where the conductors are screen printed directly onto plastic.

The polycircut plant used modern screens made from polymer filament and fine stainless steel mesh.

The ink was a functional conductive paste that contained fine particles of metal, that produced conductors after the ink was dried in an oven.

The conductive ink could be applied to almost any flat non-conductive material.

The factory's process began with a big roll of polyester film cut into sheets.  These sheets were stacked onto special pallets and moved to the screen printer. The printer would automatically pick up a sheet and move it under the screen where a squeegee pushed ink onto the film in a desired pattern.

The wet sheets were then moved into the hot zone to dry and harden the ink. The line was being tested to see if it could meet the required 1000 sheets per hour, only small test runs had been tried before then.

The machine surpassed this target but the artwork on the finished sheets did not match up. The ink patterns seemed to be distorted in some places.

What was causing this discrepancy in the artwork when the production level was increased?

Here's the rest of the story.

Not only was there a mismatch between the sheets and artwork, no two sheets were exactly alike. Those familiar with the printing process believed this should be impossible, but there was the evidence staring them in the face.

The screen image could not move since the photo-emulsion was embedded into the screen mesh. Could the screen be stretching?

The more they looked at the distorted patterns, the more everyone realized that the screen could not possibly stretch far enough to account for these results.

They needed another theory. What if air turbulence in the oven was pushing the ink around? They baked in an oven with no forced airflow. The distortion was still there. The problem was apparently occurring after printing, but not in the oven.

A sheet coming off the factory printer was taken and photographed using a manual time-lapse process. Each photograph was displayed on a large paper screen using a grid pattern.

Everyone was surprised to see the ink pattern distort in a second or two. Next, a video camera was set up and focused on an area near the edge where the distortion was very high. They were amazed as they watched the video with the ink moving around in front of their eyes.

What was moving the ink? What was the invisible force? Could it be a static charge?

The plastic sheets were being charged while the film was unrolled, cut and stacked. The stack in the printer feed would hold a random and uneven electrical charge. When the metallic ink was printed, it would move toward regions of high electrostatic charge.

The fix was easy. Add static control equipment to the line. This is a classic case of using the scientific method, applying good detective principles and utilizing forensic science.

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