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Role of Carbon Ink in Calculators

Role of Carbon Ink in Calculators
Texas Instrument's calculators made in the 80's used a flexible circuit made with carbon ink. What vital part did the carbon ink serve?
Mysteries of Science


The printed circuit used in Texas Instruments' calculators built in the 1980's, was a flexible circuit made with copper conductors and Mylar plastic. The company was scrutinizing every aspect of manufacturing in an effort to cut costs as Japanese competition was becoming more intense. 

After much scrutiny a purchasing agent began to question a black ink used in the manufacturing process. No one could quite explain what the carbon ink did, but it had always been there.

One hundred circuits were made with no black ink for testing. The calculators manufactured using this carbon inkless circuit worked, but turning them on was no small chore.

QA claimed that certification for the change would take months and be quite costly. To bypass this, the purchasing agent stated that it was a graphics change. QA allowed this to proceed without any further testing, but three months later the purpose of the black ink was clear.

What was the vital part of the manufacturing process that the carbon ink served?

Here's the rest of the story.

The carbon ink that was deleted had been put there to perform specific functions and should have never been eliminated. It created a smooth, compliant, and compatible contact to the glass LCD panel, but it also prevented the solder finish from oxidizing.

The solder oxidized so that the marginal connection points became weaker until they were insufficient to provide the proper electrical signals. Pressing on the LCD momentarily created contact, but soon, the full display would fail in the field the worst possible scenario.

The only fix was to scrap the deficient calculators at a cost of millions. There are many lessons here, but one that stands out is to get expert advice if you do not know the area; almost everything is done for a reason.


Having worked at one time for the "phone company", we saw a similar thing happen with flex circuits and silver ink. After testing revealed oxidation, we soon switched over to carbon ink and the problems went away. Great article. It brought back many fond memories.
George Kopacz, PNY Technologies, USA

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