PDF file attached - Humidity effects on board material

[EL34]

Someone sent me this attached PDF document
Some may find it useful

This may apply to Fender type board material and certain other material used for circuit boards

Anyway, instead of deleting it, I uploaded it here for you guys

[Retrovert]

That article is from 1945 and the material it covers is still used.

The material used for terminal strips, turret boards, circuit boards, etc. was a combination of paper and resin made from phenol and formaldehyde sold under the tradename "Pertinax".  (It is fully reacted, formaldehyde being a convenient chemical building block.  There's no harmful material left.)

The seal on the board's surface was poor, however, and even worse after holes were drilled in the finished board.  The now-exposed paper component—which varied between kraft paper (a technical term meaning "strong paper", as "kraft" means strong in German) and what was basically cardboard—was hygroscopic and the moisture caused all sorts of problems.  Tropical environments were particularly troublesome.

The same sorts of phenolic seal issues plagued (and continue to plague) ceramic capacitors (barium titanate is hygroscopic) and carbon composite resistors (clay is hygroscopic) which is why these components vary in value so much over time.  In resistors moisture swells the grains altering the resistance.