With the caveat that free advice is often not worth what you pay for it....

Water is a polar solvent, meaning it has a charge, the Oxygen keeps a slight
excess of electrons away from the Hydrogen, thus maintaining a charge.

This is not the case with carbon-hydrogen type solvents like say, kerosene,
or motor oil.  It is essentially non-polar, or charge free.

Polyimide is more polar than FR-4, containing Nitrogen (which tends to act
like Oxygen, and maintain a charge) and this attracts the water.  Polyimide
is more "polar", or charged, and the rule in chemistry is "like dissolves
like", so Polyimide loves water more than its uncharged cousin FR-4.

In Chemistry...

Rudy Sedlak
RD Chemical Company

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