John:

Two  reasons for overcoating a flex/solder joint interface:

1. Insulation
2. Strain relief

Two considerations:

1. Adhesion to the flex dielectric
2. Stress concentration

Most flex is polyimide-based; polyimides are very difficult to adhere.
Surface  must be extremely clean and receptive to the overcoating  material.
Pay considerable attention to the strain relief/stress concentration
issue---be sure that the added material doesn't worsen the situation!

A slightly flexiblized system is likely to be your best choice both because
these systems tend to show higher adhesion because slightly forgiving, and
because they tend to spread stress.

Tom Stearns
3 Brander Court
Nashua NH 03063
603 889 2522

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