This is BSE SEM image of a microsection of a flex circuit showing two internal copper conductor layers.
This is an area where the copper conductor is corroded. The corrosion initially attacks the grain boundaries.
This image shows that the corrosion completely consumed the copper layer at this location(top-right segment).
This is a through-hole solder joint where the internal copper conductors are almost completely gone due to corrosion as well as the nickel surface lands.
This is a higher magnification image that shows the missing material more clearly.
This is an elemental map that also shows the missing copper layers and nickel lands more clearly.
Some tips to avoid this type of corrosion include …
- chose compatible materials (adhesives and other polymers can contain chlorides, sulfur, etc. … add moisture and voltage bias and corrosion can accelerate dramatically)
- clean thoroughly & DRY at each appropriate process step (this includes bare flex fabrication and assembly processes)
- thermal damage can create paths for ionic contamination and moisture to migrate into the internal conductors, so minimize thermal exposure