Failure behaviors and mechanisms of high-ohmic resistors protected by PF/EP paint in heat and humid environment

Phenolic formaldehyde(PF)and epoxy(EP)resins are commonly used in electronic packaging. In this paper, high-ohmic resistors(2.2 MΩ, ±0.5%,)with Cr-Si film were coated by PF/EP paint, and the resulting coated resistors were used for heat and humid(HH)experiments. The experimental results show that the corrosion of band-like resistive films is selective and isotropic, and that the corrosion spots in resistive films all form along grooves and extend in the same direction. It is revealed that OH- ions are generated due to the electrochemical reactions of resistive film in HH experiments, so a NaOH aqueous solution with pH about 10 was used to study the effects of absorbed water and OH- ions on PF/EP polymer film. The results indicates that the color of some part on PF/EP polymer film changes due to corrosion, and that the corrosion part of the polymer film is easy to be peeled off. It can be inferred that OH- ions generated in HH experiments may play a catalytic role in the chemical reactions between polymer film and the absorbed water, which accelerates the degradation of PF/EP protection film for a resistor.