Effect of Silicone Conformal Coating on Surface Insulation Resistance (SIR) For Printed Circuit Board Assemblies

摘要

Conformal coatings are considered a method of providing corrosion protection to electrical assemblies used in high-humidity or harsh environments. They are applied to PCBs for various reasons: to protect from moisture and contamination, to minimize dendritic growth, to provide stress relief, and for insulation resistance. These contribute to more durable handling, enhanced device reliability, and reduced warranty costs. Increased miniaturization of new circuit board designs requires flexible, low stress coating material to protect delicate components and fine-pitch leads. Silicone conformal coatings offer many advantages that address the general trend of ongoing PCBs designs, such as: high flexibility and low modulus to reduce stress on'rJelicate or small components; high humidity resistance; wide operating temperature range that makes them suitable for extreme temperature cycling applications; excellent electrical properties; UV resistance; good chemical resistance, and adhesion to many common substrates used in electronics. Printed circuit board (PCB) coupons coated with three different silicone conformal coating formulations were exposed to temperature/humidity/bias conditions (85 °C / 85% RH/ 50 v) for 500 hours in order to evaluate the effects of conformal coating on surface insulation resistance (SIR). The goal of SIR testing is to identify possible failures in the functioning of printed circuit boards due to electro-chemical failure mechanisms, such as unacceptable electrical leakage under high humidity conditions, corrosion and metal migration, before they can occur on actual parts in the field. Results from this SIR testing are reviewed and discussed through this paper.