Author :
Zhan, Sheng ; Azarian, Michael H. ; Pecht, Michael G.
Author_Institution :
Dept. of Mech. Eng., Maryland Univ., College Park, MD
Abstract :
Printed circuit board (PCB) specimens containing three different IPC-B-25 test structures were exposed to temperature/humidity/bias conditions in order to evaluate the effects of conformal coating, conductor spacing, voltage bias, flux chemistry, and test environment on surface insulation resistance (SIR). Results indicate that conformal coatings improve reliability, provided that sources of contamination on the PCB and within the coating are minimized. The presence of fibrous contaminants within the coating represented a preferential medium for moisture adsorption and ion transport, leading to accelerated reduction of SIR. In the absence of contamination, PCBs with conformal coatings were found to be less susceptible to SIR degradation than uncoated PCBs, with silicone providing better protection than urethane, and acrylic providing the least protection of the three coating materials evaluated. Conductor spacing was observed to represent a factor in the electrochemical migration (ECM) process independent of electric field, indicating that updated test structures are required to predict reliability of today´s high-density assemblies. The SIR failure rate with rosin-based no-clean flux was observed to be greater than that with aqueous-based no-clean flux. A higher failure rate was also observed for tests conducted at 40 degC/93% RH than for 85 degC/85% RH. Due to the more rapid evaporation of weak organic acids in the flux residues at higher temperatures, test results obtained at 85 degC/85% RH will not accurately predict reliability at lower temperatures for PCBs processed using no-clean flux. PCB specimens were exposed to temperature/humidity/bias conditions in order to evaluate the effects of conformal coating, conductor spacing, voltage bias, flux chemistry, and test environment on reduction of surface insulation resistance. Results indicate that, in the absence of contamination, conformal coatings improve reliability
Keywords :
conformal coatings; insulating coatings; insulation testing; printed circuit testing; reliability; surface contamination; surface resistance; IPC-B-25 test structures; PCB; SIR failure rate; acrylic; aqueous-based no-clean flux; coating materials; conductor spacing; conformal coating; electrochemical migration process; fibrous contaminants; flux chemistry; high-density assemblies; ion transport; moisture adsorption; organic acids; printed circuit boards; reliability; rosin-based no-clean flux; silicone; surface insulation resistance; urethane; voltage bias; Circuit testing; Coatings; Conductors; Contamination; Humidity; Insulation testing; Printed circuits; Surface resistance; Temperature; Voltage; Accelerated testing; conformal coating; contamination; dendrite; electrochemical migration; filament; no-clean flux; reliability; surface insulation resistance; temperature–humidity bias (THB);
