Rapid Assessment of Solder Resist-Related Electrochemical Reliability Issues
In comprehensive humidity tests for the release of materials and processes for the assembly and interconnect technology of PCBAs of the automotive industry, the solder resist has emerged as a particular focus of defects with regard to electrochemical migration. Anodic migration phenomena inside the solder resist in high voltage applications or even insufficient hydrolysis resistance at GND lines with electrochemical migration through the solder resist are recurring failure patterns. Typical SIR tests, even when extended to 504h, have insufficient sensitivity to these failure patterns. Neither suspected material influences nor differences between process variants in solder resist application or between PCB manufacturers could be clearly detected with standard SIR tests.
With this in mind, a solder resist stress test (SRS-test) was developed to provide a simple means of detecting the isolation effect of solder resist as required by IPC-SM-840 [1]. It is based on the use of the IPC-B53 (IEC TB144) test board [6], where the test structures have been alternately covered by solder resist. Each GND line thus represents an isolator. The test is now accelerated by partially immersing the B53-test PCB in a water bath and applying a voltage. The electric current flow in the test medium is now recorded as a function of time. A failure of the insulation effect is detected by a sudden increase in the measuring current, which occurs within 3h to 300h depending on the solder resist material and processing. A good sensitive resolution between different solder resist materials and processes at the PCB supplier was thus achieved, as well as a reduction of test times compared to standard SIR test. A classification of the solder resist quality into resistance classes is possible by this procedure.
With this method, dependencies on the test voltage, the test temperature, different types of solder resist, but also their application processes were worked out. A tool was established with which solder resist suppliers and PCB manufacturers can optimize their formulations and application processes for the automotive industry without having to publish the usually sensitive data on the formulation and processes. In addition, the tool offers the possibility to detect deviations from the required resistance class (SRS class) by means of a recurring qualification. This simple test closes the currently existing comprehensive specification gap with regard to solder resist quality in terms of insulation effect.