MITIGATING CREEP CORROSION WITH EFFECTIVE TEST
Creep corrosion is a common mode of corrosion-related failure. It occurs when metallization corrodes and the corrosion products migrate across printed circuit board (PCB) surfaces, causing electrical short circuit failures when they bridge neighboring features on PCBs.
Creep corrosion is not a new phenomenon, but it has become a greater reliability concern in recent years for two reasons. First is the elimination of lead from electronics. Lead-based solders and surface finishes provided excellent corrosion resistance. However, the higher melting temperatures of lead-free solders and the poor wetting of copper metallization on PCBs forced changes to PCB laminates, surface finishes and processing temperature-time profiles. These changes resulted in a higher risk of creep corrosion on PCBs.
Second, the electronics market has expanded significantly in regions such as Asia where the environments, often humid and with high levels of sulfur-bearing pollution, have a high propensity for creep corrosion.
The Need to Test for Creep Corrosion
Reliability concerns — especially for equipment in data and telecom centers — have created the need for a corrosion test to determine the robustness of products in environments where there is a high risk of creep corrosion.
iNEMI began work on creep corrosion in 2009. Early efforts focused on improving industry understanding of the factors affecting creep corrosion on PCBs and printed circuit board assemblies (PCBAs), such as surface finishes, soldering fluxes, design features and the environment.
The iNEMI Qualification Test Development for Creep Corrosion project then turned its attention to developing a qualification test to determine whether products would suffer creep corrosion in the field. A series of experiments led to the development of an innovative flowers-of-sulfur (FoS) corrosion chamber and a test process that has been very successful in reproducing creep corrosion on PCBs from lots that suffered creep corrosion in the field.
iNEMI's FoS chamber uses flowers of sulfur as the source of sulfur gas, a saturated salt solution to maintain constant humidity and a household bleach as the source of chlorine gas. It is inexpensive to build and easy to maintain and can serve as a useful, general-purpose corrosion chamber for testing electronic hardware.
During the research effort leading to the FoS chamber design and creep corrosion test procedure, the iNEMI team made several discoveries:
- Chlorine concentration in the chamber decreases dramatically with increasing relative humidity.
- The corrosivity of the chamber air decreases dramatically with PCB loading.
- Prebaking the test circuit boards was found to be a necessary condition in this test for creep corrosion occurrence because it resulted in significant reduction of volatile organic compounds (VOCs) coming off the PCBs.
- Creep corrosion can occur on some PCB finishes at very low relative humidity levels and on others at high humidity levels.
- Creep corrosion emanates primarily from where the solder mask overlaps the copper metallization.
- A two-year storage period may reduce the propensity to creep corrosion. This discovery was based on two production circuit boards that had suffered creep corrosion in the field and in the iNEMI FoS chamber when the PCBs were new. After being stored for two years in an office environment, the circuit boards stopped suffering creep corrosion in the iNEMI FoS chamber.
Test Process Developed
Based on these findings, the team developed a qualification test that includes the following:
- Prebake the test PCBs under flowing nitrogen gas at 100°C for 24 hours to eliminate the impact of VOCs.
- Run three five-day tests in an iNEMI FoS chamber, inspecting the PCBs after each test run for creep corrosion. Each five-day test is run at a different relative humidity using a different saturated salt solution.
- Establish pass-fail criterion based on the occurrence of creep corrosion bridging the gap between neighboring metallization features on the test PCB.
White Paper & Meetings
iNEMI has published a white paper highlighting some of the key findings from its extensive creep corrosion work. The paper also describes the FoS test chamber developed and discusses how it is used. iNEMI is holding a seminar on this topic in Taipei on September 7, and webinars are scheduled September 25 & 26.
Download the white paper.
Information about the seminar (September 7, 2018).
Register for a webinar (September 25 & 26, 2018).