Creep Corrosion of PWB Final Finishes: Its Cause and Prevention

As the electronic industry moves to lead-free assembly and finer-pitch circuits, widely used printed wiring board (PWB) finish, SnPb HASL, has been replaced with lead-free and coplanar PWB finishes such as OSP, ImAg, ENIG, and ImSn. While SnPb HASL offers excellent corrosion protection of the underlying copper due to its thick coating and inherent corrosion resistance, the lead-free board finishes provide reduced corrosion protection to the underlying copper due to their very thin coating. For ImAg, the coating material itself can also corrode in more aggressive environments, which has become an issue for products deployed in environment with high levels of sulfur containing pollutants encountered in the current global market. In those corrosive environments, creep corrosion has been observed and led to product failures in very short service life (1-5 years). In some of the worst cases, creep corrosion failures within one year of product deployment have been reported. This has prompted an industry-wide effort to understand creep corrosion although minimal progress has been made in this effort. This lack of progress has been due to the inability of reproducing creep corrosion in the lab using realistic accelerated aging tests. In this paper, we will demonstrate that the creep corrosion on PWB is highly surface sensitive. Neither clean FR4 nor solder mask surfaces support the creep corrosion. In general, the board assembled with rosin wave soldering fluxes and solder paste containing rosin flux is also resistant to the creep corrosion. The residues left on the solder mask surface by the organic acid flux, on the other hand, are highly active for supporting the creep corrosion of copper sulfides. The right choice of the assembly flux can eliminate the product failure due to the creep corrosion associated with the ImAg plated circuit boards deployed in the global environment. Furthermore, mixed flowing gases (MFG) provides a realistic accelerated test for simulating the creep corrosion in the laboratory without requiring condensing conditions.