Some finishes are more resistant than others, but lack of a standard hinders testing.
A coworker asked yesterday why he was chosen to work on a defect that was less than a fraction of a percent in occurrence. It was like finding a needle in a haystack, he said. I know he was looking for sympathy, but I could not offer it, as my entire team has been working on creep corrosion for the past four years. For some, creep corrosion also occurs at less than a fraction of a percent defect rate. There is no industry standardized test to replicate it, and during the onset, the only thing known was that it predominately occurred in paper mills, fertilization plants, tire factories and car modeling studios. The answer to the question is even one unhappy customer is reason for concern.
When this column was turned over to me by John Swanson, he wisely instructed me not to tackle anything crazy on my first attempt. “Nothing like creep corrosion,” he said. This is the day I offer about 1000 words on the matter. I can and will talk at great lengths on this subject, so catch me at the next conference for the full dissertation.
Creep corrosion is a migration of copper sulfide across a circuit board. The SEM and EDS images (Figure 1) show a section of an annular ring. The pad is plated with silver (green) creep corrosion growing over the soldermask. The creep analyzes as rich in copper (red) and void of silver. Additional EDS mapping explains the creep area also is rich in sulfur.
Figure 1. SEM (left) and EDS photos of annular ring show creep corrosion (in green) growing over soldermask. The creep analyzes as rich in copper (red) and void of silver.
The defect reveals itself as an electrical short or open. Shorts occur when the volume of migrated copper has reduced the volume of a copper trace or via. A short occurs, of course, when copper sulfide travels to its neighboring trace or pad. For creep corrosion to occur, there must be exposed metal area in the final circuit. This is normally an area that was not soldered or not completely covered by solder. The environmental exposure must be high in sulfur and humidity for the corrosion to manifest. As stated, the greatest challenge is that a standardized test does not exist to recreate creep. IPC conducts a biweekly meeting to attack this very issue. After researching the environments where electronics were located, even those expected to be “clean” (such as data storage centers), it was determined there is a lot of bad air out there.
Initial attempts to create a controlled test for creep corrosion will be conducted in mixed flowing gas (MFG) chambers. The test centers around Battelle Class 4 conditions, but with almost 10x the concentration of hydrogen sulfide. Battelle uses 200ppb of H2S. This test uses 1500 to 2000ppb. Also, the operating temperature will be increased from 35° to 40°C. Unfortunately, not every MFG chamber can handle these test conditions. Some detectors are not calibrated to measure high levels of sulfur properly; some equipment cannot resist corroding under these conditions, and some lab managers just don’t want to expose their workers. At this point, a red flag should go up, and the alarm should be ringing in your head. It is difficult to accept, but our environments are getting worse, and these aggressive conditions are turning into reality. Electronics life expectations are increasing, so prolonged exposure will ultimately lead to high levels of corrosion on unprotected parts.
Of the surface finishes, immersion silver sees the greatest threat from creep corrosion. Topcoats have been formulated to mitigate creep. These topcoats act as barriers for the metal from condensation and contamination in the air. The topcoats are designed to protect both the silver surface and any copper metal that may be exposed. All these attributes have been achieved without detriment to the other functional performance characteristics of immersion silver. The immersion silver plus topcoat parts withstand all traditional MFG exposure. They even resist corrosion in more aggressive versions of MFG that veer from traditional conditions. Due to the limitations of some MFG chambers and the high cost of the test, OEMs and chemical suppliers have created internal chambers to recreate creep corrosion in a lab environment. These tests accelerate the conditions of mixed flowing gas and introduce sulfur in particulate form. Elemental sulfur is common to many of the environments where creep corrosion has occurred. It is a point that should be taken into consideration during test development.
The performance and acceptance of these topcoats has been well received, but some will hesitate until the real-world environments are truly understood and can be replicated. Questions are continuously raised about the ISA classification these coatings withstand, but perhaps the better question is, Should these classifications be revisited?