Client Case Study: The Terminal Blocks Were Clean, but Not Now
In this abbreviated case study, we explore a client who contacted us when 70% of their electronic assemblies, taken from new inventory, had corroded terminals on the terminal blocks. There were no signs of corrosion or visible residues anywhere else on the assemblies. Was the problem contaminated terminal blocks from their supplier? What to do now?
Testing Methods and Results
We used both ion chromatography (IC), per IPC-TM-650 2.3.28, and surface insulation resistance (SIR) testing, per IPC-TM-650 3.2.3.3a, to evaluate the situation. The IC testing showed, among other things, chloride (Cl-) and weak organic acid (WOA) levels that were potential threats to product quality. The SIR results showed the negative effects of surface residues on electrical performance between leads under power in high temperature and humidity conditions (85oC/85% RH).
Chloride is one of the more detrimental materials found on printed circuit assemblies. Chlorides can come from a variety of sources, but are most often attributable to flux residues. Chlorides will generally initiate and propagate electrochemical failure mechanisms, such as metal migration, electrolytic, and galvanic corrosion, when combined with water vapor and electrical potential. The as-received assemblies had chloride levels above Foresite-recommended limits, especially in the terminal block area. Remedial cleaning of the connector areas showed drastic reductions in those levels on the surface of the assemblies - well below our recommended limits and would pose no corrosion or electromigration risk.
Weak organic acids (WOAs), such as adipic or succinic acid, are used as activator compounds in many fluxes. Our findings showed high levels of WOAs on these assemblies, indicating poor performance of their cleaning system. The high WOA levels can cause electrical leakage problems. Remedial cleaning of the assemblies greatly reduced the WOA residues, minimizing the risk of electrical leakage problems.
Six as-received and six Foresite-cleaned assemblies were SIR tested. All of the former failed the IPC requirements while all of the cleaned assemblies passed.
Conclusions Derived After Testing
Our conclusion: when the final assemblies (with terminal blocks installed) were in-line cleaned, heavily flux-laden wash water sprayed onto the terminal blocks, which were subsequently not properly rinsed. A galvanic cell was created between the two dissimilar metals on the terminals (brass and the tin-plated screw) in the presence of the chloride. The resultant corrosion on the brass was insulating, causing poor connections. In addition, the flux residues on the assembly (away from the terminal block) would become a problem in the field when powered.
The End Result: An Improved Process
The terminal blocks had to be removed, the assemblies properly cleaned, and new terminal blocks installed. The assembly process on new product was changed to install the terminal blocks after assembly cleaning, with the improved process.