Thanks Wes for the good advice.
I have shared your message with some of our QA team. They corrected me with the functional test we did before approving PSW. The PSW was approved about 5 months ago when terminal pin with tin-only passed our functional testing (welded to other parts and tested). Due to other issues with the project, production got delayed until last month. Five months later, the welding started to fail significantly. The reason could be oxidation.
The spec is very clearly stated as "Finish: A. Copper flash .0001 max thick before tin-lead plate. B. Tin-lead plate (electrodeposited) per mil-P-81728 .0003-.0005 thick bright finish."
Therefore our QA team believes that the supplier should be solely responsible for this issue. Since the supplier is the one who submitted PSW, they carry the liability for any nonconformance. As a customer we can't analyze and verify all the aspects such as material and plating detail. If the material fails and we find the material isn't as specified, the supplier will be responsible. The plating is similar. Although the ideal case is that we analyze the plating detail, we rely on supplier to conform to our spec. FYI, during the PPAP, we did obtain all the material certification on plastic and pin except the plating certificate. And the supplier should require a certificate of plating from its subcontractor.
Back to the legal issue, if the current issue caused multi million dollar loss, wouldn't the supplier be liable?[/quote]That's why they have trials - to determine who is liable. That said, I know plenty of attorneys who would take the supplier's case with an expectation of winning or forcing a favorable settlement.
One of the things you haven't produced yet is expert testimony from some metallurgists who can demonstrate the failure is solely due to the lack of copper flash. There are many mitigating factors for the supplier inherent in the data you lay before us. If it were my call to make, I'd look at making the supplier an ally and jointly look for the root cause of the failure - plating error? or too long between plating and production? improper storage at customer's inventory prior to production? etc. rather than waste time, money, and energy looking for a fall guy.