Our Customer had detected tarnished pad on ImAg PCBA and had conducted several SEM-EDX test at the soldermask below some of the chip resistors. High concentration of sulfur was found and it can be significantly reduced after further chemical cleaned - as verified again by SEM EDX.
Our Customer requires us to wash clean the PCBA due to risk of metal migration should such 'free' sulfur being transferred onto the pads later which may induce creep corrosion.
Need advises on: a) is there such thing as 'free' sulfur on top of the soldermask after it was cured by the PCB supplier? b) if these 'free' sulfur were the metal sulfate (eg: barium sulfate, Silver sulfate...), what is the risk creep corrosion should it migrate onto the ImAg pad? c) is it possible for the soldermask surface having such sulfur residue after it was cured at the PCB supplier? d) what is the acceptable limit for sulfur presence on bare PCB and finished goods (PCBA) based on SEM EDX test and Ion Chromatography test.
The immersion silver is an active surface and readily combines with sulfur from the environment. Silver sulfide tarnishes the surface and creates doubt about the integrity of the finish at inspection. [uyemura.com/library-4.htm]
Your customer's boards are collecting sulfur somewhere in the chain between your ImAg tanks and their finishing process. It could be any place. I've seen sulfur on blue ESD mats in hand assembly areas.
I think you and your customer need to work together to find the source of this material. You need to show them that your boards are sulfur-free when you ship them, rather than accepting some new mandate, which we assume is unfunded by your customer.