Hi, Expert, Currently, I'm facing a very critical problem on my HAL leadfree finishing PCBs at my customer end. We run the HAL leadfree (Tin, cu and ni) board by using HAL leadfree machine (temperature about 270 deg.C) and go for normal water post clean line after leveling process and follow by PCB subsequent processes. Before leveling process, had sent the boards to post clean line and fluxing. And, there is no abnormal sign on the PCB. The finishing (appearance) of PCB pad look O.K.
However, we sent the boards to my customer, they run SMT and Reflow oven (peak temperature about 240 deg.C). After this process, it can be clearly saw the pad became yellowish (especially on the bigger pad). At the same time, they also run same product same finishing board at the same time, but there was no abnormal. And, the appearance look more shinning than our boards. We were wondering what could cause yellowish?
We had seeked help from our tin bar and flux suppliers. They mentioned is post clean line water not clean. So, we cleaned the post clean line and top-up 100Kg of fresh Tin bar and use fresh water soluble flux (leadfree). The appearance also look good and we even took some boards to others customer side (same SMT leadfree process condition). The boards O.K.
Unfortunately, we shipped out those fresh batch to my customer and they still found yellowish after thei reflow oven process. They were questioning what is the yellowish? Will be it harmful to the board and component? Will it be corrosive? I really need some info which can help us to solve this problem immediately. Thanks.
What alloy are you using for Pb-free HASL? You say tin copper nickel, but is it SN100C?
Against manufacturing engineering's recommendation, my company specified simply "Pb-free HASL" as the finish for our Pb-free PCBs. One of our suppliers sent us SAC105. It worked fine for single reflow boards, but if we tried double-sided reflow... It was not good. The pads that were not pasted during the first reflow pass would turn "brassy", especially like you said on the larger pads. Trying to run the second reflow was a nightmare. The solder would not wet to the pads, and would ball up in the middle. Or, it would just wick up onto the leads of the components and off of the pads.
I don't know why the pads turned "brassy" looking. Maybe it was corrosion... Maybe the HASL was very thin and we were starting to see the intermetallic after the first pass through reflow... All I know is we didn't see this issue with our other suppliers who use Cobalt 995, SN100C, or even SAC305.
The interesting thing is that this particular supplier first told us the boards were SN100C (which I didn't believe by looking at them), then some other alloy... and in the end it turned out they didn't even really know what was being put on the boards.