HASL PCB non-wetting

Dear all, Recently I have a problem of non-wetting at our wave soldering process. It appears only on the SMT pads for the chip components (at the solder side) of the HASL PCB. With all the same parameters on the wave soldering machine, it was found that only one of our PCB suppliers has this problem. But under the microscope, no apparent sign or indication of contaminations on those non-wetted pads. I have discussed this with other people within the industry and I got various causes such as tin oxide, tin-copper intermetallic or organic acid tin(?) formation on the solder pads. And I was also told the HASL coating may be too thin (about 1um). In order to get a better understanding of the cause and before it is sure to point the finger to the quality problem of the PCB supplier, I would need some help to answer the following questions: 1. Does anybody encounter similar problem before? 2. How to identify the cause of it? 3. How to fix it on the raw PCB? 4. Have you heard about the term Organic Acid Tin & how does it formed? 5. What is the optimum thickness for HASL on the SMT pads for reflow soldering & wave soldering? All opinions and comments are welcome. (N.B. a small batch of the non-wetting PCB was sent to another PCB supplier to do HASL again and it seems to get rid of the non-wetting problem.) Thank you in advance & best regards, Victor Ng

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