Faults Associated with Pumice Scrub

Our conformal coat area has recently requested changes to our bare board houses to perform a pumice scrub operation on the PWB prior to HASL plating and delivery to us. Our first lot received caused us to have void faults under multiple component types (passives, SOIC, BGA, etc.) using an oven profile that has been used on the same assembly (no pumice scrub) for the last 2 years on 5000+ assemblies with no faults. We are being told by the board house that this scrub operation is becoming a standard for their customers. It has also significantly improved our coating operation faults. Has anyone else seen this type of fault at the SMT or X-Ray level? What is the cause/potential causes? What have other people done to minimize/fix the problem? Does it drive back to something in the board house process?

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