RC Filter

We're using a RC filter network that we're having solderability problems with. There are 4 RC pairs with 2 ground connections (this is a leadless component). The problem we're having is non-wetting of one of the castellations (9 of the 10 scallops will have solder wicked to the top of the part, but one will form a large ball ont he pad with absolutely no wicking). This is an intermittent problem, 20 networks per board 1500 units per day and maybe 1 to 2 non-wetting problems per day. We've checked the placement and profiles and everything is OK.

We believe it's contamination or a plating problem on the component (electroplated tin over nickel). The manufacturer does QC checks on the plating and doesn't see a problem. We've had and independant lab run thickness measurements on the plating (cross section and XRF) and they show below min thickness (5 micron). We're going to use low voltage SEM with EDS to see if the nickel underneath or the tin is oxidizing, but the tests haven't been run yet.

Has anyone experienced this problem? Another company uses the same RC network with no reported problems. They use Pb-free (we're still SnPb), and they're consumer products.

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