OK, Darren. It is not easy to be categorical but I suggest you are
losing in at least one of two directions, maybe both.

1) So-called "no-clean" fluxes usually have relatively small amounts of
activators which tend to sublime/evaporate at temperatures exceeding ca.
50°C. It sounds like you may simply not have enough left on the board
when it hits the wave. Try wicking down the preheat a little at a time
until the results improve, if this is the case. Perhaps a little more
flux sprayed on would help, as well.

2) It may also be that your solder mask is unsuitable or incompatible
with the flux or has been insufficiently cured. This is quite a common
problem, especially with some of the glossy masks and dry-film types.

This list is not exhaustive, but you have a 95+++% chance that the
answer lies above.

Brian

Darren Goh wrote:

>
>
> Hi,
> I would really appreciate some help here..
> I am running some mixed board (thru holes with SMD) over wave.. after
> the wave, I discovered almost 80% of the boards are spatter with small
> lumps of solder dross on the under side... (between three to five
> counts of them)
>
> I am using Tamura no-clean flux and on board preheat temp is peak at
> 100degree for 10 sec. the whole preheat zone runs at 80 sec..
>
> am using Alpha Metal solder bar ..and adopting spray fluxer
> technology, no clean process..
>
> I hope that someone could highlight and share with me the reason (if
> possible solution) for the formulation of solder dross on the under
> side of mixed technology boards after wave.
>
> thank you very much
> Darren
>
> DID:- (65) 2493448

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