The problems I mentioned earlier are purely physical. One of the
functions of the flux is to provide a molten barrier between the
insulating substrate and the solder. A certain amount of dross is +/-
inevitable and it is important that this cannot stick to the substrate.
It is therefore a question of the surface tensions of the substrate, the
flux (which must wet the latter), the molten solder and the dross.
Having a matt mask will allow better wetting, not because the surface
tension is higher, but because of the much greater surface area and the
physical capillary forces. If there is insufficient flux left at the
moment of entering the wave, disaster in inevitable and becomes apparent
even before bad wetting, because the flux has already activated the
metal surfaces before it has evaporated.

There may also be some chemical aspects, in that no component of the
flux should have a deleterious effect on the mask. Such effects are
quasi-inevitable if the mask has not been properly cured, as the
prepolymers will almost surely be soluble in the flux solvent. This will
leave a surface of doubtful quality with unpredictable effects, but all
bad!

Hope this helps.

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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