Hi Technet,

Today I did some tests on a sample-board from a  potential future bare board supplier. It was a thin flex-rigid boards with a smooth and shiny leadtin-plating (probably HASL) on the SMT-pads.
After passing the board through our reflow oven (forced convection, nitrogen atmospere, no solder paste, no components) I noticed excessive dewetting on all of the lands, except for those that had been in contact with the center board support rail.
During wave soldering (again no components used, RMA flux) the dewetted pads picked up fresh solder from the wave without any problem, after which the soldered surfaces look smooth and shiny again.

I don't understand how a smooth leadtin plating can dewet during heating-up, and the dewetted surface can still have a good solderability. 
Does anyone have ideas about what's happening here ? Will solder joints to such a board be reliable ?

Regards,

Daan Terstegge
Signaal Communications

* Unclassified * 

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