REGARDING           Reflow problem

Hello Technetters,

I am experiencing a 'little reflow' problem with an assembly.

The PCB is quite large 21" x 21", this restricts the supplier in using an all
over solder reflow process during manufacture, as apposed to SMOBC.
There is no active devices on the board, all are passives.
I use a vapour phase machine for reflow.

Heres the problem.
Tin/lead is wicking from under soldermask near plated thru holes and forming
solderballs/bridges between conductive elements. I am seeing this on both
sides of the board, in particular the bottom side as Tin/lead seems to be
accumulating on the underside (assumming gravity is taking its toll on the
process). This seems to suggest an excessive amount of tin/lead (thickness)
under the soldermask.
I am told by the supplier that it is very difficult to control this amount of
tin/lead and SMOBC is not feasible due to board size.

Any suggestions, comments.

Can I do anything in reflow process to reduce this, anybody else come accross
similar issue in the past - with an identified solution.

Regards
John O'Sullivan.

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