This problem is likelu a process control problem when  the board was 
manufactured.  From your description, I suspect the boards had solder 
applied first then the plugs.

If you LPI first with the pads open and  then HASL and then screen print 
via plugs the HASL process must blow out all via holes, otherwise there 
may be air trapped between the ink  plug and the solder plug.  When you 
heat it up at wave solder the air expands and the solder blows out.

Problems can be worse if you plug the vias first and t hen HASL.   This
creates a dead-end hole with solder at the top and plug  ink at the
bottom
that often traps flux or in the worst case, pre-clean micro etch
chemistry.
This can blow out at wave solder as you describe or even worse cause 
long term opens in  the plated holes.

Your designer can help by either making the vias large enough to blow out

more easily at HASL or by not requiring the vias to be plugged.  This may

require not using metal jacketed components over  the vias or using 
stand-offs to prevent shorting..


On Sat, 14 Sep 1996 23:28:52 +0800 Poh Kong Hui <[log in to unmask]>
writes:
>Hi,
>
>Could anyone please tell me why a bare PCB with plugged via-holes, has
>solder in the via-hole. ???
>
>When I reflow the board, the solder in the via-hole explode out of the 
>via
>hole and fly practically anywhere on the board or somewhere in the 
>oven.
>
>The solder has formed a ball and even shorted some area
>
>Thanks .. Regards.
>
>
>Poh Kong Hui 
>Nera Electronics
>Fax: (65) 7765492
>
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