Poh Kong Hui wrote:
> 
> 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

Here is my attempt:

I assume:
1) The PWB is plugged with resin (LPI or SR1000, or the like).
2) The vias are plugged from both sides.
3) The boards are .062" thick.

Q. Why is there solder in the holes?
A. When plugging vias from ONE side only, it is common to plug the holes 
AFTER Solder Coating, this assures that all the copper in the vias are 
coated.  It is my assumption the supplier followed the same process when 
they plugged BOTH sides of the hole.

Facts:
1) When plugging from both sides of the hole, there remains an air 
cavity in the middle hole.
2) When the PWB is heated up, the gases in the air cavity tries to 
expand.  Being prevented by the resin plug firmly attached to the solder 
in the hole, the cavity builds up pressure.
3) When solder melts, as in the solder wave or the convection oven, 
things firmly attached to it (ie solder plugs) are no longer firmly 
attached.
4) The solder plugs, once firmly attached are suddenly floating on a 
solder film, and the air cavity trys to push the solder plug out of the 
hole.
5) The solder mask cracks, allowing the liquid to squirt out, like a 
small fountian.  [ I am surprised the entire plugs don't fly out.]
6) The sudden release of pressure is one definition of an explosion.

Tell your supplier/designers:
1) Do not plug from both sides, or
2) Plug from both sides before solder leveling, or
3) Tent both sides only with DRY FILM Solder Mask.

There are other problems with these suggestions which have to be worked 
through, depending on why your design requires tented vias.


Best of Luck
George Franck
Falls Church, Va.

***************************************************************************
* TechNet mail list is provided as a service by IPC using SmartList v3.05 *
***************************************************************************
* To unsubscribe from this list at any time, send a message to:           *
* [log in to unmask] with <subject: unsubscribe> and no text.        *
***************************************************************************