Ron,
	You are probably right by saying that the solder coming up the vias is causing the solder joints to reflow.  Even if the solder isn't entering the vias, on an FR4 board, the vias will act as a heat conductor between the bottom and top sides of the board.
	To be sure this is your problem, I would instrument one of these component leads that are reflowing and profile the process.  You can probably get one of your reflow guys to do this.
	If this is the case, unless you can change the board layout, you may just have to live with the problem for place and reflow this component after wave soldering.

Hope this helps,

John Guy
American Competitiveness Institute (EMPF)
317-655-3673 x130
[log in to unmask]


----------
From: 	Ron Hollandsworth[SMTP:[log in to unmask]]
Sent: 	Thursday, March 20, 1997 8:24 AM
To: 	[log in to unmask]
Subject: 	Assy:  QFP Reflow


     Hello TechNet:
     
     We have a .062", board with a top side QFP (SMT) that is reflowing as 
     the board crosses the wave.  Our top side preheat temps have been 
     checked and double checked and is in the 230 degree "F" range.  Peak 
     temps shown as the board crosses the wave are well below the 361 
     degree "F" mark.  We have vias in the pads on some of the leads.  The 
     QFP is plastic.  The board is approximately 3"x5" in size.  Density is 
     maybe a little over average.
     
     Has anyone out there experienced the same thing with some of your 
     products?  We are not seeing a blatant open solder situation, but 
     suttle fractures that will eventually turn to a open situation.
     
     Supposedly the SMT guys profiles are ok.  They are pretty reliable.  
     The boards are inspected after SMT reflow.
     
     What do you think?  Is the reflow caused by the solder coming up 
     through the vias?  
     
     R. Hollandsworth
     [log in to unmask]


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