Hi Mark -
     
     A lack of coplanarity becomes a critical issue as the component lead 
     pitch decreases. An example that is widely used to illustrate the 
     coplanarity problem is that as the board pads decrease in size to 
     match the component leads, the surface tension of solder applied 
     during HASL or Hot oil fusing results in a crowned pad. The crowned 
     pad isn't ideal for the solder paste operation and the component leads 
     tend to "slide" off of the solder crown which leads to poor placement 
     and bridging. 
     
     
     Dave Hillman
     Rockwell Collins
     [log in to unmask]


______________________________ Reply Separator _________________________________
Subject: HASL thickness...
Author:  [log in to unmask] at ccmgw1
Date:    6/14/97 8:45 AM


My curiosity is killing me.  I have been reading about Horizontal HAL 
thicknesses ranging from coverage on the low side to .0015 on the high 
side.  My experience reveals a somewhat tighter range on most QFPs.  
     
Can someone in assembly tell me what the problems are associated with 
this lack of coplanarity?  Is this phenomena creating issues about 
planarity of the site, or variances in the amount of free solder left on 
the pads?
     
Thanks in advance,
     
Mark
     
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