In 7 years of experience as Process Engineer for HASL I have seen both 
 sides of the coin. In the beginning, one of our major customers wanted the 
 solder as flat as we could make it. As pitches shrank, minimum solder 
 thickness requirements increased, especially for 20 mil QFP's. I, like 
 you, have never seen a soldering defect directly attributed to thin 
 solder. But, I have had boards returned for the "thin solder defect". 
 Typically solder thickness on discrete pads will be ~50 microinches. Those 
 pads tend to look dull compared to a 20 mil pitch QFP site with 400 
 microinches of solder. That visual difference can lead to rejection.
 
 
 There is always a very thin coating of eutectic solder over the 
 intermetallic layer (IMC) on HASL processed product. Granted, a thin 
 solder coating may have an increased lead content due to the consumption 
 of tin into the IMC. Assembly problems can arise if the product is stored 
 in poor conditions and a heavy lead oxide layer forms (difficult to remove 
 with today's water and no clean fluxes). 
 
 
 
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______________________________ Reply Separator _________________________________
Subject: Minimum Solder Thickness @ Assembly
Author:  [log in to unmask] at corp
Date:    11/22/95 9:23 AM


As fabricators, we have had it drilled into us that "flat solder is best" 
for assembly. We have never received a return from any customer for "thin 
solder", but I can't say the opposite is true. My process engineers desire 
to alter our hot air solder leveling parameters to eliminate any future 
chances of returns for thick solder. Our SPC data suggests that lowering the 
peak solder thicknesses results in exceptionally thin solder (IMO) on 
certain pad geometries. My question:
 
How thin is too thin? As assemblers, when (at what thickness or process 
conditions) would you return boards to the fabricator?
 
 
[log in to unmask] (Glynn Shaw)