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). [log in to unmask] ______________________________ 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)