Hi Werner! I agree with your assessment/description of the solderability degradation for a thin solder finish but I think that you are giving the Pb oxide too much credit as a contributing underlying mechanism. Published data within the last 2 years has shown that Pb oxide is present in only small volume/area in comparison to Sn oxide or CuxSnx intermetallic oxide. The Sn and/or CuxSnx oxides dominate the wettability causing the solderability problems with the increased liquidus temperature issues of the Pb rich layer being a bigger contributor than any Pb oxide issues. HASL finishes are definitely headaches if good process controls are not followed and maintained. Dave Hillman Rockwell Collins [log in to unmask] ______________________________ Reply Separator _________________________________ Subject: Re: RE: IPC A 600 E:vert hasl Author: [log in to unmask] at ccmgw1 Date: 5/29/97 2:03 PM Hi Frank, You responded to my message of: >>Be careful about thin HASL solder layers; below 5 micro-meters (200 >>micro-inches) solderability can decrease rapidly with shelf life. with a message dated 05/27/97 11:31:19, [log in to unmask] (frp) : >I once read about a study in which the shelf life of 1 year was >simulated by a baking process. Then the grow of the intermetallic layer >Cu/PbSn was measured. The outcome of this investigation was that the >intermetallic layer increased by 0.8 microns. So to have a good >solderability after storage 1 year >(normal conditions) you only need a minimum of 1 micron of PBSn. >So I don't think that having less than 5 microns will give problems as >long as it is not below 1 micron. > >Kind regards >Frank Petit I was perhaps too brief in my response--I would wish things were quite that simple. I have clients that wound up with non-solderable soldering pads specing a minimum of 0.1 mils (2.5 microns) of HASL coating thickness, whereas specing 5 microns eliminated the problem. While IMC growth at low temperatures, like 20 C, are indeed slow, it rapidly increases with increasing temperature. While it takes about 1 year to grow 0.8 microns of IMC at room temperature, the same thickness is obtained in about 3 weeks at 40 C. It is also a matter of solder composition; the higher the Sn content the faster the IMC growth. Further, one has to consider what the HASL layer actually consists of. The copper pad is covered by (1) a thin layer of Cu3Sn IMC which in turn is covered by (2) a thicker Cu6Sn5 IMC layer, which is the layer that grows; adjacent to the Cu6Sn5 layer is (3) a layer from which the Sn has been depleted to form the IMC layers (this layer has a higher Liquidus temperature because of the higher lead content); this layer is (hopefully) covered with (4) a layer of the regular solder, which in turn is covered with (5) a layer containing non-solderable lead oxides. If the HASL process removes too much solder, the solder layer (4) becomes very thin or non-existent. The solder lay er (4) with its low Liquidus temperature is required during reflow soldering to break up the Pb oxide layer (5) as well to help liquify the Pb-rich layer (3). Colin Lea's book 'A Scientific guide to Surface Mount Technology is a good source for more detailed information. Therefore, a total HASL layer thickness of 2 microns or less is sure to get you in trouble unless you have a perfect JIT capability. Werner Engelmaier Engelmaier Associates, Inc. Electronic Packaging, Interconnection and Reliability Consulting 23 Gunther Street Mendham, NJ 07945 USA Phone & Fax: 201-543-2747 E-mail: [log in to unmask] *************************************************************************** * TechNet mail list is provided as a service by IPC using SmartList v3.05 * *************************************************************************** * To subscribe/unsubscribe send a message <to: [log in to unmask]> * * with <subject: subscribe/unsubscribe> and no text in the body. * *************************************************************************** * If you are having a problem with the IPC TechNet forum please contact * * Dmitriy Sklyar at 847-509-9700 ext. 311 or email at [log in to unmask] * *************************************************************************** *************************************************************************** * TechNet mail list is provided as a service by IPC using SmartList v3.05 * *************************************************************************** * To subscribe/unsubscribe send a message <to: [log in to unmask]> * * with <subject: subscribe/unsubscribe> and no text in the body. * *************************************************************************** * If you are having a problem with the IPC TechNet forum please contact * * Dmitriy Sklyar at 847-509-9700 ext. 311 or email at [log in to unmask] * ***************************************************************************