Having been involved as a solder mask supplier for almost 20 years, one of the technology changes that has tended to simplify my life (most others have made it harder) has been the switchover from reflowed Sn/Pb to SMOBC. Let me explain, Going back 20 years, a high percentage of PWB's were made with fused Sn/Pb circuitry. This was back when surface mount was in its infancy, most boards were strictly plated through holes (double sided), high density was 10 mil lines and spaces and holes were big (ie, low aspect ratios). The general feeling was that Sn/Pb was more reliable than SMOBC and most end users felt more comfortable using it. Generally, these fused Sn/Pb boards were coated with solder masks (back then the most common were thermal or UV cure types, screen printed). Solder masks do not adhere very well to fused Sn/Pb (compared to copper) and when boards were run across the wave solder line, the solder would melt and break the bond of the solder mask to the solder. At that point, the sodler mask tended to blister, flake, fall off, etc. When this happened, a nasty phone call would be made to the board supplier. Since you know what flows down hill, the next phone call (even nastier) would be made from the board supplier to the solder mask supplier (poor little ol' me). As technology changed, densitites got tighter (5 mil lines/spaces), multilayers came on the scene, holes got smaller (especially vias) and aspect ratios increased and surface mount came on like gangbusters. These changes aggravated the problems with solder masks on fused Sn/Pb designs. I really believe that solder mask problems were the biggest single reason that there became a massive switchover to SMOBC away from fused Sn/Pb and surface mount technology was the main catalyst. With SMT, you have a minimum of two times where the solder will melt (once for the fusion of surface mount components on the component side and once for the wave soldering on the solder side). Whereas, you might get lucky and the solder mask will survive one "melting" of the solder without flaking, lifting, etc., it won't survive two. Therefore, Sn/Pb on SMT designs became a disaster with solder mask flaking, fusion of adjacent circuit traces, etc. By now, at least 95% of all circuit board designs are built using SMOBC technology.SMOBC has been accepted by most of the major end users who are very concerned about long term reliability (IBM, DEC, Hewlett-Packard, AT&T, Delco Electronics, Compaq, etc.). One of the real pioneers of SMOBC technology has been Odi Cardenas of Solder Station One in Santa Ana, CA. I'm sure Odi could give you a lot of information about the reliability of SMOBC and hot air solder leveling (he was instrumental in getting IBM toi switchover to SMOBC in the mid 80's), Odi can be reached at: Odilon Cardenas Solder Station One 714-558-1019 Hope this helps. Larry Fisher Dexter Electronic Materials [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] * ***************************************************************************