Having been in the solder mask business for the past about 20 years, I can give you a general overview of the evolution of SMOBC and PISM (with approximate time lines) and how these technologies have become dominate in the PWB isdustry, In the early days of circuit boards, most circuitry was Sn/Pb. Solder tends to be a very solderable surface, whereas copper becomes oxidized easily and won't solder. There were some anti oxidant treatments back then, but they were not all that reliable and had poor shelf lives. On these Sn/Pb boards solder masks were applied (usually one or two part epoxies) by standard screen printing with a pattern. Because of "bleed" large clearances around the pads had to be used; however, this was not too much of a problem back when 50 mil lines/spaces were considered "high tech" and plated through holes were 35 mils or greater. Unfortunately, during wave soldering the Sn/Pb circuits under the solder mask would melt and the mask tended to flake off. >From my perspective, this precipitated lots of nasty phone calls about lousy adhesion of solder mask, but there was litttle that I could do (once the Sn/Pb melts the adhesion is gone). Even so, this was still mostly a cosmetic problem because of the wide spaces between circuits. By the early to mid 70's circuit lines/spaces became smaller and smaller and the adjacent circuit traces with Sn/Pb circuitry would tend to fuse together when the circuits melted during wave soldering and this became a serious functional problem instead of just a cosmetic one. The advent of the hot air solder leveling (HASL) process (1970's) was the savior because now you could strip the Sn/Pb after etching, scrub and apply solder mask over the now copper circuits, apply solder to the plated through holes by HASL and have a nice sodlerable metal during wave soldering without worrying about melting the circuits because now they are copper and will not melt. So by the late 70's early 80's, the issue of melting circuits, solder mask flaking and shorts had been solved by SMOBC and HASL; however, the issue of solder mask bleed with traditional screen solder mask was still there and became more of a serious problem as plated through holes became smaller, circuits became denser and "mask on pads" became a major issue. In the USA the solution to this problem was the advent of dry film solder masks. It is interesting to note that at about the same time the solution in Europe was the advent of the Probimer curtain coated, solvent developed, liquid photoimagable solder mask and in Japan it was a number of screen applied, aqueous developed liquid photoimageable solder masks. All of these processes applied the mask to the entire surface of the circuit board and the pattern was difined by using standard UV exposure units and phototools (either silver or diazo). By the mid to late 80's Probimer came to the USA and took the market by storm at many of the larger circuit board shops. This product was cheaper, much tougher and more durable and could be applied at thinner coatings. The thinner coating became a major advantage as surface mount technology became the norm because the thicker dry film solder mask (usually 4.0 mils) caused solder shadowing and solder skip problems. By the late 80's, the aqueous PISM products from Japan made their debut in the USA. These products were cheaper, tough and durable, environmentally friendly (aqueous developed), had low VOC emissions, could be applied as thinner coatings and did not require large capital equipment investments to run. As we moved into the 90's, the aqueous developed, liquid, photoimageable solder masks became the dominant solder masks used in the USA and this market is still growing. Many equipment companies developed specialized equipment to apply and process these masks. I hope this gives you an overview of the SMOBC and PISM history. If you want to dig deeper and need more specific information, you may want to contact PC FAB magazine. Over the years they have published many articles on solder masks and would highlight solder masks in one of their issues every year. Regards, 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] * ***************************************************************************