There was a paper presented at a local (San Jose CA) section of the Electrochemical Society some years ago by a guy working at IBM. This paper presented work done on studying the effect of the "accelerator" on the quality, uniformity of electroless copper deposition. The basic thing that they found, using some very impressive research, was that the more Palladium, and the less Tin left on the surface when it went into the electroless copper bath, the faster the rate of initial deposition, and the more uniform the deposit. This probably does not surprise anybody. The surprise was when they looked into was the effect of using various different chemistries for the accelerator. And the results that they got were shocking for that day, but in light of the success of certain commercial electroless lines that do not have an accelerator, it is now expected. In essence, what they looked at was various different acids, and a dilute caustic soda bath, and to the surprise of all concerned, the dilute caustic bath worked better than any acid they checked, and they checked quite a number. The funny part of this story is that the researcher was at a loss to explain why the caustic worked better, but with some doodling around, it becomes clear what was going on chemically. First of all, it is important to understand that when you use any acid as an accelerator, you remove a lot of the Palladium that is already on the surface. Caustic removes almost none at all. However, the key feature here really is that when the catalyst goes down on the surface, the Tin is SUPPOSED to react with the Palladium, turn the Palladium from a salt into the metal, and the Tin is then converted from the Stannous state to the STANNIC state. Caustic soda, or potash, is one of the few common chemicals that will dissolve Stannic Tin, and by doing so, it drives the chemical reaction (of the reaction of the Tin and the Palladium) to completion. This is why caustic is the best accelerator. I bring all this up, because it is clear from these discussions that the substrate has a profound effect on the amount of both the Palladium and the Tin left on the surface, and this is likely the reason why the amount of electroless copper varies with the substrate. Phew, I hope the foregoing was understandable. Rudy Sedlak RD Chemical Mountain View CA