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