Definition of immersion gold vs electroless gold

Letter 18471

Definition of immersion gold vs electroless gold

++

Is there a substantive difference between electroless gold and immersion gold, and if so can you define this for me. I am not a finishing professional but I deal with components that mount to printed circuit boards.

Roger Williams
- Missouri

++

By "electroless", industry professionals really mean "autocatalytic". So your question becomes what is the difference between immersion plating solutions and autocatalytic plating solutions.

An immersion plating solution operates on the principal of displacement of the surface skin of the substrate by a more noble metal that is in solution. In science class, high school kids put iron nails in beakers of copper sulphate and watch as the outermost layer of iron goes into solution and is replaced by copper ions coming out of solution. There are several weaknesses to immersion plating. First, the thickness is limited to a few atoms: once the surface is covered by the new plating, the solution doesn't "see" the substrate anymore, so no more substrate atoms can dissolve and consequently no more noble metal ions deposit. The second problem is that you can't immersion deposit a baser metal onto a more noble metal; for example, you can't immersion deposit copper onto gold, because the built in battery that drives immersion reactions is wired the wrong way. Thirdly, immersion deposits tend to exhibit poor adhesion.

Autocatalytic solutions are sort of "spring loaded" reactions which are driven by reducing agents in the solution, but which will only take place in the presence of a catalyst; but the metal being deposited is the catalyst. So you start the reaction either with electricity or by depositing onto a substrate which is also catalytic to the solution; and the reaction continues as long as the surface being plated remains in the solution, continuing to catalyze the reaction. Consequently the thickness can be much, much greater. Autocatalytic gold plating is a recent development.

Ted Mooney, P.E.
finishing.com
Brick, New Jersey

+++

I am investigating moving to immersion silver from HASL surface finish for PWBs. I noticed Mr. Ted Mooney stated I.S. has poor adhesion. Would this be only in the as-plated state? After assembly with solder, the silver has alloyed into the joint and is no longer a layer, isn't it?

Thank you,

Lisa P Koland
- Minneapolis, Minnesota

+++

Immersion plating processes do tend to have poor adhesion. This is because the plating process is not metered or controlled by an external electrical source; rather, atoms of the plating deposit on the substrate at the rate of their choosing, often not slowly building a solid crystal structure, but instantly converting from ions to metal of a powdery form. But sometimes the adhesion it is adequate to the task. For example, virtually all plating onto aluminum begins with an immersion deposit of zinc.

As for whether immersion silver will be an improvement over HASL, Lisa, the question is not within my experience; hopefully a more knowledgeable reader will reply.

Ted Mooney
finishing.com
Brick, New Jersey

February 11, 2008

I have been operating an immersion silver line as a finish for printed circuits for the past year or so and am unaware of any adhesion problems.

David Moyer
printed circuits manufacturing - St.Paul, Minnesota

March 5, 2009

I do copper electroforming of vegetal material (leaves, flowers) and then gold electroplate it directly over copper, with very poor results. I can´t use nickel before gold plating as I produce anti-allergic jewels.

I guess my answer is going into the Electroless gold plating, directly over the copper electroformed organic object, if appropriate.

Would I need to do both steps? First do an immersion gold plating and then the autocatalytic process? Or is the autocatalytic one enough?

If so, many of my finishing quality problems would be solved.

Thanks for your comments and advise.

Hernando Durana
- Bogota, Colombia

March 9, 2009

Hi, Hernando. Gold directly on copper is not a good idea because they diffuse together. A better approach would be cobalt plating in lieu of nickel, or ideally white bronze plating in lieu of nickel.

Regards,

Ted Mooney
finishing.com
Brick, New Jersey
opt

Legal disclaimer boilerplate button