Metal finishing Q&As since 1989
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What Limits Immersion Plating Thickness?
Q. Hi,
Can you tell me what dictates immersion plating thickness. The spec I see on immersion Pd plating is 1 to 2 microinches. What determine if it is 1 or 2 or somewhere in between? Let's say I have the a circuit board called A and one called B. Does the geometry or surface area dictate how thick the immersion Pd is plated? So circuit board A is plated to a different thickness than circuit board B because of surface area? Is this true or is it that circuit board A and circuit board B will plate between 1 and 2 microinches depending on other factors? Could circuit board A plate between 1 and 2 microinches board after board after board. Does immersion plating thickness vary more because of the process than the part geometry? Does immersion plating thickness change based on metal content in the bath? As bath metal depletes does immersion plating thickness deplete? The simple for of my question is what range of thickness can I expect on a circuit board plated with immersion palladium. Same circuit board plated day after day and year after year. If I measure at a specific area every time can I expect better than a 1 to 2 microinch range.
Process Engineer - Plymouth, Minnesota, USA
August 11, 2011
A. Immersion plating thickness is generally self-regulating. Immersion plating involves a displacement reaction in which the base metal is oxidized and the metal being plated is reduced to the metal (zero oxidization) state. Once the base metal is covered, plating ceases.
James Totter, CEF
- Tallahassee, Florida
August 12, 2011
A. Hi Chris,
Unlike electroless (or autocatalytic) plating, immersion plating is self limiting (displacement reaction between substrate metal & plating metal ions) which deposition rate reduce gradually and eventually stops when substrate metal is entirely covered with plating metal. It's hard to tell exactly whether limited immersion plating thickness is 1, 1.5 or 2 microinches which depends on how well substrate metal is covered with plated metal (as long as tiny porosity towards substrate metal still exists, immersion plating reaction may still carry on until no more substrate metal is exposed).
Initial immersion deposition rate (before self limited) depends on several factors like plating temp, solution flow, plating metal concentration and even plating article (e.g. PCB) circuitry design (due to complicated PCB circuitry design, certain pads might have slightly different potential than others and record a different immersion deposition rate).
Regards,
David
David Shiu
- Singapore
August 12, 2011
Q. Hello all,
I had read some papers and textbooks recently, and having questions about the thickness a displacement plating can achieve. Since a displacement plating is self-limiting, the plating process will cease when the original substrate is fully covered by the coating. As a result, the thickness of a displacement plating should be few atom layers (about a few nm). Why is it pointed that the typical thickness of displacement plating varies from 10 to 200 µm in a textbook? Further more, what makes the thickness ranges in such wide window?
Many thanks.
- Taiwan
November 25, 2013
A. Hi Luke. We appended your question to an earlier thread which partially answers it. 10 to 200 NANOmeters is believable, for the reasons David mentioned (roughness, varying potentials at different spots, etc.). As a thought experiment, picture immersion plating a finely textured sponge -- what "thickness" of plating would you get? Well it would depend both on exact texture and how you measure and interpret "thickness" -- if you did a "strip and weigh" you'd easily calculate 200 nanometers or more.
But there is clearly either a misunderstanding or a misprint if you read that displacement plating can be up to 200 micrometers :-)
Regards,
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
November 25, 2013
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