What gold thickness should I specify on PCBs?

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I am designing an EDGE Card PCB (connector), I have gold fingers on the edge of this PCB (which wil be mating and demating on a host board connector) I am calling out minimum 15 micro inches of immersion gold over 50 micro inches of electrolysis nickel (AuNi) over copper... This covers the gold edge fingers....

Now for the rest of the board I am calling out maximum of 4 micro inches of gold (flash gold)... as the rest of the board has components on it, like resistors and capcitors...

My thinking is that lesses the gold on the pads where these components will be soldered the better, because it'll be easier for solderbility purposes....

And I do need gold on the entire board as I am running High-speed signals...

Am I correct to call out Flash gold or max. of 4 micro inches? or should I be calling out more gold?

PLEASE ADVISE

Thank you,

Atul Sharma
product designer - Dallas, Texas, USA

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Atul,
Having spent over 20 years in and around the PCB mfg market, I would suggest calling out for 150 miroinches minimum sulphamate nickel, and 50 microinches hard acid gold minimum on the edge finger connectors. You will need the added wear resistance protection in these areas because the board may have to be plugged in more than once in it's lifetime.
As far as whole board plating or "deep well Au plating" you could call out for the same type of nickel and gold plating with the only change being the Au thickness at 25 microinches. I have never been a big fan of immersion gold plated thicknesses (less than 5 microinches in this case) on PCBs. Many boards are subjected to atmospheric, humidity, and temperature fluctuations where flash coatings do not last. If you need superior solderability requirements you should call out for a pure gold on the entire board with the same thickness of 25 microinches. Good Luck!

Mark Baker
Process Engineer - Syracuse, NY USA

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Thanks for the confident and informed reply, Mark! Extensive hands-on experience beats book knowledge every time :-)

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

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Ted,
Thank you for the kind remark. It is a pleasure to be of help to anyone who is in need of an answer.

Mark Baker
Process Engineer - Syracuse, New York