I would be skeptical about claims of embrittlement with such a small amount
of gold in your solder joints. What you did not provide was the method
used to prove this embrittlement. Typically I have found that it is a
fear, not a fact, although I do get sensitive when the percentage of gold
rises over 2% in the solder connections.
Electroless nickel immersion gold is very common these days so make your
board house build it correctly.
It has become our required finish on new double sided SMT boards due to the
recent change we made where we eliminated nitrogen from our reflow ovens.
At 02:12 PM 5/3/00 -0400, <Dennis Fritz> wrote:
>Dear CL Chong:
>
>You have some of the answers already about electroless nickel gold, and you
>have some misinformation:
>
>Immersion gold plating occurs when a small amount of the electroless nickel
>is dissolved into the gold solution, and gold is deposited in place of the
>nickel (oxidation - reduction reaction). 4-10 microinches are normal deposit
>thicknesses, but not much more can be deposited since nickel stops dissolving
>through the gold layer.
>
>Yes, electroless nickel is required as a barrier for the gold (as in any
>copper/nickel/gold construction since copper and gold migrate rapidly into
>each other). Also, electroless nickel is a convenient souce of nickel ions
>to reduce the gold in solution. Electroless plating coats all the copper
>traces - top and sides, without continuous conductor attachement as in
>electroplate. Gold is most commonly electroplated when the original copper
>foil gives conductive continuity, or when the circuits are "bussed" together
>as in connector tabs.
>
>I am very surprised that 20 microinches of gold gave you embrittlement as
>gold is thought to be compatible in solder to 3 or 4%. That means that you
>were putting down less than 600 or 800 microinches of solder paste - true?
>Anyway, the thinner immersion gold plate will give you less gold in your
>solder joints.
>
>Ohter points:
>Most LPI type solder masks are now compatible with electroless
>nickel/immersion gold processing. That is feature most mask suppliers have
>been working on.
>
>Yes, phosphorus is co-deposited with the electroless nickel. But, running
>the process by the seller's instructions makes the phosphorus deposit
>consistant with bath age, bath loading, etc. Most currnent thought about
>solderability of electroless nickel/immersion gold does not focus on the
>phosphorus content of the deposit, but on the corrosion rate of caused by the
>immersion gold bath. Run properly by the manufacturers instructions, there
>should be a uniform corrosion of the nickel/phosphorus, and a uniform deposit
>of the immersion gold.
>
>4 microinches of immersion gold will have some porosity and lower shelf life.
> Deposits nearer the 10 microinch thickness should store and solder well for
>months.
>
>Please contact me off line if you want more specific information -
>
>Dennis Fritz
>MacDermid. Inc
>
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