Jim,
The nickel dissolves comparatively slowly, so it is left behind.
The inter-metallic formed is Ni3Sn4.
> -----Original Message-----
> From: Jim Kittel [SMTP:[log in to unmask]]
> Sent: Wednesday, May 03, 2000 12:32 PM
> To: [log in to unmask]
> Subject: Re: [TN] Electroless/imersion Ni/Au Vs electrolitic Ni/Au
>
> Dennis,
>
> One more question please. When the assembler applies tin/lead paste to
> the
> electroless nickel/immersion gold surface does the gold and nickel both
> dissolve into the solder during reflow or does the nickel remain behind as
> a
> barrier over the copper; and then what kind of inter-metallic would be
> formed with the tin/lead?
>
> Jim Kittel
> L-3 Communications
>
> -----Original Message-----
> From: <Dennis Fritz> [SMTP:[log in to unmask]]
> Sent: Wednesday, May 03, 2000 12:13 PM
> To: [log in to unmask]
> Subject: Re: [TN] Electroless/imersion Ni/Au Vs
> electrolitic
> Ni/Au
>
> 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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