Stephen, your message's wording indicates you are interested in flash gold
as an alternative solderable finish.
IPC D-275 paragraphs 3.8.4.3 and 3.8.4.4 do reference nickel/gold plating
requirements, but the thicknesses imposed by these paragraphs are intended
for gold plated edge connector patterns.
For gold soldering, the present recommended practice would be to communicate
your product's plating requirements directly on the fabrication drawing or
in your company's companion PCB procurement specification, if one exists.
IPC-2221, Generic Standard on Printed Board Design, is the base design
document in a series that will replace IPC-D-275. The January, 1996 draft
calls out a gold thickness of 0.8 um (microns) MAXIMUM, or approximately 30
uinch (microinch) MAXIMUM. Generally speaking, coverage of 3-7 microinch is
typical for an immersion gold (flash gold) process. Immersion gold, or
chemical gold is self-limiting in thickness. The maximum thickness is more
meaningful in the event that the gold is deposited with an electroless process.
Most drawings spec a minimum of 100 or 200 microinch for the nickel
underplate, a barrier to prevent the formation of gold/copper
intermetallics. These numbers are also used in IPC-D-275. The thickness of
nickel is dual purpose: preventing gold/copper migration/intermetallics,
and serving as a "anvil" for better edge connector performance. You might
be able to live with a lower thickness if the nickel is an electroless
coating. The IPC-2221 specifies 1.0 um (micron) = 40 microinch for class 1,
and 1.3 um (micron) = 50 microinch for class 2 and class 3 for a barrier for
copper/tin intermetallics. This thickness may be sufficient for an
immersion gold process as well -- no "anvil" effect is necessary.
One concern with gold is that in a wave solder process, the gold can
accumulate in the solder pot. If the gold concentration is not monitored,
this could lead to a significant level of gold contamination of solder
joints, which would be a reliability concern. Obviously, this is not an
issue for a board that sees IR reflow and no wave soldering.
The National Center for Manufacturing Sciences (NCMS) published at IPC
Printed Circuits Expo 1996, a summary of a five year test program on surface
finishes. In the Surface Finishes Team's paper, a reference is cited which
compared imidazole and other OSP coatings to immersion gold and HASL. It
states that the best assembly yields were obtained using an OSP coating.
The best known, most widely available OSP (organic solderability
preservative) coating is Entek Cu56 and Cu106A. The Cu106A is capable of
surviving multiple thermal excursions at assembly.
For most soldering applications, the OSP approach will cost less at PCB
fabrication and assembly and produce better yields (assuming good process
control). The OSP coating is generally more available in the domestic PCB
industry than immersion gold.
Make sure if you specify gold that you specify the gold purity. A pure gold
is desired for soldering (99.9% minimum purity, which corresponds to
MIL-G-45204 , Type III). I do not recommend using this gold mil spec unless
you want the associated mil-spec testing to be performed.
Regards,
David Estes
PCB Commodity Manager
Texas Instruments
Original message below:
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From: "Stephen Cooper" <[log in to unmask]>
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Subject: Gold Flash on PWB's
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Can anyone supply a specification to call up when requesting a Gold
Flash finish on a PWB and is it worth it today, with the advances
with the HASL finishes.
Regards
Stephen Cooper
Manufacturing Projects Manager
SCITEC LIMITED
Apollo Place
Lane Cove NSW 2066
Australia
PH : +612 428 9500
Fax : +612 428 9589
Email : [log in to unmask]
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