Hi Dave - sorry for the late reply but I have been on travel the past
week. Here is my responses to your questions:
1) What solderability test methods are recommended for these products?
Solderability testing would be per J Standard 003. This specification
works for ALL coatings provided that you and your board fabricator
come to an agreement on what degradation method to use. Example: I
wouldn't dare think about using steam aging to evaluate a gold/nickel
finish. I would work out a dry bake or some type of humid bake with my
board supplier. The 003 specification lists several solderability test
methods for the user to apply.
2) Is the IPC in the process of capturing these requirements?
The 003 specification is under revision right now. In addition the
Alternative Solderable Finishes task group is looking at several
non-tin finishes and trying to develop a conditioning procedure(s) for
these alternative finishes.
3) Is there an 'optimum' manufacturing/storage condition to minimize
or hopefully eliminate solderability concerns?
Tough question! What works for some PWA manufacturers doesn't work for
others. Look at solderability as a two mechanism problem: diffusion
and oxidation. If you finishes are thick enough to eliminate any
diffusion problems (nickel going through gold, copper through silver,
copper through tin/lead, etc.) then you only need to stop oxidation. A
dry, temperature controlled environment could give you good results.
If diffusion is a problem then all the careful storage in the world
won't help you. The simple answer to an optimum storage condition is
probably in a space orbit but that would be a bit expensive!
4) Is it implied that only ONE soldering process can be used with
immersion gold finishes thereby precluding the use of double sided
SMD?
No, many people use immersion gold and two passes very successfully.
You need to look at the process overall. How long are you storing the
boards, what flux are you using, who's plating baths are being used,
etc. A great many variables that need to be looked at, characterized,
and controlled so your process is repeatable.
5) How accurate is the measurement of gold in the ranges typically
quoted in immersion gold thicknesses (ie 2-6 microinches)? Is this measured
by weight gain? XRF?
A good thickness measurement tool is XRF and I believe that it is the
most widely used also. Weight gain, cross sectioning, and other methods are
not accurate enough for the thin plating systems.
Hope this is helpful. There is going to be some good, new data on
alternative finishes at the SMI Conference in September that may be helpful
too.
Dave Hillman
Rockwell Collins
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Subject: FAB: Immersion gold inquiry
Author: [log in to unmask] at ccmgw1
Date: 7/14/96 9:51 PM
Recently a customer of ours has inquired as to what methods are recommended
to determine solderability of non-electrolytic Au/Ni pwb's. In our
discussion this customer claimed that immersion gold boards, as manufactured
by the pwb manufacturer, soldered just fine, however during shipment and/or
storage at the assembly site, solderability was degraded to a point that the
colour of the solderable surface was, and I quote, "green in colour". I
assume that this was due to oxidized Nickle migrating through the gold.
It seems from the posting listed below that immersion gold finishes are
intrinsically flawed from the perspective of solderability.
Can anyone in electroless 'cyberspace' recommend answers to these questions? :
1) What solderability test methods are recommended for these products?
2) Is the IPC in the process of capturing these requirements?
3) Is there an 'optimum' manufacturing/storage condition to minimize or
hopefully eliminate solderability concerns?
4) Is it implied that only ONE soldering process can be used with immersion
gold finishes thereby precluding the use of double sided SMD?
5) How accurate is the measurement of gold in the ranges typically quoted in
immersion gold thicknesses (ie 2-6 microinches)? Is this measured by weight
gain? XRF?
Thanks for your feedback...
Dave Rooke
Circo Craft - Pointe Claire
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>
>| Hi!
>|
>| Accelerated aging factor for a new PCB surface finish?
>| Do you mean
>| oxidation? Intermetallic growth? What type of finish?
>| Please send
>| additional detailed info covering your request.
>|
>|
>|
>| Dave Hillman
>
>Hi Dave,
>
>In this case the new PCB suface finish is electroless Ni/immersion Au. And
>you are right, it is oxidation - under extreme and/or even normal storage
>condition, Ni has the tendency to diffuse through Au layer (if there is >any
porosity) and gets oxidized on the top of Au layer, then the >solderability
will be affected due to the existance of nickel oxide on the >top of Au
layer.
>
>So far I have not found any mathimatical model (either physical or
>experimental)for acceralated aging factor as the function of time in aging
>chamber, temperature in aging chamber, and relative humidity level in
>aging chamber, although I have found several aging method. But none of
>them provide corresponding aging factor. By aging factor, I mean the
>ratio between the time in chamber and the time in normal storage
>environment, with same level of solderability degradation.
>
>Thanks in advance for your advice.
>
>
>Nora
>
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>
>
D. Rooke
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