Hi Richard - very good discussion! I have the luxury of having a SERA
tester to measure the oxidation state of my immersion silver pwbs so I can
determine just have bad things are getting. Here is some wording we use on
our product floor to assist with the "changing visual appearance" of
immersion silver finishes. This rule of thumb isn't perfect but has been
helpful.
Immersion Silver Visual Appearance Rule of Thumb:
Immersion silver surface finishes will discolor as a printed wiring board
goes thru the assembly process. This discoloration is oxidation and not
corrosion with one exception. The typical discoloration has a yellow to
goldish tan to light brown visual appearance which is cosmetic in nature.
If the pwas have a dark brown to black visual appearance then they have
been either mishandled, were left damp going into their ESD bags or the
plating is bad. So rules of the road:
1) If the pwas have a yellow to goldish tan visual appearance in an area
that is not going to be soldered, no action required.
2) If the pwas have a yellow to goldish tan visual appearance in an area
that is going to be soldered, no action required unless we encounter
soldering issues.
3) If the pwas have a light brown visual appearance in any areas of the
pwa, then we should review our handling procedures to make sure we are not
putting pwas in bags damp or not handling the pwas by the edges. We should
also inspect the bare pwbs in stock to see if they have a light brown
appearance prior to assembly.
4) If the pwas have a dark brown to black visual appearance in any areas
of the pwa, then we have a problem and need to find root cause.
As you detailed, rubber bands and tape cause major issues for immersion
silver pwbs turning black due to sulfur and chloride contamination
Dave Hillman
Rockwell Collins
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"Stadem, Richard D." <[log in to unmask]>
Sent by: TechNet <[log in to unmask]>
02/09/2011 08:49 AM
Please respond to
TechNet E-Mail Forum <[log in to unmask]>; Please respond to
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Subject
Re: [TN] IPC-4553
In terms of losing solderability on the secondary side after a single trip
through the reflow, I think you have immersion silver finish confused with
immersion tin. Immersion silver does not usually degrade significantly
after a single reflow excursion.
But again, it depends (ka-ching).
There are many immersion silver plating products, some not as good as
others, and there is a huge variation amongst the plating companies that
apply these products. Like any other plating, it depends on which product
you use, and which plating company or PWB fabricator you use. Some IAg
products have a much superior co-inhibitor that prevents tarnishing and
oxidation.
Minor tarnishing (yellowing) of immersion silver has very little effect on
the DPMO of a given PWB. Black areas caused by direct contact with
sulfur-bearing desiccant packages, rubber bands, tape, paper (including
the Moisture Indicator cards) are a completely different story; these will
have significant impact on solderability, and the flux has little or no
effect on this.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Glidden, Kevin
Sent: Wednesday, February 09, 2011 8:33 AM
To: [log in to unmask]
Subject: Re: [TN] IPC-4553
Good point. Solderability depends heavily on your flux I would say. Those
using a high activity, halide containing formulation would have an easier
time than those with ROL0. Plus, the additional predicament is that the
oxidation may be minor when the PCBs are received, and they solder just
fine, but after 6 months or so in stock it is worse, or even after 1 trip
down the reflow oven making Side 2 unsolderable.
-----Original Message-----
From: Jack Olson [mailto:[log in to unmask]]
Sent: Wednesday, February 09, 2011 9:11 AM
To: [log in to unmask]; Glidden, Kevin
Cc: Jack Olson
Subject: Re: IPC-4553
Good question.... and when looking at an actual board, how is one to
determine
visually if we are looking at "staining" or "oxidation/corrosion"?
Inquiring minds
want to know.
(many would say that even oxidized boards are solderable and acceptable.
right? I think I just read that somewhere...)
Jack
.
On Tue, 8 Feb 2011 09:20:44 -0500, Glidden, Kevin
<[log in to unmask]> wrote:
>Morning everyone,
>I have a question on IPC-4553, specifically, section 3.1 "Visual", and
specifically regarding staining of immersion silver. The text states "The
coverage shall be complete and the finish shall be uniform on the surface
to be
plated (see Figures 3-1 through 3-5 for properties visually identified
with IAg
plated surfaces)" Figure 3-1 shows 'Example of Uniform Plating", with no
staining. Figures 3-2 through 3-5 show staining. Is this to imply
(without text
to support) that any staining of immersion silver is determined to be non-
uniform and therefore a defect?
>Thanks.
>
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