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February 2011

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From:
"Stadem, Richard D." <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Stadem, Richard D.
Date:
Wed, 9 Feb 2011 10:44:58 -0600
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You nailed it pretty good, Dave. Those are about as good of ground rules for such a subjective issue as can be found. We use similar wording, but I added pictures of heavily "oxidized" examples, such as along the edge (caused by tape residues used during fabrication), blackened areas (caused by direct contact with desiccant pouch or MIC), etc.
We don't have a SERA tester. Our PWB inspector is named Mary, and she has some bad days and good days......  Sea lo que sea, será.


From: [log in to unmask] [mailto:[log in to unmask]]
Sent: Wednesday, February 09, 2011 10:18 AM
To: TechNet E-Mail Forum; Stadem, Richard D.
Cc: [log in to unmask]
Subject: Re: [TN] IPC-4553


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
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TechNet E-Mail Forum <[log in to unmask]>; Please respond to
"Stadem, Richard D." <[log in to unmask]>


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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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