Tom,

what about printing paste on the worst PCB you have and pass it in the reflow, without any parts. What do you get?

Regards,

Ioan

-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Tom Parkinson
Sent: Monday, October 16, 2006 9:48 AM
To: [log in to unmask]
Subject: [TN] Suspect PCBs


Hello All (DN & TN),

I'm having a disagreement with one of my board vendors.  We have rejected
some of the boards for "suspect bad pads" (ENIG).  During visual inspection,
the suspect pads 'jump out' at you by looking dark in color.  At the very
least, they sure refract the light the wrong way making them look dark in
color. Rotating the board does not make them look better.  

In the past we have tried soldering to these pads and have run into problems
with wetting.  In my letter to the vendor, I mentioned the ENIG was either
too thick, or too thin, both which will cause problems. 

I'm especially worried about these when they are under BGAs.

The vendor says there is nothing wrong with the pads as-is.  In fact he said
the following:

=========================
"Boards are good.  There is no black Pad - only dull gold.  Xxxxxx (board
vendor) is "Cleaning" the gold to make it look "Pretty".  We will be
returning the boards on Monday to you.

There is always a variation in surface color with any immersion gold
process. It may be minimal in some cases or more obvious as in the boards in
question. But problem is not the correct description, variation is the
correct term. It will not effect form fit or function.
 
The color variation you refer to is not because of the variation in
thickness. Three to seven micro-inches of gold is deposited over 100 to 150
micro-inches of nickel. The gold vaporizes during your reflow process and
the adhesion is solder to nickel. There will be no adherence problem. By the
same token, there is no embrittlement problem with immersion gold as there
would be with electro-plated gold. For one thing electro-plated gold is 10
times thicker than immersion gold. Secondly the grain structure of immersion
gold and electro-plated gold is very different. Electro-plated gold having a
long grain structure in comparison to immersion gold.
 
Using a pencil eraser as you suggest only proves my point. The surface color
variation is caused by an alkaline deposit on the surface of the gold which
vaporizes on contact with molten solder. We propose to clean the boards
using a mild baking soda solution."
==========================

Looking forward to your inputs.

Thanks,

Tom



Tom Parkinson 
Quality System Manager - CIT
WinTronics, Inc.
Phone: 724-981-5770 - extension 235

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