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November 2011

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Subject:
From:
Steve Gregory <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Steve Gregory <[log in to unmask]>
Date:
Mon, 21 Nov 2011 14:06:18 -0500
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Hi Phil,

Yes, I was surprised too, especially since I've not seen this before. But I
did take a stiff bristle brush and vigorously "buffed" the area. This is
what resulted:

Before "buffing":

http://stevezeva.homestead.com/Before_Buffing.jpg

After "buffing":

http://stevezeva.homestead.com/After_Buffing.jpg

Hmmmm, I really don't think this was a residue, because we use this paste
all the time and this is the first time I've ever seen something like this.
It really has me puzzled...

Steve

-----Original Message-----
From: [log in to unmask] [mailto:[log in to unmask]] 
Sent: Monday, November 21, 2011 1:39 PM
To: TechNet E-Mail Forum; Steve Gregory
Subject: RE: [TN] Strange Gold?

Hi Steve,

I am surprised that the colored area did not turn silverish after
cleaning.  I wonder what the surface profile looks like as it seems odd
that the leading edge wave sits up higher than the colored area (or
maybe I am mistaken).  If you scratch the purple/blue, does the scratch
reveal silver colored metal below?  I have had capacitors turn similar
colors but we always considered it to be marginal plating as the root
cause.

One thing that jumped out at me is that you used electrolytic gold as
opposed to ENIG.



-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Steve Gregory
Sent: Monday, November 21, 2011 6:29 AM
To: [log in to unmask]
Subject: Re: [TN] Strange Gold?

Hi Chuck,

There was no coating on the boards. What looks like a coating being
melted
back is actually the leading edge of the solder wetting out along the
pads
(as indicated by the red arrows). The close-up photo shows the edge of
the
solder...

Steve

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Chuck Brummer
Sent: Friday, November 18, 2011 3:57 PM
To: [log in to unmask]
Subject: Re: [TN] Strange Gold?

Steve,

Great pics as usual.

What coating is over the gold that you are melting back to the edge of
the 
lands?  Looks like a conformal or some oxide prevention layer.

Chuck


Charles W. Brummer | 3M Manufacturing Engineer
3M Electronic Solutions Division
3M Canoga Park, 8357 Canoga Ave. | Canoga Park, CA 91304
Office: 818 734 4930
[log in to unmask] | www.3M.com


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From:
Steve Gregory <[log in to unmask]>
To:
<[log in to unmask]>
Date:
11/18/2011 11:06 AM
Subject:
Re: [TN] Strange Gold?
Sent by:
TechNet <[log in to unmask]>



Hi Stewart,

I think you're probably right, I just never have seen this before. 

To answer some questions others have asked, the location that I took a 
photo
of was of what it looked like after the board came out of the reflow
oven.
The part was never placed, what you see is the solder from the solder 
paste
print. I had a "window-pane" in the stencil for the thermal/ground pad. 

This is a circuit on a small board that is panelized in a 2X5 array. It
is
plated with electrolytic gold. There are two other SON8's that are
placed
right next to the empty location, and they appeared to have soldered
fine
along with everything else on the board. X-ray of the two placed SON8's
revealed nothing out of the ordinary. A few voids in the thermal pad,
but
typical for what I'm used to seeing with these kind of devices.

http://stevezeva.homestead.com/Small_Circuit.JPG

I took another photo after the boards had been cleaned. What's strange
looking is the way the solder wet out on the pad, almost like a rolling 
wave
across the pad. You can see the leading edge of the wave where I have
the
arrows pointing to:

http://stevezeva.homestead.com/Trippy_Solder.jpg

And here's a close-up:

http://stevezeva.homestead.com/Trippy_Solder_Close.jpg

I took the photos with a little Dinoscope with LED lighting, so maybe
the
LED's accentuate the purple hue.

Anyways like I said, I have never seen this before, and I'm trying to
understand why I'm seeing the appearance this time, and haven't on other
boards.

Steve

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stewart McCracken
Sent: Thursday, November 17, 2011 2:57 PM
To: [log in to unmask]
Subject: Re: [TN] Strange Gold?

Hi Steve,

Recommend SEM+EDX to be sure, but based on visuals alone there is a very
good chance that the discolouration is caused by formation of gold-tin
intermetallic compounds (IMCs). These can form some amazing colours (to 
the
point where IMC's are grown intentionally to make jewellery).

The key to the formation of visible IMC's is the ratio of tin to gold.
You'll see that the discolouration happens where the tin was thinnest.
At
this point the mix is relatively rich in gold. As a result, a number
gold
rich IMC's can form and remain visible at the surface where they produce

the
sorts of colours shown in your image. Should produce some really 
interesting
SEM shots and EDX maps. 

Hope that helps....

Stewart


Stewart McCracken
MCS Ltd.
Centre House
Midlothian Innovation Centre
Roslin
Midlothian
EH25 9RE
U.K.

t. +44 (0)131 440 9090
m. +44(0)7711 541735
e. [log in to unmask]
w. www.themcsgroup.co.uk

 



-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Steve Gregory
Sent: 17 November 2011 18:59
To: [log in to unmask]
Subject: [TN] Strange Gold?

Hi All,

 

Below is a photo of a SON8 footprint that was printed with SAC305 on a 
gold
plated PCB and reflowed. I have never seen this before. Is this "purple
plague"? If it is, exactly what causes it?

 

http://stevezeva.homestead.com/Strange_Gold.jpg

 

Steve



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