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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