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 This message (including any attachments) may contain material, non-public information or proprietary information and is for the intended recipients only. If you are not the intended recipient, you should notify the sender and delete this message. Any disclosure, copying, or use of this information is strictly prohibited and may subject you to legal liability. 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 ______________________________________________________________________ This email has been scanned by the Symantec Email Security.cloud service. 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