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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