Hi Werner!
     
I don't think there is any confusion on the intermetallic - the formation of 
copper/tin intermetallic is the metallurgical combination of tin with copper: 
whether you view tin diffusing "down" to combine with copper or copper 
"diffusing "up" to combine with tin is a mute point. We all seem to agree that 
if the intermetallic gets to the surface bad things result. The copper - tin 
binary phase diagram doesn't show any solid solution zones for copper and tin  
(for the temperatures most everyone worries about) but the intermetallic phases 
can grow at room temperature (and yes that growth is very slow). Is thinking the
copper diffusing through the tin metallurgically, exactly correct - No, but it 
gives everyone a simple basis of understanding. Good info about needing the 
intermetallic to form a sound solder joint - the email traffic didn't mention 
that topic and should have!


Dave Hillman
Rockwell Collins
[log in to unmask]


______________________________ Reply Separator _________________________________
Subject: Re: SnPb Oxidation
Author:  [log in to unmask] at ccmgw1
Date:    11/19/96 5:07 PM


There is a significant amount of confusion on the subject of intermetallic 
compounds. When soldering to copper surfaces copper/tin intermetallic 
compounds are the result of good wetting--no intermetallic compounds=no 
wetting. Next to the copper surfaces there is a thin quite uniform layer of 
Cu3Sn which is covered by a layer of Cu6Sn5 which growth with time and 
temperature and has a rough outline. While other intermetallic compounds, such 
as those with silver and gold readily migrate throughout the solder joint, 
Cu6Sn5 stays mostly in that layer on top of the Cu3Sn-thus Cu does not migrate 
to the HASL solder surface to any degree that might impede solderability. 
However, if the HASL layer is too thin the Cu6Sn5 peaks can grow sufficiently 
to reach the surface and oxidize. That together with the lead oxides which can 
no longer be washed away by liquid solder underneath it because most of the 
tin in the solder has been converted to Cu6Sn5, cause the solderability 
problems. The solution is a thicker HASL solder thickness; 60 microinches is 
the minimum and gives limited shelf life; for long shelf lives solder 
thicknesses of about 150 to 200 microinches are required. Colin Lea's book "A 
Scientific Guide to Surface Mount Technology" gives this subject a good 
treatment. 
On the subject of intermetallic compounds there is too much concern about 
intermetallic layers growing too thick. It is much more important that in 
fact all the solder joints in an assembly are properly wetted even on 
thermally massive components, which means that in some cases it is necessary 
to exceed the industry rule of thumb  of "60 seconds above liquidus. This 
rule of thumb can be pretty bad, (1) because to most people liquidus means 
183C, which is the solidus temperature (for most tin/lead solders the 
liquidus is higher), and (2) good wetting is assured (given solderability) if 
liquidus+20C is reached for 5 to 10 seconds on even the slowest heating 
solder joint. 
     
Werner Engelmaier
Engelmaier Associates, Inc.
Electronic Packaging, Interconnection and Reliability Consulting 
23 Gunther Street
Mendham, NJ  07945  USA
Phone & Fax: 201-543-2747
E-Mail: [log in to unmask] 
     
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