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] 

***************************************************************************
* TechNet mail list is provided as a service by IPC using SmartList v3.05 *
***************************************************************************
* To unsubscribe from this list at any time, send a message to:           *
* [log in to unmask] with <subject: unsubscribe> and no text.        *
***************************************************************************