Hi Jowan! Yes, you may see a difference is the ImSn degradation depending 
on which order you conduct your baking process. The ImSn surface finish 
has two degradation mechanisms - oxidation of the Sn on the ImSn surface 
and diffusion of the copper into the tin creating a copper/tin 
intermetallic (IMC) which has really bad solderability characteristics 
once it becomes oxidized. Diffusion is a time/temperature controlled 
process so if you "bake and store" the pwbs, you are effectively assisting 
the creation of the IMC, its oxidation and degrading the pwbs much faster 
than if you " store and bake". ImAg and ENIG are much more robust in terms 
of oxidation and IMC issues in comparison to ImSn but the same processes 
are going to occur just on a different time scale and with different IMC 
phase reactions (i.e. oxidized Ni based IMCs are not any fun to solder 
either). The good news is that the many of the pwb surface finishes on the 
market today have 12 months of shelf life when stored in reasonable 
temperature/humidity controlled conditions. The IPC-1601 committee is 
working on a document that addresses your question.

Dave Hillman
Rockwell Collins
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[TN] IMC growth on ImSn






Hello all,

Baking boards before SMT assembly  is very common.
Is there a difference between "baking and then storing the board for 6
months" and "storing the board for 6 months and then baking before
assembly" ?
Does the IMC grow faster on boards with ImSn finish after baking?
I'm especially talking about ImSn but I'm also interested in the effect
of baking on other finishes like ImAg and Enig.

Storage would be in a dry shield bag with silicagel or a dry cabinet.

Any suggestions are welcome.


Jowan Iven 



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