Hi Jowan - the baking process itself is accelerating the diffusion
process. Baking creates more IMC than storage and increases the
probability that the IMC will reach the finish surface. Once the IMC
reaches the surface it will continue to oxidize during storage. The
diffusion rate during storage is so low that there is minimal risk in the
IMC reaching the surface and to start oxidizing thus the reason that a
"store and bake" flow would be better.
Dave
"Iven, Jowan" <[log in to unmask]>
02/17/2010 08:35 AM
To
"TechNet E-Mail Forum" <[log in to unmask]>, <[log in to unmask]>
cc
Subject
RE: IMC growth on ImSn
Hi Dave,
Thanks for your input.
What exactly would happen with the IMC if the boards are baked first
before storage?
In other words: Why does the diffusion accelerate after baking? I would
presume that the acceleration of IMC growth is only temporary during the
baking process.
And , in fact, there would be no difference if the two steps are
performed in a different order.
Jowan Iven
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of David D. Hillman
Sent: Wednesday, February 17, 2010 2:47 PM
To: [log in to unmask]
Subject: Re: IMC growth on ImSn
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
[log in to unmask]
"Iven, Jowan" <[log in to unmask]>
Sent by: TechNet <[log in to unmask]>
02/17/2010 02:11 AM
Please respond to
TechNet E-Mail Forum <[log in to unmask]>; Please respond to
"Iven, Jowan" <[log in to unmask]>
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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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