Hi Bob - yes, using an inert atmosphere during the baking will
significantly reduce the impact of surface oxidation for a pwb surface
finish but the inert atmosphere won't change the formation and oxidation
of the IMC. The oxidation of IMC happens very quickly (within 20 minutes
once exposed to air) so even if an inert atmosphere prevented oxidation of
the IMC during baking, the IMC will oxidize during storage if it reaches
the surface. The big issue is that the baking process is the growth assist
mechanism for the IMC - if you don't bake, the probability of the IMC
becoming thick enough to reach a surface and become oxidized is low. As
for using inert atmosphere in the automated processes, many folks do
utilize nitrogen in that capacity. However, using an inert atmosphere
within a process does increase the process consumables costs and an inert
atmosphere isn't always necessary to produce the required solder joint
quality. We utilize inert atmospheres in our reflow ovens for assemblies
that have 0201s, QFNs, CSPs but have determined that QFPs, TSOPs, 0603s,
etc do not require an inert atmosphere reflow environment, providing they
have good solderability, to form acceptable solder joint quality. An inert
atmosphere is one portion of the recipe involving flux type/activity, flux
residue quantity/removability, component wettability and process cost -
depending on which process attribute is more important to a user,
different process priority choices are made.
Dave
Bob Landman <[log in to unmask]>
Sent by: TechNet <[log in to unmask]>
02/17/2010 06:44 PM
Please respond to
Bob Landman <[log in to unmask]>
To
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Subject
Re: [TN] IMC growth on ImSn
Dave,
Why not bake boards in an inert nitrogen atmosphere?
And while we're on the subject, why isn't it a requirement that all
automated soldering (wave and reflow) done under a nitrogen atmosphere?
What's the cost difference? Isn't it worth it as it reduces the need for
a flux which is highly activated and which is harder to remove after
soldering?
Bob Landman
H&L Instruments, LLC
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of David D. Hillman
Sent: Wednesday, February 17, 2010 8:47 AM
To: [log in to unmask]
Subject: Re: [TN] 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
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"Iven, Jowan" <[log in to unmask]>
Sent by: TechNet <[log in to unmask]>
02/17/2010 02:11 AM
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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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