Wow Joyce, you sent that on a Blackberry? What do you use to sharpen your
fingers??....8-)
John Burke
(408) 515 4992
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Joyce Koo
Sent: Wednesday, February 17, 2010 6:21 PM
To: [log in to unmask]
Subject: Re: [TN] IMC growth on ImSn
Bake in inert atm only limited the oxidation, but not imc formation. If you
bake 1st, the imc most likely will grow alone any defects, such as
grainboundary, or free space. It most likely not going to be layer by layer
growth. Just look at the imc would tell you the nucleation and growth. The
initial driving force could be diffusion driven, the further growth are also
aidded by volume change of imc vs the initial x'tal structure of the two
elements (if not three), that would resulted small voids. Based on my
recall, the bulk, grain boundary and surface diffusion coeff are increase
based on the element localized environment. Surface being the highest, with
or without presence of oxygen (lowest activation energy). If you have poor
plating with a lots of voids, the imc could form very rapidly with or
without oxygen.
--------------------------
Sent using BlackBerry
----- Original Message -----
From: TechNet <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Sent: Wed Feb 17 19:44:41 2010
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
[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]>
To
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cc
Subject
[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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