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February 2010

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From:
Joyce Koo <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Joyce Koo <[log in to unmask]>
Date:
Wed, 17 Feb 2010 21:21:27 -0500
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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.     
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----- 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
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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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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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