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]> To [log in to unmask] 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 ______________________________________________________________________ This email has been scanned by the MessageLabs Email Security System. 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