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December 2011

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From:
William Noel <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, William Noel <[log in to unmask]>
Date:
Wed, 14 Dec 2011 05:43:44 -0600
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Hello all - I am looking for some advice on building our standardized CAD models for our BGA's.  I have been creating our CAD models using the IPC LP Calculator as a companion tool to the IPC-7351 SM Design and Land Pattern Standard. One of the conventions that 7351 recommends is creating solder mask shapes at a one to one size relative to the pad size.  To my understanding this creates a Solder Mask Defined (SMD) solder joint.  Additionally I believe this  solder joint is termed a "non-collapsing" solder joint using the IPC terminology.  When the solder mask is relieved by a slight amount (in other words if the opening in the solder mask is slightly bigger than the pad geometry) then this creates a Non-Solder Mask Defined (NSMD) solder joint which I believe is termed a "collapsing" solder joint using the IPC technology.  Our fabrication notes indicate that the solder mask may be "swelled" by the fabrication house as appropriate to create a high quality solder joint while simultaneously insulating routes that are in close proximity to the joint itself.  The concerns we have are that now the fabrication house has possibly modified our artwork and we have no way (without close examination under a microscope) of knowing if the soldermask opening was enlarged and if so by how much.  We don't really want to leave it up to our designers to modify the soldermask opening for the "standardized" CAD models coming out of library.  We would prefer to have our CAD model fully represent what is being built.  Any advice on this subject would be appreciated.  Thank you.

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