The widow pane pattern allows for the gas generated by the consumption
of the flux to escape from the thermal pad. The treatment is similar
to SOD's and QFP's with thermal pads.
On 12/13/11, Stadem, Richard D. <[log in to unmask]> wrote:
> QFNs are very typically subjected to an aperture reduction. Usually the
> peripheral pads are reduced by 10-15%, and if a large belly pad is present,
> the stencil aperture reduction is typically between 40% to 60% of the pad
> area.
>
> To understand why, think of it like this:
> A chip capacitor, resistor, or a leaded SOIC, QFP, etc. all have someplace
> for the molten solder to flow other than the pad itself. Chips have endcap
> terminations, molten solder can wet out to the edges of the pads and run up
> the sides of these endcap terminations. Likewise with leaded components. In
> all of these cases, the solder has somewhere to go; there is no real
> constraint that would cause the surface energy of the molten solder to go
> "out of bounds" in the form of solder spattering.
>
> In the case of QFNs, if you print solder paste in a one-to-one ratio with
> the pads, the molten solder has no place to wet and flow. With the component
> body mass sitting on the resultant puddles or blobs of molten solder on the
> belly pad and the peripheral pads as well, spattering occurs, and that is
> why there are so many solder fines trapped under the QFN part. To prevent
> this, the stencil aperture is reduced such that the smaller amount of solder
> has someplace to flow.
>
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Joyce Koo
> Sent: Tuesday, December 13, 2011 2:40 PM
> To: [log in to unmask]
> Subject: Re: [TN] TI Paste Recommendations
>
> For QFN, 7093 is the guideline you want. QFN is rather different compare to
> the BGAs.
>
> Joyce Koo
> Materials Researcher - Materials Interconnect Lab
> Research In Motion Limited
> Office: (519) 888-7465 79945
> Mobile: (226) 220-4760
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Gerald Bogert
> Sent: Tuesday, December 13, 2011 3:28 PM
> To: [log in to unmask]
> Subject: Re: [TN] TI Paste Recommendations
>
> December 13, 2011
>
> Solder paste size can depend on the part manufacturer recommendations. For
> example, Freescale for their BGA P013 processor recommends that the PC board
> solder pad and stencil aperture diameters should generally be the same; a 1
> to 1 ratio.
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of William Noel
> Sent: Tuesday, December 13, 2011 2:52 PM
> To: [log in to unmask]
> Subject: [TN] TI Paste Recommendations
>
> Hello all - I am working on creating a 24 pin QFN footprint for my CAD
> library. Is there a valid reason for making the solder paste smaller than
> the actual pad outline? I notice that TI consistently seems to recommend a
> smaller paste outline than pad outline. Additionally, they recommend a
> series of small square paste squares to appear on the thermal pad rather
> than a single smaller square of paste. Essentially, IPC-7351
> recommendations are different than the TI recommendations. Any help? Thank
> you.
>
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