Solder paste size for Least

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   Hi, We usually use the "least" setting when we generate our footprints.  The solder paste size is the same as the pad size. 
My assembly team is telling me that there is too much solder on the pads and that we should reduce the size of the aperture for the solder past screen.  

The problem is more obvious for QFN parts.  When using the "least" setting the it looks like the toe is just too short and the solder creates a bubble shaped contact instead of a nice fillet.

My question is, does the IPC standard say anything about solder past aperture for QFN when using the least setting?

Thanks,
Dominic

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dcayen Members Profile Send Private Message Find Members Posts Add to Buddy List New User Joined: 06 Mar 2015 Status: Offline Points: 3	Post Options Post Reply Quote dcayen Report Post Thanks(0) Quote Reply Topic: Solder paste size for Least Posted: 08 Mar 2018 at 5:21am
	Hi, We usually use the "least" setting when we generate our footprints. The solder paste size is the same as the pad size. My assembly team is telling me that there is too much solder on the pads and that we should reduce the size of the aperture for the solder past screen. The problem is more obvious for QFN parts. When using the "least" setting the it looks like the toe is just too short and the solder creates a bubble shaped contact instead of a nice fillet. My question is, does the IPC standard say anything about solder past aperture for QFN when using the least setting? Thanks, Dominic



Tom H Members Profile Send Private Message Find Members Posts Add to Buddy List Admin Group Joined: 05 Jan 2012 Location: San Diego, CA Status: Offline Points: 5717	Post Options Post Reply Quote Tom H Report Post Thanks(1) Quote Reply Posted: 08 Mar 2018 at 9:03am
	The IPC-7093A (unreleased and in progress at this time) has lots of guidance on QFN assembly. I'm on that IPC committee and we're looking at releasing it this summer. Also, I have noticed in Texas Instruments recommended patterns for micro-miniature packages that the paste mask is reduced but the solder mask is also reduced to make the pad stack "Solder Mask Defined" - http://www.ti.com/lit/ds/symlink/tlv713p.pdf ; But less solder can also be achieved by thinning the paste mask stencil where small aperture openings are. We are going through a learning curve with small pad sizes and paste mask reduction. Some companies globally reduce the paste mask by 10% on all SMD pads. But if you are using Library Expert V2018 you will need to use the Calculator to auto-generate the QFN land pattern and then move it to FP Designer to edit the pad stack paste mask size and then save to FPX and output to your CAD tool. I always use the recommendation of the assembly shop as they are ultimately responsible for the end product reliability. Neither the IPC-7351 or the IPC-J-STD-001 have any guidance on paste mask reduction for QFN packages.
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