Screen printing QNF24 problem

I'm having a real problem with printing a QFN. It's a QFN24, 3mm across. I have to get a rather large deposit of solder for the size of the part. It seems the only way we have reliable, repeatable joints under this part is to have peaks as high as .012" on our solder paste deposit. (Why it doesn't short to everything is beyond me) Currently I'm using a .006" thick foil, with only a 1.5 mil reduction on the aperture width (to avoid shorts), and no reduction on length. (This leaves an aperture of 10X17) Still, I have to cycle the squeegees 4 times, at .2"/sec., with a snap-off of .080", and I can only get maybe three or four acceptable prints a day! Can you help offer a process to this problem?

B.D.

Expert Panel Responses

The latest revision of IPC 7525 Stencil Design Guidelines deals with QFN stencil aperture design. Specifically it is recommended to reduce the aperture sizes over the ground plane by about 50% by window payning that aperture. For Tin lead aperture reduction of 1.5 to 2 mils in width and 1.25 to 1.5 oversize of the pad in the 4 corners. For lead free 1-1 with the pads and the same overprint for the 4 corners.

Bill Coleman
Vice President Technology
Photo Stencil

For over 18 years, Dr. Coleman has been the vice president of technology for Photo Stencil, working closely with customers to understand their printing requirements. His efforts have resulted in several new stencil products.

I'm assuming the reason you changed to the process you describe is because a normal "On Contact " process caused insufficient paste on pad. By printing with the snap-off you are effectively pump printing the paste which is causing the extreme paste height you're getting. I recommend going back to an "On Contact" print process (no snap-off) and optimize the process from there. I suggest contacting the package provider to ensure the PCB Land-Pad design is within their specifications (IPC-SM-782). Texas Instruments suggests the Pad design to be 0.010" X 0.033" with a stencil aperture of 0.009" X 0.030" at 4 to 5 mil thick. This document can be found on their web site.

Michael O'Hanlon
Applications Supervisor
DEK

Mr. O'Hanlon serves as the DEK Applications Supervisor for the Americas region. He has over 20 years of electronics manufacturing experience and has spent the last 13 years at DEK providing equipment utilization and process solutions for SMT manufacturing.

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