You have 2 different pad patterns for QFNs depending on whether they have exposed side termination that start at the base or not...
For QFNs with side terminations we get filleting to the exposed edge but it is driven primarily by the oxidation level of the exposed metal, height of the exposed metal from the part base and paste composition. Secondarily it is the total solder volume available as determined by the pad extension beyond the outline of the part, the available over print area and the type of solder paste use... SO it depends...
Additional issues are lead pads that are not separated from the grnd pad with soldermask or silkscreen that will cause solder "leaching"...
Paul
Paul Edwards
Process/Quality Engineering
Surface Art Engineering
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Pete
Sent: Wednesday, October 12, 2011 7:01 AM
To: [log in to unmask]
Subject: Re: [TN] [DC] QFN layout
A couple conversations conveniently converging.......
The QFN question started on the Designer's Council listserver. More from a designer's point of view than an assembler's. So the answer there was much more generic, but still had to account for more than just a datasheet.
Paul & Bob are having a discussion about the minute amount of difference in solder paste deformation, depending upon the software used for P&P control.
From the QFN manufacturer's standpoint, any chance they considered that when proposing their footprint? Or even could?
Timely for Karen to ask where the data comes from, if not the manufacturer. Especially timely since she asked it before Paul & Bob made a great point about the many factors the manufacturer can't consider. And it makes an even better point about another discussion on another thread (more later)
To try to answer Karen - I use IPC-7351B and the calculator software to start. I can enter some data from my manufacturer into that calculation. Then I take that info and adjust it based on what we've learned from previous assemblies. I use some of the information I get from places like this listserver. Then make some compromises, since the 3 alternate sources for the part have slightly varying dimensions, and now that we no longer assemble in house, it could go to 3 different shops. All that to come up with something that still requires manufacturing engineering to tweak things like stencils and vacuum release points. But hopefully I gave them a pattern they could work with.
About that other thread. PCB Designers are a dying breed. There's another active thread on another site that is trying to figure out what to do about the trend to not educate or hire PCB designers, instead figuring EE can just press buttons in the PCB design software and make a board pop out. Management doesn't see this kind of effort, that an EE doesn't have the time or background for, then can't figure out why assembly yields are dropping. Oh well, as long as I have my job I'll keep trying to do the best I can, subscribing to threads like this, where I can learn about things like vacuum release points. It takes a villiage, or maybe a cubicle farm.
Sorry for the rant, and Karen, I hope this sort of answers your question.
Peter
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