Q for CNC geeks

Projects
#1

The chip carrier frame that I use to apply solder paste to ICs is too lose. The IC has a ± 0.15mm play in the X and Y directions. This combined with all the other tolerances (stencil, stencil holder alignment pins) is causing me a lot of grief with misplaced solder paste. For your reference, the small pads on the IC are 0.24 mm wide.

stencil_frame
stencil_frame800×1067 384 KB

I looked into getting another frame cut in the US (the one I have is from China) with the hopes of getting tighter tolerances, but they have quoted me $350 USD for one. The made in China one cost me $120. In all fairness to Chinese manufacturers, the frame I have may have been made for a previous version of the IC I’m using.

Is it possible to cut one of this frames using the VHS CNCs with better than 0.1 mm tolerance? I’m also getting another stencil cut, but those are not too expensive (~$45)

Thanks!

H

#2

That looks like something you could do with an acrylic stackup on the laser cutter. Doesn’t answer your specific question, but is an alternate possibility maybe

#3

Thanks, I thought of that, but I’m concerned about ESD issues with acrylic. The plastic carrier frames I have seen have conductive fibers embedded in the plastic to dissipate the static build up.

#4

0.1mm should be easy to achieve. The 6040 CNC at VHS would be the most appropriate for the job.

On the design though, you need to change how it works to get the precision and accuracy you’re looking for. You want a single reference surface on each X and Y, that both the IC and stencil are pushed against. i.e. the design of your IC frame should be to offset the IC from the reference planes by Nmm and Mmm only, and just have something to apply pressure to the opposite side of the IC to hold it firmly at those offsets. And same for the stencil, it should be referenced to the same surfaces in the same manner.

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#5

Simple solution, use a centre punch and make a series of divots to slightly expand the metal to make the fit a bit tighter. You might need to file afterwards to get a flat surface, but it should work

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#6

LOL … I like it! I’ll try that now and let you know how it goes

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#7

Sweet! it worked … the ic has no visible play (under the microscope) and just enough friction that it’s not lifting with the stencil. Thanks @JohnC

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#8

No problem. I love it when simple solutions work well.

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