Printed Circuit Board Assembly & PCB Design SMT Electronics Assembly Manufacturing Forum

Printed Circuit Board Assembly & PCB Design Forum

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PCB stretch

Views: 4727

#78097

PCB stretch | 6 July, 2017

We are having an issue w/PCB stretch & shrinkage over some 300mm panels. 5-up array, w/the smallest pads at 6mil (QFN). Is anyone aware of a machine that will measure the distance between features on the substrates?

Tony

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#78101

PCB stretch | 7 July, 2017

any p&p machine, just get coordinates and subtract them.

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#78140

PCB stretch | 7 July, 2017

You didn't mention why is stretch a problem? Where in the process you have difficulties?

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#78145

PCB stretch | 7 July, 2017

The problem is registration between the stencil & the smallest pads on the 5 substrates. Using SPI s/w I can overlay the paste layer over the PCB image to determine stretch/shrink. Our worst case was shrinkage of 10.2mil on a 5-up 300mm panel - smallest pads are 6mil. The offsets were using the 1st substrate as 0, -1.5mil, -3.5mil, -7mil, -10.2mil.

I'm looking for a machine that I can zoom in to the edge of a feature than determine the distance to the next substrate. P&P doesn't zoom in enough.

Tony

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#78147

PCB stretch | 7 July, 2017

Tony,

I will throw some thoughts here:

1. When we speak about shrink/stretch i start thinking about temperature right away. How do you store PCBs and or Stencils? Is your stencil getting hot or cold in the stencil printer? Are the boards on a room temperature when you print them? 2. Have you ever suspected the stencil? With accurate pick and place machine, you can grab the placement coordinates from each board(just pick C1 or a fiducial mark) on the panel and calculate distance/pitch between the boards. That way you an compare with your gerbers and see if the board is causing all the problem. But compare all results with gerbers - your SPI might have inaccuracy as well.

3. PCB - As you described your problem with the offsets keep rising, I question the panel accuracy coming from the board house. Mouse bytes or V-score might not be made precisely. Use method from point 2 or come with your own to measure incoming batch of boards. The problem might be not your environment.

4. Once you determine if it is the board, the stencil or both causing the problem we will give you more help.

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#78148

PCB stretch | 7 July, 2017

There is no temperature issue. The problem is the PCB and the PCB house has admitted to be there being stretch. I don't have a need for process advice.

My request was in relation to some equipment that will measure the offset, from the extreme edge of a pad, not from a fiducial or a pad.

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dwl

#78171

PCB stretch | 8 July, 2017

As others have mentioned, using your P&P machine is as accurate as any other solution and has the advantage of using equipment you already have. Most, if not all, P&P machines should have some sort of teach mode where you center the camera over the feature and record the coordinate. Write it down, and move on to the next point. Do a little arithmetic and there's your offset.

If you have cash to burn you could always invest in some sort of coordinate measurement machine such as this:

http://metrology.news/nikon-video-measuring-system-accelerates-pcb-inspection

And there's always low cost, manual analogue equipment such as:

https://en.wikipedia.org/wiki/Ruler

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#78358

PCB stretch | 13 July, 2017

tsauve,

didn't mean to hurt your feelings. The forum here is for sharing technical expertise between experts in the field, and that is what we do. Sometimes though, technical problem has non technical answer.

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