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

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.


PCB snap-offs

Dougie

#3138

PCB snap-offs | 29 August, 2000

Hi,

This a dull question I know, but I have a boss who want's an answer...

PCB snap-offs: * Design keep them to a minimun to save cost on the PCB * Production want more to stabilise the board when it goes through the process as some PCBs partially snap out during the production process.

My theory is that each board must have snap-offs placed dependant on what processes the board will be going through. I think/know that the boards we are having problems with have too few snap-offs. Snap-offs need to be added at corners where support is needed etc.

My boss reckons there may be some standard practice, number of snap-offs/mm or something. I agree with this that there may well be, but at the corners, only common sense gained by knowing which type of components will be being placed on an unsupported corner can tell you how many snap-offs are required.

Put one of us out of our misery, please!

Any comments greatly appreciated.

Cheers, Dougie.

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Travis Slaughter

#3139

Re: PCB snap-offs | 29 August, 2000

There are a large number of variables that determine how many tabs are needed. The biggest ones I can think of, off the top of my head are. Board thickness .031 needs more than .093 Process and equipment used in assembly some processes and equipment can be gentle and others are not. Number and types of parts on the board lots of, or big heavy ones need more support. I would tend to be with you its very product and process dependant.

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#3140

Re: PCB snap-offs | 29 August, 2000

Dougie, not a dull question at all. How about this. Get one of your engineers cad files and add a couple of snap-offs to it yourself. Put both files to a supplier and see if the price is any different. I asked two of my local manufacturers and they both said it wasn't a consideration in costing as long as it wasn't intricate stuff i.e tool on tool off etc. Do design actually know that it is going to cost more? Could be a furphy. Good luck. Darby.

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Campbell Livingston

#3141

Re: PCB snap-offs | 29 August, 2000

One of the major problems with snap off's is heat from the wave weakens the joints. Try using more holes but smaller size. V-grooves may also be an option, but depth control is a problem as most are not NC machine controlled. Direction through the wave may also impact how a board sags when being heated. After boards come out of the wave, make sure they stay flat until cool.

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#3142

Re: PCB snap-offs | 30 August, 2000

you also may find that varying the width/spacing of your breakaways affects how they run through a pass-through system, if you utilize one. They can raise he!! with conveyor sensors, throw timing off, send printers into a new cycle before the board exits, etc.

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Chris May

#3143

Re: PCB snap-offs | 31 August, 2000

Dougie,

If you have square, rectangular etc; shaped PCB's maybe you could have routing between circuits. "V" shaped grooves top and bottom.

I found this extremely good especially if the assemblies are wave soldered. No gaps for flowing up and over.

To help in de-panelising, you can get a "Slicer" which will partially cut through the groove allowing easy breakout.

One thing to be aware of if you go this route, would be that some operators may be tempted to snap the boards apart without first using the slicer.

I had this problem which manifested itself by a batch of boards having cracked chip caps along the edge due to the force applied during manual breakout.

Hope this helps.

Regards,

Chris.

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Dougie

#3144

Re: PCB snap-offs | 31 August, 2000

Thanks for your help guys.

We've V-scored boards in the past and it led to difficulties, as mentioned with the depth of groove etc...

Our PCB's vary in shape and size drastically, and I mean drastically. So it looks as if a little more though on what's on the board, where it is on the board and the processes the board will go through is the best way to determine snap-off positioning.

The boos man is going to love this!!

Cheers, Dougie.

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