Is De-paneling PCBs by Hand Acceptable?



Is De-paneling PCBs by Hand Acceptable?
Is it acceptable to hand break when de-panelizing circuit boards that contain QFN and SMD components? Are there specs for hand breaking?
Board Talk
Board Talk is presented by Phil Zarrow and Jim Hall of ITM Consulting.
Process Troubleshooting, Failure Analysis, Process Audits, Process Set-up
CEM Selection/Qualification, SMT Training/Seminars, Legal Disputes
Phil Zarrow
Phil Zarrow
With over 35 years experience in PCB assembly, Phil is one of the leading experts in SMT process failure analysis. He has vast experience in SMT equipment, materials and processes.
Jim Hall
Jim Hall
A Lean Six-Sigma Master Blackbelt, Jim has a wealth of knowledge in soldering, thermal technology, equipment and process basics. He is a pioneer in the science of reflow.

Transcript


Phil
Welcome to Board Talk. This is Jim Hall and Phil Zarrow, the Assembly Brothers, also known as ITM Consulting, here to talk about SMT assembly process problems, solutions, and predicaments.

Jim
Today's question comes from R.M., and it's about de-paneling. The question, "Is it acceptable to hand break when de-panelizing our circuit boards. Are there any resources or papers that quantify the type of failures caused by hand breaking and stress?"

Phil
I have to say that is something we've encountered quite a bit in our consulting time. We've come across all types of de-paneling methods, and methods to avoid. I would have to say that, in our experience, hand breaking is about the worst method we've seen.

This calls to mind the experience I had a number of years ago. We had a client, an American company, building satellite down-converters. The boards were being built for them in Taiwan. They were having a capacitor cracking problem. This pre-dates the days of common use of QFNs.

They sent me some photographs and also some cross sectioning. Looking at the crack, it wasn't a thermal crack. It wasn't the wave solder or the reflow process, which they originally thought. Maybe it's dwelling on the wave too long, or something along those lines.

It was definitely a mechanical crack, so I said, "Can you send me more slides of your process?" And all of a sudden, there was one where a guy is holding the board, it was a four-up, and he was doing a karate chop to break it apart.

It wasn't a good thing for the boards. So it was, "There's the problem." Common sense will tell you you're putting undue stress on a board, breaking them that way, even if you've scored the boards properly.

The IPC spec, if I recall, in "Best Practices," is about a third down from each side. You're inducing stress. Most of the pizza cutter manufacturers recommend a keep-away distance from scored area of at least five millimeters. They're addressing specifically capacitors, but the same thing goes with regard to any kind of device, especially something as tender, and vulnerable, if you will, as a QFN, or even an area aray.

Jim
Most of the issues we see are with cracked chip components, resistors and capacitors too close to the breaking zone because of localized flexing. Certainly we're concerned with QFNs, having the weakest, least robust solder joints of any of the IC configurations.

I saw a presentation at a local SMTA chapter meeting, in Boston. It was by DFR Solutions, and they showed pictures of horizontally cracked joints in QFN components due to flexing of the board. I don't recall that they were specific about whether it was de-panelization, but we know that de-panelization can be one of the biggest sources of flexing in the assembly process.

Phil
So there you go, hopefully that answers your question.

Jim
In the meantime, whatever you do, do not solder like my brother.

Phil
And don't solder like my brother.

Jim
And keep the kids away from the flux pot.



Comments

Routing is likely the least stressful method of depaneling, but you can accomplish the routing before PCB assembly. For perforated tab panels, keep the tabs away from sensitive parts. Have open routing adjacent to the sensitive parts instead.

For V-groove, you also have the option to add an open route adjacent to sensitive parts, flush to the final edge of the PCB. It won't weaken the panel noticeably, and you will avoid the surface transmitted stress from depaneling.

I put together a few notes on panelization and got it published in Electronic Design. It's far from perfect, but it can help you develop your own comprehensive panelization policies.

http://electronicdesign.com/boards/pcb-designers-need-know-these-panelization-guidelines
Jack Lucs, AMETEK PPD
My company, Pioneer-Dietecs Corporation has a depaneling methodology utilizing opposed Knife Edged Blades that allow safely cutting tabs or v-scores within 1/2 mm of ceramic components. The process is virtually stress free. It is a press based system that can be used in a cleanroom for BioMed PCBAs.
Jerold B. Larson. Pioneer-Dietecs Corporation
We have a V-score depaneler made by Olamef (I believe Fancort and Manncorp have them also) that consists of a fixed lower blade and a motorized pizza-cutter type upper wheel. This runs along the v-score to separate the boards.

This can work well, although sometimes we are faced with panels that either don't meet the 5mm clearance recommendation the brothers mentioned, or there are overhanging parts that would interfere with machine operation. In these cases, we are still faced with the substandard hand breaking method, although there is no karate involved.

Another option we've seen is a router (specifically for tab routed boards), but that method seems to be much slower. With tabs, there are specific hand tools designed to cut the tab out and minimize stress on the board, like the CHP DP-20-N.
Mark Daley, SMTNW, Inc.
If you orient the SMD capacitors parallel to the the board edge then they are less likely to get cracked when de-paneling.

Cracking is more likely if they're perpendicular to the board edge.
Steve OBrien
At a previous company I found many instances where manual breaking of boards resulted in cracked chip caps. For V-score panels long blade separation was the lowest strain inducing process (<200 micro strain). If using mouse bites or tabs, routing was a good choice followed by punch press (only if tools are maintained).

Also, we found that capacitors could crack at strain levels lower than specified by the manufacturer. Our corporate limit for ceramic capacitors was 1,000 micro strain.
Brian Chandler, Schlumberger
If hand breaking is not acceptable than what method is acceptable for scored boards?
John Dammeyer, Automation Artisans Inc.
The only way to say "yes" or "no" for hand depanalization is perform strain gage in these components and check in datasheet if the results are acceptable.

You can find on IPC/JEDEC-9702 the strain characterization guidelines and IPC/JEDEC-9704 focuses on printed circuit board strain gage tests. Strain gage tests results normally impacts process engineers, but permit to learn a lot about PCBA production processes.
Glayson Figueiredo, Philips Medical Systems
We specify hand de-panelization for many of our critical assemblies using a "mousebite" pattern like what Mr. Darner describes above, and we keep sensitive components away from the mousebites. We've seen no issues with board stress caused by this method of de-panelization.

One question not addressed by Phil and Jim in the original transcript: If hand de-paneling is not recommended, then what is? There are many methods, and some I've seen put stresses on the boards that are arguably worse than hand de-paneling with well-designed mousebites. If de-panelization options exist that are superior to our current methods, I'd certainly like to learn what they are.
Grayson King, LORD MicroStrain Sensing Systems
While the original question may have had more information, I see nothing to show that RM was using scored-both-sides panels. We have many panels where divisions are cut clear through (routed), leaving short support tabs. These, in turn, have a hole pattern to control break location. I developed the pattern for a board where the break HAD to be sub-flush due to clearance issues. Hand de-paneling works fine IN THIS CASE - but the karate-chop would probably not.
Ronald G. Darner, Mortara Instrument, Inc.

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