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V-Score and Depanel in One Step



V-Score and Depanel in One Step
Can we score and de-panel assembled PCBs in one step to create a card edge connector on the PCB. Is there a precedence for doing this? Jim Hall and Phil Zarrow, The Assembly Brothers, answer this question.
Board Talk

Transcript


Phil
Welcome to Board Talk with Phil Zarrow and Jim Hall of ITM Consulting, the Assembly Brothers. Today, we're coming to you from the ITM Elegante Ballroom high atop Mount Rialto. 

We're here to talk about electronic assembly; materials, equipment, components, practice and procedures, and who knows what else.

Jim knows what else because he's got today's question.  What is it, Jim?

Jim
It comes from B.S. I am considering scoring a PCB with a 90 degree scoring blade after SMT placement and reflow to create a card edge connector on the PCB. 

I would score it deep enough to bevel and de-panelize in one step. Is there a precedence for doing this and are there any horror stories of failures?

Phil
This is an interesting situation because we're not just talking about scoring and singulation of boards. We're talking about where an edge connector is ultimately going. 

I'm trying to contemplate your board layout and figure you probably have some sort of interconnectors, either fingers or holes or pads. So we're assuming the area is clear in the vicinity of the score of components, particularly those wonderful, very crackable ceramic capacitors.

In terms of the stress of the wheel or the pizza cutter it will probably be somewhat nominal. I would also hope that you've had these PCBS pre-scored in your fab shop to IPC-782 specifications.

However when you actually do separate the boards, what should he look out for?

Jim
Certainly you don't want to flex the board in any way that would crack solder joints. The classic ones are your ceramic chip capacitors which can be near the edge or can be anywhere on the board. 

If they're in the area and the board gets flexed, you can crack the joints. Most panels that we've seen like this are pre-scored at the fab shop before the components are placed and the solder joints are created. 

Supporting the board properly so that it doesn't flex and crack your solder joints is the horror story. There are horror stories probably around any process in SMT or electronics assembly because, we're all human and if it can go wrong, it has gone wrong.

Phil
Hopefully we've answered the question and given you the proper caveats. 

Remember, whether you agree or disagree with Board Talk, the secretary disavows any knowledge of our actions. This recording will self-destruct in ten seconds, good luck Jim. 

Whatever you do, don't solder like my brother.

Jim
And don't solder like my brother.

Comments

I agree with the respected Ms. Chonis, with one caveat: one must consider ESD. Huge tribolectric charges can occur during any mill-routing process. This is the single reason nearly all routing and scoring is done at the bare PWB level, prior to assembly and reflow of sensitive parts, and singulation is done after. However, in certain situations there can be a payoff if done properly using an ionizer and a well-grounded router and worktable/fixture inside the machine. Some form of humidity control inside of the machine helps as well (40 to 55%RH) and other mitigation methods I cannot disclose.

A much better option is to review the PWB pallet design (use a team approach) to obtain a combination of scored edges, routings, and mousebite tabs to minimize both the cost of singulation as well as the shock, stress, strain, and charge buildup that could lead to latent damage. Consider for example, a rectangular or square individual PWB.

The pallet layout has the boards facing each other on the connector ends. With a router bit shaped like a cylinder with a cone on the bottom, the first pass of the bit the cone cuts the beveled (connector) edge for two pwbs at the same time (the point of the cone just passes through the thickness of the PWB), then the bit is raised slightly in the z-axis to score the long sides from one end of the pallet to the other. Routing the connector edge slightly past the PWB sides obtains the bevel for two boards with a single pass, and the opposite (non-connector) end can be routed to leave a flat edge for two PWBs with the bit extended down slightly so the cylinder edge does the routing.

After assembly, entire rows of finished CCAs can be removed from the pallet frame by simply running the pallet through the pizza cutter, right through the routings on the short sides. For a pallet with 3 rows of 4 PWBs, all 12 PWBs can be singulated with just 4 passes through the score lines, provided Row 1 shares a score line with Row 2, and ditto with Rows 2 and 3.

Keep the ceramic chip caps and resistors, BGAs, glass diodes, and other leadless parts on the inside, away from the scored edges, and let the compliant-leaded SOICs, QFPs, etc, be placed along the long edges but provide at least .100" from the scored edges to allow the pizza wheel to pass through. With this example one can see that a little bit of thought during the pallet design and component layout process can pay off big time when it comes to automated depaneling and elimination of shock, stress, etc.

A good training program for something so simple as depaneling is often overlooked. Don’t let that happen; the operators need to know WHY they must NEVER press up or down or attempt to “fold” the boards out of the scored pallet (cringe).

And one more thing, IPC-TM-650 is a free download of test methods, and the depaneling strain can be measured using the strain-gauge methods documented there, including sources for all of the equipment. You can actually test your designs during development, and it really is not expensive at all compared to the proposition of reworking/scrapping hundreds or thousands of boards whose caps cracked but went undetected at visual inspection and test, only to fail in the field later.

Ditto for any latent ESD damage.
Richard Stadem, Analog Technologies Corp.
Using Pizza cutters can cause unnecessary stresses so we would recommend a semi-automated Depaneling machine. We offer ones which have both the capabilities to cut V-scored boards as well as tabs or mouse bites by utilizing milling bits and saw blades all in one machine with no changeover needed.
Tom Herndon, SCHUNK Electronic Solutions
As an electronic contract manufacturer we work with a wide variety of PCB's and often are challenged to depanel boards without causing damage to the boards. As board size continues to shrink and density of components continue to increase it is becoming even more critical for us to be able to utilize as much of the PCB as possible. We have installed a Hylax Technology 20-Watt UV laser and a Epilog Fusion M2 40 75-Watt CO2 laser which allows us to depanel PCB's automatically and without stressing the PCB and causing damage or latent failures.
Steve Kelly, Z-Axis
The answer to the question is yes. A pizza cutter to depanel scored boards can work, but the better question should be "how much strain is being induced upon my circuit during the depaneling process?"

The cost to repair a soldered PCB is considered to be 10X that of a non soldered PCB. Choosing the right tool to accurately remove the circuit while protecting the value added investment of parts and process cost down stream is critical, and often an afterthought.

Today's lead free alloys and ultra fine components compound the issues and challenges of limiting the mechanical stress due to being less ductile and malleable.

Strain tests that we performed comparing "pizza style" devices vs routing show an exertion of mechanical strain more than 10X that of a more controlled process such as routing where the parts are mechanically secured onto the routing table, with each circuit being accurately and precisely cut to .005"; preventing any possible error due to improper handling by the operator.

Using a board router the individual circuits in the array are not induced to undue stress and vibration. Furthermore, with Servo Z control, boards with overhanging components and parts near the edge can still be singulated without special jigs or post operations. The routed board leaves a smooth fray-free edge that doesn't require any further manual operation or handling.
Patty Chonis, A-Tek Systems Group
There are always horror stories. When using V-Score cutters, the simplest machine is the open wheel with a short guide where the wheel is fixed (and rotating) and the operator moves the board. This means the operator and the adjustment of the machine, controls exactly where the cut is made. Some skill is required.

If you wish to improve your odds, use a V-Score machine that allows for fixing the board on a jig and aligns the V-Score of the board on a long fixed blade or rest for alignment. These types of V-Score cutters have a moveable cutter wheel on a slide and remove a large portion of the alignment away from the operator providing a lower defect rate and improved quality of cut.
Bruce Webster, Iridium Communications, USA

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