Wayne,
I don't know about "Shock Waves" though I can image that a board dropping after cutting could generate vibrations. I would not term a vibration as a shock wave though it is a propagation of a vibration. The terms stress, strain, compression, tension and shear all have well defined meanings. Exactly what is a "Shock Wave"? I know what it is in the detonation of explosives.
If you have ever run a material tester and pulled a 1/4 inch steel sample to where it fails in tension you get a heck of a bang, a shock wave for sure as the sample fails. And I imagine any cutting action will generate vibrations as the material shears. But even routers generate a heck of a lot vibration. Magnitudes, frequencies and durations are all different.
Pizza cutter or router I just cringe when I see an operator grab a loaded board and bend it until the score line fails. Anything is better. It is the strain (deformation caused by a bending stress) that breaks the brittle MLCC. Watchout for stress levels to avoid breaking things. Just consider the magnitude of stress as an operator bends a board, terrible. Do pizza cutters generate more or less vibrations (Shock Waves?) than a router? If I had a crystal on a board I would worry about the router, high frequencies for a long duration.
Bob K.
-----Original Message-----
From: TechNet <[log in to unmask]> On Behalf Of Wayne Thayer
Sent: Wednesday, August 15, 2018 1:38 PM
To: [log in to unmask]
Subject: Re: [TN] EXTERNAL: Re: [TN] MLCC spacing/orientation to V-score or Perforated Tabs
Hi Bob-
I used to think that pizza cutters didn't induce stresses away from the cut area either. So you can imagine my surprise when I got the failures on an
0805 cap about 0.125" away. There was no bending applied.
No matter how you separate, you either have to plan for a shock wave as the material fails or a whole bunch of debris from a cutter (of course, these are just differences in scale because I'm sure the cutter is also violent at the microscopic level!). The magnitude of the shock wave is proportional to the energy needed to create the failure. The typical V-score leaves 20mils of material on a standard thickness board. Depending on the speed of separation the power level in the shock waves can be large. You can also imagine that the PCB is under some bizarre stresses: Let's say you draw a pair of 1" long lines on the pcb perpendicular to the path of the cutter.
And let's stop the separation process when the cutter is centered on the first line it encounters. Now the distance between the line endpoints where the cutter is equals 1" PLUS the width of the cutter, but the line that hasn't been cut yet is still 1" long. The smaller the diameter of the cutting wheel, the more acute the stress in the board to accommodate this issue.
Wayne Thayer
On Wed, Aug 15, 2018 at 9:23 AM Robert Kondner <[log in to unmask]>
wrote:
> Hi,
>
> I haven't played with V score machines, can they be set to completely
> separate the board as they are scored? I would think it requires a
> couple passes?
>
> Don't know if I would call then stress waves. The stresses in the PCB
> material would be located at the wheel edges during scoring. A simple
> failure in compression. (Compressions failures are typically shear
> failures.) If the board is flat as it goes through the wheels to
> competition there should be no moment applied. Bending a board to
> break a rat bite or to separate two V scored items is totally
> different, big time bending moment which eventually cause a failure in tension.
>
> You want to score a PCB deep so very little bending is required. A
> flat edge at the V score increases the moment at the score line.
>
> Bob K.
>
> -----Original Message-----
> From: TechNet <[log in to unmask]> On Behalf Of Wayne Thayer
> Sent: Wednesday, August 15, 2018 11:20 AM
> To: [log in to unmask]
> Subject: Re: [TN] EXTERNAL: Re: [TN] MLCC spacing/orientation to
> V-score or Perforated Tabs
>
> Thanks Scott.
>
> I'll see what I can divine from CALCE.
>
> I have a hard time imagining the "stress lines" for the V-score (aka
> "Pizza
> Wheel") singulation process. With tabs, bending is clearly the way
> shock waves propagate, so you'd think aligning parts with the long
> access parallel to the edge would be the orientation for minimal
> differential stress. But the "Pizza Wheel" is trying to rip the board
> apart in a very different way. I'm suspecting the stress waves would
> be of similar shape to the waves around the bow of a canoe when you're moving through the water:
> They start out as propagating in the same direction as the bending
> waves, but end up turning nearly 90 degrees as the wheel wedges the board apart.
> Probably CALCE has modeled that.
>
> Wayne Thayer
>
> On Wed, Aug 15, 2018 at 7:47 AM Decker, Scott UTAS <
> [log in to unmask]> wrote:
>
> > Wayne,
> > We have requirements here also regarding the location of MLCC's
> > near the edges and areas of stress, like mounting holes, mouse
> > bites, scoring, etc. and the electrical engineers are urged to use
> > flexy leaded type caps when possible. Like Steve mentioned, .200"
> > from a lot of things mentioned is a good start along with
> > orientation of the components along the stress lines, etc. We also
> > have restrictions on the soldering of boards with MLCC's on them as
> > far as temp rise rate to avoid thermal shock. This is also related
> > to the size and soldering
> type used, wave or re-flow.
> > Something that might help also is to check into The Center For
> > Advanced Life Cycle Engineering which is a research center at the
> > University of Maryland. They have a calculator that you can use to
> > help predict cracking issues with the parts. I can't share exact
> > numbers and other related information without congressional approval
> > from people I don't even know, but I will say that the .200" number
> > is pretty good and Steve said the same thing. These parts have
> > really been a thorn in the side for designs with always having to
> > remember
> which way, and how far, etc. but it is what it
> > is... :-/ Good luck.
> > Later...
> >
> > Scott Decker – Staff Engineer, PCB Design Services CID+ – Electronic
> > Systems Center UTC AEROSPACE SYSTEMS
> > 3445 S. 5th Street, Suite 170, Phoenix, AZ 85040 U.S.A.
> > Tel: 602 308 5957 FAX: 602 243 2347 KE7MWT AKA:PadMasterson
> > [log in to unmask] www.utcaerospacesystems.com
> >
> > CONFIDENTIALITY WARNING: This message may contain proprietary and/or
> > privileged information of UTC Aerospace Systems and its affiliated
> > companies. If you are not the intended recipient please 1) do not
> > disclose, copy, distribute or use this message or its contents, 2)
> > advise the sender by return e-mail, and 3) delete all copies
> > (including all
> > attachments) from your computer. Your cooperation is greatly appreciated.
> > This Document Does Not Contain Export Controlled Technology Or
> > Technical Data.
> >
> > -----Original Message-----
> > From: TechNet [mailto:[log in to unmask]] On Behalf Of Wayne Thayer
> > Sent: Wednesday, August 15, 2018 6:22 AM
> > To: [log in to unmask]
> > Subject: Re: [TN] EXTERNAL: Re: [TN] MLCC spacing/orientation to
> > V-score or Perforated Tabs
> >
> > Hi Steve-
> >
> > Thanks for the info. That sounds very reasonable, but it would be
> > nice to have more analytical data such as -Which orientation is more
> > susceptible to damage? (seems obvious with the tabs but I'm not
> > certain on the scores) -It seems obvious that smaller parts would be
> > less susceptible, so maybe they can be closer to the edge?
> >
> > Wayne Thayer
> >
> > On Tue, Aug 14, 2018 at 11:49 AM, Vargas, Stephen M <
> > [log in to unmask]> wrote:
> >
> > > Wayne:
> > >
> > > We recommend keeping larger size ceramic style parts at
> > > least
> .200"
> > > from the PCB edge where depanelization is required. For V-Scores,
> > > we ask that a route in the breakaway be added to minimizes the stresses.
> > > For mouse-bites, we like to see the tabs as far as possible from
> > > the edge parts.
> > >
> > > Regards,
> > > Steve Vargas
> > >
> > > Lockheed Martin RMS-Rotary and Mission Systems Polaris Contract Mfg.
> > > 15 Barnabas Rd
> > > Marion, MA 02738
> > > 774-553-6192
> > > [log in to unmask]
> > >
> > > Please consider the environment before printing this e-mail
> > >
> > > -----Original Message-----
> > > From: TechNet <[log in to unmask]> On Behalf Of Wayne Thayer
> > > Sent: Tuesday, August 14, 2018 2:36 PM
> > > To: [log in to unmask]
> > > Subject: EXTERNAL: Re: [TN] MLCC spacing/orientation to V-score or
> > > Perforated Tabs
> > >
> > > That doesn't give me "warm fuzzies"! I have personally seen pizza
> > > wheels result in a broken cap 0.125" away (no visual cue, 10% of
> > > parts had the problem). Leaving a potential reliability issue to
> > > each pcb assembly house is a real problem for me. And not just me,
> > > for all of us who use PCBs. I did find a paper from Kemet which
> > > touched on the issue, but not in any clear, useful guideline kind
> > > of
> way.
> > >
> > > Wayne Thayer
> > >
> > > On Tue, Aug 14, 2018 at 11:03 AM, John Maxwell <[log in to unmask]>
> > wrote:
> > >
> > > > Depend on vendors recommendations for best results
> > > >
> > > > Sent from my iPad
> > > >
> > > > > On Aug 14, 2018, at 1:24 PM, Wayne Thayer
> > > > > <[log in to unmask]>
> > > > wrote:
> > > > >
> > > > > Hi-
> > > > >
> > > > > Is there an IPC standard for distances from components to
> > > > > board edge for the V-score or perforated tab techniques? I've
> > > > > been able to find several recommendations from manufacturers,
> > > > > but I couldn't find the applicable
> > > > IPC
> > > > > doc.
> > > > >
> > > > > Wayne Thayer
> > > >
> > >
> >
>
>
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