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August 2018

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From:
Robert Kondner <[log in to unmask]>
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Date:
Wed, 15 Aug 2018 12:23:24 -0400
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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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