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Subject:
From:
Wayne Thayer <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Wayne Thayer <[log in to unmask]>
Date:
Thu, 5 Mar 2015 17:14:38 +0000
Content-Type:
text/plain
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text/plain (263 lines)
Yes, Reuven, you should take care about spacing from the edge:
-First, you shouldn't put traces closer the edge than you would to a via hole 
wall (and as with vias, don't forget about targeting of milling operations to 
traces, and layer-to-layer alignment.
-Next, consider sub-panelization and singulation operations: If the adjacent 
edge is to be routed, no further backoff is necessary.
-The final depanelization is usually mostly uncontrolled and kind of violent. 
It has the potential to damage laminate and break mounted ceramic parts. I 
believe there are good component placement specifications available which take 
this into account. As to the amount and depth of laminate damage due to final 
singulation (for which some kind of edge repair technique might be 
applicable-but NOT conformal coat!), there is a huge variance which is a 
combination of the process design (usually specified on the customer-provided 
fabrication notes), the performance of the PCB fabricator to meet the design 
specs, the skill of the singulation operator, and the condition of the 
singulation equipment.

Wayne

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Reuven Rokah
Sent: Thursday, March 05, 2015 10:11 AM
To: [log in to unmask]
Subject: Re: [TN] Required Conformal Coating of Edges of PCB's

You should take care:
1. During the design stage, for a space of traces from the edges.
2. The process of milling / cutting the edge.
3.  The thickness of traces affect the bonding of the laminates at the edges.

On Tue, Mar 3, 2015 at 5:06 PM, Dale Ritzen <[log in to unmask]> wrote:

> Seriously folks... is this a real or imaginary problem? We have seen
> both "yes" and "no" opinions over the last several days. Is anyone
> basing their opinions on a specific standard that states what
> conditions require the PCB edges to be coated, or is it simply a
> matter of the customer specifying that requirement on their S.O.W. for
> the product? Are manufacturers to second guess the customer about the
> places the product will be used and the environmental conditions it
> will be placed in that might warrant coating of the PCB edges, or does
> that really matter anymore with the state of the art PCB manufacturing
> processes - regardless of the condition of the material left on the
> edges (with the possible exception of PCBs made of Polyimide materials)?
>
> So far we have seen opinions - not quotes from any standard that
> covers this. Does that exist, or is it something that the IPC
> technical committees need to look into for further definition? Sounds
> to me like it needs some definition in a standard so we all have something 
> to march to...
>
> IMHO,
> Dale Ritzen, ASQ CQA
> Quality Manager / ISO Management Representative
> ___________________________ Austin Manufacturing Services
>
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>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Yuan-chia Joyce
> Koo
> Sent: Monday, March 02, 2015 5:20 PM
> To: [log in to unmask]
> Subject: Re: [TN] Required Confromal Coating of Edges of PCB's
>
> also there are cut off edges of the PWB like using shear... really bad
> with all the fiber glass stick out.... there is not enough coating can
> fix that... (don't laugh, those are real surprise you get from far)...
>          jk
> On Mar 2, 2015, at 10:04 AM, Stadem, Richard D. wrote:
>
> > You lost me on that post, Wayne.
> > Not sure what you are trying to describe when you talk about
> > drilling thousands of overlapping holes as a method of routing out a
> > PWB?
> > The edge-coating being discussed was conformal coating, not plating
> > of the edges?
> > Sorry if I am slow on the uptick today.
> > dean
> >
> > -----Original Message-----
> > From: TechNet [mailto:[log in to unmask]] On Behalf Of Wayne Thayer
> > Sent: Monday, March 02, 2015 8:44 AM
> > To: [log in to unmask]
> > Subject: Re: [TN] Required Confromal Coating of Edges of PCB's
> >
> > I feel the need to point out the obvious:
> >
> > -Suppose we singulated a board by using a pcb drill, drilling
> > thousands of overlapping holes. It's hard to keep drills sharp, so
> > let's assume we just swap out bits after the same number of "hits"
> > that we deem the bit good for via hole drilling. Now how 'bout we
> > coat the exposed edge with plated metal, just to make sure that if
> > there's a problem with the drilling/routing process, we have a real
> > good chance of making a short.
> >
> > Any reason that should be dis-allowed? Try calculating the exposed
> > area on the edge vs. all of the via circumferences you've got!
> >
> > Therefore, it's ridiculous to specify coating of routed board edges,
> > which if anything, have less potential to damage fiber bundles than
> > a drill. Snapped areas are a different category because
> > drilling/milling has a limited capability to damage the laminate.
> >
> > Wayne
> >
> > -----Original Message-----
> > From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D.
> > Sent: Monday, March 02, 2015 9:26 AM
> > To: [log in to unmask]
> > Subject: Re: [TN] Required Confromal Coating of Edges of PCB's
> >
> > Rich,
> > In addition to what Dr. Pauls has detailed below, I also want to
> > point out that whether or not any specification "recommends" or
> > "does not recommend"
> > coverage on certain areas of components and PWBs, it is the assembly
> > drawing which has historically taken precedence over all standards
> > when it comes to defining conformal coating coverage.
> > This is because every assembly and PWB has different design
> > requirements and it would be too difficult to document all of the
> > exceptions to the different rules for each type.
> > Many circuit boards are simply blanked out on a press, leaving
> > exposed fiberglass edges, but these are also typically high-volume,
> > low-reliability PWBs used only for consumer electronics. All other
> > PWBs are typically routed or laser cut, and as Doug stated those are
> > typically sealed by the singulation process. Depending on the type
> > of PWB material, the method of singulation, and the application,
> > there may be no need to coat the edges.
> > Or there might be, but then one would expect this to be detailed as
> > part of the assembly requirements on the drawing.
> >
> > dean
> >
> > -----Original Message-----
> > From: TechNet [mailto:[log in to unmask]] On Behalf Of Douglas Pauls
> > Sent: Sunday, March 01, 2015 8:07 PM
> > To: [log in to unmask]
> > Subject: Re: [TN] Required Confromal Coating of Edges of PCB's
> >
> > Rich,
> > MIL-I-46058 is simply a materials qualification document.  It does
> > not address the coating of board edges.  I would disagree with
> > Graham and I do not believe that conformal coating edges of boards
> > is a value added process.  Most boards in high performance
> > electronics have routed edges.
> > The routing process tends to smear the resin over the glass
> > reinforcement, sealing the edges.  And since most design standards
> > do not allow internal circuitry closer than 25 mils from the edge of
> > the boards, water or external contaminants would have to penetrate
> > 25 mils of epoxy resin to get to circuitry.  If the edges of the
> > boards were sheared or snapped, where the resin did not seal the
> > ends, then perhaps the sealing would be justified.  I can say that
> > Rockwell has coated some board edges and left other edges free.
> > We have no field failure, ever, that can be traced to lack of
> > coating the board edges.
> >
> >
> > Doug Pauls
> > Principal Materials and Process Engineer Rockwell Collins
> >
> > On Fri, Feb 27, 2015 at 10:51 AM, Richard Kraszewski <
> > [log in to unmask]> wrote:
> >
> >> Does anyone recall  which  MIL document calls out the requirement
> >> to cover  the PCB edges of  assemblies?
> >> I have been led to believe that one exists but that  more than
> >> likely it is not MIL-I-46058C.
> >>
> >> Any thoughts??
> >>
> >> Rich Kraszewski
> >> Plexus
> >>
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-- 

Best Regards,

*Reuven Rokah*

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