Chris,

Design software default seems to identify the adjacent layer as "Must
Remove" because blind via software was modified to create the design
program's "backdrilling" software feature (rather than starting from
scratch).  In reality the "Must Remove" layer should be replaced by
"maximum stub length", where the margin between "Must Penetrate" and
maximum stub length must be enough to allow for board thickness
tolerance, layer depth tolerance, drill bit tip geometry and drill
wander considerations (did I miss anything?).  Therefore, unless the
backdrilling depth is fairly shallow or the layers far apart, any "Must
Remove" layer would not be immediately adjacent to the "Must Not
Penetrate" layer.

Regards,
Karl Sauter


On 7/1/2010 10:04 AM, Chris Mahanna wrote:
>
> Karl,
>
>
>
> Those are excellent points.  I'd not seen an OEM put a number on it
> before.  Thanks for the information.
>
> Assuming that the adjacent layer is a "Must Remove" are we looking at
> design standard 0.2 mm dielectric minimum?  Or greater?  I don't
> recall if the design standards address this...?
>
>
>
> Another concern is that if there is poor hole-wall adhesion the "must
> not penetrate" now becomes the end-stake... with no wrap.
>
> In B.R.s case below he mentions rough hole-wall.  Assuming this means
> good adhesion, I would suspect that this would actually increase the
> reliability of the backdrilled hole.
>
>
>
> In any case, I would suggest that B.R. use convection reflow
> simulation as thermal stress prior to SI assessment.  Method 2.6.27
> has already proven itself to impart cycling (copper) damage mechanisms
> not seen in multiple solder floats.
>
>
>
> Chris
>
>
>
> *From:* [log in to unmask] [mailto:[log in to unmask]]
> *Sent:* Thursday, July 01, 2010 10:59 AM
> *To:* TechNet E-Mail Forum; Chris Mahanna
> *Subject:* Re: [TN] Single PCB backdrilling
>
>
>
> Chris,
>
> How reliable a back-drilled hole is depends upon how close the
> drilling gets to the "Must Not Penetrate" layer.  The back-drill bit
> geometry, drilling parameters and hole size affect how much copper is
> pushed into the hole ahead of the tip and how much the PTH hole wall
> is stressed near the tip of the drill bit.  A conservative rule of
> thumb is that the stressed area not be closer than 0.1 MM to the
> connecting or "Must Not Penetrate" layer.
>
> Regards,
> Karl Sauter
>
>
> On 7/1/2010 6:33 AM, Chris Mahanna wrote:
>
> Backdrills are generally considered a unique structure for which structural integrity after thermal stress shall be assessed to prove conformance.
> However, this is not 100% agreed upon and the specifications are rather mute/ambiguous.
> Some procurement activities allow conformance assessment from simple electrical test of properly designed test coupons.
>
> I personally have not seen mandated reliability testing on backdrills.  I suspect the lack of mandates indicates a general consensus that, in typical use, backdrills are more structurally reliable than their non-backdrilled counterparts.
>
>
> Chris
>
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of ??
> Sent: Thursday, July 01, 2010 12:01 AM
> To: [log in to unmask] <mailto:[log in to unmask]>
> Subject: [TN] Single PCB backdrilling
>
> Dear Technos,
>
> Anyone use backdrilling tech in a single pcb? We backdrilled a lot of backplanes, but none of them should survive a second reflow process and maybe a wave process.
>
> Here is the questions:
>
> Does the high temperature stress affect the backdrill holes reliability?  Two reflow processes and a wave process, Consider the PTHs' drilled edge are rough, maybe they will easy to be damaged by thermal stress.
>
> Will the the rework process of the connector damage the backdrilled holes?
>
> Anything we can do? No HALS? More inner layer connection? Other suggestions?
>
> Thank you so much.
>
> B.R.
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--
Oracle <http://www.oracle.com>
Karl Sauter | Principal Engineer
Phone: +14082763207 <tel:+14082763207>
4150 Network Circle, MS: SCA12-306 | Santa Clara, California 95054
Green Oracle <http://www.oracle.com/commitment> Oracle is committed to
developing practices and products that help protect the environment


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