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April 2008

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Subject:
From:
Lee parker <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Lee parker <[log in to unmask]>
Date:
Tue, 22 Apr 2008 14:18:25 -0400
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Ryan

This could be caused altogether by straining the copper during assembly or 
perhaps the copper plating in the barrel is inadequate. To start the 
investigation I would measure the electrical resistance of other vias from 
this supplier prior to assembly. If you have boards with the same date code 
that would be preferable. You might want to thermally stress a few boards 
and measure the electrical resistance before and after stressing. The change 
in resistance should be minimal.

Best regards

Lee

J. Lee Parker, Ph.D.
JLP Consultants LLC
804 779 3389

----- Original Message ----- 
From: "Ryan Grant" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Tuesday, April 22, 2008 1:42 PM
Subject: [TN] Via barrel cracks


Hi Technetters,

I have a case of via barrel cracks (E crack) and I would like to know
possible causes.

The crack is circumferential and occurs at the center of the via.  The
hole appears smooth, i.e., no glass nodules or tearout and there is
neither positive nor negative etchback.  Of course there is some
wicking, but only about 25um max.  The copper thickness is 20um at the
crack and roughly 35-40um at the knee.  Although, at the initiation
point of the crack, the copper thickness is only 10um, but that could be
due to stretching and subsequent thinning of the copper.  Around the
periphery of the crack are micro-voids in the resin about 30um.  It is
triangular shaped, so I can't tell if it is resin recession or a
laminate crack.

In other words, this via pretty much looks normal, except for one thing,
which might be an artifact of stretching and cracking of the via.  For a
span of about 60um on the vertical of the via, at the crack, there is a
roughness that slightly reduces the copper thickness.  The surface of
the copper at this point appears both rough and black in color.

This board has seen two reflows and a hot functional test before final
failure.

The board shop's initial response was speculation of the OSP soft etch
attacking the copper, but this particular via has solder mask plugging
both sides of the via; therefore chemicals after solder mask cure can
not get into the via.

In case it matters, at one time, we have had occurrences of this board
shop "re-working" panels after copper plating by placing the panel in a
tank of etchant, then re-running the board through copper plating.
(They do panel plating).

I'm a little suspicious of their FA, so I would like some independent
opinions and places for me to dig into.

Thanks for any help,
Ryan

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