I've never seen that. Usually the flux trapped in the partially sealed
cavity can perform that function!
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Ryan Grant
Sent: Tuesday, April 22, 2008 5:45 PM
To: [log in to unmask]
Subject: Re: [TN] Via barrel cracks
Hi Wayne,
Understood. Can the solder mask solvents cause corrosion too? (Just
checking everything...)
Thanks,
Ryan
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Thayer, Wayne
Sent: Tuesday, April 22, 2008 3:42 PM
To: [log in to unmask]
Subject: Re: [TN] Via barrel cracks
Hi Ryan!
Werner gave a good rundown.
I just wanted to question your comment about "plugged with solder mask
from both sides". Solder mask is not designed for plugging at all. You
need to specify a Pb-free compatible via plugging paste. Solder mask is
loaded with solvent and rarely makes a decent "tent" over a via.
Wayne Thayer
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Ryan Grant
Sent: Tuesday, April 22, 2008 1:43 PM
To: [log in to unmask]
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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