Hi Pete,
If the path exists, CAF can be instantaneous with the application of a potential and in the presence of moisture.
Werner
-----Original Message-----
From: [log in to unmask]
To: [log in to unmask]
Sent: Fri, Oct 30, 2009 4:50 pm
Subject: Re: [IPC-600-6012] Latent defects on PWA
To answer your questions:
The overall thickness of this board is .091" (nominal)
aterial is polyimide.
his through hole pattern is a .050" pitch
inal finish is ENIG
aiyo 4000 BN soldermask
don't have the specific assembly profiles but this went through a wave
nd convection oven (for surface mount). Note - the boards pass when
hey come out of assembly.
The board was assembled. There are both through hole and surface mount
omponents. There were two assembly heat cycles.
he boards are tested immediately after assembly and they pass.
Originally we were running the boards through our standard ESS screen
58C to +60C 20 cycles (powered) and we started to see fall out. We
mplemented a single cycle (same profile) and we see fall out - with
ust one cycle.
I was ruling CAF out for a couple reasons. 1. This is instantaneous. My
nderstanding is CAF requires a period of time to 'grow'. 2. And the
eometries are huge. These holes are on a .050" pitch. If it were CAF
rowth I would expect much more time that we are seeing.
If it is CAF is there a test that would define positively that it is
AF?
To help understand the cracking we are measuring the Tg and running a
288 test.
Thanks again.
Pete
-----Original Message-----
rom: IPC-600-6012 [mailto:[log in to unmask]] On Behalf Of Chris
ahanna
ent: Friday, October 30, 2009 11:23 AM
o: [log in to unmask]
ubject: Re: [IPC-600-6012] Latent defects on PWA
Fritz,
vidently I need a faster server.
Pete, this is rather common with lesser resin materials. Polyimide
etted nicely to the glass shouldn't do this, unless you tortured it.
ooks to me like you must have had some dryweave. Drill induced
racture doesn't look to be an big issue. For those purest out there-
eep in mind that we don't have enough information yet to call it CAF.
t could be another flavor of internal migration.
Chris
-----Original Message-----
rom: IPC-600-6012 [mailto:[log in to unmask]] On Behalf Of Francis
yle
ent: Friday, October 30, 2009 11:10 AM
o: [log in to unmask]
ubject: Re: [IPC-600-6012] Latent defects on PWA
Hi Pete,
hat's causing the low resistance is probably CAF, which stands for
onductive Anodic Filament. Electrically it acts like dendritic growth
n a surface, but it's a somewhat different physical process.
he separation along the glass bundles is what allowed the CAF to
appen. It looks like the laminate specified is not up to the processes
t is exposed to. A few questions:
- What laminate is being used?
- What's the overall PWB thickness?
- What is the PWB final finish?
- What thermal processes does the PWB see in assembly? Peak
emperatures, number of exposures, etc.; specifically, how many SMT
eflows at what peak temperature, and any selective or wave soldering,
hat pot temperature and contact time, any rework and if so by what
rocess and at what temperature?
Regards,
Fritz Byle
-----Original Message-----
rom: IPC-600-6012 [mailto:[log in to unmask]] On Behalf Of
[log in to unmask]
ent: Friday, October 30, 2009 9:34 AM
o: [log in to unmask]
ubject: [IPC-600-6012] Latent defects on PWA
ll:
I have run into an anomaly that I am having trouble understanding. I am
ncluding a couple cross section pictures to show the anomaly.
After as few as one thermal cycle (-55 to +60C) the boards are failing.
he failure is a reduced resistance between isolated circuits. The
oard shown in the pictures was 160 ohms.
I had the failed connection cross sectioned (both horizontal and
ertical views) and it revealed a crack in the prepreg. This is a 14
ayer board and the crack is showing up between layers 7 and 8 - it is
-stage that is cracking. The laminate is polyimide.
As seen in the cross section pictures it appears to me that the crack is
appening after plating - most likely during my thermal cycle. I see no
vidence of plating or any contamination in the crack.
So, my question boils down to why is the crack manifesting itself as a
hort or reduced resistance? Has anyone ever come across something like
his or have an explanation?
Thanks for your help.
Pete Menuez
upplier Quality Engineering Manager
-3 Communications Cincinnati Electronics 7500 Innovation Way Mason,
hio 45040 [log in to unmask]
13-573-6401 Voice
13-573-6767 Fax
<<Cross section.jpg>>
<<Z axis view.jpg>>
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