Interesting... I would not rule out CAF just yet. Confirming CAF with
high confidence can be difficult, but here's a thought: since the
geometry is as large as it is, why not try drilling out the area of the
visible glass bundle between the holes (do this carefully with the
appropriate bit and equipment, i.e. carbide PWB drill in a high-speed
drill press. If removing the suspected conductive path opens the
circuit, you've confirmed growth along that path.
The fact that polyimide laminate is suffering delamination raises
questions as to processing. Polyimide is moisture-sensitive. Are the
boards being baked prior to assembly?
Regards,
Fritz Byle
-----Original Message-----
From: IPC-600-6012 [mailto:[log in to unmask]] On Behalf Of
[log in to unmask]
Sent: Friday, October 30, 2009 10:51 AM
To: [log in to unmask]
Subject: Re: [IPC-600-6012] Latent defects on PWA
To answer your questions:
The overall thickness of this board is .091" (nominal)
Material is polyimide.
This through hole pattern is a .050" pitch
Final finish is ENIG
Taiyo 4000 BN soldermask
I don't have the specific assembly profiles but this went through a wave
and convection oven (for surface mount). Note - the boards pass when
they come out of assembly.
The board was assembled. There are both through hole and surface mount
components. There were two assembly heat cycles.
The 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
implemented a single cycle (same profile) and we see fall out - with
just one cycle.
I was ruling CAF out for a couple reasons. 1. This is instantaneous. My
understanding is CAF requires a period of time to 'grow'. 2. And the
geometries are huge. These holes are on a .050" pitch. If it were CAF
growth 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
CAF?
To help understand the cracking we are measuring the Tg and running a
T288 test.
Thanks again.
Pete
-----Original Message-----
From: IPC-600-6012 [mailto:[log in to unmask]] On Behalf Of Chris
Mahanna
Sent: Friday, October 30, 2009 11:23 AM
To: [log in to unmask]
Subject: Re: [IPC-600-6012] Latent defects on PWA
Fritz,
Evidently I need a faster server.
Pete, this is rather common with lesser resin materials. Polyimide
wetted nicely to the glass shouldn't do this, unless you tortured it.
Looks to me like you must have had some dryweave. Drill induced
fracture doesn't look to be an big issue. For those purest out there-
keep in mind that we don't have enough information yet to call it CAF.
It could be another flavor of internal migration.
Chris
-----Original Message-----
From: IPC-600-6012 [mailto:[log in to unmask]] On Behalf Of Francis
Byle
Sent: Friday, October 30, 2009 11:10 AM
To: [log in to unmask]
Subject: Re: [IPC-600-6012] Latent defects on PWA
Hi Pete,
What's causing the low resistance is probably CAF, which stands for
Conductive Anodic Filament. Electrically it acts like dendritic growth
on a surface, but it's a somewhat different physical process.
The separation along the glass bundles is what allowed the CAF to
happen. It looks like the laminate specified is not up to the processes
it 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
Temperatures, number of exposures, etc.; specifically, how many SMT
reflows at what peak temperature, and any selective or wave soldering,
what pot temperature and contact time, any rework and if so by what
process and at what temperature?
Regards,
Fritz Byle
-----Original Message-----
From: IPC-600-6012 [mailto:[log in to unmask]] On Behalf Of
[log in to unmask]
Sent: Friday, October 30, 2009 9:34 AM
To: [log in to unmask]
Subject: [IPC-600-6012] Latent defects on PWA
All:
I have run into an anomaly that I am having trouble understanding. I am
including a couple cross section pictures to show the anomaly.
After as few as one thermal cycle (-55 to +60C) the boards are failing.
The failure is a reduced resistance between isolated circuits. The
board shown in the pictures was 160 ohms.
I had the failed connection cross sectioned (both horizontal and
vertical views) and it revealed a crack in the prepreg. This is a 14
layer board and the crack is showing up between layers 7 and 8 - it is
B-stage that is cracking. The laminate is polyimide.
As seen in the cross section pictures it appears to me that the crack is
happening after plating - most likely during my thermal cycle. I see no
evidence of plating or any contamination in the crack.
So, my question boils down to why is the crack manifesting itself as a
short or reduced resistance? Has anyone ever come across something like
this or have an explanation?
Thanks for your help.
Pete Menuez
Supplier Quality Engineering Manager
L-3 Communications Cincinnati Electronics 7500 Innovation Way Mason,
Ohio 45040 [log in to unmask]
513-573-6401 Voice
513-573-6767 Fax
<<Cross section.jpg>>
<<Z axis view.jpg>>
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