Under the right circumstances, CAF and other flavors can occur very quickly. The reason most people think of them as being so slow is because of our sensitivity of detection. They can burn themselves up many, many times before we detect them, just like surface dendrites.
Using current limited event detection, or feedback bias they can be found much more quickly.
As for determining whether you have CAF- If it is not complete, bake it and watch the resistance go up dramatically. When probing, curve tracing... be VERY careful to voltage clamp or current limit, or you'll lose your evidence. If you have a good polarizer/analyzer, or DIC you should be able to see them below the surface of the mount. Be careful about misinterpretation of light refraction off of the dry glass. Which leads you to the last step: grind the other direction and look for the filament around the glass fiber + SEM/EDX...
Fritz's recommendation sure sounds like it would work, but I've never tried it.
BTW, PCQRR has got a sweet new test coupon for CAF testing.
Chris
-----Original Message-----
From: IPC-600-6012 [mailto:[log in to unmask]] On Behalf Of Francis Byle
Sent: Friday, October 30, 2009 12:35 PM
To: [log in to unmask]
Subject: Re: [IPC-600-6012] Latent defects on PWA
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
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