In looking at the pictures of the board, I agree that the resistors have been replaced. I have seen many antique boards that look like this with the lands fried and the solder slumped, however the resistors usually have a white to gray discoloration on them.
1. The resistors are too closely spaced for that much heat dissipation.
2. The resistors should be further spaced off the board at least half their diameter, with stress relief bends on the leads.
3. The crack to me looks physical not thermal, yes the substrate was most likely hardened and when the resistors were handled the board was damaged.
4. Was the solder crystalized? Was there evidence of solder reheating. I find that when this occurs the area around the land is smoked, it does not appear in these photos.
5. What are the resistors used for? Are they limiters for stepper motor drives? Are they sized correctly, too small ohm wise for the application.
Pat Goodyear
Control Technician
PG&E
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Guy Ramsey
Sent: Monday, September 24, 2012 6:23 AM
To: [log in to unmask]
Subject: Re: [TN] What could have caused the PWB laminate cracks???
It looks to me like the resistors are sitting on the board. If so, this should be corrected. Look close at image 3026.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D.
Sent: Monday, September 24, 2012 8:40 AM
To: [log in to unmask]
Subject: Re: [TN] What could have caused the PWB laminate cracks???
It is pretty obvious that the three power resistors ran very hot. I do note that you have them lifted up off of the PWB per IPC 610, so that is good, but they still ran so hot they cooked the board and some of the surrounding components.
The reason the CCA cracked is simple. When only a portion of the CCA is heated, in this case the area around the power resistors, but the rest of the CCA is relatively cool, the portion that is hot is expanding at a much faster rate than the rest of the CCA. If the difference (delta T) in the rate of thermal expansion (CTE) is great enough, something has to give, and in this case the PWB cracked.
You may wish to consider placing each of the power resistors into its own radiator heatsink.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Steve Gregory
Sent: Saturday, September 22, 2012 5:33 PM
To: [log in to unmask]
Subject: Re: [TN] What could have caused the PWB laminate cracks???
Hi Gerald!
I still maintain my web page for images and I have yours posted. They are here:
www.stevezeva.homestead.com/IMG_3021.JPG
www.stevezeva.homestead.com/IMG_3022.JPG
www.stevezeva.homestead.com/IMG_3023.JPG
www.stevezeva.homestead.com/IMG_3024.JPG
www.stevezeva.homestead.com/IMG_3026.JPG
www.stevezeva.homestead.com/IMG_3029.JPG
Good lord, that looks pretty bad! I think you meant R15 as being the one with the cracks, I’m surprised that there aren’t more cracks. That laminate is starting to come apart from the heat. Are the other boards that don’t have cracks as discolored as this one is beneath the three resistors ? Do the resistors really run that hot ?
The resistors in the photo look brand new to me, like they’ve been replaced, even the sleeving on the leads looks pristine. Look at all the powdery deposits around in the area. I think that something special happened with this board and it smoked R15 big time, which led to the laminate breaking down and failing. I think something catastrophic happened with this particular board and it has been reworked with new resistors...
Steve
From: [log in to unmask]
Sent: Friday, September 21, 2012 4:22 PM
To: [log in to unmask]
Subject: What could have caused the PWB laminate cracks???
September 21, 2012
We have an OEM who is having problems with cracks in board laminate. The six PIX are for the CCA involved. If you look at Image 3026 and view the area around R13 you will see the cracks radiating from around the PTH. The CCA is FR4 epoxy 0.062 in thick with Type UR conformal coating. The area under the power resistors is not conformal coated. You can easily see this since the other areas are conformal coated and have brown color. This brown color is due to the operating temp which is most likely > 100C.
This CCA design was fully qualification tested and passed all tests in 50C ambient temp environment. The CCA runs hot as you can see from viewing the other images.
We have fielded many of the same CCAs previously with no evidence of cracking.
Any input on what could have caused the cracks would be appreciated.
Not sure if you are still the go to person who maintains the photo library for tech net; if so, can you please post the PIX and send out e-mail to tesn net on my behalf. Thanks.
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