Electronics > Repair
Furnace circuit board repair - How stupid should I feel?
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Excavatoree:
My A/C (aircon, air conditioner) stopped working earlier this week.   The blower (air handler, really my furnace that I use as a blower in the summer) wasn't working.  I found the furnace was pretty simple  - one circuit board.  I found the motor relay.  I toyed with the idea of taking the board out and seeing if I could fix it.

Instead, I decided that because I'm not an A/C technician and never have been, I'd call in a pro.  The guy came out, and agreed with me that the board was bad.   I had them order a new one.  It was expensive, almost 600 USD.  (I bought one from the furnace manufacturer, via an authorized dealer.  I didn't want to chance an aftermarket, used, or "ebay new" board, although there was an e-bay seller selling one.)

When he came back to install it, I asked if I could have the old board.  He said the old one wasn't required, and I could keep it.  It had a plastic bracket that I had to drill off to get to the trace side of the single sided board.  The attached picture is what I found.  It's not the best picture, but I hope you can see what looks like part of the trace that has broken off and still attached to the relay pin, and an annular air gap between that and the board.    The edges are jagged and there is evidence of heat with some black residue on the board.

However, when I removed the relay, I found that wasn't the case.  It was just a solder ball.  The board is fine.  I could have fixed the thing with less than a minute of work.  (desolder, clean, re-solder) 

I suspect that the solder joint was poor from the factory, causing a high resistance that heated the solder and caused it to drip, but I really don't know.  The rest of the components in the blower motor circuit (board terminals, other component leads) are undamaged with no evidence of heating from over current.   So, I'm really at a loss to explain what happened.

At least I have what I believe to be a good spare.  I won't know unless it fails again, as I have no intention of taking apart my working furnace.  (yes, I know.  I'm turning it on instead of taking it apart.)

I suppose I should have been bolder. 

IanB:
This failure mode is extremely common on all sorts of mains powered devices (dishwashers, washing machines, ovens, etc.).

It is a symptom of cost cutting in design at the expense of the consumer.

Robust design says that mains voltage parts of a circuit should use appropriate mechanical connections (flexible wires, crimped lugs and spade terminals, for example).

What you don't do is run mains voltage on traces on a circuit board with solder connections. The reason why is illustrated in the photo. If low voltages run on a circuit board, then any weak joint causes a high resistance that reduces the current, but if the current is already small this may not escalate. If a mains voltage, high current path runs on a circuit board, then any weak joint will not reduce the current, it will simply get hot. This heat will further weaken the joint and will accelerate the problem towards total joint failure.

It's always worth looking for such a failure on a control board before ponying up hundreds of dollars for a replacement.
Stray Electron:
   The first thing that I was taught about soldering is that you need to have a good strong (and clean) mechanical connection on any joint before you solder it.  Pretty much every consumer grade gadget out there violates that rule!  Just an example, when I installed CO equipment for the Tel Cos we had to make ONE wrap of the insulated wire around the terminals on the Frame Blocks and then exactly one and a half wraps of bare wire around them before soldering the joint.  Joints made like that will NEVER come part on their own.
calzap:
First thing to do when an electrical device fails is visual inspection.  I’ve found somewhere around half the time, something wrong can be spotted.  Next thing is check and clean contacts and connections.  Then bring out the multimeter, scope, etc.

Mike
blue_lateral:
Often resistive contacts on a relay create the heat, which travels down the pin and unsolders it from the board. I would replace the relay, or alternatively if you can get the lid off, clean the contacts.
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