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Author Topic: Dry solder joints - how to fix?  (Read 836 times)
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Captain_Boblo
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« on: May 21, 2020, 02:56:53 14:56 »

Hello all,

I have just spent a hour or two repairing an old (20 years) ignition board for a gas central heating boiler.  It is a single sided and all thru-hole board, (all leaded components).  Some diodes and resistors looked like they had been quite hot over the years and darkened the PCB slightly in places and the solder joints under those areas looked particularly suspect. 
I decided to apply flux to ALL the joints and retouch every one of them with fresh solder and the board now works.   Smiley

Is there an easier way?  I have a reflow oven but I don't think this board would be suitable for reflowing as there are several relays and other plastic-bodied parts. 
Has anyone done something similar (renewing 100+ old solder joints) and found an easier way?

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Sideshow Bob
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« Reply #1 on: May 21, 2020, 04:08:44 16:08 »

No I do not think so. Unless you have access to wave soldering. However electronic solder wire do contain flux. So pre fluxing might be reduntant. I would have cleanded the board before soldering. Anyway prefluxing will not hurt in any way. What you also should consider is to swap out all electrolytic caps. They also age
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millegps
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« Reply #2 on: May 21, 2020, 05:49:31 17:49 »

I agree with Sideshow Bob about capacitors. I usually replace also components darkened: you can replace resistors with a better power rating and the same for diodes. It is cheaper to replace in advance while you have the board on your hands than disassembling later again

No I do not think so. Unless you have access to wave soldering. However electronic solder wire do contain flux. So pre fluxing might be reduntant. I would have cleanded the board before soldering. Anyway prefluxing will not hurt in any way. What you also should consider is to swap out all electrolytic caps. They also age
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norkimo
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« Reply #3 on: May 21, 2020, 05:59:23 17:59 »

No I do not think so. Unless you have access to wave soldering. However electronic solder wire do contain flux. So pre fluxing might be reduntant. I would have cleanded the board before soldering. Anyway prefluxing will not hurt in any way. What you also should consider is to swap out all electrolytic caps. They also age

I think he meant that he added flux to an existing joint and reflowed the joint without adding more solder wire, hence the need for pre-fluxing.  I've done this often when I don't want to remove old solder first and I don't want to add excess solder.
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metal
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« Reply #4 on: May 21, 2020, 06:05:45 18:05 »

replace resistors, generally leave like 5mm between the resistor and the board for all components beneath which the board has darkened.
« Last Edit: May 22, 2020, 02:47:41 02:47 by metal » Logged
papy_bidouille
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« Reply #5 on: May 21, 2020, 06:09:14 18:09 »

completely agree on the replacement of chemical capacitors. in addition I will check on the resistors which have heated that their value corresponds well to the circle of colors which can have aged orange for brown for example. and finally a small photo of the card which works to keep in memory the value of the resistances we do not know what can happen later small smoking for example. I already practiced it and it was very useful to me.
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millegps
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« Reply #6 on: May 21, 2020, 06:37:23 18:37 »

About caps again: most of the time, if you have extra space, you can replace them with an higher voltage too.
A lot of times I saw failures because operating voltage was too close (or even a bit over) to the nominal voltage of capacitor. You should check ESR, as higher voltage means lower ESR.
Another thing you should consider if you are going to buy new caps for replacement is operational lifetime and operating temperature
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Parmin
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« Reply #7 on: May 21, 2020, 10:30:57 22:30 »

 I would use hot air to reflow the board for the dry joints.
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mars01
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« Reply #8 on: May 22, 2020, 01:50:54 13:50 »

Since everybody is pitching in, I will just throw my 2 cents Smiley
It is well known that a slight increase in temperature will dramatically decrease the component life. Also, while some components (like resistors) are artificially aged before are sold, they still change values in time and the culprit is also the raised temperature.
Therefore I would also act preventive and try to lower the ambient temperature. Perhaps adding an air flow (cooler) will improve things. Also check for wiring impeding the current airflow.
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Captain_Boblo
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« Reply #9 on: May 22, 2020, 05:45:53 17:45 »

All very good advice, thank you all! 

I didn't replace any components, just metered the resistors and checked against the color codes, and checked the diodes and removed and checked the triac that drives the pulse transformer (for the ignition). 

There are two non-sealed DPDT relays on the board, so I was trying to avoid using the hot air gun (risk of flux getting in), but that is certainly a good option to save time if I had better flux applicator skills Smiley
Thanks again. 
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Vineyards
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« Reply #10 on: May 23, 2020, 01:01:25 01:01 »

In what part of the circuit do you have the darkened area? If it is close to the dry contact side of the relay where the igniter filament is switched off and on then it might be due to occasional spikes in the current due to the sticking contactors of the relay or due to the degradation of the filament. Depending on its design an igniter can consume up to 700-800 watts of energy and most designers would use a relay that can handle several times more power making a compromise between cost and unit life.  There is a concept called inrush current stabilization which can take anywhere between a few miliseconds to a couple of minutes where the current can temporarily stay up to 20 times higher than the rated value and slowly stabilizes back to the normal level. During this period all the components including the relay contacts,  the copper lines and the solder points on the PCB and everything in the way is exposed to an enormous stress.

Actually, you can't blame the design all that much  because the board has lasted 20 years. The only solution to make it last longer would be redesigning the whole board for wider stress margins including larger PCB lines, brazing the critical parts rather than soldering, a larger relay unit preferably of a easily replaceable type etc.

Maybe you can solder the parts using a large solder pot but that requires a lot of manual dexterity as well as quite a few pieces of equipment and would certainly be an overkill for a single unit. Plus you would still need to hard solder certain points.
 
« Last Edit: May 23, 2020, 05:11:25 17:11 by Vineyards » Logged
chicowood
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« Reply #11 on: May 25, 2020, 07:23:55 19:23 »

Has anyone done something similar (renewing 100+ old solder joints) and found an easier way?

I've wondered that many times. Sometimes I find old transistor radios (like from the 1960s) at garage sales and try to make them work again.

As mentioned above, the caps should be replaced, resistors measured, diodes checked, etc. But it's always good to go remelt all the solder joints. If they don't turn into a nice shiny solder joint, I add a small amount of new flux-core solder. I can almost always get it running again with these techniques.

But for me, since I haven't found a way to do this quicker, I just turn on some good music, settle in and make the job as pleasant as possible. If I'm in the right mood, hitting a few hundred old solder joints isn't too bad, and the results are satisfying...
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