Electronics > Repair

Desoldering advice

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strawberry:
lead solder. 80W should be minimum
if relay do not operate and coil is not open then something is wrong with control circuit

tooki:
You absolutely need auxiliary heat!

Think of it like this, assuming the original lead-free solder: you need to raise the joint temperature by over 200°C (ambient 20C, melting point 227C). Right now, ALL of that energy has to flow in through the soldering iron tip, and you’ve got a big board sucking heat away on all sides. Even if your iron can reach 450C at the tip, all this means is that you’re overheating one spot on the board, while still not heating the joint as a whole sufficiently.

Now suppose you use a small oven or a hot air gun at a gentle temperature (say, 150C) to heat the entire board to 100C. Now the iron only needs to increase the joint temperature by 130C. Or preheat to 150C and the iron only needs to add 80C to the joint. Those are MUCH more reasonable numbers, and mean you’re less likely to damage the board.

And for sure, use a bigger tip. Bigger tip = better thermal conductivity = lower tip temperature needed = less risk of board damage.

antenna:
I thought I would get to it today, but I didn't.  I did manage to find my big soldering iron tips for the propane torch, so I will give that a try.  My soldering station has an IR preheater, so I will do as suggested and get it warmed up before taking the soldering torch to the leads.  I may just replace the electrolytics and not worry about overheating them.  Tomorrow is supposed to rain all day and I need a break from what today has been, so hopefully I will have that relay out.

Given my next attempt will be with a much larger soldering tip and using preheating, what would be better, adding regular solder to transfer and hold the heat or trying more low MP alloy? 

shapirus:

--- Quote from: antenna on April 27, 2024, 03:03:20 am ---Given my next attempt will be with a much larger soldering tip and using preheating, what would be better, adding regular solder to transfer and hold the heat or trying more low MP alloy?

--- End quote ---
It depends if anything in that area of the board is expected to get hot during normal operation. If it is so, then I would only use low-melt solder when there's still not enough heat transfer for the regular solder to melt. The remaining traces of bismuth can potentially lower the melting point of the post-repair joints enough to cause reliability issues if they get hot enough.

fmashockie:
After seeing the photos, the reason you are having so much trouble desoldering this relay is because look how beefy the traces are that it connects to.  You need a lot of heat to get those relay leads removed.  When I am working on boards like this (with thick power traces) I use a combination of hot air station and my iron.  I hold the iron on the lead to be desoldered, and hover the hot air over it.  This probably isn't good for the tip (I haven't seen any issues with my tips and I've done this fairly often), but this works.  Of course using a larger iron could possibly work.  But this method works fine with a Hakko FX-888 station and a decent hot-air station.  When I am dealing with multi-lead components like a relay that you can't just pull out one leg at a time, I also use solder wick to suck up the solder at each leg.  Again, you are only going to be able to do this with enough heat.  Use flux as well and continue to wet the wick with flux to get the solder to adhere to it and away from the leg to be desoldered.

The same goes for getting that relay back on the board.  You're going to need a lot of heat to get it soldered back on.  Same method works.  Add solder with the iron, then heat it up with both iron and hot air to get it to form a nice joint.  Obviously do this for as short a time you can to not overheat the component.

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