I obtained an old electronics board to practice SMD soldering on it. I only have an Iron, a heat gun and a thermometer to solder/desolder it. It has 4 components very close together (3 mm apart) with plastic connectors also 3 mm apart. They have pads on the bottom, soldered on without any visible solder. As I have already damaged some connectors and components, and damaged some parts of the board by desoldering and soldering. The thinnest nozzle I have is 1 cm in diameter. Do I need to use the thinnest mouthpiece I can find? I need tips.
You just can not deliver very focused heat with a hot air blower. The usual method is to use aluminium foil and kapton tape to keep most of the heat away from parts you do not want heated, then warm up the “general area” to about 150c and use more focused heat to heat the part you want to (de) solder.
There are also quite a lot of soldering tutorials on youtube, and also people with a lot of experience (de) soldering stuff as a part of some other topic.
And it’s also off topic for this forum, and such topics tend to get closed quickly here. There are better suited forums for generic electronics or soldering questions.
It’s more advantageous to use a thinner mouthpiece, am I right?
It’s very hard to avoid damaging plastic components when using a hot-air gun.
My suggestion: pre-heat the board from the bottom. Get it almost hot enough to melt the solder, then just hit it with the hot air gun from the top for a few seconds to remove the component, and move the board away from the pre-heater.
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I don’t use hot air for desoldering if I can help it, where I need to remove a dead component I use this:
Chip Quik SMD4.5NL
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A good hot tweezer can be really nice for some parts, but they are a little expensive. You can also use two soldering irons for some parts. For example you can probably desolder a SOT323 with one wide tip soldering iron and another one with some other reasonable size.
Suplement on what bob says: you can do this with an upside down iron (those things some people use on their laundry)
I have not used it myself, but there also things like a hotplate such as the mhp30. They are around 100 bucks but from what I read they really handy
I hardly ever desolder and I don’t have anything but an iron, tin pump and litz wire. If I desolder something it is SOIC or bigger. I cut every lead with a knife and remove each lead using the iron and a tweakers. Afterwards I remove redundant tin with the litz wire.
Bas
To desolder plastic connectors, you want to apply heat from the bottom of the board. For big THT parts, you might also want to use low-melt solder sticks.
The nozzle of a hot air gun has to be a medium size. When it is too small, you have too low heat output, so you’d need to heat too long (and cook the board) or never finish, when it is too big, you also have too much heat. It requires experience. But as written above, you don’t want to to use a heat gun on plastic parts (using it on the other side of the board is probably fine). and when using it near plastic parts or other delicate electronics (like memory chips) you want to apply metal shields to them to protect them from the heat. This can be a part of a soda can or some other piece of metal.
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Smallest nozzles cán be used more closest right?
Sure, but when you want to work on a small, focused area, a soldering iron might be a better choice as you can be even more precise. But again, it depends on your preferences and experience. Some prefer to work with an iron, some with a heat gun and tweezers, others with heated soldering tweezers.
But generally, a heat gun is probably more difficult to use as you aren’t as precise, and as you can blow away components with the air stream, and if you’re inexperienced, you can easily burn the board.
I use two soldering irons. Even if one is a cheap one you can easily remove any two leaded part with minimal risk to other parts.
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From what I’ve seen in YouTube, sometimes is easier to desolder the plastic connector to work on the component that you need and then soldered again at the end.
The guy at NorthbridgeFix (not affiliation) uses low temp solder to aid with desoldering and lots of flux. Many useful videos there to get some pointer, but mostly you have to do it to get experience.
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That stuff may work nicely, but I find it offensively expensive. But on the plus side, you can probably recycle it and re-use it a bunch of times. It does not have to be a nice wire when you first apply it. You can for example forcibly throw TQFP packages into a glass jar to shake of the still molten stuff and then pick it up with a hot tip of your iron.
And also, that chip quick stuff is nothing special. It just contains a lot of bismuth, and bismuth costs around EUR20 to EUR30 per kg.
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I don’t think the bismuth alone is what makes it expensive. If you’re paying around €30 for 80g of desoldering wick, the raw price of the bismuth in it should be around €1.50. Of course they need to make a profit, but €30 is still a lot.
My use of it has already save me more than that in time . . . and I probably have 90% of it left.
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