Basic Desoldering Tecniques
Desoldering: An Introduction
Please be careful not to burn yourself.
Not everything permanent has to be permanent forever. Our intentions may be that we should solder a part into place and never touch it again, but that didn't stop us from putting the wrong part in place or having the wrong part to begin with. This is where desoldering comes in. Desoldering is the process of removing solder OR components from a solder-situation you'd like to change. The techniques and concepts involved in desoldering can also be applied to soldering things together and will promote a better feel for what works with soldering in general. Practice may also increase your confidence. Lets take a look!
Basic Principles of Desoldering
Lets talk about solder! For the rest of this article we will be referring to the properties of leaded solder. We're not going to get technical here, but lets start out by saying that solder loves to stick to and spread out over hot copper. This is called wetting.
When your copper is the right temperature, the solder will melt onto it and will try to spread itself to everywhere that the copper is at that critical wetting temperature. It will tend to spread itself out in such a way as it can creep into through holes or fully cover a copper pad. These are helpful properties! Another property of copper is that when copper surfaces are exposed to air they the surface will oxidize. This oxidization will form a layer of unhelpful oxidized copper which will prevent the solder from sticking to the copper and will inhibit wetting. We commonly get around this with flux, a substance (usually an acid) meant to remove this oxide layer and inhibit it from reforming. Without this oxide layer the flux will wet again! Many preparations of solder meant for use with electronics will have a flux core meaning the solder has flux inside of it to help remove the oxide layer as you go.
So as a recap, solder wants to pile up and stick to itself unless you give it something it would rather stick to, like hot, clean copper. If you have hot, clean copper available solder will prefer to try to spread itself out over all the areas of copper which are hot and clean enough. So where does that leave us with our task of removing solder? Sometimes you can use your soldering iron's tip as a hot enough, clean enough surface to pick up solder with (especially if you frequently clean your iron's tip), but that is not really a very effective tool. Instead we should use our first tool of desoldering, copper braid.
An important consideration before beginning: How many leads do you have to desolder? You really only need to remove solder from N-1 joints, where N is the number of legs on your component. The last leg of the component can be removed by heating the joint to the point of liquid solder and pulling the component away with pliers. The difficulty of removing a part from a PCB is that solder cools and hardens too quickly to allow keeping more than two joints molten for long enough to remove said component.
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| Solder Wetting |
When your copper is the right temperature, the solder will melt onto it and will try to spread itself to everywhere that the copper is at that critical wetting temperature. It will tend to spread itself out in such a way as it can creep into through holes or fully cover a copper pad. These are helpful properties! Another property of copper is that when copper surfaces are exposed to air they the surface will oxidize. This oxidization will form a layer of unhelpful oxidized copper which will prevent the solder from sticking to the copper and will inhibit wetting. We commonly get around this with flux, a substance (usually an acid) meant to remove this oxide layer and inhibit it from reforming. Without this oxide layer the flux will wet again! Many preparations of solder meant for use with electronics will have a flux core meaning the solder has flux inside of it to help remove the oxide layer as you go.
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| Flux Core Solder, 1 |
So as a recap, solder wants to pile up and stick to itself unless you give it something it would rather stick to, like hot, clean copper. If you have hot, clean copper available solder will prefer to try to spread itself out over all the areas of copper which are hot and clean enough. So where does that leave us with our task of removing solder? Sometimes you can use your soldering iron's tip as a hot enough, clean enough surface to pick up solder with (especially if you frequently clean your iron's tip), but that is not really a very effective tool. Instead we should use our first tool of desoldering, copper braid.
An important consideration before beginning: How many leads do you have to desolder? You really only need to remove solder from N-1 joints, where N is the number of legs on your component. The last leg of the component can be removed by heating the joint to the point of liquid solder and pulling the component away with pliers. The difficulty of removing a part from a PCB is that solder cools and hardens too quickly to allow keeping more than two joints molten for long enough to remove said component.
Copper Braid
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| Solder Braid Wick, 2 |
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| Solder Braid Wick saturated with Solder, 3 |
Copper braid is a strip of braided woven copper which is produced and sold for the purposes of removing solder. It achieves through its high surface area (and copper-y nature). There is a lot of exposed copper surface area on a string of braided copper and solder wants to wet out onto hot, clean copper so the great surface area will tend to pull solder onto itself. We can use braid with a soldering iron to remove solder. Flux is also needed when working with copper braid. The great surface area of the braid promotes lots of oxidization which the flux will remove and protect against.
The procedure is simple! You should be prepared to hold the solder braid with some pliers or some other instrument as the copper will get hot and we want to help you not burn your fingers.
1) Pre-Heat your iron: We have more thermal mass than a normal soldering situation so a higher temperature than you would usually use is appropriate. Start anywhere above 650 degrees Fahrenheit.
2) Cut a small sub-section of braid from the reel: A larger cut for more desoldering or a smaller cut for less, ~2 inches may be a good starting place. We cut a section off to reduce the total thermal mass of the working bit of braid. In order to melt the solder you will have to get the copper up to temperature and the more copper there is the longer that will take any time you apply or remove your iron. The copper braid is no longer useful once it is filled up with solder so you should be prepared to trash it.
3) Apply flux to the braid: the braid's woven nature will help keep the flux inside and all over (It will act like a flux wick).
4) Place the braid on top of the solder joint you want to remove solder from: Try for a length of the braid with no solder stuck on it.
5) Press the braid into the solder joint with the soldering iron: Now is the time to get everything hot. Expect lots of flux to burn off and smoke up (do not breathe these fumes in). You should see solder seep up into the braid from below once everything is at temperature.
6) Remove the braid and the iron: be sure not to burn yourself!
7) Inspect the joint: Is there still solder in? Can you remove the component you meant to? You may need to try again and press deeper into the joint, or try from the other side if it is a through-hole design.
8) Repeat as needed until the component can be removed!
The procedure is simple! You should be prepared to hold the solder braid with some pliers or some other instrument as the copper will get hot and we want to help you not burn your fingers.
1) Pre-Heat your iron: We have more thermal mass than a normal soldering situation so a higher temperature than you would usually use is appropriate. Start anywhere above 650 degrees Fahrenheit.
2) Cut a small sub-section of braid from the reel: A larger cut for more desoldering or a smaller cut for less, ~2 inches may be a good starting place. We cut a section off to reduce the total thermal mass of the working bit of braid. In order to melt the solder you will have to get the copper up to temperature and the more copper there is the longer that will take any time you apply or remove your iron. The copper braid is no longer useful once it is filled up with solder so you should be prepared to trash it.
3) Apply flux to the braid: the braid's woven nature will help keep the flux inside and all over (It will act like a flux wick).
4) Place the braid on top of the solder joint you want to remove solder from: Try for a length of the braid with no solder stuck on it.
5) Press the braid into the solder joint with the soldering iron: Now is the time to get everything hot. Expect lots of flux to burn off and smoke up (do not breathe these fumes in). You should see solder seep up into the braid from below once everything is at temperature.
6) Remove the braid and the iron: be sure not to burn yourself!
7) Inspect the joint: Is there still solder in? Can you remove the component you meant to? You may need to try again and press deeper into the joint, or try from the other side if it is a through-hole design.
8) Repeat as needed until the component can be removed!
Solder Sucker
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| Disassembled Solder Sucker, 4 |
Another, perhaps trickier, tool you can use to remove solder from solder joint and circuit boards is the solder sucker. This is a spring loaded momentary vacuum designed to suck molten solder into itself and off of your board. The tricky part is keeping the solder molten long enough to vacuum it.
Lets look at the procedure!
1) Pre-Heat your soldering iron: Needs hot!
2) Check the nozzle of your solder sucker for obstructions: a clogged nozzle is an excellent way to become frustrated with your solder sucker. However, this is not our goal today. Engage the spring on the sucker. Stick the tip of your soldering iron into the hole of your sucker, wait for a few seconds and release the spring. Hopefully this has melted the solder in the nozzle and sucked it into the chamber of the solder vacuum and this will usually clear out a clog.
3) Load the spring on the sucker: prime that vacuum!
4) Apply the soldering Iron to the joint and wait for the solder to melt. Once melted, keep the iron placed in the molten solder and apply the nozzle trying to cover the soldering iron tip and joint firmly with the nozzle.
5) FIRE: Release that spring and suck the solder in.
The preceding technique will work for through hole and SMD board types but often with the solder sucker you will need to try multiple times to get a sufficient amount of solder removed from the joints. The following technique is slightly more effective but only works with through hole boards. This technique, like many related to soldering, is best performed by someone with 3 or more hands and arms. (A joke)
1) Pre-Heat your iron!
2) Check and clear the nozzle of your solder sucker.
3) Load the spring on your solder sucker.
2) Check and clear the nozzle of your solder sucker.
3) Load the spring on your solder sucker.
4) Place the loaded nozzle on the through hole you wish to remove solder from from the back side of the PCB (opposite where the components are)
5) Place the tip of the soldering iron on the joint from the top side. (The side with the components present)
6) Fire the vacuum!
This should be very effective at clearing out through holes, however it may still take a few attempts.
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| A through hole style hole with a cold solder joint, 5 |
General Tips
Lets cover a few more techniques before we end our post today!
A technique which sounds wrong but will often work out positively is to add fresh and extra solder to a joint you want to remove solder or a component from. This is helpful for several reasons. One is that solder in a solder joint can age and oxidize in a way that makes it harder for solder to flow and makes it somewhat harder for it to absorb heat from your iron. Fresh solder can add some flux the mix and help it melt easier. Another helpful aspect to adding solder is there is more solder around to be in contact with your iron. More solder around your iron's tip means that it can transfer more heat more easily to the solder in the joint once all the solder is connected and melting together.
A technique which sounds wrong but will often work out positively is to add fresh and extra solder to a joint you want to remove solder or a component from. This is helpful for several reasons. One is that solder in a solder joint can age and oxidize in a way that makes it harder for solder to flow and makes it somewhat harder for it to absorb heat from your iron. Fresh solder can add some flux the mix and help it melt easier. Another helpful aspect to adding solder is there is more solder around to be in contact with your iron. More solder around your iron's tip means that it can transfer more heat more easily to the solder in the joint once all the solder is connected and melting together.
Another technique involves using your oven! There are many different designs for printed circuit boards, and some use more copper than others. The more copper or whichever conductive metal there is on a circuit board the more thermal mass it has and the more thermal mass it has the more tricky it will generally be to solder onto. Remember that the local temperature at the location you want solder to wet to has to be quite high. PCBs with a large thermal mass will take longer to get to that temperature, and particularly may be good at diffusing the heat and decentralizing it. A way to fight this tendency is to put your board in an oven and heat it up to 170-200F for perhaps 30 minutes. If you're concerned about getting PCB waste inside your oven (Although there should be none) you can wrap it up in aluminum foil beforehand. Pre-heating your board will generally make it easier to bring your solder joints to a point of liquidity. Be careful not to burn yourself when handling a hot board.
Craft Yourself a Nice Day!
- William
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