All You Need to Know About Soldering

Soldering has been around for thousands of years but soldering as we know it today with electric soldering irons was invented in the late 1800 hundreds like many other electrical tools.
Soldering is when you attach 2 metal surfaces together by using a joining material that has a lower melting point then the 2 metals you want to join to have good electrical conductivity.
You can attach 2 wires this way or attach components to a PCB(printed circuit board).
Brazing and welding are very similar to soldering, but the main difference is temperature: for welding the 2 metal pieces are heated to there melting point, for Brazing and soldering only the joining filler melts. Soldering is used for delicate and thin metal(so mainly electronic components) while brazing is higher temperature and used for mechanical applications.
Not all metals can be soldered because the joining metal/alloy needs to have a lower melting point then the 2 metals/alloys that will be attached together. Some metals oxidize very easily and then the surface does not want to react with anything anymore so the solder joint metal will not beable to attach to it. Also not all metals chemically like each other. For example copper and tin like each other but stainless steal and tin do not. Instead, silver needs to be used for soldering stainless steel.
In this article I will focus on soldering electronics(so copper soldered with tin).
For pcb soldering there are 2 types of soldering: smt where components lay on the board, and Through hole where component legs go through the pcb to the other side.
Soldering is a skill that like biking is learned with lots of practice. That is why I wrote down everything I know about soldering in text format so that the details can easily be looked up. So, feel free to skip ahead to any specific section(equipment, polarity, soldering wires, through hole, SMT, SMT with Metal Bottom Pad)

Equipment

• Soldering Iron

This is your main tool. Its like a pencil or paint brush, just really hot.
Soldering irons have different tips (not sure if they have tricks to though :) )
Once you've played around with them a bit it becomes pretty intuitive which one to use when. But in general if need more heat use a bigger one, if need precision work use a smaller one.
To clean the tip you can either use a sponge or metal wool. Metal wool is more aggressive.
You can only clean the tip when hot because otherwise tin(aka solder) wont come off properly since the solder is not liquid. After cleaning (and done removing excess solder on component legs) always put new tin on the tip because otherwise the tip will oxidize and will no longer take tin which means it becomes unusable.

How a soldering iron works:
It has a heating element inside or close to the tip. The amount of heat generated depends on the current through the element to its resistance ratio. The tip is made of a highly heat conductive metal, usually copper. However, copper oxidizes at high temperatures and then wont take tin, so the copper is plated with other metals like iron so that it oxidizes less quickly.
Tip activator(also called tip tinner) can be used to remove some oxidation on the tip, but it only works to a certain extent. It is highly aggressive acidic flux with solder paste. The aggressive acidic part eats away on the oxidized tip and the solder/tin then sticks to it again. However, if overused it can eatway on a perfectly fine tip shortening its life.

• Tin

Solder is often called tin because it is the main ingredient to the alloy joining material. There are many different solders with different concentrations, but the 2 main categories are leaded and unleaded tin.
Leaded tin melts at slightly lower temperatures but is banned in the EU due to the toxicity of lead. It depends on concentration ratio but usually leaded tin melts at ~200°C and unleaded at ~250°C. In order for the solder to be fully liquid it is best to go a bit above the melting point. Some people like to solder at 400°C for unleaded, but I think 300°C is completely fine. The hotter the quicker you can melt it fully(get in quick, get out quick) but also higher chance to ruin your tip due to oxidation while cleaning.
By the way in the rest of the article when refering to temperatures I am talking about unleaded tin since nowadays it is more commonly used.

	Leaded Tin	Unleaded Tin
conventional spool color	white	green
melting temperature (depends on concentration ratios, so these are rough estimates)	~250 degrees celcius	~300 degrees celcius
composition	Tin and lead	mostly Tin, other metals like silver and copper in small amounts


Wire Tin is a tube of solid solder and the inside of the tube is filled with flux.

For manual soldering wire tin is used, but it can also come in paste form. It is made of powdered solder and lots of flux. It is mainly used by smt machines.

Specific to Soldering Wires

• wire strippers

They are special cutters that only cut to a certain diameter because the blade is on the inside of a circle. You put the wire into the hole of the right diameter and pull(if it falls out you picked the wrong hole, if you cut the full wire you probably picked a too small hole).

Specific to Through Hole

• Desoldering Pump

A desoldering pump(also called solder sucker) is used for removing remaining tin in through holes.
It is called solder sucker because it acts like a syringe. To use it you first push down the end, this compresses the spring inside. When the spring is released by pressing the button on the side the spring goes back to its original size and pulls along a piston attached to its front end. As the piston moves upwards as the spring pulls it, air rushes into the created gap/vacuum causing a suction effect, sucking the molten solder up and into the pump.
Since the solder is still liquid it does not seal the tip of the pump but rather flies further and solidifies further in. When you push the back of the pump down to load the pump the piston pushes some of that sucked in solder out so that the pump does not clog.

Specific to SMT

• Tweezers

There are special smt tweezers, but normal ones work just fine. You just need to be able to comfortably hold the component in place with them.

• Flux

Soldering flux is used when several legs are attached together by tin and you want to remove this excess tin so that each leg is only attached to the pcb.
Flux helps the tin flow better because like soap on water, flux reduces the surface tension of the tin.
Tin prefers to stick to metal rather than plastic so if it is able to flow nicely it will go towards the metal legs making each leg attached to the pcb individually rather then together with a big lump of solder.
Flux prevents new oxidation by shielding the fresh solder from the surrounding air. But it can also remove existing oxidation because it is a reducing agent meaning it donates electrons. The oxidized metals got electrons from the oxygen in the surrounding air but when they meet the flux they prefer to take there missing electrons from the flux instead of oxygen.

There are many different flux mixtures.


• Rosin Fluxes
The oldest type of flux is made from tree resin(rosin). It is dissolved in alcohol and then filtered or distilled. Nowadays some rosin fluxes are synthetic and might also contain other chemicals to further improve its reducing agent properties.

• Low-Solids/No-Clean Fluxes
No clean flux leaves behind less residue because the mixture is such that it is mostly not solid(everything else evaporates away upon heating).
• Water Soluble Fluxes
These types of fluxes are chemical mixtures that are more corrosive so it can better remove and prevent oxidation but the pcb needs to be carefully washed with water so the components dont get corroded over time.
• Rubbing Alcohol

Pure isopropyl alcohol(IPA) or rubbing alcohol(less concentrated, rubbing alcohol=isopropyl alcohol + other stuff like water) is used to clean off flux after done soldering smt. Flux is alcohol soluble so the isopropyl alcohol dissolves the flux allowing it to be rubbed away with a cloth.

• Desoldering braid(also called wick)

It's a pre-fluxed copper braid that when heated makes the tin stick to it allowing excess tin to be removed from pcb leg pads.
It does not have to be, but usually it is pre-fluxed because otherwise it oxidizes more quickly when heated(and then wont take tin anymore). When you use it, and you pay attention to it, you can see small amounts of flux evaporating. If the braid is no clean then it means it was fluxed with no clean flux.
It removes excess tin from the pads even though the pads are also copper because of capillary action. This is why the braid is a braid and not a copper sheet.

• Heat Gun

A heat gun, used to remove smt, is a fancy hair dryer. The main difference is that the heat gun is alot hotter.

General tips:

• Be patient
• Heat first then do stuff
• Always think about how to get to each leg without burning anything else
• Be gentle, use soldering iron like a paint brush, dont destroy your canvas.
• To remove parts from a pcb (both smt and through hole) with a soldering iron add more tin because then there is a much better heat conduction.
  • When using a heat gun adding more solder just adds more surface and therefore is not effective since the heat can dissipate better over this bigger surface.
  • When using a soldering iron however its different because the extra tin increases the heat transfer between the iron and the thing that needs heating. The heat gun uses air which does not conduct heat better if add tin while the soldering iron uses the metal at its tip to transfer the heat. Also, the soldering iron can compensate for the extra surface area by drawing more power and can therefore keep the same temperature with more tin.

This stuff(tin, flux, heated components, ...) is all a little toxic so good air ventilation and an open window is best.

Polarity

Some components have polarity.
Usually the start of the writing or a dot or a notch or a line is the start of the chip.
Some chips have writing one way and then the same writing upside down written the other way. Those do not have polarity.
Resistors usually never have polarity even if the writing is only written one way.
For through hole LEDs, the shorter leg is the negative terminal.

Soldering Wires

Attach

1)Remove plastic insulation(using wire strippers) at the ends of the wires where you want to attach them together

2)Hold the wires ends in place in the air with something because you need both your hands in the next step(for example clamp them to the table(so that the wire ends stick out))

3)Tin the wire ends
Because then in the next step you only need to heat the wires. Its way more difficult to put tin and attach the 2 wire at the same time.

4)Slide the insolation tube onto the wire

5)Hold the tinned wire ends together with something or your hand(I usually keep one clamped and hold the other one with my hand). Heat the joint with the soldering iron so that the tin melts and the wires attach together

6)Slip the insulation tube onto the joint.

• Some insulation tubes are heat shrink so then heat them with a hot air gun or hairdryer
• Insulation is important not just to stop shorts but also because the insulation tube gives the joint stability. The tube stops the wire from bending at high angles meaning the wire wont easily break at the solder joint.
  Tape is not good because it does not have this anti-bending property, it comes off easily and it leaves a sticky mess when removed.

Remove

1)Remove the insulation by sliding or cutting it off.

2)Heat where the 2 wires are joint together until they separate.(honestly it's easier to unsolder then to solder wires).
If for some reason someone twisted the wires together then you might need to pull a little. If that does not work just cut the entire joint part off instead.

Soldering Through Hole

Through hole components are components where the legs go through the pcb to the other side.

Attach

1)Put the component legs through the holes from the topside of the pcb. Watchout, some components have polarity.

2)Then turn the pcb over. If necessary(and legs long enough and bendable enough) bend the legs of the component a bit outward(after putting it through the hole) so the component does not fall out when turning the pcb over.
Also, always start with the least high components because once flipped over the table will hold them. If this is not possible because you are replacing a component then put something underneath the replacement component so that it does not fall out and sits straight.

3)Put tin all around the legs so that each hole is filled.

4)Cut any excess leg wire off with a wire cutter or the cutting part of pliers.

Remove

Option 1: heat and pull legs out

1)Put more tin on the legs

2)Heat with soldering iron.

• If 2 or 3 legs:
  Heat with soldering iron. Preferably heat all legs at the same time and when tin is liquid pull the component out with your hand or pliers. For some components you can heat one leg at a time and wiggle the component up until it comes out.
• If to many legs to heat at the same time or legs are too far apart:
  destroy the component by cutting the legs off and then heat with soldering iron and pull each leg out individually with pliers.
  Header pins are usually made such that the metal pins just sits in the plastic through friction so you can heat the solder joint of each and pull them out individually.

Soldering SMT

SMT(Slightly More Tricky :) ) no I'm joking its easy once you get the hang of it. It actually stands for Surface Mount Technology because these components are laid on the pcb. Because these components can be very small(some resistors and capacitors can be sand grain sized!) a sinner soldering iron tip can make this type of soldering easier.

Remove

1)Use a Heat gun (~350°C) and heat the component

2)Be patient, small components come off quick while bigger ones take longer. For very big components you might need to crank up the heat to like 400°C otherwise it will take forever.

3)Remove the component with tweezers once possible (aka when tin is liquid). I find for small components grabbing it by the housing is easiest but for big components grabbing it by one of the legs works best.

4)Check if during the removal process any other components where accidentally moved out of place. If so either also remove them with a heat gun and reattach or replace them, or if it only has 2 legs/attach points you can carefully heat and nudge it back into place with the soldering iron.

5)Use the desoldering braid to clean the pads: Press the soldering iron onto the desoldering braid and gently wipe along the pads to remove the tin. This is necessary to not have shorts and because if you want to place a new component there then the component will not lay flat if there is still tin.

If while attaching a component you have trouble because one side is too high its likely because the pads where not properly cleaned.

If you dont have a desoldering braid you can also use the desoldering pump(do like with through hole, heat and then right away trigger the pump to suck up the tin) However, it's not as effective and I find its way more effort.

A bit more messy but only the component and its direct surroundings get heated(with a heat gun a bigger surface of the board gets heated which reduces chances of local overheating but increases overall thermal stress).

1)Put tweezers away. If you try to heat one side with the soldering iron and pull with tweezers you risk ripping off a leg pad from the pcb.

2)Make sure that the component does not have a metal pad below. If it does but you did not know you wont break anything, its just that option 2 will simply not work. (smt components with a bottom pad have to be removed with a heat gun)

3)Put a lot more tin on the legs

4)Heat both sides repetitively with the soldering iron until on both sides the tin stays liquid long enough that you can move/push the component off with the soldering iron. You can also just use 2 soldering irons and heat both sides at the same time.
You can do this for all components with legs on 2 sides even if it has many many legs. However, if it has legs on 4 sides you can not because it will be very very difficult to move fast enough to heat such that the tin is liquid on all sides at the same time inorder to move the component(maybe you could if you use 2 soldering irons but I've never tried it).

Attach

1)Make sure pcb leg pads are clean(no solder on them). If not use desoldering braid(see above in removal section).

2)Put tin on one leg pad

3)Then place the component. Watchout, some components have polarity.

4)Hold the component in place with tweasers(if it's a very big component, holding it with a finger is easier) and heat the leg pad with the tin on it to attach the component

5)Check that the component is aligned on all sides and that the legs sit flat on the pcb leg pads, if not then heat the attached leg to remove/move the component and do any necessary corrections

• Especially for components with many legs close together it is important to have them perfectly aligned because otherwise you wont beable to have each leg separate from the others at the end
• Once more then one or two legs are attached you can not easily move the component. At that point you have to remove the component completely and start over.

• If legs are far enough apart or only 2 legs/contact points you can attach each leg by individually tinning them.


• For components with many legs you instead put a blob of tin on them and then gently go along the legs and spread the tin to the other legs(use soldering iron like a paint brush). A bigger soldering iron tip or even the super big rectangular one might make this easier. Then using lots of flux brush down and outward to remove excess tin on the legs. Sometimes going sideways(to the legs) helps to clump the tin together so you can more easily remove it. Clean the soldering iron tip every time you brush down and out so that you dont reapply the excess tin you just removed.
  • NEVER try to go between the legs to remove tin even if its just 2 legs because you risk scratching the pcb and potentially ripping the wire structure(this will be more apparent if you ever unsolder the component because at the scratched spot the leg pad might come off meaning that the pcb is ruined because the leg cannot have contact anymore, and you cannot really fix ripped pads especially if lots of other leg pads next to it).

Soldering SMT with Metal Bottom Pad

This is very difficult because these components are meant for machines not humans.
These components are called BTCs(bottom terminated components)(not bitcoin lol :) ).
The pad is usually GND(ground) and sometimes additionally used for heat dissipation or mechanical support(to hold the component in place more easily). This is why it is so difficult to do by hand, the pad is meant as a heat sink so it's hard to heat it enough for the component to get attached.

Remove

1)remove like normal smt, only thing is it will take longer to heat enough to beable to remove it. You can also crank up the heat on your heat gun to 400°C if it takes to long. Like with normal smt, the bigger the component the longer it takes to heat enough to remove it.

Attach

1)check the datasheet of the component to see if you really need to solder the pad. If it is only for mechanical support you can get away with just soldering the side pins.

2)If you need the pad then Option 1: tin

  1)Put a little bit of solder on the pad. Try to make the layer as thin as possible otherwise the component will not lay flat. (if in the next step you want to use a heat gun at normal temperatures you need to also put tin on the component otherwise it wont stick. However, at these normal temperatures(350°C) it takes forever so not recommended).

  2)Crank up the heat on your heat gun to like 400°C otherwise it will take forever(also it is usually better for components and board health to have higher heat for a short amount of time rather than lower heat for a long time). Heat the pcb pad and hold the component right next to it so that it also heats but only place it once hot enough(if you place it right away the chip itself insulates the pcb pad, and it takes forever for the tin to melt. You need to heat the component and the pad at the same time because you cant put a cold component on a heated pcb pad, it wont hold. And even if it does hold the tin will cool immediately upon contact creating a bad connection(called cold solder).

  This step might need several attempts before the component is not only attached but also aligned properly. If you need more then 2 or 3 attempts take a new component because long heat exposure can/will damage the component, so it wont function properly anymore and the bottom pad wont take tin properly anymore either. If you need lots of attempts(usually 6 or more) the pcb will eventually have heat damage and a new pcb will be needed(you can see it, if the pcb turns brownish black its burnt). If the component is attached on a part of the pcb which sticks out from the rest of the pcb or that the pcb is very small, the pcb will overheat more quickly because less mass and cant cool as much since only attached to main board at one spot. If you heat the main board the heat can more easily dissipate to the rest of the pcb.

  3)Solder the legs like normal smt. Some have only small contacts on the sides(no real legs) but it's the same thing, solder them like you would a chip with many legs.

Option 2: solder paste

  I have never tried it, but I heard that using solder paste for this makes it easier. I think it's because you can place the component properly while still cold and the solder paste will hold it in place. Also since solder paste has lots of flux it will flow better.

  1)Apply solder paste to the pad by using tweezers.

  3)Place component

  4)Heat with heat gun(heat gun needs to be at higher temperature so like 400°C). When the paste starts flowing out on the sides a tiny bit then you know you are done heating.

  5)Solder the legs(or small contacts) like normal smt, I think you can use the solder that has flown out in the previous step and just add more solder.

Soldering Machines

In a factory the soldering process is usually mostly automated. Through hole machines are called wave machines because the pcb is passed over a wave/waterfall of liquid solder(is it called tinfall in this case? :) ). Smt machines are actually 3 machines: smt stencil printer, pick and place machine and reflow oven.

First a metal mask which has holes where the tin should go is put over the pcb(the pcb board itself is already made so it already has the leg pads and traces) Then tin paste is applied by the stencil printer. Then the machine places the smt components. Then the pcb is heated in a reflow oven to attach the smt components. Then it is cooled for inspection. The cooling is not necessary just that you can not inspect the boards for faulty soldering if the tin is still hot and since on a production line there is no time to wait, the boards are cooled.

Some boards have smt on both sides. First one side is done then the other side is done the same way. (Usually the back is done first because it usually has less components, so less components are stressed twice and heavier smt parts which could fall off despite tin not being fully liquid are also less likely to be on the back).
The components dont fall off when the other side is done because the board is only heated on one side and the heat is not high enough and the pcb insulates enough that the otherside is not hot enough to melt the tin completely.

Then the through hole parts are placed. After that the boards go into the wave machine where first flux is sprayed onto the bottom of the board in the areas where the through hole components need to be attached. Then the board is heated again so that there is no cold solder(if board still cold when tin is applied the tin will cool to quickly resulting in a badly electrically conducting joint(this is called cold solder)). Flux is put before heating because we want the contacts to not oxidize when they are heated(oxidation happens more at higher temperatures). Once warm(no need to go to the full 300 degrees celcius of the tin but like 150 or 200 is fine) the bottom is coated in tin by passing the board over liquid flowing tin.
Since the tin is fully liquid the flux has to be applied separately beforehand and can not be in the tin mixture cus it would evaporate when the tin is heated and never actually get to the board.

If the board has smt on both sides then the pcb is placed in a special tray that covers the smt parts as it goes through the tin to attach the through hole parts.

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