Soldering: Everything a beginner needs to know

[mariushm]

 

If you don't want to read a wall of text, scroll down to the second post where you will see VIDEO TUTORIALS about soldering

 

I saw another person start a thread about soldering but felt like my long posts would be lost in the middle of pages among other messages.

Maybe this can be made sticky by a moderator or something like that.

 

What's soldering? Basically it's a chemical reaction between metals ... the process in which two metals are joined together using a metal filler called solder.

This chemical reaction that binds the metals together (similar to welding) happens at specific temperatures and that's why you need some soldering equipment to solder.

 

You have three major kinds of soldering tools:

1. Soldering guns - very basic soldering irons that usually plug directly in mains socket and always run at same temperature

2. Soldering irons with power control - these soldering irons give you some control over the tip temperature by allowing you to adjust how much power is sent to the heating element in the tip, but don't actually "read" the temperature and have any feedback mechanisms to dynamically control the tip temperature

3. Soldering stations - soldering irons which have a temperature sensor in the tip and allow you to configure a temperature, and then automatically control the amount of power sent into the heating element in the tip to keep the temperature within a very narrow range of your set temperature.

 

Temperature is the most important thing when soldering, so obviously having good control of temperature is important. 

Some components are very sensitive to heat (LEDS, some sensors, solar cells, various ICs) and flux (i'll talk about it further down) is also very affected by temperature.

 

The temperature of the tip is affected by two things : the size of the soldering iron tip and how big the "heat reservoir" is.

Depending on how the soldering iron tip is, less or meat heat can be transferred to the metals that the tip touches in a unit of time. For example, a conical tip will have less surface area in contact with the metals compared to a chisel (like flathead screwdriver) tip... the chisel tip will have bigger contact area so more heat will be transferred to the metals.

At the same time, the metals that touch the soldering iron tip also act like heatsinks, so they suck away heat from the soldering iron tip, cooling it down which is not good, because you get a good solder job when you have the right temperatures.

The heat reservoir is basically just how well an soldering iron tip holds temperature. A small heat reservoir means the iron tip cools faster when it touches what you want to solder, so a soldering iron would have to pump a lot of energy into the heating element to quickly recover. A bigger heat reservoir means the temperature will fluctuate less.

 

The enemy of soldering is oxidation ... the formation of oxides on the soldering iron tip and metals you want to solder.

Oxides form naturally wherever there's oxygen, so any metal leads or pads on the circuit boards will have oxides on the surface (well, maybe less or none if you have gold plating or something like that but in general it's safe to assume there's oxides on metals)

You need a high enough temperature to solder, but too high temperature also accelerates the process where oxides are formed. 

Oxides block transfer of heat, and blocks that chemical reaction between metals, so you would not get a proper soldering job if the surfaces are oxidized.

 

In order to deal with oxides and various other crap that's on leads and soldering iron, there's flux used.

Flux is basically an ACID which activates when it reaches a certain temperature and quickly attacks the surfaces moving away oxides and crap from the area, giving the soldering iron tip and metals a very clean contact point, and that's where the chemical reaction between metals and solder happens.

Most soldering wire you can buy has some amount of flux built in ... think of it like a semi-solid or solid line of flux in the middle of the soldering wire (sometimes there's several such lines inside a thin soldering wire).

The flux has a lower melting temperature, so when the soldering iron tip starts to heat up the soldering wire, the flux melts first and pours over the metal parts you want to solder and at the same time it activates doing it's "acid" job, to quickly corrode the oxides and crap on the metals and as the flux evaporates, it binds with those oxides and they go away from the surface.

At the same time, some amount of flux survives and more or less covers the iron tip and the point where the tip makes contact, preventing air from coming into that point of contact.

 

So, this is another reason why temperature control is very important. If the tip temperature is too high, the iron tip can basically burn that flux too quickly, and the flux will have no time to actually activate and corrode the oxides on the surface and clean the area, because the tip burns it out too fast.

 

Now let's discuss those three soldering tools.

 

1. The cheapest are the soldering "guns" or irons that plug directly in the mains socket and offer no temperature control or very little adjustment options, which look like this:

These are very basic and simple. They don't control temperature and have no feedback mechanisms, there's just a heating element inside connected directly to mains, which pumps constant amount of energy and raises the tip temperature to around 400-450 degrees celsius, which is too high and that means tips of such soldering guns quickly form oxides which makes it even more difficult to solder. You're supposed to plug them in, wait 5 minutes or so to get up to temperature, quickly solder and then unplug, to "save" the tip from being corroded and damaged by the heat.

The tips are most often conical which means you have little contact between the iron tip and the metals you want to solder, further making it harder to solder.

 

Because of the small heat reservoir, as soon as you connect the tip to the parts you want to solder, those parts will suck heat away from the tip cooling it down... and because they pump a constant amount of energy in the heating element you will then have to wait some time for the temperature to come back to where it should be.

So for example, your tip may be at 300 degrees Celsius which is more than 190-230 degrees you need to solder, but you connect the tip the leads or wires you want to solder and the temperature almost instantly drops to 150-200 degrees and you have to wait seconds with the tip touching the parts for it to go above the soldering temperature of 190-230 degrees at minimum.

 

Nobody should buy these because the tip is bad, they'll quickly damage the tip if you leave them plugged in for long time, and they result in poor soldering jobs because of tip shape, small heat reservoir and high temperature which can burn up the flux inside solder before it can actually work.

 

2. The soldering irons / stations with power control / "fake" temperature knobs which look kinda like this: 

 

 

These are a bit better, as in they usually have a better soldering iron tip (bigger heat reservoir) and for some models you can actually change the tips to have various shapes that are better suited for your particular need at that time.

These don't control temperature using feedback, they just control the constant flow of energy to the heating element.  As an analogy, think of it like the dimmer for your lights, or switching between 40w , 60w, 75w and 100w light bulbs.

The knob just controls the amount of energy going into the tip, but the actual tip temperature will still vary depending on tip shape.

The benefit is that you can keep the amount of power low, to keep the iron tip at a lower temperature and reduce the amount of oxidation that forms on the tip and extend the life of the tip, and you can raise the power just a few seconds before you actually decide to make the connection.

You can also keep the power lower for small soldering jobs like soldering thin leads of a led to a circuit board, or you can raise power high if you have two thick wires you want to solder.

You can get used to one of these.. as in by repeated soldering, you would figure out that the iron tip would be at around 300 degrees with a chisel tip and knob at 60% which works for thin leads, or that for thick wires you'd want at least 75%

 

3. The soldering stations with temperature control which look like this :

 

 

 

These are the better of the three.  The iron tips can be changed, to use the best tip shape for your needs, and the heat reservoir is bigger because the heat element is often much bigger)

Also, inside the tip there's a temperature sensor which "tells" the unit the tip temperature right there at the tip, and that allows the soldering station to quickly react to changes in temperature and either pump more energy into the heating element or stopping energy when the temperature is too high.

The knob on the unit is actually calibrated to be reasonably precise (typically give or take 10-20 degrees celsius), so for example if you set the knob to 300 degrees, the unit will constantly read the tip temperature and adjust the heat element's power to keep the tip temperature constant.

When the tip touches some leads or metal parts and heat is sucked out of the tip (because those metal parts act as heatsinks, cooling the tip), the unit detects that and quickly pumps a lot of energy in the heater, raising the temperature back much faster.

Unlike previous soldering stations with power control, the flow of energy into the heating element is not constant, it's always adjusted to keep the temperature where you want it.

 

This is what you should buy, they're more expensive, but can be bought for as little as 20-30$ these days and they will last you at least 10-20 years if you take care of them.  Yeah, they're more expensive than 3-5$ you'd pay on ebay for the mains soldering irons (1) but so worth it.

 

Soldering alloys...

 

Soldering soldering alloys are :

1. lead free or lead based solders. 

2. low melting point solders or regular melting point solders

3. euctetic and non-euctetic solders

3. without any flux or with various kinds of flux inside

 

Lead based solders are much better for beginners, as they melt at lower temperatures and therefore you can use cheaper soldering irons.

Because of the lower melting point, these often also have less aggressive fluxes so you don't have to be worried as much about hurting your lungs (over time)

Another reason they're good for a beginner is because if you do a proper soldering job, the surface of the solder joint will be shiny. A disturbed solder joint will not be shiny, will be matte, opaque. Lead free solders will not have this behavior, they won't be shiny, reflective etc... so you can't use this property as a quality indicator with lead free solders.

 

Lead based solders may be harder to find in the future, for example in Europe they already don't allow regular persons to buy them, but again, it's much better than lead free solder.

 

Why should you use Leaded solders?

 

Well, mainly because they have a lower melting point of around 180-183 degrees Celsius, compared to Lead free solder which usually has a melting temperature of around 217 degrees Celsius.

Because of the higher melting temperature, companies often have to use much stronger fluxes inside lead free solders, as people use much higher temperatures on the solder iron tips. Those higher tip temperatures will burn through the fluxes much faster, so the fluxes have to be stronger acids to turn to liquid and attack surfaces before the heat from the tip evaporates / burns it.  The fumes from the flux being burnt can be worse for you, and it's often recommended to use at least some kind of fan (a case fan is enough) to move the fumes away from you.

 

2.   Low melting point  solders are solders which are made of various metals like Indium or Bismuth which melt at much lower temperature than normal (let's say 80-150 degrees Celsius) and which are often used in the process of repairing things. For example, this low melting point solder can be added to existing solder in order to reduce the original solder's melting temperature and make it possible to desolder a component without thermally shocking it  (as I said, some components are very sensitive to heat, and don't like 200-250 degrees Celsius on their leads)

 

What's Euctetic and non-euctetic solder? 

 

Well, solders become liquid when they're heated, but not all solder allows behave the same. 

 

Some solder alloys  are euctetic, which means at a very precise temperature they'll turn from liquid to solid ... for example 183 degrees Celsius +/- 1..2 degrees, the 63/37 alloy snaps from liquid to solid. This is great, especially if you're soldering in places where there's vibrations or other things that can perturb the joint, what you solder. However, if you use a very cheap soldering iron that can't keep up the temperature properly, this type of alloy can be harder to work with.

 

Other alloys are non-euctetic, which means that between the liquid and fully solid ranges, there's a range of temperature where they are "plastic", like a semi-solid metal.

For example, the most common lead solder alloy 60/40 , will start to melt at around 180 degrees , and will stay in that "plastic region" for around 5-10 degrees Celsius. 

This makes it more beginner friendly, but like i said not as good if you have vibrations or shocks or whatever ... if the soldering joint  is disturbed while the solder is in that plastic region, semi-liquid, the solder joint will be bad, so you'll have to redo it.

 

So the most common lead based alloys are:

63/37  - 63% Tin, 37% Lead - euctetic, 183 degrees, that's what I recommend to everyone

60/40  - 60% tin , 40% lead - non-euctetic, 180 degrees, a bit more beginner friendly and cheaper but not as great as 63/37

 

There's some variations which include 1-2% Cu (copper) or silver (Ag) ...

Copper is added because it can help the iron tips, extending their life, but otherwise don't really have big benefits for the solder joints.

Silver was introduced when surface mounted components (resistors, capacitors, chips) had their terminals / leads plated with silver. This extra 1-2% of silver in the solder alloy would help create a slightly better reaction between the solder and those plated leads/terminals making a strong connection.

These days, few components actually use silver on them, and there's minimal benefit to use solder with silver in it.

Audiophiles still suggest it but it's mostly placebo, presence of silver doesn't really help sound in any way.  

 

Lead free solders will typically be 99%...99.7% Tin, followed by some additives that help to some degree. Minimal benefits to lead free solders that have 1-2% copper or silver, they may help extend the life of your soldering iron tip a bit, but nowadays solder iron tips are cheap.

 

Solder diameters

 

You can buy them in various diameters ... my advice is to get something like 0.7-1mm for soldering thicker wires and 0.56mm or less for surface mount stuff and small things. If you don't have lots of money, go with 0.56mm or a bit smaller diameter, as you can always cut a length of solder and double it to make it thicker.

 

I personally use 63/37 0.56 mm thick solder with 2% of no-clean flux from Multicore, ad this solder has multiple strands of flux spread throughout the lengths of the solder. It's a bit more expensive, but a spool of 250-500g will probably last you more than 5-10 years.

 

Flux amount and flux types

 

Look for Rosin based , Rosin Activated (RA) or Rosin Mildy Activated (RMA) or no-clean fluxes.

These are milder fluxes, which do a good job and residue of such fluxes can usually be left on the circuit board without any bad effects. If you want, you can clean with isopropyl alcohol or water or other solvents.

Some RMA fluxes can be strong enough that they should be washed off with some isopropyl alcohol or just plain water, because if they're not cleaned over time they can continue to corrode the metals (as they're acids)

 

You may be tempted to go with "water based" or "organic" fluxes, thinking it's more eco friendly, but really don't, 

These fluxes are much stronger acids which will make more toxic fumes when burnt and most of them MUST also be cleaned from the surfaces, otherwise even when cooled down they will continue to attach the metals and corrode them. For some water based fluxes, the recommendations are to put the whole solder joint under isopropyl alcohol and then clean the area with distilled water or specific flux solvents to make sure the acid is neutralized.

As for "organic" fluxes, they're just worse for your lungs, should not inhale the vapor of such fluxes. 

 

The percentage of the flux inside wire makes a difference... for 0.56mm or less, aim for 2-3% flux and for 0.7mm or higher, you can do with 1-1.5% flux.

 

EXTRA FLUX always helps. Even if you have new components with clean leads and good solder, it never hurts to add a drop or two of liquid flux. Flux pens are expensive, but you can buy fluxes in small 100ml or 250ml bottles for a few dollars, and they really do make a difference.

 

 

Other notes

 

Low melting point solders should NOT be used for permanent jobs, as they don't have the same properties (strength, resistance to vibrations, don't handle flexing so well).

After using low melting point solder, the solder should be removed using solder wick or other methods and fresh regular solder should be used.

 

In general it's not a good idea to mix lead free solder with leaded solder, but for hobby projects, home repairs, it will be fine. Wherever possible, remove the old solder and use your new solder kind.

It's common practice to add leaded solder to a lead free solder joint in order to reduce the melting point temperature and make desoldering something easier, but once part is desoldered, the "mixed" solder should be removed from the board.

 

Common misconceptions and myths

 

Lead solders are toxic, bad for you, you'll breathe lead and get poisoned.

 

Unless you EAT the solder wire or you have open wounds on your hands, you will be fine.  If you are handling lead solder with your hands, some particles of lead can "chip" off the wire and stay on your skin, but washing your hands with soap after you solder is enough to be clean.

You can not breathe in lead, because it doesn't "float" in that smoke produced when soldering. It would require way higher temperatures than 250-400 degrees Celsius for lead or tin to become airborne, in the smoke.

In fact, the smoke is mostly the acid flux being burnt or evaporated by the soldering iron tip, and as it's an acid it can hurt your lungs, if you are exposed to such smoke for a long time (think weeks, months, years of daily soldering). 

Lead free solders are actually worse for your health for this reason: because the iron tip temperature is much higher, manufacturers have to use fluxes inside the solder which melt and have higher acidity so they'll do the cleaning job in a shorter time (because the hot tip will quickly evaporate the flux) 

 

 

[mariushm]

Video tutorials about soldering:

 

Pace Soldering Lessons

 

Looks a bit outdated, but don't be put off by it. The information and techniques presented are STILL valid and correct and everything is very well presented.

I recommend watching at least the two lessons I embed below as they're most useful to a beginner.

 

These lessons are also available in other languages (French,Spanish, Swedish,Chinese) as separate playlists :

 

Playlist : https://www.youtube.com/watch?v=vIT4ra6Mo0s&list=PL926EC0F1F93C1837

 

Lesson 1 - "Solder & Flux" - explains  solder types, role of flux, about temperature and heat reservoir of soldering irons and other bits

 

 

Lesson 6 - "Component Soldering" - shows the proper technique of soldering leads which you'll use often

 

 

 

Dave Jones from Eevblog has a series of 3 videos about soldering, explaining about tools to use and materials  (soldering stations, solder wick, solder alloys etc)

He explains it with easy to understand words and has plenty of demonstrations... I'm gonna embed just the first two parts, as the third is less important (surface mount soldering) :

 

 

 

 

I also want to embed the following, which shows the proper technique of cleaning the iron tip before using, tinning the tip, applying tin on the tip to prevent oxidation when you're done soldering.

 

The rest in the video is somewhat over the top, paranoid level cleaning of parts, "avionics" grade / nasa certified soldering... a beginner or someone just soldering some components on the board doesn't need to be as thorough with cleaning everything

 

 

 

If anyone has some suggestions for soldering tutorials, mention them or send me a message and I'll add them to the post.

 

 

 

[greenflights]

On 4/17/2020 at 10:46 AM, mariushm said:

You can not breathe in lead, because it doesn't "float" in that smoke produced when soldering. It would way higher temperatures than 250-400 degrees Celsius for lead or tin to become airborne, in the smoke.

In fact, the smoke is mostly the acid flux being burnt or evaporated by the soldering iron tip, and as it's an acid it can hurt your lungs, if you are exposed to such smoke for a long time (think weeks, months, years of daily soldering). 

Rosin core flux is pretty bad for you, and can cause occupation asthma.

 

If you avoid intentionally inhaling it and solderin a well venitlated place infrequently it will be fine, but if you begin to solder quite frequently or in a poorly ventilated room a cheap extraction system is a very good idea.

The UK Government publishes this helpful set of recomendations for employers who expose employees to soldering:

 

https://www.hse.gov.uk/pubns/indg249.pdf

 

The recomendation is pretty common sense. Use extraction for rosin core solder, don't breath the smoke.

I find a fan blowing over the top of the board I'm soldering to be adequate for keeping the smoke out of my face, and I normally solder in a well ventillated place.