lead free soldering
Purpose: To be able to teach kids how to solder, using lead free solder and materials with low toxicity or other health risks for safer handling.
This is not meant as a guide for professional soldering, since leaded solder is currently (2023) still unmatched in terms of both quality of solder results as well as ease of handling. Even for adult beginners I still recommend using leaded solder (if you can get your hands on any, since they have mostly been removed from the private market).
Introduction & Basics
The following guide is intended for experienced people with advanced soldering skills, who would like to learn how to solder lead free and also teach kids or others. With underage teens & kids being the main target group, the products I recommend have been selected with a focus on health aspects (low toxicity and safety in handling), acceptable costs (non-professional equipment) and teachability.
Of course this guide can also be used by beginner adults to learn how to solder lead free. However, I must note that lead free soldering is much more difficult if you're lacking both experience and knowledge. It is difficult to judge the success of your lead free soldering if you don't know what a good solder joint should look and "feel" like while soldering. This is much easier to learn with leaded solder. Since leaded solder has basically been removed from the market and is only available to professionals and companies, beginners will be forced to use lead free solder.
If you're a beginner who wants to learn how to solder, please be aware that it might be necessary to get help from someone with advanced experience.
Speaking of experience, professionally I am surrounded by both hobbyist and professional solderers. Even though I asked around for help or advice, yet nobody knew how to solder lead free or could offer little more than theoretical knowledge. I only found one person with hands on experience and very basic but helpful advice (thanks for the tip about the liquid flux!). Lead-free soldering is extremely unpopular and I have found that there is a severe lack of professional knowledge on the subject.
I have started to research this topic in order to be able to teach kids in our student lab of the institute I work at. And although the target group is kids, we wanted to satisfy high professional standards in terms of quality and especially safety. Using toxic liquid flux where there data sheet requires the use of gloves and safety goggles but then not actually using them would simply be unprofessional and irresponsible. When it comes to the safety of kids, in my opinion the hacker mentality of "anything that works goes!", which I often live by in my private life, is simply not good enough.
If you're here to teach yourself how to solder, please put in some extra time for reading and researching the topic on the internet. Here are some simplified(!) basics you need to understand:
- Soldering is connecting two metallic parts both mechanically and electrically. Unlike welding, the metals do not melt themselves, instead another metal alloy (solder wire) with a low melting point is added to connect the pieces together (much like "glue").
- Soldering temperatures typically lie between 300°-400°C and are applied using a soldering iron.
- The solder nowadays contains flux in the core of the wire, making it very easy to use.
- Flux is a helper material that will remove oxidation on the surfaces of the metals. Oxidisation is problematic, because it will prevent the heat from the soldering iron to be transferred into the electrical components and prevent the metals from connecting with one another. Flux will help remove oxidisation and therefore enables the heat to enter the metals. Without flux soldering is almost impossible.
- Flux will also evaporate at typical soldering temperatures (300°-400°C), where the higher the temperature the quicker the flux will burn or evaporate. Therefore the soldering job should be finished by the time the flux has disappeared. Lead-free solder requires higher soldering temperatures, which means the flux will disappear more quickly than with leaded solder. Therefore soldering has become a little more difficult.
- When flux is heated it will release smoke. There is a wide variety of different fluxes with different properties. Generally speaking the main health hazard comes from the soldering smoke (the flux). Lead-free solder typically requires flux that is more toxic than the flux used in leaded solder, making lead-free solder more environmentally friendly but also a bigger health hazard.
- There are VOC-free and halogen-free fluxes available also for lead-free solder, making them less of a risk factor. They do create less smoke but also leave sticky residue behind.
- There are so called "no-clean" fluxes that leave almost invisible and non-corrosive residues on the solder joint environment. Make sure to either use solder wire with "no-clean" flux, or buy an appropriate flux residue remover for your solder wire.
... no, I'm not done yet, there's more:
- Many people are used to using wet sponges to clean their solder tips. Some beginners don't even know you're supposed to wet the sponge and instead actually burn the sponge with their iron. Wet sponges cause mechanical stress to the iron due to the big and sudden temperature drop from above 300°-400°C down to 100°C. Please don't use wet sponges. Use so called "dry sponges" that look like metallic wool ("Reinigungswolle").
- A dull looking or dark/brown/black soldering iron tip is not usable. It is heavily oxidised and needs cleaning. Sometimes tinning the tip multiple times fixes the issue, but if this doesn't solve the problem you either need to get a special cleaning product or exchange the tip for a new one. This problem can be avoided by taking good care of your tip, not leaving it running unused at high temperatures and regularly tinning the tip. Switch it off when you're done and use a standby temperature when currently not in use.
- A so called cold solder joint ("kalte Lötstelle") happens when solder sits on top of the metals it is supposed to connect to, but it didn't connect. Enough flux, heat and time will cause the solder not only to melt but also to "flow". When it flows it will visibly "creep" along the metals and the surfaces will properly connect to one another. A good solder joint look shiny with a smooth surface, a cold solder joint is dull and rugged.
- Fresh air and an air filter is advisable for health reasons, but please be aware that cold airflow should never be present at the soldering area. Filter the air above the work area. Cold airflow will cool down the parts to be soldered, meaning it will counteract the heat you are trying to introduce into the metals.
... there is still more to know about soldering, but this should get you started :-)
Leaded Solder Wire
While lead is a heavy metal and is a concern for both the environment and our health, it is worth noting a few things.
- Professional soldering on a large scale using automated equipment is an easy to control environment. It is therefore both feasible and absolutely necessary to use lead free solder for environmental long term reasons.
- Soldering by hand is only a tiny fraction of all the soldering happening world wide. Given how little most people solder per year (even professionally), my personal opinion is using leaded solder is justifiable for private/personal purposes. On a professional level there are laws mostly prohibiting the use of lead  .
- There is little to no lead that enters the human body while soldering neither by breathing in fumes nor by touching it with (unbroken) skin. But please don't stick solder wire into wounds or your mouth ;-)
- Measurable lead contamination shown in blood levels stem from a lack of hygiene. People often touch solder wire and then touch their face/mouth/eyes, which is how lead enters the body. Don't touch your face or food while working and always wash your hands with soap when you're done.
- Using lead in jewellery is prohibited... don't solder LED-earrings with lead! 
- Lead will harm your fertility  .
- Typically the flux and it's fumes (especially for lead-free wires) are a bigger health risk than lead, provided you wash your hands and watch your personal hygiene. Use a fume extractor for the fumes.
- Fumes will be carcinogenic and allergy inducing.
Blei und seine Verbindungen sind als reproduktionstoxisch Kategorie 1 A eingestuft. Dies bedeutet, dass Blei und Bleiverbindungen beim Menschen bekanntermaßen fruchtschädigend (entwicklungsschädigend) wirken (RD1 A) und wegen ihrer möglichen Beeinträchtigung der Fortpflanzungsfähigkeit des Menschen Anlass zur Besorgnis geben (H360FD, bzw. H360Df). Auch bei Einhaltung des Beurteilungsmaßstabes kann eine Fruchtschädigung bei einer Exposition Schwangerer nicht ausgeschlossen werden (siehe dazu auch Nr. 6.3).
Die im Rahmen einer umfangreichen arbeitsmedizinischen Studie gewonnenen Messergebnisse zeigen jedoch, dass sich die Blutbleiwerte von Kolbenlötern nicht von denen der Allgemeinbevölkerung unterscheiden.
Ein Unternehmen hat auf der Grundlage des Ergebnisses der Substitutionsprüfung nach GefStoffV vorrangig eine Substitution durchzuführen und dadurch die Gefährdungen der Gesundheit und der Sicherheit der Beschäftigten bei Tätigkeiten mit Gefahrstoffen auszuschließen. Es hat Gefahrstoffe oder Verfahren durch Stoffe, Gemische oder Erzeugnisse oder Verfahren zu ersetzen, die unter den jeweiligen Verwendungsbedingungen für die Gesundheit und Sicherheit der Beschäftigten weniger gefährlich sind. Ist dies nicht möglich, hat es die Gefährdung auf eine Minimum zu reduzieren.
TL,DR; Lead-Free Soldering Quick Guide for Hobbyists
Read the more in depth explanations below for how I came up with these recommendations.
Disclaimer: Most links I provide link to conrad.de, because of its accessibility to private customers. I do not endorse the shop and most products can be found cheaper on ebay.de or other sources.
For private or hobby purposes I recommend using a soldering station that can be set to at least 450°C, ideally with enough wattage (e.g. 100W or more) to heat up the iron quickly. The working temperature should always be below 400°C, but heating up the iron quickly is essential both for an acceptable handling experience as well as minimizing high temperature exposure and oxidisation of the soldering iron tip.
Additionally at least two programmable buttons for temperature presets is very useful. I also recommend using a dry cleaning sponge (Reinigungswolle) instead of a wet sponge, since this will minimise temperature loss and temperature strain on the solder tip. Please don't use wet sponges.
Product example: Toolcraft ST-100D SE from conrad.de, max. temp 450°C, 3 programmable buttons, 100W power, dry cleaner sponge (Reinigungswolle) included. Pricing ca. 130EUR (incl. MwSt, in 2023).
Personally I prefer small flat soldering tips over the typical rounded ones. If your soldering station comes with a fine enough soldering tip, you don't need to worry about buying additional tips.
Product example: Lötspitze Meißelform TOOLCRAFT T-0,8D Spitzen-Größe 0.8 mm Spitzen-Länge 17 mm
I also recommend getting at least one PCB-holder ("Dritte Hand", a simple one without a magnifying glass is sufficient) and a good light source on a flexible stand/arm. They can also be bought as a combined tool ("Lupenleuchte").
Product example: Tisch-Lupenleuchte with pcb-holder from conrad.de
Plastic cutting mats can cause ESD damage to electronic components, which is why I advice against using one to protect your table surface. The cheapest option is to use a large piece of cardboard, or if you want to go fancy use a proper soldering mat.
Product example: Soldering mat (Lötablage) from conrad.de
Depending on your application some tweezers or special PCB-holders and flat pliers can be useful too. Here's a list of equipment we use in our student lab:
flat self-closing tweezers / flache Lötpinzette / flache Kreuzpinzette
Entlötlitze/Desoldering Braid (recommended) or Entlötpumpe/Desoldering Pump (causes mechanical stress, but is easier to handle for beginners)
Disclaimer: I focused on products by Stannol for my research for good products. There may be other useful products on the market.
Solder wire with silver (Ag) component has a lower working temperature than without silver. The price is slightly higher but it's worth the cost.
Recommendation: Use 0.5mm diameter solder wire, useful not only for SMD but regular THT soldering.
Product example: Stannol HS10, 95.5% Sn, 3.8% Ag, 0.7% Cu (Sn95,5Ag3,8Cu0,7)
Alternatively the Stannol HF32 variant (Sn95,5Ag3,8Cu0,7) uses flux that is halogen-free and produces less smoke. It's worth noting the noticeable higher price and sticky residue it leaves behind. If health risks are a major concern, the HF32 wire can be recommended instead of HS10.
Additionally for lead free soldering using additional liquid flux ("Lötwasser") is essential for correcting mistakes or handling thicker wires. Thick wires require more time for heating up, which will burn through the flux included in the solder wire more quickly. Thus extra flux is required to be able to introduce enough heat.
I have found one liquid flux that is surprisingly low in toxicity but also works nicely with different solder wires that I tested. I recommend liquid flux with a brush included in the bottle cap.
Product example: Edsyn FL 112, VOC-free No-Clean rework flux.
How to Solder Lead Free, Step by Step Beginner-Guide
Soldering lead free is more difficult than with leaded solder wire, so following procedure more closely is essential for achieving good results.
Step 0: Health & Safety Stuff
- When soldering, never touch your face, mouth, drinks or food. When done soldering wash your hands thoroughly with soap.
- If you have long hair tie it back. Hair on fire is a bad hairstyle.
- Leave a window open or better: leave the windows closed but use an air filter to filter out the soldering fumes. Fumes can sting your lungs and eyes, close your eyes and hold your breath if fumes blow into your face.
- Regularly take breaks to open the windows and ventilate.
- If you use toxic flux, read the data sheet and use appropriate gloves and eye protection.
- Don't burn the cable of your soldering iron with the soldering iron ;-)
Step 1: Preparations
- Take time to prepare your PCB or soldering job before you switch on the solder iron.
- Program button 1 on your soldering station for standby temperature (150°-180°C).
- Program another button for your lead-free solder ideal working temperature (380°-390°C)
If you're using a different station or different solder you need to experiment to find the correct settings for your station. Be aware that lead-free solder without silver requires about 10°C higher temperatures, but the temperature should never exceed 400°C. I found that silver-free solder requires at least 390°C to be able to properly work with it. Lower temperatures are indeed possible but not advisable. You will run into problems if your temperature is too low.
Choosing a higher temperature however increases the oxidisation speed of your soldering iron tip as the flux will evaporate more quickly. Figuring out ideal temperature settings for your station is very difficult for beginners, so get yourself some help from an experienced soldering expert.
Lead-free soldering is more advanced than using leaded solder and not beginner friendly!
Using ideal temperature settings is essential for the lead-free soldering experience, but difficult for beginners to figure out. If you're struggling with soldering, come back to this step to reconsider your temperature settings!
Step 2: Tinning & Cleaning the Tip
Every(!!!) time you pick up the iron for soldering, you need to tin the tip first. Press your working temperature button (button 3 in my example). Wait for the tip to heat up, hold solder wire against the tip until it begins to melt. Cover the tip all around from all sides with solder.
Make sure the tin doesn't only sit on one side, rotate the tip while tinning.
Dip the tip into the dry cleaning sponge once(!!) to remove excess solder, do not thoroughly wipe the tip in the sponge and do not "clean off" the solder too well. The solder is actually good maintenance for the tip, wiping too much off creates microscopic scratches in the tip.
Your tip should be silvery shiny now. Do not wait, immediately start soldering. If you wait too long, you need to repeat this step before soldering. This is what a good soldering tip ready for work looks like:
Your soldering iron tip must always(!!) look silvery and shiny when you begin soldering!
If your tip looks dull, dark, brown or black, you will not be able to solder. This layer is called oxidisation. Remove the oxidisation (aka "clean the tip") by applying new solder all around the tip and carefully removing excess solder using the dry cleaning sponge.
This is what an oxidised tip looks like: