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.

Soldering Basics

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:

... no, I'm not done yet, there's more:

...  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.

Sources (in German): [1] TRGS 505 [2] DGUV Information 213-714

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.

Tool 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.

Soldering Station

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).

Soldering Tip

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.

The Toolcraft ST-100D station includes a fine tip (not pre-mounted) useful for most THT soldering jobs. For SMD soldering we ordered a flat tip for our student lab.

Product example: Lötspitze Meißelform TOOLCRAFT T-0,8D Spitzen-Größe 0.8 mm Spitzen-Länge 17 mm

Other Tools

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:

Material Recommendations

Disclaimer: I focused on products by Stannol for my research for good products. There may be other useful products on the market.

Solder Wire

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.

Liquid Flux

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

Step 1: Preparations

I use button 2 for leaded solder (350°C) and button 3 for lead-free solder with silver (385°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.

iron_tip_tinning.jpg

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.

iron_tip_wiping.jpg

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:

iron_tip_shiny.jpg

iron_tip_shiny2.jpg

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: