Solder is Solder, right?

I mean you heat it up, it melts, you remove heat and it hardens.

There’s not much more to it than that.

However, if you’ve spent any amount of time Soldering you’ll soon realize that not all solders are created equal.

There are hundreds of different combinations of alloys, wire sizes and flux types to choose from that can greatly affect your soldering experience.

Solder Alloys

Solder comes in a variety of different alloys. Every alloy has a different composition of metals in it. The 2 main categories of alloys are Lead solders and lead-free solders.

Until a decade or so ago, lead based alloys were the most prevalent and were very reliable.

However, as we have learned more about the harmful effects of lead in our environment, more and more companies have been switching over to lead-free solders. Most of this is due to new regulations (such as RoHS in Europe) that have been adopted.

Lead based solder comes in a number of different alloys. The most common alloys you will see are 60/40 or 63/47. These numbers tell you what percentage of tin (Sn) and lead (Pb) are in the alloy. So 60/40 has 60% tin and 40% lead.

The main thing to know about tin-lead solders is that most of them don’t switch directly from solid to liquid as you heat them up. An alloy that does this is called “eutectic.” Most of these alloys are not eutectic – instead they go through a transition period where they become a kind of “pasty” material – neither solid nor liquid.

However, there is one eutectic alloy that transitions directly from solid to liquid – 63/37.  (The phase diagram above shows this point at 62% tin/38% lead.  A lot of the charts I’ve looked at say the same thing.  Yet most resources say eutectic solder is 63/37.  If anybody knows why this is, please leave a note in the comments.) It also has the lowest melting point (183 degrees Celcius) of all the tin-lead alloys. This makes it very useful for people who solder RC electronics (or any type of electronics).

Lead-free solders also come in a number of different alloys. Many of these alloys are made up of varying amounts of Tin-Silver-Copper (Sn-Ag-Cu). They are also known as “SAC” alloys.

SAC305 is probably the most common lead-free solder alloy. It consists of 96.5% tin, 3% Silver and 0.5% copper.

Wire Size

Wire solder comes in a number of different sizes, sometimes called gauges. Usually you see it listed as a diameter in inches or millimeters. For general, through-hole soldering 0.031″ (1/32″) is a good size to use. If you solder bigger lead wires to connectors (say a battery lead wire to an XT60 connector) it may be helpful to have a bigger solder wire size on hand (0.062″) but it isn’t necessary.

Flux

Flux helps solder flow and stick to the surface you are soldering. It is slightly acidic and removes any oxidation from the surface of the metal you are soldering.

Most solder wires come with a rosin flux in the center of the wire. You can see the rosin in the picture on the right.  You can get different amounts of flux in the center but I would look for 2% or 3% for through-hole soldering.

Rosin core flux wire usually is adequate but it can sometimes be helpful to have separate flux available as well. Why? When you let solder sit on the tip of your soldering iron, it will burn away the flux. So when you go back to start soldering the flux isn’t there anymore. Adding just a bit of flux to the surface of what you are soldering can be helpful in this case.

Lead or no lead – That is the Question

There’s a lot of debate among RC and electronics hobbyists about what solder alloy to use, especially when it comes to lead vs no lead.

Actually there isn’t that much debate … as I read around forums almost everybody is against lead-free and thinks it’s really difficult to work with.

There is some truth to that but lead-free solders are quickly becoming the industry standard and thousands of companies around the world use them every day without any issues.

My theory is it comes down to a few different things:

• Technique – Tin-lead solder is more forgiving than SAC solder so if you don’t use good soldering technique, you can still get pretty good results with lead-based solder.
• Equipment – Likewise, because it is more forgiving if you aren’t using the right soldering iron tip or your temperature regulation isn’t very good you can still get good results with lead based solder.
• Materials quality – Cheap materials, whether lead-based or lead free, will cause anybody issues. If you buy a random/no-brand/cheap solder on Ebay or BangGood, you don’t know what you are getting. Check out Oscar Liang’s post about this. He blames it on the solder being lead free but I think it is quite clear that it is a material quality issue, not an alloy issue. I’ve heard a lot of these cheap solders use recycled materials so they can have impurities. I only use Kester brand solder but there are other good brands, which I’ll list later in this article.

My Advice for the Best Solder to Use for RC/electronics Hobbyists

If you have experience soldering and have a good soldering station, I would go with a lead-free SAC305 alloy from a reputable brand. If you are a beginner or have cheaper soldering iron, then I’d suggest using either a 60/40 or 63/37 tin-lead solder. The rosin core should be 2-3%, which is pretty standard. I prefer 1/32″ (0.031″) diameter wire for most of what I do.

As for brands, stick to Kester, Multicore/Loctite, MG Chemicals or any brand that has a website where you can go find a datasheet for the solder. The datasheet should tell you the allow, other various properties, what standards it has been tested to, etc. If they have this, they are a legitimate company and you can be pretty sure you’ll get what they say you’ll get.

So my recommendation for lead wire solder would be the Kester 60/40 0.031″ or the Kester 63/47 0.031″ solder.

My recommendation for non-lead wire solder would be the Kester SAC305 0.031″ solder.

One note about the environment: You, personally, won’t use enough lead in everything you solder to have any noticeable effect on the environment. It’s when hundreds of thousands of people just like you also use lead solder, then crash your quad and then throw it away that we might start having issues. This is no different than any other environmental issue. If you throw some trash along the side of the road, it won’t make much of a difference. It’s when millions of other do the same thing. That’s why I’d recommend using lead-free if possible.

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