Solder: What it is and how we use it

Solder

In a nutshell, solder is a metal alloy that can be melted and fused with components to join them together. To melt solder, you usually need a torch to create a high temperature heat source and not all solders have the same melting point, nor are they all used for the same applications.  There are three melting ranges for the different kinds of solder1:

  • Soft Solder: 90-450°C (190-840°F)
  • Mid-range: 180-190°C (360-370°F)
  • Hard Solder: +450°C (+840°F)

For those of you who have some knowledge of solder, or experience using it, you may be familiar with the top and bottom types, as well as what they’re typically used for. We’ll cover that later, for now just know that alloys at the low and mid-range melting temperatures are the most commonly used for soldering projects. When soldering, there are multiple components to consider such as the kind of flux it uses and what the alloy is made of a.k.a. how pure is the solder you are using, and how does it react to heat and metal?

 

Flux

Every stick, coil, or strip of solder has a core made of flux that causes the solder to spread out during application. Before it can fuse with materials like metal, an oxidized layer needs to be removed, seeing as how nearly all metals oxidize and create a barrier of sorts that keeps solder from “wetting” and bonding to them. Because of this, you need flux and basically, flux is a generally weak acid that removes this barrier of oxides to allow the solder to adhere to the metal as well as spread out, transfer heat, and blankets the component to keep oxygen out until the solder cools2. Ultimately, flux is a core component of solder and is essential to the soldering process.

Most of the flux you will be getting to work with is also known as “rosin core”. Be careful when buying your solder, especially if you’re shopping in a hardware store; more so, keep your eyes open for what it is exactly that you’re buying. If the solder is not rosin core, and it says “acid core,” that’s not the kind of solder you want for your electronics project. Acid core is corrosive and will destroy printed circuit board (PCB) traces, erode lead, and can create a conductive layer that will lead to electrical shorts3. Acid core is meant for soldering plumber pipes; rosin core is made to use for soldering electrical components.

Now, it might all sound a little confusing. We have flux which is a weak acid, and then acid-core solder which you don’t want to use. The good flux is usually referred to as rosin-flux and that is how I will refer to it in this blog. Anyways, rosin-flux has its uses when soldering; not only for actually soldering but also when wetting the tip. Rosin-flux is not as aggressive as some flux, and when wetting the tip of the soldering iron, you are what they call “tinning” or covering the tip in a thin layer of solder to increase the transfer of heat while soldering. This is actually the first step in preparing to solder electronics.

 

Alloys

As mentioned above, not all solder is the same and is usually made of different materials. They are usually made with one or more of the following: lead, tin, silver, and copper.

  • Lead is common because it is inexpensive and suitable to use.1
  • Tin is usually the base metal of the alloy; it is strong and has good wetting properties.
  • Silver provides a greater conductivity and mechanical strength, but it’s more expensive.
  • Copper lowers the melting point and improves the wetting properties of solder.

When it comes to leaded solder, the most common type of solder, it will usually have one of two popular ratios: 60/40 or 63/37 (lead/tin). I’ve seen lead-free solder made with 99.3% tin and 0.7% copper, and lead-free with silver made out of 96.5% tin, 0.5% copper and 3% silver. As you can guess, there are multiple kinds of solder. As discussed here, they are: leaded, lead-free, silver bearing, and hard solder.

 

Types of Solder

Standard leaded solder is made of tin and lead in one of the above ratios. Either ratio is fine for soldering electronics but each ratio has a different melting point. 63/37 melts at one temperature, whereas 60/40 goes through a pasty stage where it has to hit a range of temperatures to completely melt2. Picture it like this: 63/37 is like melting frozen water, at a certain temperature it will melt; 60/40 is like cooking frozen food, through a range of heat, some parts of the food will cook while other parts are still cold and need to hit that higher temperature.

Lead-Free Solder came about for two main reasons: RoHS/WEEE and competition of better products. If you read about RoHS and WEEE, you’ll know that those are two directives that limit the amount of lead that can be used to make any type of product, mainly electronics. This is because electronics are typically thrown away, filling landfills, and as they decompose, it is feared that they will leak lead into our drinking water underground. That’s really the main reason why lead-free solder was made. In terms of competition, that may have something to do with silver used in solder composition.

Silver bearing solder is pretty popular because it is said to flow better, has a lower melting point, is stronger, and has a higher conductivity2. It is also believed that having silver in a solder will create stronger joints, though there is no real data to support this. As mentioned above, this solder is made out of three elements and not two like other types of solder.  I’m not quite sure why, but from what I’ve read it seems to have to do with compatibility of different elements and the strengths they bring to the table.

Hard solder was mentioned earlier in this post. It is mainly used in brazing, silver-smithing or jewelry making, and melts at a much higher temperature1. It does not contain lead and is mainly made of metal. It is used in a lot of welding projects such as mechanical repairs, plumbing, and really anything that requires a tight hold that has high resistance.

 

Uses

I’ve basically covered or mentioned most of the uses solder has. Plumbing, mechanics, electrical repairs, making PCB connections, and soldering wire connections; off the top of my head, those are the projects that come together when I think of soldering. Are you limited by these? Of course not, you can use soldering for a lot of things, and there’s actually a lot of websites out there that show off arts and crafts that can be done with solder. Now that you know a little more about solder and the types there are, next time you have a soldering project you’ll have some ideas of what type to use for your needs and specifications.

This is just an informative blog; I didn’t want to get too involved with what you can do with solder. It feels limiting to just name off a few things. However, if my how-to’s have been getting your attention, maybe next month I’ll do a weekly how-to solder tutorial. Any comments are appreciated so I can hear back from my audience, what they think of this new blog style, and if they find my recent posts helpful.

 

1 http://en.wikipedia.org/wiki/Solder

2 http://store.curiousinventor.com/guides/how_to_solder/kind_of_solder/

3 http://www.aaroncake.net/electronics/solder.htm

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