Solder is a metal alloy with a relatively low melting point that’s used to create strong, electrically-conductive joints to bond with other metals.
It has many applications, but the type of solder you use in a project depends on its composition. This begs the question, what is solder made of?
Traditionally, solder is made of a mixture of tin and lead. However, there’s a current shift toward lead-free solder that uses various other metal alloys. There are also solder alloys with an added flux core to improve wetting and prevent metal oxidation.
So let’s delve deeper into what solder is and what it’s made of. Stick around!
What Is Solder?
When compared to other metals, solder has a low melting point, from 190 to 840 °F, due to the unique properties of the metal alloys used to make the solder.
Such a melting point range allows us to easily melt the solder with a soldering iron to connect different metals together. Depending on the alloys used, we can even maintain and improve the joined metal’s electrical conductivity.
The process of soldering is often referred to as either:
-Soft soldering; if the solder has a melting point between 194°F-842°F
-Hard soldering; if the solder has a melting point above 842°F
Brazing is another method of joining metal with solder. It’s commonly used with solder alloys that have an even higher melting point.
Solder has a wide range of applications. It’s commonly used with Printed Circuit Boards (PCBs) and other electrical applications. It’s also used in mechanical, plumbing, and aerodynamic applications, among other things.
Solder alloys are sold in a variety of shapes, the most common of which is a wire spool. Less common forms include ribbon solder alloys, solder foil, and solder rods. They’re also sold in thicknesses ranging from 0.01 inches to 0.250 inches.
What Is Solder Made Of?
Now that we know what solder is, let’s see what it’s made of that gives it its unique qualities. There are different types of soldering paste.
Lead-Based Solder Alloys
Until recently, almost all solder alloys were made with an alloy of lead and tin. Tin ensures that the metal alloy has a low melting point, while lead prevents tin whiskers from forming.
This low melting point, which is a little under 400 °F, makes lead-based solder alloys easy to work with and form strong bonds with other metals.
Any lead-based solder alloy is a mixture of varying ratios of tin and lead, such as:
-60% Tin, 40% Lead or Sn60/Pb40
-63% Tin, 37% Lead or Sn63/Pb37
-50% Tin, 50% Lead or Sn50/Pb50
-60% Lead, 40% Tin or Sn40/Pb60
They have a lower melting point compared to lead-free solder alloys
They’re the more reliable option compared to lead-free solder alloys. That’s why they’re still used in the aerospace and medical industries
Lead-based solder alloys flow better than their lead-free counterparts
Lead is a toxic substance. If ingested, it’s extremely dangerous. In some cases, even excessive exposure to it is harmful. What’s more, lead waste can contaminate the soil and groundwater
Lead-Free Solder Alloys
These solder alloys rely on metal alloys that are free of lead, such as copper, nickel, and silver. However, tin is still a staple in most of them.
The Shift to Lead-Free Solder Alloys
Because lead is toxic, there have been bans on lead-based solder, which shifted the attention toward using lead-free solder.
The European Union ban was first enacted in 2006 as part of the Restriction of Hazardous Substances Directive. However, some exceptions were made, such as the exemption for the aerospace and medical industry.
As for the U.S., the use of lead solder alloys in pipes or plumbing is banned. The amendment to the Safe Drinking Water Act that requires access to lead-free water set this in motion. This motion, however, applies only to plumbing and pipes.
In most cases, the use of lead solders is permitted in other productions, but manufacturers who switch to lead-free solder alloys may be eligible for tax benefits in the U.S.
Examples of Lead-Free Solder Alloys
Each metal in the alloy contributes a distinct characteristic to the mixture. Still, that doesn’t mean we should mix random metals, since mixing dissimilar metals may cause brittleness and corrosion.
96.5%Tin, 3% Silver, 0.5% Copper or SAC 305
This was the first lead-free solder ever introduced. The silver reduces oxidation of metals, and copper improves electrical conductivity. This solder melts at 422.6°F, and it’s still one of the more commonly used lead-free solder alloys.
99.25% Tin, 0.7% Copper, 0.05% Nickel, 0.01% Germanium or Sn100c
This lead-free solder performs similarly to lead-based solder alloys. The nickel prevents the formation of tin whiskers, and the germanium increases the surface tension. Its melting point is 440.6°F and it flows well.
Pros and Cons of Lead-Free Solder Alloys
Doesn’t expose you to harmful metals
Silver lead-free solder alloys form stronger bonds
More difficult to work with due to their higher melting point
Forms surface oxides and alloy deposits, which can impair electronics
More expensive than lead-based solder
The flux core acts as a solder-reducing agent. So regardless of whether they’re lead-based or lead-free, it’s common for solder in the form of wire spools to have a flux core.
This is because the solder wires are hollowed out, and the hollow is filled with flux. During the soldering process, the flux releases, improving the flow, electrical conductivity, and wetting ability of the metals. It also reduces oxidation.
The Two Types of Flux
Rosin Flux is commonly used with electronics like PCBs due to their relatively low corrosiveness. However, make sure to clean the residue with isopropyl alcohol to further prevent corrosion.
Acid Flux is commonly used in plumbing or metal joining, but because it’s highly corrosive, it should never be used with electronics.
So, what is solder made of?
The simple answer is that solder is made of metal alloy mixtures. The most reliable solder alloys are made from lead and tin, but because lead is hazardous, its use is no longer recommended.
Other solder alloys are made from silver, tin, copper, nickel, and a lot of other safe substances. It’s also common for them to be made with a flux core, which further improves the solder’s quality.
Keep in mind that while lead-free solder alloys don’t perform as well as lead-based solder, the risk of lead exposure isn’t worth it. However, if you must use lead-based solder, make sure to take all necessary safety precautions.
We have discussed the difference between soldering and welding here.