What is solder and how does it work?

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Solder is the material that provides a mechanical as well as an electrical joint between components and the circuit board on which the components are being assembled. A good electrical joint is important for the circuit to work and for the signals to pass through. A mechanical joint is important to keep in small electrical components in place.

In order to achieve these two types of joints, the solder should be of a robust material and be a good conductor of electricity at the same time. It must be resistant to the effects of oxidation and corrosion, which may compromise the joint over time. It must also be easily available, easy to use, and convenient to store. It should have reasonable operating parameters and must melt at a temperature which does not require special expensive equipment. In other words, it should melt at a reasonably low temperature so that it can be used in a basic workshop setting.

The materials used to make solder allow it to be produced in a wire form. Hence, solder is available commercially in varying gauges of wire-thickness. One can get a wire in the range of 18AWG to 22AWG quite easily. The lower the AWG number, the thicker the solder wire. Thinner solder is good for working with very small components and very fine dimensional designs, while thicker solder wires are good for making large joints.

Traditional Solder Composition and Properties

Traditional solders are made of two metals, tin and lead. The ratio is the mixture is often 60% tin and 40% lead. This 60/40 solder, as it is often known, melts at a temperature which is much lower than the equivalent temperature of its individual components tin and lead. In chemistry, there is a term for this, it is called a “Eutetic mixture”. It must be noted that the tin-lead solder is used in the electronics industry. Other industries use other types of solder. There are different types of solder with different ratios of metals.

Solder CompositionTensile Strength
(N mm-2)
Density (g cc-1)

Electrical Conductivity
(% IACS)
Pure Tin1210.213.9
96S (96% Tin, 4% Lead)5410.013.9
95A (95% Tin, 5% Lead)317.310.8
Sn63 (63% Tin, 37% Lead)678.511.9
Sn60 (60% Tin, 40% Lead)488.511.5
50EN (50% Tin 50% Lead)478.510.9
40EN (40% Tin 60% Lead)479.110.1
30 EN (30% Tin 70% Lead)499.39.3
20/80 (20% Tin 80% Lead)519.78.7
15/85 (15% Tin 85% Lead)497.58.5
HMP (5% Tin 95% Lead)367.28.0

The use of Flux in Solder

In addition to tin and lead, one more ingredient used in solder is called flux. Flux is an acidic mixture that assists in the removal of oxides in the area where the joint is being made with the solder. Flux is often made up of boric acid and denatured alcohol. Flux is the brownish liquid that you see ooze out when you heat the solder wire. The solder used in electronics has flux already mixed in the wire, while other solders such as the ones used in plumbing have a separate flux. Flux allows the liquefied solder to flow freely over the area of the joint and form a strong bond. Flux can sometimes give out a pungent odor and an irritating smoke fume. Hence, soldering must always be done in an area which is well-ventilated.

Lead-Free Solder

While the use of lead used to be acceptable a few decades ago, there is a lot of focus on sustainability in current times. Lead is not considered to be very environmentally friendly. Hence, there are now lead-free solder wires that are also available as an option. It may be an option in some countries, but in some other countries (especially in Europe), lead-based solder is banned for commercial sale.

Lead-free solder includes a wide variety of compositions. One of the most common is tin and a hint of copper. The ratio of tin to copper is 99.3% to 0.7%. Changing the composition of the solder from a traditional tin-lead make to a tin-copper make does not change the melting temperature by much. Lead-free solder melts at around 227oC.

There is a lead-free solder type which has silver in it. It is known as lead-free silver solder. The actual content of silver is very small. Reels of this silver solder are available in the markets in a similar way to reels of traditional solder wire. The melting point of silver-based solder is slightly lower than that of traditional solder.

Sometimes, bismuth is used along with tin, sometimes with the tin and lead combo. Bismuth lowers the melting point, something which is very useful when soldering mainframe computer parts. Hence the Bismuth-based alloy is used in low-temperature applications.

Indium is added to the solder alloy in order to solder non-metals such as glass, alumina, and mica. Indium based solder is also used for soldering gold. Gold tends to dissolve in the presence of tin. It does not dissolve as much in Indium.  Indium-based solder is vulnerable to corrosion.

Zinc may also be used in solder. It is very economical and can lower the melting point of the soldering alloy in which it is used. Zinc-based solder is great for soldering aluminum. It is susceptible to oxidation and corrosion. Hence, zinc is not a good choice for wave soldering. A solder paste with Zinc will have a much shorter life than a paste without Zinc.

At times, Antimony is added to increase strength without affecting wettability. It also helps prevent tin pest. Antimony-based solder is used in the plumbing industry because it can provide good resistance to thermal fatigue while offering good shear strength. Antimony-based alloy is also used for soldering iron and steel.

Properties of commonly used solder compositions

MaterialMelting Point
(oC)
Thermal Conductivity
(W/m-k)
Aluminum660.1240
Copper1083393-401
FR-4
(fiberglass epoxy mixture)
n/a1.7
Lead327.335.0
Pb-5Sn31063
Sn-2.5Ag-0.8Cu-0.5Sb215-21757.26
Sn-2.8Ag-20.0In175-18653.5
Sn-37Pb18350.9
Sn (Tin)231.973.0