Solder

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Summary: Solder is used to join two or more wires together for electrical connectivity, apply detail parts brass locomotives, and fasten rail to printed circuit board ties while hand-laying track.

Selecting Solder

Solder on a roll, and in a dispenser.

Solder for electrical and electronic applications comes in several alloys. See the table below for more details.

The most common is tin/lead (Sn/Pb). The lead component is being phased out due to the toxicity of lead, in particular its oxides. These solders are labelled as "lead free".

The ratio of the alloy is important, as it determines not only the melt point, but also the freeze point. For electrical work, the most common solders are 60/40 and 67/37. The 60/40 alloy is a little cheaper, but the 63/37 solder is a little easier to use. The difference is the 63/37 is eutectic, meaning its freeze and melt points are almost the same. The plastic range of 60/40 is much larger. While in production soldering this isn't an issue, when soldering with an iron the very small plastic range of 63/37 makes it much easier to get a good joint. If the work being soldered moves during the plastic state, a cold solder joint can result.

The price difference between the two is not that great, the additional expense of 63/37 is worth it. For certain applications, a solder containing a small amount of silver (Ag) is desirable, as the joint will be stronger and more resistant to heat.

Flux Core.

For electrical soldering, the only type of flux to use is Rosin Flux. Most solders sold for the purpose have a rosin core. Light gauge solders may not incorporate rosin flux, as the wire is too small. For these solders an additional flux is required.

Gauge

Solder comes in a number of forms, such as bars and wire. With wire solder, the gauge is important. Heavy gauge solders are not really suitable for electrical work, as you can add too much solder to the work. For electrical work, especially when soldering fine gauge wires to small pads or together, a light gauge solder is better, as it allows more control over the amount of solder used. For most purposes, the amount needed is very small. This becomes more important if the desired result is to be as inconspicuous as possible.

Components of Solder

Most solders for electronic and other purposes are an alloy of lead (Pb) and tin (Sn). Some solders also contain Silver (Ag), and others are lead free.

The solder used for electrical work is a tin/lead alloy. Tin has a melting point of 232° Celsius, and lead melts at 325.5° C. When alloyed 60/40 (SnPb) the melting point is 188° C. The most common solder alloy used for electronic/electrical work is 63/37 SnPb, which is a eutectic solder, meaning its melt/freeze point is 183° C, it also has the lowest melting point found in Tin/Lead alloys.

Some solders contain Silver (Ag) which is used to reduce the erosion of gold or silver plating, or for joints which have to withstand higher temperatures. Some silver solders are also used with integrated circuits during manufacturing to solder microprocessors and other ICs to the PCB. Silver solders require more heat, and make a stronger joint as well.

(De)soldering a contact from a wire.

Soldering, brazing, and welding are techniques used to join metals together. Of the three, soldering is arguably the weakest. It is strong enough for the needs of Digital Command Control wiring.

What is Solder?

60-40 Solder.jpg

Solder is an alloy of Tin (Sn) and Lead (Pb) for electrical connections, although the lead component is being phased out because the lead ends up in landfills and can leach into the ground water supplies. The melting point of solder depends on the metals involved and their ratios. Lead free solders have higher melt points.

Pure tin melts at 232°C while lead melts at 328°C . An alloy of 63% Tin and 37% Lead is ‘’’Eutectic’’’, which melts and freezes at 183°C, the lowest temperature soldering alloy composed of just tin and lead. Yes, the temperature at which the solder melts is BELOW the temperature at which either component will melt! The more lead, the higher the melting point; adding tin lowers the melting point.

A 60Sn Alloy has a working range of 183-238°. There is a lower temperature alloy Sn43Pb43Bi14 has freeze/melt temperatures of 144-163°.

Some solders include Silver (Ag), which have a much higher melting temperature, and some formulations include other metals. Only Tin/Lead solders should be used for electrical connections. Silver based solders are used for other purposes where strength is important, such as track work and specifically, soldering the switch rails to the throwbar when making your own turnouts. The only application of silver solder for electrical purposes is where there will be a lot of heat, enough to melt solder alloys used for electrical work. Some examples: connecting wires to a heating element, or a shunt (a device used to measure large currents.) The additional cost of silver solder isn't justified for typical electrical soldering.

Rosin core electrical solder.JPG

A 60/40 or preferably 63/37 tin/lead solder should be used for electronics, and only Rosin Flux core solders. Solder is available in a solid wire, or with a rosin (flux) core. Solid wire solder requires flux, or it will not work. For electrical work, a rosin core solder is preferred.

A 60/40 solder is a little cheaper than 63/37 solders. For volume soldering, the cost may make enough of a difference.

The rosin flux removes the surface oxidation of the metals, allowing them to bond cleanly. Flux should be cleaned from the work after soldering using a flux remover.

Rosin Core Solder, such as those alloys in the table below, and related fluxes, have a shelf life of three years from date of manufacture.

Solder Alloys for Electrical and Electronic Applications

Alloy Freeze/Melt, °C Eutectic Comments Sn Pb Ag
Sn60Pb40 183/190 No Sn60, ASTM60A, ASTM60B. Common in electronics, most popular leaded alloy for dipping. Low cost and good bonding properties. Used in both SMT and through-hole electronics. Rapidly dissolves gold and silver, not recommended for those. Slightly cheaper than Sn63Pb37, often used instead for cost reasons as the melting point difference is insignificant in practice. On slow cooling gives slightly duller joints than Sn63Pb37. 60 40 -
Sn63Pb37 183 Yes Sn63, ASTM63A, ASTM63B. Common in electronics; exceptional tinning and wetting properties, also good for stainless steel. One of the most common solders. Low cost and good bonding properties. Used in both SMT and through-hole electronics. Rapidly dissolves gold and silver, not recommended for those. Sn60Pb40 is slightly cheaper and is often used instead for cost reasons, as the melting point difference is insignificant in practice. On slow cooling gives slightly brighter joints than Sn60Pb40. 63 37 -
Pb92Sn5.5Ag2.5 286/301 No For higher-temperature applications. 5.5 92 2.5
Sn62Pb36Ag2 179 Yes Sn62. Common in electronics. The strongest tin-lead solder. Appearance identical to Sn60Pb40 or Sn63Pb37. Crystals of Ag3Sn may be seen growing from the solder. Extended heat treatment leads to formation of crystals of binary alloys. Silver content decreases solubility of silver, making the alloy suitable for soldering silver-metallized surfaces, e.g. SMD capacitors and other silver-metallized ceramics. Not recommended for gold. General-purpose. 62 36 2
Pb97.5Ag1.5Sn1 309 Yes Ag1.5, ASTM1.5S. High melting point, used for commutators, armatures, and initial solder joints where remelting when working on nearby joints is undesirable. Silver content reduces solubility of silver coatings in molten solder. Not recommended for gold. Standard PbAgSn eutectic solder, wide use in semiconductor assembly. Reducing protective atmosphere (e.g. 12% hydrogen) often used. High creep resistance, for use at both elevated and cryogenic temperatures. 1 97.5 1.5
See note above regarding shelf life of solders.

(Reduced version of the table found on Wikipedia)

One source (http://www.tpub.com/neets/book4/12n.htm) indicates that soldering is more than just melting-there is a chemical change that occurs as well.