Tin-Lead Solder Alloys: 63/37, 60/40, 50/50 and More

Tin melts at about 232 °C and lead at about 327 °C, yet a mix of roughly 63 % tin and 37 % lead melts at 183 °C, lower than either. That point is the eutectic. An alloy of exactly that composition goes from solid to liquid in one step, with no mushy range.

Move away from 63/37 in either direction and the alloy melts over a range. 60/40 starts melting at 183 °C and is fully liquid at about 190 °C. 50/50 runs from 183 to about 216 °C, and 40/60 from 183 to about 238 °C. During that pasty range the joint must not move, or it freezes with a rough, cracked surface known as a disturbed joint.

• 63/37: fine electronics and PCB work, where a sharp freeze prevents disturbed joints and keeps heat on components low
• 60/40: general electrical and electronics soldering, cable lugs, tinning wires; slightly cheaper than 63/37 and nearly as easy to use
• 50/50: general sheet metal, radiator and roofing work, and non-potable plumbing
• 40/60 and 30/70: wiping joints and filling, where the long pasty range lets the solder be shaped
• High-lead solders such as 10/90 and 5/95: high-temperature joints, since they melt above 260 °C

The EU RoHS Directive (2011/65/EU) limits lead to 0.1 % by weight in each homogeneous material of electrical and electronic equipment placed on the EU market. India's E-Waste (Management) Rules, 2022, in force from 1 April 2023, require producers of covered electrical and electronic equipment to meet the same kind of limits, including 0.1 % for lead. So tin-lead solder cannot be used in most new consumer and IT products sold in India or Europe.

Leaded solder remains legal for repair and maintenance of older equipment, for many industrial uses outside the scope of those rules, for some exempted applications, and for non-electrical work such as radiators, sheet metal and stained glass. Before switching or continuing, check which rule applies to your product, and keep leaded and lead-free solder, tips and wire clearly separated on the shop floor.

The usual lead-free replacement for electronics is SAC305, which is 96.5 % tin, 3.0 % silver and 0.5 % copper and melts at about 217–220 °C. It needs higher iron temperatures, wets more slowly, leaves duller joints and costs more because of the silver. For plumbing, tin-copper and tin-silver alloys are used. Tin-lead remains easier to work with, which is why many repair shops still prefer it where it is permitted.

Solder only bonds to clean metal, and flux is what removes the oxide film as the joint heats. Rosin-based fluxes are used for electronics because their residue is not corrosive. Acid fluxes based on zinc chloride work well on sheet metal, brass and galvanised steel, but their residue is corrosive and must be washed off; never use them on electronics or fine wire. Most tin-lead problems we hear about, dull grey joints that crack or do not stick, turn out to be dirty surfaces or the wrong flux rather than bad solder.

Do not use tin-lead solder on pipes that carry drinking water. Lead can leach from the joint into water standing in the pipe. Many countries ban it for potable plumbing; in the US, solder for drinking water systems must contain no more than 0.2 % lead. Use a lead-free plumbing solder there, and keep 50/50 for drains, heating systems and other non-potable lines.

Tell us the ratio, the form (bar, ingot or wire), the quantity and whether you need flux-cored wire or solid. If you melt and cast your own solder, you may also want tin ingots and lead sheet or pig lead separately. For tinning copper conductors, compare with our tinned copper range. The test certificate we provide, issued through a Govt. recognised test house, states the actual tin and lead content of the lot.