Low Temperature Solder Alloys Compared With SAC305

A lead free assembly is normally soldered with an alloy of tin, silver, and copper that melts at about two hundred and seventeen degrees Celsius, which forces a peak reflow temperature above two hundred and forty. That temperature is close to the limit of many laminates and it is well above the limit of some components. A low temperature solder alloy melts between about one hundred and thirty eight and one hundred and seventy degrees, which allows the same assembly to be built with a much cooler profile and a much smaller thermal load.

This article explains what the low temperature alloys are, what the cooler profile buys, what it costs in reliability, and how a mixed alloy joint behaves.

What The Alloys Are

The common low temperature family is based on tin and bismuth. A tin bismuth alloy at fifty eight percent bismuth melts at one hundred and thirty eight degrees, which is the eutectic of that system, and the addition of a small amount of silver raises the melting point slightly and improves the mechanical behaviour. Indium containing alloys melt lower still, at around one hundred and seventeen degrees, and are used for temperature sensitive assemblies, while an alloy of tin, silver, copper, and a few percent of bismuth reflows near two hundred degrees and represents a compromise rather than a low temperature solution.

The alloy datasheet gives a solidus and a liquidus temperature rather than a single melting point, and the difference between them is the pasty range in which the joint is partially liquid. A narrow pasty range is important for a reflow process, because a joint that is partly solid while the board is cooling can be disturbed by vibration or by differential movement, which produces a rough or cracked fillet.

<img src="https://www.gopcba.com/wp-content/uploads/2026/08/Turnkey-PCB-Assembly.jpg.webp" alt="Low temperature reflow profile plotted against a SAC305 profile” />

What The Cooler Profile Buys

The first benefit is a lower risk of damage. A peak below two hundred degrees keeps the assembly further from the glass transition temperature of the laminate, which reduces warpage and the associated defects in fine pitch arrays. It also reduces the thermal stress on plastic packages, on optical components, and on parts that are rated for a lower reflow temperature, and it reduces the growth of the intermetallic layer at every joint, which is one of the mechanisms that limits the life of a solder joint.

The second benefit is process latitude. A cooler profile heats and cools a board faster in absolute terms for a given oven length, reduces the energy consumed, and reduces the thermal mass that the oven has to control. For a thin board or a module that carries a large metal stiffener, the reduction in warpage alone can justify the change, because the board stays flat through the process rather than bending at the peak.

What It Costs In Reliability

Bismuth makes a solder joint harder and more brittle than a tin silver copper joint. The brittleness of the alloy is the price of the lower melting point, and the consequence is a reduced resistance to mechanical shock and to drop, and an area array joint that is cycled mechanically can crack at the interface between the solder and the copper pad rather than deforming. Where an assembly must survive an impact, that difference has to be evaluated in a drop test rather than assumed from a datasheet.

The low melting point also sets a service temperature limit. A joint that melts at one hundred and thirty eight degrees cannot be used in a product that will operate near one hundred degrees, and the derating has to allow for the self heating of the components as well as the ambient. The thermal fatigue behaviour also differs, and the alloy has to be qualified by cycling the actual assembly rather than by comparing handbook figures.

Contamination And Mixed Alloys

Contamination matters more with a bismuth alloy than with a conventional one. Lead forms a low melting phase with bismuth, and a joint that contains both can have a melting point well below either, so a leaded surface finish or a leaded component termination introduces a phase that softens at a temperature the product may reach. The same concern applies to a board that was previously soldered with a leaded alloy and is then reworked with a bismuth material.

The mixed alloy situation is the other important case. When a low temperature paste is used to attach a ball grid array whose spheres are a tin silver copper alloy, the joint that forms is not uniform: the ball remains a higher melting alloy and the paste forms a lower melting phase around it. The joint is sound as made, but it has a low remelt temperature in the paste region, which means a subsequent reflow, a rework step, or a hot service condition can soften it again. The result has to be evaluated as a new joint, and the process window for any second reflow has to be set from the remelt temperature of that mixed structure rather than from the original alloys.

Process Considerations

A low temperature profile still needs a proper soak and a proper flux. The peak is between about one hundred and seventy and one hundred and ninety degrees, so the flux has to activate at that temperature rather than at the higher temperature it was designed for, and a paste formulated for a tin silver copper profile may not perform correctly. The soak length, the ramp rate, and the cooling rate all have to be tuned to the alloy, and the profile is normally developed with a thermocouple on the board rather than from the oven settings.

Inspection criteria also change. A bismuth alloy joint has a dull, slightly grainy appearance, and a criterion written for the shiny fillet of a tin silver copper joint will reject sound joints. The acceptance description in the work instruction has to be written for the actual alloy, and where a joint is reworked, the soldering iron temperature and the alloy of the added wire have to be consistent with the original joint. Electroless nickel and gold finishes are common in these assemblies because they tolerate the lower temperature without the surface changes that a hot air levelled finish shows.

Where The Choice Fits

The alloy is a good fit for a product that is temperature sensitive, thin, or mechanically constrained, and for a manufacturing flow that benefits from a fast profile and a lower energy cost. It is a poor fit for a product that will see impact, high vibration, or a high operating temperature, unless the qualification tests show otherwise for that specific assembly and geometry.

The decision is therefore a system level one, made with the reliability requirement, the component ratings, the laminate, and the service environment in view. The alloy families and their processing are compared under lead free and leaded soldering, the assembly consequences under component shift during reflow, and the fabrication context under PCB design and fabrication.

FAQ

Why is bismuth used in a low temperature alloy? Because it lowers the melting point of tin substantially while remaining compatible with a reflow process. The cost is a harder and more brittle joint than a tin silver copper alloy produces.

Can a low temperature alloy be reworked with a normal iron? It can, but the temperature has to be set for the alloy and the added wire has to match it. A hot iron and a higher melting alloy produce a joint that is a mixture of two materials.

What happens when a low temperature paste meets a SAC ball? The joint is not uniform. The ball stays at its own melting point while the paste forms a lower melting phase, which can soften again during a later reflow or in a hot application.