Low Temperature Solder Alloys and Their Behaviour

A low temperature solder alloy melts well below the standard lead free range, which allows an assembly to be reflowed at a temperature that the components and the laminate can tolerate more easily. The benefit is real and the trade offs are equally real, because the alloy that melts lower also behaves differently in service. This article explains what changes and where the substitution is appropriate.

Why Lower the Reflow Temperature

The peak temperature of a reflow profile is set above the alloy melting point with a margin for thermal mass differences across the board. Lowering the alloy melting point lowers the whole profile, which reduces the thermal load on every component, reduces the warpage of the package and the board, and reduces the intermetallic growth at the interfaces.

The benefit is largest for assemblies with a wide spread of thermal mass, for thin and large boards that warp at high temperature, and for components with a limited thermal rating. It is also useful for a second reflow on a double sided assembly, where the first side has already been exposed to a full profile.

Melting Range and Alloy Families

Low temperature alloys are typically based on tin with bismuth and often with additional elements to adjust the melting behaviour and the mechanical properties. The melting point can be lowered by a considerable margin compared with the standard lead free range, which is what makes the lower profile possible.

The melting range matters as much as the melting point. An alloy with a wide pasty range leaves a joint that is partially liquid over a range of temperatures, which can be disturbed during cooling and which behaves differently under thermal cycling. A narrow range is preferred for a consistent joint, and the choice should be based on the measured range rather than the nominal melting point.

Bismuth Content and Its Consequences

Bismuth lowers the melting point efficiently, and it also changes the mechanical behaviour. A bismuth containing joint is stronger at low strain rate and more brittle under impact, and its behaviour changes with the bismuth content and with the presence of other elements. The alloy is therefore not simply a lower melting version of the standard one.

Bismuth also interacts with other alloys. Where a bismuth containing joint is later mixed with a lead containing alloy, the mixture can form a low melting phase that liquefies at a temperature far below either alloy. That is why the use of a low temperature alloy has to be recorded and controlled, because a later repair with the wrong wire can create a joint that melts during normal operation. The alloy compatibility considerations are described alongside solder joint acceptance criteria.

Mechanical Behaviour and Thermal Cycling

The mechanical properties at temperature are the main limitation. A low temperature alloy is operating closer to its melting point in absolute terms, so creep is more significant and the joint deforms more readily under a sustained load. A joint that is adequate at room temperature may be marginal in a product that runs warm.

Thermal cycling behaviour depends on the alloy and on the joint geometry. Some low temperature alloys perform well in accelerated cycling because their lower modulus reduces the stress per cycle, while others fail earlier because their strength is lower. The comparison should be made by test on the actual assembly rather than by extrapolation, following the approach used for thermal cycling test design.

Process Differences

A lower profile is not simply a scaled version of the standard one. The flux has to activate at a lower temperature and clean the surfaces in a shorter time, and the paste chemistry is usually matched to the alloy rather than interchangeable with a standard paste. The stencil and the paste volume also change, because the alloy flows differently and the joint geometry can differ.

The cooling rate matters more at a lower peak, because the margin above the melting point is smaller. A slow cooling can leave the joint in the pasty range for longer, which increases the risk of a disturbed joint. The profile should be developed with thermocouples on the assembly and verified on the actual product.

Reliability and Qualification

Qualification has to cover the conditions the product will see. A low temperature joint should be tested in thermal cycling, in a sustained high temperature test and in any mechanical test that represents the application. The results should be compared against the standard alloy on the same assembly, so that the trade is quantified rather than assumed.

The thermal limit of the joint is the temperature at which the mechanical behaviour becomes unacceptable, and it is usually well below the melting point. Where the product’s internal temperature approaches that limit, the low temperature alloy is the wrong choice regardless of the process benefit.

Where the Trade Is Worth It

The substitution makes sense where the process benefit is large and the thermal demand is moderate. A thermally sensitive module, a double sided assembly, a large thin board that warps and a product that will never run hot are all reasonable candidates. A power product, a product with a wide temperature range or one that will be reworked with a different alloy are not.

The gopcb engineering team decides on a product specific basis, comparing a low temperature process against the standard one on the same design. The comparison covers the profile, the joint appearance, the void content and a thermal cycling result, and the decision is recorded with the alloy so that a later repair or a second source does not introduce an incompatible material.

FAQ

Is a low temperature joint weaker at room temperature? Its behaviour differs and it is more sensitive to load and strain rate. It is not simply weaker, but the design should not assume the same margins.

Can a low temperature joint be repaired with standard wire? Mixing alloys can create a low melting phase. The repair material should be chosen with the original alloy in mind, and the mixture effect should be evaluated.

Is the paste the same as a standard paste? No. The flux has to be active at the lower temperature, so the paste is normally matched to the alloy.