Lead-Free Solder and Tin-Lead Assembly: What Changes

Assembly with tin-lead solder and assembly with lead-free solder look like the same process with a different alloy, and they are not. The alloy melts at a different temperature, which changes the entire thermal profile; the joints that form behave differently under stress; the flux chemistry needed to make the joint is different; and the materials on both sides of the joint have to tolerate conditions they were not originally designed for.

The Alloys and Their Melting Points

The traditional tin-lead alloy used in electronics is close to the eutectic composition, which melts sharply at around 183 °C. It is forgiving: the joint passes from solid to liquid over a narrow range, wets readily and produces a bright, smooth fillet.

The common lead-free alloys are tin-based with small additions of silver and copper. Their melting point is roughly 217 °C, around thirty degrees higher, and they do not melt quite so sharply. That thirty-degree difference propagates through everything downstream of the alloy choice.

Consequences for the Reflow Temperature

Because the alloy melts higher, the process must run hotter. A tin-lead profile peaks somewhere a little above the alloy’s melting point; a lead-free profile peaks around 235 to 245 °C to ensure the alloy fully melts and wets before cooling.

The higher peak changes several things at once. The process window narrows, because the upper limit is set by what the components and the laminate can survive and the lower limit by what the alloy needs, and there is less room between them. Components specified for a leaded process may not be rated for the higher temperatures, which makes the maximum temperature rating a selection criterion rather than a footnote. The laminate is heated closer to its transition and decomposition temperatures, which is why the material choice for a lead-free assembly has to account for the process temperature rather than only the operating temperature.

Flux chemistry follows the temperature. Exposed to more heat for longer, the surfaces oxidise more, so the flux has to be more active to keep the surfaces clean enough to wet. That activity has consequences for residue, for the cleaning process and for the risk of leaving corrosive material on the board, and the compromise between activity and residue is one of the reasons lead-free processes are less forgiving of poor storage and out-of-date material.

What Changes in Joint Reliability

A finished lead-free joint is different from a tin-lead joint in ways that matter both to inspection and to reliability.

Mechanically, the lead-free alloy is harder and less ductile. The practical consequence is that it resists fatigue better under thermal cycling, which is an advantage in applications with repeated temperature excursions, but it accommodates mechanical strain less well, which makes joints more sensitive to bending of the board and to mechanical loads on connectors.

Visually, the joint is duller and has a grainier surface than the bright, smooth fillet of tin-lead. This matters because appearance is one of the first things an inspector uses, and a lead-free joint that is perfectly sound can look less acceptable than a leaded one that is not. The wetting angle also differs, so a fillet that looks slightly more convex than expected may still be correct. Inspection criteria for lead-free assemblies therefore have to be stated in their own terms rather than inherited from leaded ones.

Materials and Compatibility

Two compatibility questions arise, and they point in different directions.

The board has to tolerate the process. That affects the laminate choice, as noted above, and it affects the surface finish on the copper, since some finishes degrade at the higher temperatures and others are specifically designed for them. A finish chosen for a leaded process may not be the right one for a lead-free assembly.

The components have to be compatible as well. A component whose terminations are finished with a lead-free coating is suited to a lead-free process. A component with a traditional tin-lead finish can be soldered with lead-free paste, because the small amount of lead from the termination dissolves into the joint without changing its behaviour in a way that matters. The reverse situation — lead-free terminations soldered with tin-lead paste — is the one that has to be considered more carefully, because the melting point of the joint is affected by the mixture and the result depends on the proportions involved.

What follows for the design is that the bill of materials and the process have to be stated together. A board built with an alloy that the components were not specified for is a risk that appears later, at a joint that has to be reworked or in a field failure that cannot be explained by the design.

Cost

The alloy itself costs more than tin-lead, and the processing costs more for several reasons. Higher temperatures consume more energy and shorten the life of equipment components such as heaters and conveyor parts. The flux chemistry is more expensive. Solder pots and tools need to be dedicated, since contamination between alloys is not acceptable. Rework is more demanding, because more heat has to be applied to the same component and the risk of damaging it is higher.

The total increase is usually quoted in the range of twenty to thirty per cent for the assembly step. What has to be balanced against that is the market access the process provides, since many jurisdictions restrict the use of lead in electronic products — the RoHS directive among them — and products that cannot be placed on those markets at all are not cheaper at any price.

The regulatory position is not uniform, and some applications — certain high-reliability, medical, aerospace and defence products — remain permitted to use tin-lead alloys under exemptions. Where an exemption applies, the argument for leaded assembly is usually mechanical reliability in environments with severe thermal cycling or high vibration, where the ductility of the leaded joint is an advantage.

What the Design Has to State

Three items belong in the fabrication and assembly data rather than in a conversation.

The alloy and the process, stated explicitly, since they determine the profile, the finish and the component temperature ratings. The maximum temperature the components and the board will see, so that the parts can be checked against it before the order is placed. And the inspection criteria that apply, expressed for the process being used, because applying leaded criteria to a lead-free joint produces rejections of sound work and acceptances of joints that are not sound.

The preferred sequence and the defects each stage can produce are covered in the article on common soldering defects, and the process controls that keep the result consistent are described in our overview of SMT PCB assembly and the quality management behind it.

FAQ

Why does lead-free assembly need a higher temperature? Because the alloy melts around thirty degrees higher than tin-lead, and the profile has to exceed that by enough to ensure full melting and wetting.

Are lead-free joints less reliable? Not in general. They resist thermal fatigue better and are less tolerant of mechanical strain, so the answer depends on what the product experiences.

Can leaded and lead-free materials be mixed? Lead-free terminations with lead-free paste is the standard case. Mixing in the other direction, or mixing alloys in a pot, changes the melting behaviour and should only be done where the combination has been defined and validated.