Solder Alloy Selection: SAC305, Low Temperature and High Temperature

The alloy in a solder joint is chosen once and then inherited by every product that follows. It determines the reflow temperature, the joint strength, the behaviour at high temperature and the cost, and the trade is different for each product.

What the Alloy Determines

The melting range sets the reflow temperature and therefore the components that can be used and the process window available. A higher melting alloy demands more heat and leaves less margin below the component limits.

The mechanical properties determine the joint’s resistance to fatigue and to creep. Creep matters at high temperature, where a joint that is strong at room temperature deforms slowly under load.

The alloy also determines the intermetallic compounds that form at the interfaces with the copper and the component finish. These compounds are necessary for a joint to form and are brittle if they grow too thick.

The Common Choices

A tin lead alloy, typically sixty three tin thirty seven lead, melts at a single temperature of one hundred and eighty three degrees Celsius and is the most forgiving alloy available. Its use is restricted by regulation in most consumer products.

SAC305 is the standard lead free alloy, with a melting range beginning around two hundred and seventeen degrees. It has good mechanical strength and a higher reflow temperature.

A low temperature alloy containing bismuth melts around one hundred and thirty eight degrees, which allows temperature sensitive components to be used and reduces the thermal load on the board. Its mechanical properties are weaker and it must not be mixed with lead containing alloys.

Mixing Alloys and the Consequences

A board that is soldered with a lead free alloy and then reworked with a tin lead alloy, or the reverse, produces a joint whose composition is a mixture. The melting behaviour of the mixture is not the simple average, and a low melting phase can form that melts far below either alloy.

Bismuth and lead are the dangerous pair. A small amount of lead in a bismuth containing joint produces a phase that melts at about ninety five degrees, and the joint can fail in service at a temperature the design never considered.

The rule is therefore that the alloy must be consistent across the process, including rework. The rework station, the hand soldering iron and any solder used for repair must use the same alloy family, and this should be stated in the process documentation.

High Temperature Applications

Where the product operates above about one hundred and twenty five degrees, the standard lead free alloys creep and the joints relax. The answer is an alloy with a higher melting point, such as one containing antimony or a high lead alloy.

The higher melting point requires a reflow temperature that many components cannot tolerate, so the choice of alloy interacts with the choice of components. The design must be made with both in view.

Where the assembly cannot tolerate the temperature, the alternative is a mechanical fixing that carries the load while the solder provides only the electrical connection. Our solderability notes describe how the alloy and the finish interact at the interface.

Intermetallic Growth

At the copper interface, a layer of copper tin compound forms during soldering and continues to grow slowly at operating temperature. The layer is needed for bonding and becomes a weak point when it is thick.

The growth rate depends on temperature and time, so a joint that experiences a long reflow profile and then runs hot accumulates a thicker layer. The layer is also harder and more brittle than the solder, so a crack tends to propagate along it.

This is the mechanism behind the degradation of joints in high temperature service, and it is why the maximum operating temperature is a joint design parameter rather than only a component parameter. Our solder defects notes describe how the failure appears in a section.

Contamination and Impurities

Small amounts of certain elements change the alloy significantly. Gold dissolves into solder and produces brittle joints at low concentration, and zinc is worse. Both arrive from plated finishes and from hardware that touches the molten solder.

The alloy in a solder pot accumulates these elements over time, which is why the pot composition is monitored. Our quality notes describe the sampling and the limits.

In paste, the impurities arrive from the powder and the flux, and the paste specification includes limits. A paste that is outside its specification produces joints that differ from the qualification sample.

Choosing for a Specific Product

The choice follows from four inputs: the maximum temperature the components will tolerate, the maximum temperature the product will see in service, the mechanical environment and the regulatory requirement.

A consumer product with a benign environment uses the standard lead free alloy. A product with a high service temperature uses a high temperature alloy and pays for it in component cost and process difficulty.

A product with temperature sensitive components and a benign environment is the case where a low temperature alloy is justified, and the restriction on mixing it with lead must be enforced. Our surface finish notes describe how the finish choice interacts with the alloy.

Additional Considerations for This Build

Practical attention to low temperature solder pays for itself here, because it decides whether the finished board behaves as the drawing intended. Where the requirement is not stated on the fabrication drawing or in the assembly notes, the shop has to assume a default, and that default is rarely the value the design was simulated with. Stating low temperature solder explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.

Process Control and Verification

On a design of this kind, intermetallic is the item that decides how the rest of the board is arranged. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance. Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance.

A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used. Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design.

FAQ

Can a lead free assembly be reworked with tin lead solder? It can be, and the result is a mixed alloy joint whose properties are not those of either. For a bismuth containing alloy it is not acceptable.

Is a low temperature alloy as strong as SAC305? No. Its fatigue strength and creep resistance are lower, which limits it to benign environments and to assemblies that are supported mechanically.

What does gopcb provide for alloy selection? We provide a selection matched to the component temperature limits and the service environment, process windows for the chosen alloy, analysis of pot and paste contamination against the specification, and sectioning of joints in high temperature service.