How to Use Low-Temperature, Lead-Free Solder For Heat Sensitive Assemblies

Lead-free soldering is used to comply with environmental and safety regulations, like the original WEEE and RoHS initiatives. A low temperature solder process will deliver a lower total cost of ownership, while ensuring reliable joints during the soldering process. Lead-free, low temp solder might be useful in a through-hole step soldering, replacing two surface mount technology steps after wave soldering, so you can save on materials, cost, labor, and energy. This product technology may add value to your electronic assembly process, especially in the manufacture of heat sensitive assemblies. Here is a look at low-temperature soldering:

The process — Low-temperature, lead-free solder paste has an SnBiAg alloy system to meet the 138°C- melting point, which enables the peak reflow temperature ranging between 170°C and 180°C. It also meets reflow parameters. Despite the lower melting point of the lead-free alloy, it bears excellent resistance under accelerated thermal cycling tests, which are often administered in consumer electronics.

Some manufacturers of low temp solder have come up with a dual alloy SMT process, introducing a SAC alloy over the assembly. When boards are flipped, they are printed with the lead-free solder. Solder preforms may be needed to ensure proper hole-fill of plated through-hole components. To keep the process successful, solder paste is used as an alternative to the wave soldering stage.

When is the low-temperature solder necessary? — Low temp solder in the SMT process uses a low melting point alloy to remove the wave soldering step in the manufacturing of mixed technology assemblies. This way, it is possible to establish more value when beginning to implement the use of low temperature alloys. This can be critical in a multi-technology process where there are one or two SMT steps before a selective or wave soldering stage. With a dual alloy process, a low temperature surface mount technology process is introduced.

The SMT-only process encourages higher throughput and streamlined processes, and the possibility of enabling rework on select components, such as RF shields, for instance, must be soldered separately.