What is Low-temperature Solder Paste?

What is considered to be a low temperature solder paste? Why do we use low-temperature solder pastes?

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Editorial Team - PCB Directory

Jan 17, 2021

One of the most important parameters when selecting a solder paste is its melting temperature. A Low-temperature solder paste can be broadly classified as one that melts at a temperature below 180° C.

Low temperature soldering is required in a number of cases specially when dealing with heat-sensitive devices, parts with significant differences in their coefficients of thermal expansion, components exhibiting severe thermal warpage, or products with highly miniaturized design.

Low Temperature Solder paste is also required in step soldering where we need to solder components on a board that already as some components soldered on to it. Using low temperature solder paste to solder new components/joints will ensure that the previously soldered are not disturbed as this solder paste will melt at a lower temperature.

Many applications need a low-temperature solder that will reflow below 180° C. For example, LED attach, optics assembly, and MEMS mounting all of these require low temperature soldering. There are more than 100 metal alloys that melt below the temperature of the conventional Sn/Pb alloys.

But there are two metals, in particular, that help fulfill these needs. One is indium and the other is bismuth. Lead free solders like Tin / indium 52% and tin / bismuth 58% have significantly lower melting points than tin / lead 37% solder. The SnBiAg alloy system has a melting point of 138°C which enables a peak reflow temperature between 170 - 180°C. These low peak temperatures allow for soldering of thermally sensitive assemblies. Having said so, tin, bismuth, and lead can form an alloy that melts around 95°C. This could potentially lead to solder joint failure due to natural heating of the assembly during use. Therefore, we need to be careful when selcing these alloys to ensure that the melting temperature is not too low. Tin / bismuth alloys are generally used in combination with other lead-free tin-based alloys.

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