Low Temperature Solder Paste
Low temperature solder paste is becoming more common in surface mount assembly. Certain assemblies cannot handle the temperatures used for lead-free soldering which typically reaches 240 – 250 °C. Sensitive components might be damaged by these high temperatures. High temperatures can also cause warpage and other damage to the assembly. Low temperature solder alloys can be used in many forms including solder paste, wire solder, pre-forms, and bar solder. Low temperature solder paste is the focus of this page.
Lead-free solder alloys have high melting points as compared to leaded solder. The process temperatures required to solder with tin-based lead-free alloys are challenging for some assemblies. Indium and bismuth can be used to lower the melting points of tin-based solders. The melting ranges and peak reflow temperatures of several solder alloys are shown in the table below.
Tin / indium 52% and tin / bismuth 58% are lead free solders that have substantially lower melting points than tin / lead 37% solder. Tin / bismuth and tin / bismuth / silver are the most common choices for low melting lead free solder pastes. The peak reflow temperatures provided by tin / bismuth alloys are low (160 – 170 °C) as compared to tin / lead solders (210 – 220 °C). These low peak temperatures allow for soldering of thermally sensitive assemblies.
Caution must be used when using tin / bismuth alloys. It is dangerous to mix tin / bismuth with lead containing alloys. Tin, bismuth, and lead can form a very low melting combination that melts around 95 °C. This could potentially lead to solder joint failure due to natural heating of the assembly during use.
Tin / bismuth alloys are safe to use in combination with other lead free tin-based alloys. In some cases surface mount assemblies are made with SAC305 on the first side and tin / bismuth on the second side. The low reflow temperature for tin / bismuth minimizes intermetallic growth in the SAC305 solder joints and allows for soldering of thermally sensitive components.
Tin / bismuth solder pastes are readily available from many sources. FCT Assembly offers a no clean tin / bismuth solder paste called NC722. Please contact your FCT Assembly representative for more details.