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What do you think of low temperature lead free solder paste?

The use of low temperature lead free solder paste is becoming more common within the surface mount industry. Certain components cannot handle the peak temperatures necessary for lead-free soldering which typically reaches 240 – 250 °C.  These high temperatures can also cause damage to the PCB material such as delamination.  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.
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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.

Low Voiding

Void formation under components with large soldering planes and low stand-off heights, such as DPAK components, is a well-known phenomenon. These air pockets can reduce mechanical strength as well electrical and thermal conductivity.  For Sn(Ag)Cu-based alloys, voiding levels are typically in between 10% and 35% but can be greater than 50%.  By using low temperature lead free solder voiding levels can be reduced to less than 10% as can be seen below - click here for more details on voiding.

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High level Voiding
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Low level Voiding

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