Solder paste is a combination of small metal solder particles and flux used in printed circuit board assembly & manufacturing. The purpose of solder paste is to create a stable bond between the plated connection pads on the PCB board and the electronic components being added to form a working electrical circuit. Size, composition, and flux density play significant roles determining the effectiveness of manufacturing process.
The exact solder paste reflow temperature is essential to ensure certain components do not excess their maximum temperature rating during assembly. Low-temperature solder pastes that melt at a heat below 180° C are used for such situations, as “typical” pasts generally reflow at 250° C or higher.
Low temperature pastes can also be used in conjunction with higher temperature ones for applications requiring two-stage assembly. That is, some may require components to be attached first, and then additional ones added in a second step. By using two different reflow temperature pasts, the parts attached in the first step with a higher temperature paste, can be unaffected by the second lower temperature application.
Solders generally melt between 90° C and 450° C.The melting point of solder is engineered via selection of the solder blend and the heat resistance power of the particular devices being attached.
|
Alloy |
Melt Range |
Peak Reflow |
| Sn63/Pb37 |
183C |
210-220C |
| Sn96.5/Ag3.0/Cu0.5 (SAC305) |
217-221C |
240-250C |
| Sn99.3/Cu0.7/Ni0.05/Ge0.005 (SN100C) |
227C |
245-255C |
| Sn89.0/Ag3.0/Bi0.5/In8.0 |
200-205C |
220-230C |
| Sn88.0/Ag3.0/Bi0.5/Cu0.4/In8.0 |
197-202C |
215-225C |
| Sn48/In52 |
118C |
140-150C |
| Sn42/Bi58 |
138C |
160-170C |
| Sn42/Bi57/Ag1.0 |
138-140C |
160-170C |
Tin/bismuth, tin/indium, or tin/bismuth/silver are the most common “low melting point” solders (150-170° C).
Two metals help to reduce the melting point, indium, and 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 160 - 170°C. These low peak temperatures allow for soldering of thermally sensitive assemblies.
For all your sensitive electronic assembly needs, including specialized processing such as low temperature applications, please send us your project requirements now, either through our online quote tool, or to orders@optimatech.net . We strive to produce the highest quality PCBs and assemblies to meet all our customer needs.
