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In simple terms solder paste is solder in paste form. Solder paste is an alloyed metal which when heated, liquefies and melts to flow on to solder pads or into the space between two close fitting parts in the case of industrial soldering.
There are several different heating methods like hot air soldering, furnace soldering, resistance soldering and torch soldering. Paste doesn't work well with a soldering iron because the paste is attracted to the tip and doesn't flow where you want it to go. Paste is also much easier to use because there are tools that dispense paste making repetitive tasks less daunting.
Solder paste is made from a solid alloy or multiple alloys that get melted down. Once the alloy is molten, it gets finely atomized with air to separate the alloy into particles. Once the particles cool down, they are put through a screen to separate by mesh size. Then a flux and a binder would be added to create the final paste-like consistency. The binder keeps the paste from separating and the flux cleans the surface so the alloy will bond to it.
There are three well known types of solder paste T3, T4, and T5. Most people who work with solder recognize T3 is the most commonly used in electronics but T5 is becoming more popular with fine pitched components. Each of these pastes are different based on the size of solder particles.
Type | Powder Size (um) | Minimum Needle Size | Mesh Size |
1 | 75-150 | 18 | -100 |
2 | 45-75 | 18 | -200/+325 |
3 | 25-45 | 23 | -325/+500 |
4 | 20-38 | 25 | -400/+635 |
5 | 10-25 | 27 | -500 |
Industrial solder paste
Standard or industrial solder paste alloys can includes tin, lead, silver, bismuth, copper and aluminum. These are used in many industries for a wide range of applications. Some of the common uses are costume jewelry, lighting, medical, automotive, air conditioning, heat sinks. Typically industrial pastes are T1 or T2. These pastes have very different flux types which are usually acid based and should not be used on circuit boards. When using an iron or torch to melt the paste, you want to try to heat the parts and let the solder flow to the joint for best results.The industrial pastes are usually corrosive and will need to be washed after soldering.
Solder paste consists of solder particles in flux with a binder that turns the solid metal into a flexible liquid, making it easier to do specific jobs. The flux and binder burn off when the paste is heated leaving your with a reliable solder joint on your parts.