How to use solder paste
- tutorial, information, article about what is solder paste, how to use solder paste and gain the most from the PCB assembly process.
Solder paste is a form of solder that is used in PCB assembly, and including prototype PCB assembly, especially when using reflow soldering techniques. The solder paste is a mixture a solder spheres and a specialized form of flux. This means that the solder forms a form of paste that can be printed onto the surface of the printed circuit card in the required There are a number of types of solder paste, but that used for electronics manufacture is a form that can effectively be "printed" onto the surface of the electronic printed circuit board in the required place.
Solder paste basics
The solder particles are a mixture of solder. Traditionally this used to be tin and lead, but with the legislation being introduced around the world, there is a move to lead free solders. These may be made from a variety of mixtures. One is 99.7% tin and 0.3% copper, whereas there are other mixtures that include other metals including tin.
Solder paste storage
In order to ensure that the solder paste is suitable for proving the highest performance for PCB assembly it is necessary to ensure that it maintains the required properties. To achieve this it is imperative that the solder paste is stored correctly. It should always be stored in an airtight container to prevent oxidation. The very large surface area of the minute solder spheres, means that oxidation can present a very great problem.
Additionally, the solder must be stored at low temperatures. Not only does this reduce the rate of any oxidation there may be, but it also reduces the rate at which the flux degrades. While a low temperature is imperative, it should not be stored at a temperature below freezing.
In view of the way in which solder paste can degrade, it also has a defined shelf life and it should not be used after its end date. If old solder is used there is a distinct risk of a much higher defect rate, and the cost of any rework incurred would be well beyond the cost of replacing the solder paste.
How to use solder paste
When solder paste is used in mass PCB assembly as well as prototype PCB assembly there are a number of stages that are undertaken. First solder paste is applied to the printed circuit boards. The solder paste is only applied to the areas where solder is required. This is achieved using a solder paste stencil that only allows the solder paste through in certain areas. (Further information is available on another page in this section of the website).
Once the solder paste has been applied to the printed circuit board, it is then passed into the pick and place machine where the components are added. The solder paste has sufficient tension that it holds the components in place. However care should be taken not to knock the board at this stage otherwise the components may move of fall off. Additionally the board should be soldered within a few hours of being placed, otherwise the solder paste may deteriorate.
Solder paste is widely used in PCB assembly - both in mass production and also for prototype PCB assembly. When used with care it enables very high quality soldered joints to be produced, however very careful control of the process is required if this is to be maintained. It is necessary to apply the correct amount, and in the correct place. Additionally the solder paste must be within its "use by" date.
By Ian Poole
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