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.
Summary
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.
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