What is Solder Paste & How to Use It

Solder paste is applied to surface mount boards prior to pick and place and when passed through an infra-red reflow machine, melts to provide the soldered joints.

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Solder paste is the form of solder that is used in association with infrared reflow machines during the PCB assembly process.

Solder paste is used in PCB assembly as it provides significant advantages and its form enables the process to become a simple and easy process.

The solder paste can be sued within large scale PCB assembly, or even within prototype production. In fact it is used in most forms of SMT assembly, proving an easy to use medium for the solder.

What is solder paste

Solder paste is a mixture of minute solder spheres held within a specialised form of solder flux. As the name indicates it has the texture of a paste, and hence the name.

The fact that it is a paste means that it can be easily applied to the board during PCB assembly.

The solder particles are a mixture of solder. Traditionally this used to be tin and lead, but with the legislation has been introduced around the world, to only use 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 grades

There are various grades of solder paste and the required type can be selected to fit in with the PCB assembly process used. The solder paste is graded according to the size of the small solder balls. As they cannot be exactly graded, the different types have a band of solder ball sizes between which 80% of the minute solder balls fall.

IPC Type Designation	Particle Size (µm) *
Type 1	75 - 150
Type 2	45 - 75
Type 3	25 - 45
Type 4	20 - 38
Type 5	10 - 25
Type 6	5 - 15
Type 7	2 - 11
Type 8	2 - 8

* 80% minimum between the stated sizes

Solder can also be categorised according to the type of flux used:

Rosin based solder pastes: Rosin based pastes are made of rosin, a natural extract from pine trees. These fluxes can be cleaned if required after the soldering process using a solvent (potentially including chlorofluorocarbons).
Water soluble flux based solder pastes : Water-soluble fluxes are made up of organic materials and glycol bases. There is a wide variety of cleaning agents for these fluxes.
No clean solder paste: A no-clean flux is made with resins and various levels of solid residues. No-clean pastes save not only cleaning costs, but also capital expenditures and floor space. Although the no-clean flux based solder pastes sound attractive, they need a very clean assembly environment and may need an inert reflow environment.

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 paste 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.

There are many ways of achieving this, but typically a stencil is placed over the board, and the paste is applied though this, ensuring that the required amount is applied - too little and the joints will have sufficient solder - too much and the joints will be too large and there may be the possibility of poor joints and even shorts between adjacent pads, etc.

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.

Purchasing solder paste

Solder paste can obviously be bought in industrial quantities for large PCB assembly plants, but it can also be bought in smaller qualities. It can be purchased in tubs and syringes. These are particularly useful to applications like general or BGA rework areas or for small prototype assembly.

Solder paste is widely used in PCB assembly - both in mass production and also for prototype PCB assembly. It provides an excellent method of applying the solder that is applicable to large and small forms of 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, and any issues discovered need to be fed back into the process to correct the issue as quickly as possible. In particulart is necessary to apply the correct amount, and in the correct place. Additionally the solder paste must be used within its "use by" date to ensure that joints are of the required standard.