Solder paste

A solder paste usually consists of 80% of the desired alloy in the form of metal powders. The remaining 20% are fluxes, resins and organics to ensure processability.

Solder paste

The metal powders are divided into different grain sizes or are standardized according to DIN standard 32513 or JEDEC standard J-STD-005.

TypeSize (80% of the solder balls) in µm

As a rule of thumb, the size should be selected so that at least five balls fit into the smallest opening of the stencil. The lower the layer thickness to be printed, the higher the grain size type must be selected.

Organic composition

The fluxes are specified according to the military standard QQ-S-571 or using the IPC flux evaluation system. The fluxes are divided into natural (rosin, RO for short) and artificial (resin, RE for short) resins.

The best-known natural flux is colophony (R). It only has a low surface activation and can therefore only be used on materials that are easy to solder, such as copper.

Another group of fluxes are no-clean fluxes (NC), i.e. fluxes that cannot be cleaned. These leave a kind of seal on the surface and thus trap their residues automatically.

There is also the group of mildly activated rosin (RMA). This also consists of solvents and a small amount of activators.

If a larger quantity of activators is added to the collophony, the term rosine activated (RA) is used.

Another group to be mentioned are the water soluble fluxes (Water soluble WS). These are divided into different activation levels L,M,H.

Another important property of solder pastes is the halide content (1) or whether they are halogen-free (0).

Solder pastes in combination with formic acid or hydrogen

A number of pastes have recently come onto the market for power electronics and flux-free soldering with formic acid. These differ from conventional pastes in that they have a low resin content or are resin-free. The fluxes used do not contain any activators and are highly volatile to ensure that the formic acid can reach the surface of the components. These pastes can be soldered such as preforms in vacuum chambers and leave virtually no residue in the oven.

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