Solder paste

Often missed during inspection, a head-in-pillow (HIP) defect appears like a head resting on a pillow with a visible separation in the solder joint at the interface of the BGA sphere and reflowed paste deposit.

[1] An electronics manufacturer needs experience with the printing process, specifically the paste characteristics, to avoid costly re-work on the assemblies.

The paste's physical characteristics, like viscosity and flux levels, need to be monitored periodically by performing in-house tests.

A solder ball is spherical in shape; this helps in reducing surface oxidation and ensures good joint formation with the adjoining particles.

Irregular particle sizes are not used, as they tend to clog the stencil, causing printing defects.

To produce a quality solder joint, it's very important for the spheres of metal to be very regular in size and have a low level of oxidation[citation needed].

Solder pastes are classified based on the particle size by IPC standard J-STD 005.

No-clean pastes save not only cleaning costs, but also capital expenditures and floor space.

As well as forming the solder joint itself, the paste carrier/flux must have sufficient tackiness to hold the components while the assembly passes through the various manufacturing processes, perhaps moved around the factory.

The main requirement is a gentle rise in temperature to prevent explosive expansion (which can cause "solder balling"), yet activate the flux.

The main reason why evaluation of solder paste is necessary, is because 50-90% of all defects result from printing problems.

This procedure is quite thorough, yet minimizes the amount of testing required to differentiate between excellent and poor solder pastes.