Solder Balling

What is it?

Solder balling is the formation of undesirable solder balls in areas where there is not supposed to be any solder. Larger balls that are found in close proximity to discrete components such as chip resistors and capacitors are known as solder beading.

Why does it happen?

Solder balls can be formed in a number of ways. For example, in circuit boards where etch resist has been allowed to remain on the conductor tracks, it is possible for this metal to fuse during soldering operations and to squeeze out at the solder mask to track interface and form a ball on the surface. These balls may be deposited in areas away from the conductor tracks and components. They are formed due to gassing and flux spitting on the surface of the solder wave or by solder itself which comes from the wave.

Solder beading is caused when the molten solder material squeezes out of the gap between the component and the board to a form a freestanding soldering ball. Specific factors that lead to solder beading include component placement pressure that is too high and the dispensing of too much solder paste. The latter can be addressed by modifying the stencil apertures accordingly. Most stencil manufacturers will be able to offer advice.

What problems can it cause?

Solder balling can cause reliability problems. It can create short circuit if it occurs between two conductors. The balls can also be dislodged during handling, which can impact on the performance of adjacent assemblies or components. Its significance as a reliability issue has increased as device sizes and interconnection dimensions have continued to get smaller.

The use of no-clean solder fluxes means that flux residues are not removed from the circuit board surface after soldering and therefore any solder balls formed remain trapped. Solder balls that are formed need to be removed and this will lead to an increase in the overall cost of the assembly process. If a solder ball is missed and then subsequently causes a problem in service the economic impact may be even greater.

How can it be prevented?

The occurrence of solder balling is influenced by a number of factors. These include the type of solder mask used as well as the soldering materials, equipment and the process conditions. It can be reduced by optimizing process conditions as well as by choosing materials that are specifically formulated with this problem in mind.

If solder balling still occurs it can be removed after soldering. If the balls do not adhere strongly to the solder mask they can be easily removed and the problem is effectively eliminated. Hence the choice of solder mask is an important factor. For example, VOC-free, no-clean liquid fluxes and special solder masks that have been designed to reduce solder balling defects are now commercially available. Other materials related factors include the viscosity and oxide content of the solder paste, as well as the type of solderable finish used on the board.

From a processing perspective, key influencing factors include the type of reflow oven used, the reflow profile and the processing environment. Infra-red ovens generally give inferior performance to forced convection ovens since they have more homogeneous heating that minimizes solder spattering. They can also spatter solder paste by heating less densely populated areas faster than more densely populated areas.

Optimisation of the reflow profile is also important since, if the ramp rate is incorrect or, if the peak temperature is too high, the paste can spatter, causing solder balls to form.

The level of humidity in the soldering environment is also important because, as it increases, materials such as the solder paste flux chemistry can absorb more moisture. During subsequent heating this may lead to solder segregation, spattering and eventual balling.

To eliminate solder balls it is important to determine their size and location and then to work closely with solder paste suppliers, board fabricators and stencil makers, who should each be able to offer specific advice.