Design Improvements for Selective Soldering Assemblies

Selective soldering, along with pin-in-paste reflow and press fit, is the primary assembly method for through-hole components. The reflow process is limited by component dimensions and heat resistance. Press fit becomes expensive when defects that can’t be repaired. Electronic manufacturing services realize that SMT can’t replace the through-hole technology completely. The selective soldering process offers opportunities to make solder connections on different levels, connecting housings, junction boxes, aluminum parts, stacking PCBs and more. Designers of new board assemblies can benefit from the specialized soldering nozzles and robotics capabilities that modern selective soldering machines offer.

Selective soldering can be achieved under an angle (tilt) as in wave soldering, or horizontally with different shaped nozzles and nozzle materials. All have different properties and can be applied to successfully soldering the most complex assemblies. In order to optimize production and soldering efficiency, assembly engineers should be involved in the Design for Assembly process. Knowledge of the selective soldering process and nozzle technology may offer competitive advantages when implemented in new design and assembly processes.

Studies have been done to determine minimum distances to adjacent components, especially SMDs. Questions asked include ‘What pin to hole ratio provides the best hole filling? and ‘How much influence has flux selection on soldering results, and which nozzles should be used?’ Historical data is combined with several Design of Experiments, looking for soldering defects such as bridging, but also seeking process optimization to achieve the best hole filling. Hole filling is critical for high thermal mass boards. The thick copper layers absorb a great deal of heat from the preheating and liquidous solder. Special design modifications will result in more heat in the solder barrel, which will guide the solder to the solder destination side of the board. In combining the right nozzle selection with correct solder acceleration and deceleration will ensure that even the most difficult to create joints will meet the IPC-A-610 classification.