Reduction of voids in solder joints an alternative to vacuum soldering

Voids in solder joints are representing one of the main problems especially for power electronics. A low and homogeneous thermal resistance of solder joints is demanded for a quick and uniform conduction of the heat loss from the power chip. The same applies for the electrical conductivity of solder joints. Enclosed voids can cause a displacement of electrical and thermal paths and a local concentration of power and heat. In addition, gas voids are anxious to form spheres in the solder gap, which could be a cause for tilting of chip components and a wedge-shaped solder gap. This is tightening the problem of patchy distribution of current or heat and is causing stress and cracks. The amount of voids can be influenced by different measures, e. g. a good wettability of metallization, solder pastes with special adopted solvents and an adequate preheating profile. However, a special vacuum process step during soldering is demanded for absolutely void free solder joints. But this vacuum process is associated with some essential disadvantages. Besides of the technical expenses for vacuum pumps and additional locks, the vacuum process excludes the use of gas convection for heating and cooling. Apart from a special vapour phase-vacuum technology, most machines are using infrared radiation or heat conduction for soldering. The same principles as used in vacuum soldering technology are applicable also for a higher pressure level. If the void in the solder joint is arising for an excess pressure, the normal atmosphere pressure could be sufficient for escaping of enclosed gas. Essential for this effect is the pressure difference between inside and outside of solder joint. A benefit of soldering with excess pressure is the possibility of gas convection for heat transfer. This allows the application of conventional components and the realization of the usual temperature distribution and profiles