Electronics Assembly Knowledge, Vision & Wisdom
Reduction of Voids in Solder Joints
Reduction of Voids in Solder Joints
This paper reviews an alternative to vacuum soldering for the reduction of voids in solder joints.
Analysis Lab

Authored By:
Rolf Diehm
SEHO Systems GmbH, Kreuzwertheim, Germany

Mathias Nowottnick
University of Rostock, Rostock, Germany

Uwe Pape
Fraunhofer IZM, Berlin, Germany
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Transcript
Voids in solder joints represent 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 power chips.

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 can be a cause for tilting of chip components.

The amount of voids can be influenced by different measures, including 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 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 vapor phase-vacuum technology, most machines are using infrared radiation or heat conduction for soldering.

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.

This paper reviews an alternative to vacuum soldering for the reduction of voids in solder joints.
Summary
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.
Conclusions
The smaller process window for lead-free soldering does not allow much clearance for optimizing a void-free process. With best possible conditions including optimized fluxes and surfaces showing a good wetting behaviour, the amount of voids can be reduced.

A nearly void-free solder joint, often demanded for assembling of power electronics components, definitely only can be realized by variation of the atmospheric pressure. A variety of vacuum soldering processes were established in the past which use infrared radiation, heat conduction or the combination with vapour phase soldering for heating.

The same principle of pressure difference is used for soldering with elevated pressure and a sudden pressure release to normal level (hyper pneumatic soldering process). This new technology allows forced convection for heat transfer during the reflow process with all its well-known advantages.
Initially Published in the IPC Proceedings
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