Development of interconnecting materials

Fluxless SAC305

In general, residues remain after the soldering process on the substrate when using solder pastes with liquid flux. These residues can be very corrosive. With the encapsulation of acid and salts in the resin of the “no-clean” solder paste, the residues are less aggressive for the PCB and components, but still existing and visible. With a fluxless solder paste no residues occur. Fluxless soldering, i.e. residue-free soldering with the aid of gaseous activation, is known for many years, but only well established in the field of opto- and microwave electronics. In low-cost high-volume applications this technology has not yet become mainstream.

Our research group has developed a fluxless solder on which various tests were applied and different properties of the developed solder were investigated.

For wettability and melting experiments and the demonstration of the functionality of the gaseous activator, fluxless solder paste was printed on a test board for ceramic packaged LEDs. The functioning of the reductive atmosphere can be seen comparing the reflow soldered test board in the figure beside.

The investigation on the void formation during the soldering of the developed fluxless SAC305 shows the level of the void formation is acceptable.

The mechanical integrity of the solder joints were tested by shear strength tests. Here the fluxless solder joints shows a similar shear strength compared with the flux-included solder. The concept and process proposed for soldering the Fluxless SAC305 can reduce the production costs of electronic systems and improve the quality of soldered electronic modules.

 

Copper sintering

The increasing trend of electrification in automobiles has escalated the miniaturization of power electronic systems. Conventional Si-based power converters no longer meet the high requirements and wide band-gap (WBG) devices have gained momentum to address the shortcomings.

However, WBG devices still continue to be packaged through conventional established packaging technologies limiting their overall performance. Conventional lead free solders such as Sn based solders have a relatively low melting point of 220-230°C, thereby limiting the reliable temperature use to below 150°C.  High temperature solders have been proposed but they have their limitations with AuSn being too expensive, Bi-based suffering from low thermal conductivity and Zn-based suffering from brittleness and low wettability.

Therefore, new packaging technologies are the need of the hour.

Silver sintering has gained prominence in the last decade with a number of players entering the market and commercial silver sinter pastes offering low-temperature sintering (250°C), high thermal and electrical conductivities and reliability at high temperatures. However, the high cost of silver is a deterrent and challenges with pressureless silver sintering have limited the penetration of silver sintering into a mass production solution.

Copper offers a low cost alternative to silver. It has a lower coefficient of thermal expansion than silver and nearly the same electrical and thermal conductivities. Further, it is a ~100 times cheaper than silver and more abundant.

However, one major challenge with copper is its high chemical reactivity, specially its affinity to oxidize readily under air. Further, by virtue of the fact that the melting points and the sintering temperatures are correlated, copper sintering is foreseen to be at higher temperatures compared to silver.
Within our research group, we are working on novel solutions to develop copper sinter pastes by employing a multi-dimensional approach.

First results return shear strengths of ~80MPa for sintering under pressure and ~25MPa for pressureless sintering, which are comparable to commercially available silver sinter pastes.

 

Contact

Head of Research Group Microelectronics Packaging
Prof. Dr. Gordon Elger
Phone: +49 841 9348-2840
Room: A114
E-Mail: