Robust power electronics through improved mounting technologies

Pressure sintering: The alternative to solder joints

By contrast, diffusion processes compress the silver particles when soldering using sinter paste and applying heat (above 220°C) and pressure (5-30 MPa). Pressure sintering helps reduce coarseness in the mounting layer, specifically with large-surface semiconductors. Unlike with soldering, cleaning is unnecessary after sintering. Joining using sinter adhesive is another distinctive feature. Here the adhesive cures using heat and sinters the silver particles. No external pressure is necessary. As above, no cleaning is necessary after the curing process. Sinter adhesive combines the advantages of the materially-bonded connection of silver (Ag) sinter pastes and the strong adhesive powers of Ag conductive adhesives into one paste system.

Sinter adhesive versus sinter paste

Compared to a soldered connection, a joint produced with sinter adhesive is more flexible. This enables it to better compensate for the thermo-mechanical loads that arise from differences in the thermal expansion coefficient between the substrate and the semiconductor, which can be particularly relevant for large-surface semiconductors.

However, the silver adhesive exhibits a lower thermal and electrical con-ductivity than sinter paste. By contrast, sinter paste can produce joints that are thermo-mechanically stable even at extremely high temperatures (300 °C).

Increasing the temperature resistance and power cycle performance through sintering, for example, also requires the bonding wire connection to be optimized. Ultrasonic wedge bonding with aluminum fixed wires with a gauge of up to 500 μm represents the standard process. The power cycle performance can be slightly improved by using aluminum ribbons with an optimized geometry. Using Cu wires and ribbons enables a significant improvement in power cycle performance. Furthermore, copper possesses markedly better electrical and thermal conductivity than aluminum. Copper (Cu) is also substantially harder, which requires greater forces during bonding and rules out its use with sensitive semiconductors.

Aluminum-coated copper bonding wires/ribbons represent a compromise: The aluminum coating allows these bonding wires to bond with significantly less applied force than pure copper wires, and the copper core provides better thermal, electrical, and mechanical properties – and therefore better power cycle performance than pure aluminum wires.

Clad strips and press-fit pin

According to Heraeus, its AlSi Bond:CuNiSi clad strips have proven espe-cially effective for integrated circuit and mounting technology. Due to the material combination of bondable aluminum (AlSi) and an alloy comprised of copper (Cu), nickel (Ni), and silicon (Si), it is especially well-suited as a high-performance material for press-fit technology applications in power electronics. This unique pairing of materials combines an extremely reliable aluminum-clad bonding surface with a copper alloy that provides
optimized stress relaxation in the area of the press-fit pins, even at elevated temperatures.

According to Heraeus, this material’s
exceptional potential can be fully exploited in the robust and secure production of joints for ceramic hybrids or LTCC substrates. Lead frames can be produced from AlSi Bond:CuNiSi with a bondable zone and a flexible press-fit zone, called the press-fit pins. The pins are designed so that they form a durable connection with a metallic or metallized substrate without a soldering process.

The combination of press-fit pins and bond surface opens up new design possibilities for power modules in challenging environments, such as high temperatures (above 150 °C). The relaxation stability of the base material (CuNiSi) allows the production of reliable, durable systems, while the AlSi coating generates robust bonds.

(ID:43634735)