Hitachi, Ltd. announced the use of it at the "PCIM Europe 2016" juxtaposed meetingCopper aluminum glass sintered packagePower component packaging technology. The feature of this technology is that although it is a lead-free packaging material, it can reduce material costs and improve reliability.

As a result of the measurement of the "power cycle tolerance" as a reliability index, the number of cycles has reached more than 10 times that of conventional lead-containing solder. Specifically, when the power cycle test was carried out under the conditions of a power element maximum junction temperature (Tjmax) of 175°C and a ΔTj of 125°C, the conventional lead-containing solder was up to 50,000 times, and when sintered copper was used, it reached 550,000 times. . When the thermal cycle test from -40°C to +200°C was carried out, the number of cycles reached 1000 without any problems.

In addition, high-output power modules that have not yet completed the lead-free process (such as railway train modules) also require the use of lead-free solder, and as the output power density of power modules increases year by year, the operating temperature of power components is also rising. Therefore, the use of highly reliable welding consumables is required. Candidate materials include gold, silver, copper, etc., which have high thermal conductivity. One of the most common is the sintered solder of silver nanomaterials.

Nanoparticulate copper

Hitachi chose copper. Because copper has a low coefficient of thermal expansion and high Young's modulus, the company believes that its mechanical reliability is excellent, so it uses copper. In addition, low material cost is also one of the reasons for choosing copper.

However, the melting point of copper is as high as about 1100°C, and it is difficult to use it as a packaging material when used directly in bulk. Therefore, Hitachi decided to use copper as nanoparticles with high surface energy. The nano-particle copper is coated between the power element and the insulating plate under the power element, and sintered by applying temperature and pressure to package.

With the above measures, the physical properties can reach the same level as that of bulk copper and maintain a stable state. The specific sintering conditions have not been announced, but according to published materials, it is estimated that the encapsulation will be implemented between about 250°C and 400°C. As mentioned in the article, after reaching a temperature of about 250°C or higher, the bonding strength after sintering is 20MPa, which can reach a practical level (the result of a shear test after the power element is packaged on nickel-plated copper).

No need for gold plating (Au)

A higher bonding strength than nickel is also an advantage of sintered copper. When using ordinary lead-containing solder, gold plating is applied to the nickel electrode of the power element and then packaged on the insulating substrate. That is, there is lead-containing solder between the gold plating layer and the insulating substrate. And the same is true when using sintered silver.

When using sintered copper, gold plating is not required, and the nickel electrode and the insulating substrate can be directly joined. One of the reasons is that the lattice constants of copper and nickel are close, so it is easy to achieve orientation matching. Since gold plating can be reduced, it helps reduce costs.

The sintered copper is used for the power module this time, and the results show that the module can smoothly realize the switching operation. The withstand voltage of the module is 3.3kV and the rated current is 1800A. The maximum junction temperature of the power element is set to 175°C.

The sintered copper developed this time is mainly used in the power modules of railway trains. Hitachi has not announced a timetable for using sintered copper for power module products, but at the PCIM Europe conference, many technology announcements are aimed at practical use in 1 to 3 years, so sintered copper technology is expected to be put into practical use as soon as possible.