Low temperature and low pressure die-attach bonding of high power light emitting diodes with self reducing copper complex paste

The automotive high power light emitting diode (LED) market is expected to grow to ~ US$ 30 billion by 2024 [1] and new interconnect technologies that enable reliable operation are required. Traditionally AuSn soldering or Ag sintering have been the interconnect technologies of choice. However, the constantly increasing price of Au and Ag have been a motivation to find low cost alternatives offering similar or better performances. Cu offers the next best electrical and thermal characteristics among all metals and is also cheaper by a factor 100 compared to Ag. Nevertheless, oxidation of Cu particles, long sintering times and comparatively high sintering temperature are major challenges that need to be addressed. In the present study, a low temperature, low pressure, rapid sintering process for high power LEDs is developed. A self-reducing Cu paste composed of commercially available Cu(II) formate tetrahydrate particles dispersed in a binder combination of α-terpineol and polyethylene glycol 600 (PEG600) is developed. Die-attach bonding is achieved by sintering Cu nanoparticles realized in-situ through the thermal decomposition of the Cu(II) formate molecules. Shear strength values of ~70 MPa are achieved while sintering at 275 °C for 5 min in an open bond chamber under a constant flow of nitrogen with an application of 10 MPa bonding pressure. A dense and homogeneous interconnect with low porosity (< 30%) is achieved. Additionally, the developed Cu paste is observed to be compatible with standard metallization, such as Ag and Au, apart from sintering on bare Cu substrates, and to be suitable for standard stencil and screen printing applications. The analyzed solution offers a promising die-attach bonding technology that can be easily integrated into existing Ag sinter production lines without the need for high capital investments for extensive equipment redesign and recommissioning, thereby offering the manufacturers the flexibility of running both Ag and Cu sintering on the same production lines based on their specific product requirements.