A reliable and environmentally friendly packaging technology-flip-chip joining using anisotropically conductive adhesive

There is an increasing demand to move the radio base station closer to the antenna for future mobile telecommunication systems. This requires a significant reduction in weight and volume and increased environmental compatibility. This work provides an evaluation of environmental impact and reliability when using anisotropically conductive adhesives (ACA) for flip-chip joining in radio base station applications. Conventional FR-4 substrate has been used to assemble a digital ASIC chip using an anisotropically conductive adhesive and flip-chip technology. The chip has a minimum pitch of 128 μm with 7.8 mm in chip 8 and has in total 144 bumps with a bump size of 114×126 μm². Bumping was made using electroless nickel/gold technology. Bonding quality has been characterized by optical and scanning electron microscopy and substrate planarity measurement. The main parameters affecting quality are misalignment and softening of the FR-4 substrate during assembly, leading to high joint resistance. Reliability testing was conducted in the form of a temperature cycling test between -40 and ±125°C for 1000 cycles, a 125°C aging test for 100 h and a 85/85 humidity test for 500 h. The results show that relatively small resistance changes were observed after the reliability test. The environmental impact evaluation was done in the form of a material content declaration and a life cycle assessment (LCA). By using flip-chip ACA joining technology, the content of environmentally risky materials has been reduced more than ten times, and the use of precious metals has been reduced more than 30 times compared to conventional surface mount technology