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

There is increasing demand to move radio base stations closer to the antenna for future mobile telecommunication systems, which requires significant reduction in weight, volume and environmental compatibility. In this work, an evaluation of environmental impact and reliability when using anisotropically conductive adhesives (ACA) for flip-chip joining in radio base station applications has been performed. Conventional FR-4 substrates were 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 a chip size of 7.76 mm and has a total of 216 bumps with bump size of 108×120 μm using electroless nickel/gold bumping technology. The bonding quality was characterized by optical and scanning electron microscopy and substrate planarity measurement. Main quality-affecting parameters are misalignment and softening of the FR-4 substrate during assembly, which lead to high joint resistance. Reliability testing was conducted in a temperature cycling test between -40 and +125°C for 1000 cycles, a 125°C aging test for 1000 hours and a humidity test 85°C/85% RH, 500 hours. The results show that relatively small resistance change was observed after reliability testing. The environmental impact evaluation was conducted as a material content declaration and as a life cycle assessment (LCA). By using flip-chip ACA joining technology, the environmentally hazardous materials content was reduced more than ten times and the use of precious metals was reduced by more than 30 times compared to conventional SMT