High temperature potential of flip chip assemblies for automotive applications

Flip chip technology has been widely accepted within microelectronics as a technology for maximum miniaturization. Typical applications today are mobile products as cellular phones or GPS devices. The upper temperature limits for such applications range from 80 °C to a maximum of 125 °C. To widen the application range of flip chip technology and to address the volume market of automotive and industrial electronics, the development of high temperature capable assemblies is crucial. Typical scenario for the integration of electronics into a car is a control unit in the engine compartment, where ambient temperatures are around 150 °C, package junction temperatures may range from 175 °C to 200 °C and peak temperature may exceed these values. When using flip chip technology under high temperature conditions, major challenges are found in the application of interconnect media and supporting polymers. At elevated temperatures, the intermetallic phase formation of lead-free solders might lead to a reliability decrease, where polymeric materials as substrate and encapsulant do potentially show mismatched thermo-mechanical properties or material degradation and thus reliability is reduced. Literature does typically describe flip chip technologies behavior on organic substrates for consumer applications, but almost no information is available on the performance at temperatures beyond 125 °C. Within the European project HOTCAR, dealing with high temperature electronics for automotive use in general, a German consortium consisting of an IC manufacturer (IFX), two technology users (Siemens VDO & Temic) and a research institute (Fraunhofer IZM) have cooperated to evaluate the high temperature potential of lead-free flip chip technology for automotive applications. According to automotive demands, an experimental study on the suitability of advanced Underfill encapsulants for high temperature has been performed. With the outcome of this pre-study, two promising underfill materials were selected and used in a test run with an automotive test vehicle. This comprises an automotive grade μController mounted on a substrate manufactured according to automotive standards, as the major system components. Solder material used wa- s SnAg with a Ni UBM in combination with two different substrate finishes NiAu and immersion Sn. These test devices were submitted to temperature cycles according to automotive specifications with a maximum temperature of 150 °C. Intermetallic phase formation was studied after high temperature storage by cross sections and shear tests. Typical failure modes for flip chip failure have been identified and are described in detail. The experimental reliability investigations were backed by thermo-mechanical simulations. Taking advantage of the so-called submodelling technique, the solder joint behavior could be studied in detail for lead-free solders. Starting stress-free at 150 °C, the calculations followed the real thermal cycling regime. As primary results, the accumulated equivalent creep strain and creep strain energy distributions were obtained. Based on Manson-Coffin-coefficients from recent experiments at IZM, mean cycles to failure (MCF) have been estimated for solder joint fatigue and compared with observed failure. In summary, a status of the high temperature potential of lead-free flip chip technology under automotive conditions is given and a first design guideline for high temperature automotive flip chip applications is provided.