High-temperature reliability of Flip Chip assemblies

Abstract Flip Chip technology has been widely accepted within microelectronics as a technology for maximum miniaturization. Typical applications today are mobile products such as cellular phones or GPS devices. For both widening Flip Chip technology’s application range and for addressing the automotive electronics’ volume market, developing assemblies capable of withstanding high temperatures is crucial. A typical scenario for integrating electronics into a car is a control unit within the engine compartment, where ambient temperatures are around 150 °C, package junction temperatures may range from 175 °C to 200 °C and peak temperatures may exceed these values. If Flip Chip technology is used under harsh environment conditions, it is clear that especially the polymeric materials, i.e., underfiller, solder mask or the organic substrate base material, are challenged. Generally, the developmental goal for encapsulants compatible with high-temperature applications are materials with high Tg and low degradation even at temperatures >200 °C. According to these demands, a test group of advanced underfill encapsulants has been used for assembling Flip Chip devices. These test vehicles were built using lead-free and lead-containing solders such as SnAgCu and eutectic PbSn and standard FR4 substrates, for evaluating the reliability potential of state-of-the-art underfillers. Material analysis is performed for studying both material degradation as well as temperature-dependent thermo-mechanical and adhesive properties. For assessing reliability, temperature cycling is performed with different maximum test temperatures ranging from 150 °C to 175 °C. The device status is intermediately analyzed by using electrical measurement for detecting bond integrity and acoustomicroscopy for determining the occurrence and growth of delaminations. Extensive failure analysis is added to investigate device failure mechanisms, especially related to the respective test temperature. In summary, an empirical status of the high-temperature potential of state-of-the-art underfillers and material combinations is attained and an outlook on future demands and developments is provided.