A manufacturing approach to reducing underfill voiding on large die (> 18 mm) flip chip organic laminate packaging

The rising production levels on flip chip large dies (15 to 26 mm square) and low stand off solder C4's has brought to light greater process sensitivity with respect to the formation of voids in underfills. The formation of voids is a result of flow limitations and / or moisture diffusion from the laminate during the capillary underfill dispense and cure processes. Voiding propensity depends on a number of factors, such as underfill type and multiple variables from the laminate and chip construction and designs. Another set of factors however are controllable through process, and include preparatory bakes, preheat steps, adequate temperature control during the dispense process as well as underfill dispense pattern strategy. This paper summarizes a series of experiments conducted to better understand the interplay of several of these factors and the validation of best practices through controlled experiments on the production line. The diffusion behaviour of moisture in laminates can be roughly anticipated with standard models of moisture diffusion in epoxy and offers insights to proper adjustments of preparatory thermal treatments. The data indicates that appropriate thermal treatments to, minimise the possibility of moisture absorption and a good control of dispense parameters are key elements to reducing the occurrence of voids in underfills.