Advanced encapsulant systems for flip-chip-on-board assemblies: underfills with improved manufacturing properties

Underfill materials for flip chip on board (FCOB) were developed to address the issues observed during assembly of consumer electronic products on a high volume manufacturing FCOB/surface mount technology (SMT) line. The development of underfill materials with enhanced flow properties and faster curing kinetics is critical to continue the move toward the integration of FCOB assemblies as an alternative packaging system in electronic products. The results from this study showed that materials with enhanced flow properties were enveloped and. Some approached a 10X reduction in the time to underfill a flip chip when compared to the control underfill. The viscosity, surface tension, and filler particle sizes were studied in an attempt to correlate these properties to the recorded underfill times. Also, materials characterization studies were performed to determine the glass transition temperatures (Tg), tensile elastic and loss moduli (E\´ and E"), flow profiles, coefficients of thermal expansion (CTE), and apparent strengths of adhesion (ASA). Thermodynamic and microdielectrometry studies were conducted to identify the optimal thermal cure schedules of the underfills. These kinetic studies identified materials which completed curing in 6× shorter times as compared to the control underfill. In addition, reliability tests were performed using FR4 substrates and continuity die to determine the relationship between materials properties and reliability responses. The experimental results showed that there was a strong potential to develop materials for FCOB assemblies with enhanced flow properties and shorter cure schedules without compromising reliability performance