Encapsulant materials systems for flip‐chip‐on‐board assemblies: addressing manufacturing issues

Abstract

Encapsulant materials for flip‐chip‐on‐board (FCOB) were developed to address issues that have been observed during assembly of consumer electronic products on a high volume manufacturing FCOB/SMT line. The viscosity, surface tension, and filler particle sizes of several encapsulants were studied in an attempt to correlate these properties to their recorded underfill times and to observe their flow properties under the gap. Materials characterization studies were performed to determine their glass transition temperatures (Tg), tensile elastic and loss moduli (E′ and E′′), coefficients of thermal expansion (CTE), and apparent strengths of adhesion (ASA). In addition, reliability tests were conducted, and several promising materials were identified. The ASA of the encapsulant to the die passivation and the printed circuit board (PCB) is critical to the robustness of the assembly. Studies were conducted to observe the ASA as a function of FCOB assembly conditioning prior to underfilling and the degradation of the ASA as a function of humidity exposure. The ASA of the FCOB encapsulants was highest when the assembly was “baked‐out” prior to underfilling. Conditioning the assemblies for 24 hours at 23°C/85 per cent RH, to simulate the “worse case” factory environment, reduced the ASA. The ASA was also reduced when the “baked‐out” assemblies were placed in the 85°C/85 per cent RH chamber after underfilling. Although the ASA was decreased when the boards were not “baked‐out”, the reliability performance was not affected during air to air temperature cycling (AATC). A new class of low stress encapsulant materials systems were developed to reduce the stress state of the backside of the die. Studies showed that for specific materials compositions, the stress was proportional to the glass transition temperature of the encapsulant. In addition, it was observed that the stress state was a function of humidity, temperature, and time. FCOB assemblies were built with several low stress encapsulants and placed in reliability testing and they performed as well as assemblies underfilled with the qualified encapsulant.