Advanced encapsulant materials systems for flip-chip-on-board assemblies. I. Encapsulant materials with improved manufacturing properties. II. Materials to integrate the reflow and underfilling processes

Abstract

Encapsulant materials for flip chip on board assemblies (FCOB) were developed to address the issues observed during assembly of consumer electronic products on a high volume manufacturing FCOB/SMT line. The development of encapsulant materials with enhanced flow properties and faster curing kinetics is critical to continue the move towards 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 developed and some approached a 10/spl times/ reduction in the time to underfill a flip chip when compared to the qualified encapsulant system. The viscosity, surface tension, and filler particle sizes were studied in an attempt to correlate these properties to the recorded underfill times. 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. In addition, reliability tests were conducted using FR4 substrates to determine the relationship between materials properties and reliability responses. The experimental results suggested that there is a strong potential to develop materials for FCOB assemblies with enhanced flow properties and shorter cure schedules without compromising reliability behavior. In addition, unique encapsulant materials systems with sufficient fluxing activities to remove the metal oxides on the die and/or substrate bumps and assist in the formation of metallurgical interconnects were developed: reflowable encapsulants. The experimental process flow was as follows, a finite volume of reflowable encapsulant was dispensed on the PCB at the die site, the die was aligned over the bond pads, and the die was placed into the encapsulant. Next, the FCOB assembly was transferred to a reflow furnace and subjected to a standard SMT eutectic Pb/Sn reflow profile, the solder was reflowed, interconnects were formed between the die and PCB, and the reflowable encapsulant was partially cured. Promising reliability results were obtained warranting further evaluation of the reflowable materials systems and process.