Underfilling Chip Scale Packages with Reworkable Underfills for Consumer Product Applications

ABSTRACT The introduction of Flip Chip underfills provided the solder interconnection technology with mechanical robustness and a significant increase in flip-chip low cycle solder fatigue resistance. Offspring of this technology like plastic ball grid arrays (PBGA), ceramic ball grid array (CBGA), chip scale packages (CSP), etc., were introduced subsequently. These new technologies are meant to mimic and improve product robustness and reliability of the original controlled collapse chip connection (C4) first used by IBM on solid logic technology (SLT) for flip chips in the early sixties. These were developed to increase both mechanical robustness and fatigue resistance. For electronic packaging, the use of these new interconnections has been as successful as the original C4 technology. However, the environments for these packages became increasingly demanding. The introduction of CSP and High Density Interconnect (HDI) motherboards into hand-held electronic devices has produced a number of new device failure mechanisms and failure modes. The integrity and reliability of the components are being threatened by some of these mechanisms. This paper reviews the need for CSP reinforcement. Two main failure modes must be addressed to ensure reliability of CSP devices: low cycle thermal fatigue due to temperature excursions and temperature gradients, and the brittle fracture caused by static and dynamic loads including, impact shock, vibration, torque and bending for various encapsulant materials including reworkable underfills. The current investigation provides results of a study aimed at enhancing mechanical and thermal shock reliability for CSP assemblies in mobile phone applications including experimental results with reworkable underfills. Reliability results of underfilled CSP compared to no-reinforced assemblies are presented. Issues surrounding development of effective materials, processes and equipment for reinforcing board-mounted CSP's in high volume production environment are discussed in this paper. Analytical models and simulation of these thermo-mechanical loading are proposed. Package response to dynamic loading, high strain rates, is calculated including, random vibration spectral analysis and mechanical impact shock. A nonlinear 3-D transient solution is proposed. Four reworkable underfills, A, B, C, and D were evaluated in order to find a suitable system for CSP assemblies. Material properties such as coefficient of thermal expansion (CTE), glass transition temperature (Tg), modulus, strength of adhesion, etc were obtained. Tests that simulate the production process and product reliability tests have been conducted with these materials. A comparative study of the performance of Circuit Card Assemblies (CCA's) with four reworkable underfill materials, snap-cure (non-reworkable) underfill & no-underfill materials are reported. Results of mechanical & thermal shock tests, cycle time studies, ease of reworkability, and cure times are discussed. Underfill A was demonstrated to be a better choice for the application under consideration, based on the current results.