(Georgia Institute of Technology, 2002-04)
Wong, C. P.; Zhang, Zhuqing
Lead-free solder reflow process has presented challenges to no-flow underfill material and assembly. The currently
available no-flow underfill materials are mainly designed for eutectic Sn–Pb solders. This paper presents the assembly
of lead-free bumped flip-chip with developed no-flow underfill
materials. Epoxy resin/HMPA/metal AcAc/Flux G system is developed
as no-flow underfills for Sn/Ag/Cu alloy bumped flip-chips.
The solder wetting test is conducted to demonstrate the fluxing
capability of the underfills for lead-free solders. A 100% solder
joint yield has been achieved using Sn/Ag/Cu bumped flip-chips
in a no-flow process. A scanning acoustic microscope is used to
observe the underfill voiding. The out-gassing of HMPA at high
curing temperatures causes severe voiding inside the package. A
differential scanning calorimeter (DSC) is used to study the curing
degree of the underfill after reflow with or without post-cure.
The post-curing profiles indicate that the out-gassing of HMPA
would destroy the stoichiometric balance between the epoxy and
hardener, and result in a need for high temperature post-cure.
The material properties of the underfills are characterized and the
influence of underfill out-gassing on the assembly and material
properties is investigated. The impact of lead-free reflow on the
material design and process conditions of no-flow underfill is
discussed.
(Georgia Institute of Technology, 2002-03)
Wong, C. P.; Zhang, Zhuqing; Fan, Lianhua
Most no-flow underfill materials are based on poxy/anhydride chemistry. Due to the sensitizing nature, the use of anhydride is limited and there is a need for a no-flow underfill using nonanhydride curing system. This paper presents the development of novel no-flow underfill materials based on epoxy/phenolic resin system. Epoxy and phenolic resins of different structures are evaluated in terms of their curing behavior, thermo-mechanical properties, viscosity, adhesion toward passivation, moisture absorption and the reliability in flip-chip underfill package. The influence of chemical structure and the crosslinking density of the resin on the material properties is investigated. The assembly with nonanhydride underfill shows high reliability from the thermal shock test. Solder wetting test has confirmed the sufficient fluxing capability of phenolic resins. Results show that epoxy/phenolic system has great potential for an environmental friendly and highly reliable no-flow underfill.