Title:
Assembly of Lead-free Bumped Flip-Chip with No-Flow Underfills
Assembly of Lead-free Bumped Flip-Chip with No-Flow Underfills
Authors
Wong, C. P.
Zhang, Zhuqing
Zhang, Zhuqing
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Abstract
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.
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Date Issued
2002-04
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272158 bytes
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