Novel high performance no flow and reworkable underfills for flip-chip applications |
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Authors: | C P Wong Lejun Wang Song-Hua Shi |
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Affiliation: | (1) School of Materials Science and Engineering & Packaging Research Center, Georgia Institute of Technology, Atlanta, GA 30332-0245, USA Fax: +1-404-894-9140, GE |
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Abstract: | Flip-chip interconnect is the emerging technology for the high performance, high I/O (Inputs/Outputs) IC devices. Due to
the thermal mismatch between the silicon IC (CTE=2.5 ppm/0 C) and the low cost organic substrate such as FR-4 printed wiring board (CTE=18–22 ppm/°C), the flip chip solder joints experience
high shear stress during temperature cycling testing. Underfill encapsulant is used to couple the bilayer structure and is
critical to the reliability of the flip-chip solder joint interconnects. Novel no-flow underfill encapsulant is an attractive
flip-chip encapsulant due to the simplification of the no-flow underfilling process. To develop the no-flow underfill material
suitable for the no-flow underfilling process of flip-chip solder joint interconnects, we have studied and developed a series
of metal chelate latent catalysts for the no-flow underfill formulation. The latent catalyst has minimal reaction with the
epoxy resin (cycloaliphatic type epoxy) and the crosslinker (or hardener) at the low temperature (<180° C) prior to the solder
reflow and then rapid reaction takes place to form the low-cost high performance underfills. The effects of the concentration
of the hardener and the catalyst on the curing profile and physical properties of the cured formulations were studied. The
kinetics and exothermic heat of the curing reactions of these formulations were investigated by differential scanning calorimetry
(DSC). Glass transition temperature (Tg) and coefficient of thermal expansion (CTE) of these cured resins were investigated
by using thermo-mechanical analyzer (TMA). Storage moduli (G′) and crosslinking density of the cured formulations were measured
by dynamic-mechanical analyzer (DMA). Weight loss of these formulations during curing was investigated by using thermo-gravimetric
analyzer (TGA). Additionally, some comparison results of our successful novel generic underfills with the current commercial
experimental no-flow underfills are reported. Additionally, approaches have been taken to develop the thermally reworkable
underfill materials in order to address the non-reworkability problem of the commercial underfill encapsulants. These include
introducing the termally cleavable blocks to thermoset resins, and adding additives to thermoset resins. For the first approach,
five diepoxides containing thermally cleavable blocks were synthesized and characterized. These diepoxides were mixed with
the hardener and the catalyst. Then the properties of these mixtures including Tg, onset decomposition temperature, storage
modulus, CTE, and viscosity were studied and compared with those of the standard formulation based on the commercial epoxy:
ERL-4221D. These mixtures all decompose at lower temperature than the standard formulation. Moreover, one mixture – Epoxy5
– showed acceptable Tg, low viscosity, and fairly good adhesion. For the latter approach, two additives were shown that after
added to typical cycloaliphatic epoxy formulation, do not interfere with epoxy curing, and do not affect the typical properties
of cured epoxy system, yet provide die removal capability to the epoxy. Furthermore, the combination of the two approaches
showed positive results.
Received: 28 September 1998 / Reviewed and accepted: 1 October 1998 |
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Keywords: | Flip-chip No flow underfills Reworkable underfills Latent catalyst Metal chelates Thermally degradable epoxies Epoxies and anhydrides |
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