A Novel Anisotropic Conductive Adhesive for Lead-Free Surface Mount Electronics Packaging

[+] Author and Article Information
S. Manian Ramkumar

Center for Electronics Manufacturing and Assembly, Rochester Institute of Technology, 78 Lomb Memorial Drive, Rochester, NY 14623smrmet@rit.edu

Krishnaswami Srihari

Systems Science and Industrial Engineering Department, State University of New York, Binghamton University, Binghamton, NY 13850srihari@binghamton.edu

J. Electron. Packag 129(2), 149-156 (Aug 14, 2006) (8 pages) doi:10.1115/1.2721086 History: Received October 04, 2005; Revised August 14, 2006

The electronics industry, in recent years, has been focusing primarily on product miniaturization and lead-free assembly. The need for product miniaturization is due to the continuous demand for portable electronic products that are multifunctional, yet smaller, faster, cheaper, and lighter. This is forcing the industry to design and assemble products with miniature passive and active devices. These devices typically have fine pitch footprints that provide a very small surface area for attachment. The solder attach technique relies primarily on the formation of intermetallics between the mating metallic surfaces. With a reduction in the surface area of the pads, the ratio of intermetallic to solder is very high once the solder joint is formed. This could result in unreliable solder joints, due to the brittle nature of intermetallics. In addition, the need to eliminate lead-based materials as a means of interconnection has renewed the industry’s interest in exploring other means of assembling surface mount devices reliably. The use of a novel anisotropic conductive adhesive (ACA) as a means for assembling surface mount devices, the ACA’s performance characteristics, and preliminary research findings are discussed in this paper. Typically, ACAs require the application of pressure during the curing process to establish the electrical connection. The novel ACA uses a magnetic field to align the particles in the Z-axis direction and eliminates the need for pressure during curing. The formation of conductive columns within the polymer matrix provides a very high insulation resistance between adjacent conductors. The novel ACA also enables mass curing of the adhesive, eliminating the need for sequential assembly. The novel ACA was found to be very effective in providing the interconnection for surface mount technology (SMT) passives and leaded, bumped, or bumpless integrated circuit packages. The requirement for precise stencil printing was eliminated, as the application of magnetic field aligned the conductive columns in the Z-axis direction eliminating any lateral conductivity. The ability to mass cure the adhesive while applying the magnetic field reduced the assembly time considerably. Placement accuracy was still found to be very critical. Shear testing of adhesive joints after thermal aging showed significance past 500 h and after temperature–humidity aging showed significance within the first 100 h. IV characteristics of the daisy chained ball grid array devices assembled with and without bumps revealed considerable difference in the breakdown current. The correlation between initial contact resistance of the daisy chain and the final breakdown current was also determined. Preliminary experiments and findings, discussed in this paper, show the viability of the ACA for mixed SMT assembly. Further experimentations will include in situ contact resistance measurements during thermal aging, temperature–humidity aging, drop testing and thermal shock.

Copyright © 2007 by American Society of Mechanical Engineers
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Figure 8

BGA without bumps completely misaligned

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Figure 9

Shear force variation for 0603 components under thermal aging

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Figure 10

Shear force variation for 0805 components under thermal aging

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Figure 11

Shear force variation for 0603 components under temperature/humidity aging

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Figure 12

Shear force variation for 0805 components under temperature/humidity aging

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Figure 13

I–V characteristics for BGA joints assembled with and without bumps

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Figure 14

Initial joint resistance versus maximum breakdown current for adhesive joint

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Figure 7

Adhesive voids underneath the components (BGA without bumps)

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Figure 6

Column of ACA particles around the BGA bump

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Figure 5

BGA daisy chain used for novel ACA joint I–V characterization

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Figure 4

Test vehicle for studying novel ACA joint shear strength

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Figure 3

Functional test vehicle assembled with novel ACA

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Figure 2

Test vehicle to evaluate functionality of ACA joint

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Figure 1

Novel ACA before and after cure




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