Adhesive bonding and miscellaneous chemical manufacture – Methods – Surface bonding and/or assembly therefor
Reexamination Certificate
2000-07-07
2002-07-23
Dixon, Merrick (Department: 1774)
Adhesive bonding and miscellaneous chemical manufacture
Methods
Surface bonding and/or assembly therefor
C156S275500, C156S272400, C156S305000
Reexamination Certificate
active
06423172
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method of coating particles as well as a method of forming a coating onto an array monolayer of particles, which may be in an adhesive form or previously formed onto a film, and products formed thereby. Such adhesives and films whose uniformly separated particles have now been coated with a conductive interconnection material, such as solder, possess anisotropic conductive pathways and enhanced interconnective capabilities, which are particularly attractive in many commercial applications, such as in the electronics industry.
2. Description of Related Technology
The advent of miniaturized electronics devices has spurred the development of numerous techniques and devices for electrical interconnection, many of which use regular-shaped metallic particles in the assembly process.
The trend towards miniaturized ball grid arrays (“BGA”) was initiated in the 1960's by International Business Machines Corporation (“IBM”) through its controlled collapse chip connection process (“the C4 process”) . The C4 process initially used arrays of highly regular copper microspheres—of about 1000 microns in diameter—which were subsequently electrodelessly plated with nickel and then with gold [see U.S. Pat. No. 3,303,393 (Hymes) and P. A. Totta and R. P. Sopher,
IBM J. Res. Devel
., 226 (1969)].
Microsphere development for BGA and so-called &mgr;BGA now seeks such spheres with diameters as small as 100 microns, and accurate control of particle diameter distribution (or dispersity). Surface tension properties of molten metals and alloys have been recognized in the production of regular solid gold and solid solder microspheres with narrow spreads of particle diameters [see K. Tatsumi, et al., Int'l Pack'g Strategy Symp. '96, 4-1, Kudan Kaikkan Japan (1996)]. Attachment of gold spheres of such diameters to specific locations on substrates often requires attraction of the particles onto a template of undersized holes (relative to the sphere diameter), with subsequent alignment onto the substrate electrode pattern achieved using a vision system. The gold spheres themselves are bonded to the substrate by thermocompression; in contrast, solder particles are attached to previously-fluxed electrode substrates.
The microelectronics industry has moved generally from conventional surface mount technology (“SMT”), which uses packaged integrated circuits (“IC”) with peripheral pin-type connections, to more advanced, yet well-known, technologies, such as BGA, chip scale or chip size packaging (“CSP”), and now direct chip attachment (“DCA”) or flip chip (“FC”) .
CSP uses compact arrays of connection (i.e., inputs/outputs or “i/o”) to connect to the IC, BGAs or pin grid arrays (“PGA”), depending upon whether the i/o's have metal spheres or metal pins for connectors. For miniaturized electronics devices, CSP enables higher densities of interconnection in smaller spaces than SMT. The resulting devices can thus be made thinner and more compatible with tape-based manufacturing methods, similar to those developed for tape automated bonding (“TAB”) technologies.
DCA, or FC, involves direct attachment of the bare IC face down on the substrate. In certain situations, FC uses microscopic solder joints to mediate electrical currents to and from the IC and the board. The solder is reflowed onto metalised bumps already built up on the chip during the fabrication processes—of course, in many instances thermocompressed gold joints may be used in place of the reflowed solder.
Chips attached to ICs by such microscopic joints should maintain or enhance the integrity of their attachment by underfilling the space beneath the chip (which is not occupied by discrete microsolder joints) with a durable adhesive (applied in a liquid form), which is wicked in under the chip and subsequently cured to a durable solid. The so-formed solid serves to bond the chip to the board and protect the microscopic electrical interconnections. Without such an underfill sealant, chips attached by DCA or FC tend to exhibit a higher rate of failure.
Anisotropically conductive adhesives (“ACA”) and anisotropically conductive adhesive films (“ACF”) are well-known for their use with electronic device, such as FC, interconnection. [See J. H. Lau, Flip Chip Technologies, McGraw-Hill, NY, Ch. 8-10 (1995), and International Patent Application WO 95/20820, the disclosures of each of which are expressly incorporated herein by reference.] Of course, with ACA/ACF technology underfill sealants are no longer necessary to provide the added benefits of chip bonding, sealing and shock absorbing properties to which reference is made above.
ACAs and ACFs are loaded (at about a 10% wt/wt level) during their formulations or fabrication with conductive fillers. These fillers are typically compliant crosslinked polymer microspheres coated first with nickel and then with gold, or simply with a nickel coating. The latter can further be subjected to electrodeless plating of metals such as copper, palladium, platinum, and the like. These spheres are manufactured commercially in a range of sizes, but within each size range the particle diameter is precise, varying only by fractions of a micron (and accordingly may be considered “monodisperse”) . However, such spheres are not available commercially with a further coating of a material such as solder.
Metal-coated polymer spheres offer advantages over solid metal spheres with regard to the control of diameter range and accuracy, as well as the compliant nature of the sphere and control of such compliance.
The ability to control sphere diameter with high precision in these products is a consequence of the way in which they are manufactured. Typically, the spheres are grown from a polymer seed, with the growth reaction being terminated precisely thereby controlling the particle geometries through the propagation of units at the molecular level. Such polymer spheres are then subjected to chemical processes to enhance adhesion of a thin metal seed coating (ordinarily, nickel) deposited by electrodeless plating. Gold is subsequently electrodelessly plated over the nickel seed layer. Electrodeless plating typically deposits metal layers up to about 500 Å in thickness; it is not a commercially practical method for metal coating deposition of any significant thickness. In addition, to date, it is not believed that solder or resinous materials may be coated onto such spheres in a commercially practical manner by electrodeless techniques.
Other approaches to enhancing polymer particle conductivity include doping their core material with conductive materials, such as silver filled thermoplastic resins. [See U.S. Pat. No. 5,531,942, (Gilleo).] The melting point of metal alloys is much higher than for thermoplastic resins; hence, the thermal performance of electronic devices using solder interconnection materials is superior to electronic devices using metal-filled thermoplastics, particularly in more demanding applications.
Arrays of solder-tipped metal conductors are known. [See U.S. Pat. No. 5,681,647 (Caillat).] However, the system described in the '647 patent is produced by a complex process involving cathodic sputtering of metals, multilayer film depositions, lithography and electroplating. Such a complex process is likely to be commercially unattractive at least because of the many process steps, and the choice of available chemistry is limited due to thermal sensitivity of the so-described process.
There are, however, disadvantages of metal-coated polymer spheres, including their typical limited current carrying capability by virtue of the small quantity of conductive material present. The metals traditionally used to coat the surface of polymer spheres are not fusible, and hence cannot wet substrate metalisations to make efficient joints. While low current carrying capabilities in particles bearing thin metal coats may be circumvented by increasing particle density, a bett
Barnes Rory B.
McArdle Ciaran
Bauman Steven C.
Dixon Merrick
Loctite (R&D) Limited
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