Adhesive bonding and miscellaneous chemical manufacture – Methods – Surface bonding and/or assembly therefor
Reexamination Certificate
1999-08-05
2001-09-04
Ball, Michael W. (Department: 1733)
Adhesive bonding and miscellaneous chemical manufacture
Methods
Surface bonding and/or assembly therefor
C156S275300, C156S275500, C156S275700, C029S832000, C029S831000, C349S150000, C257S783000
Reexamination Certificate
active
06284086
ABSTRACT:
INTRODUCTION
The present invention relates to electronic circuit packages and more particularly to a direct chip attachment (DCA) method incorporating anisotropic conductive adhesives.
Increasing circuit element density in integrated circuits as well as the increasing number of addressable cells in current generation Liquid Crystal Displays, and the requisite increase in input/output (I/O) terminals of these and other current generation microelectronic devices has caused the electronics industry to move away from traditional peripheral wire bonding between the microelectronic device and its mounting substrate (e.g. chip carrier, circuit card, flex circuit, etc.) to interconnection schemes capable of supporting greater densities of I/Os per chip such as tape automated bonding (TAB) and various “flip chip” bonding methods. In the flip chip process, a two-dimensional array of solder wettable terminals are provided on the bottom surface of the microelectronic device. A mirror image array of solder wettable terminals are provided on the mounting substrate. Solder bumps are deposited on the solder wettable terminals on the chip. The chip is then turned upside down (hence the name flip chip) with the solder bumps on the chip aligned with the terminals on the substrate and the chip is joined to the substrate by the reflow of the solder bumps. An underfill of epoxy is then provided to secure the chip to the substrate and to limit moisture infiltration. The flip chip process accommodates very high I/O density because the entire bottom surface of the chip is made available for I/O connections. The solder bumps used in the flip chip process ordinarily comprise a low tin solder alloy having a relatively high melting point. Low tin alloy solders have superior thermal fatigue characteristics and the high melting point reduces the risk that subsequent soldering operations on the substrate will loosen the flip chip bond. The relatively high joining temperature (between about 340° C. to about 380° C.), however, precludes the use of direct chip attachment methods with temperature sensitive components such as Liquid Crystal Displays (LCD's).
A prior art method for adapting the flip chip bonding approach to polyimide flex circuit substrates is the Fiber Push Connection (FPC) method illustrated in FIG.
1
. In the Fiber Push Connection method, an Nd: YAG laser having a wavelength of 1,060 nanometers is directed through a glass fiber contacting the back side of the polyimide flex circuit. Since the transparency of polyimide is about 88% at the wavelength of the Nd: YAG laser, the majority of the laser energy passes through the flex circuit and is absorbed by the copper leads on the flex circuit. The copper leads are heated by the laser energy until the solder bump on the flip chip melts and reflows, thereby establishing a connection between the flex circuit lead and the terminal on the flip chip. A fairly complex feedback control system including an infrared detector must be used in order to avoid overheating the copper lead and causing it to delaminate from the polyimide backing.
For high impedance tolerant devices such as liquid crystal displays, a highly cost effective alternative to flip chip attachment has been the use of anisotropic conductive adhesives (ACA's) to bond the display chip to the substrate. In a conventional anisotropic conductive adhesive bonding process, as shown in
FIG. 2
, an anisotropic adhesive comprising conductive particles suspended in an insulating matrix is applied between the bonding pads of a microelectronic device and the substrate. The microelectronic element is then pressed against the substrate until the gap between the bonding pads of the element and substrate is less than the diameter of the conductive particles. Some of the conductive particles are trapped between the bond pads of the microelectronic element and the bond pads of the substrate, thereby completing the circuit. If the pitch of the bond pads and the diameter and density of the conductive particles are properly controlled, circuits will be properly made between opposing bond pads without shorts forming between adjacent pads.
Standard ACA attachment methods comprise thermal compression bonding, in which the microelectronic device and substrate are pressed together, typically at a force of at least one-half ton per square inch of bump area and typically about five tons per square inch of bump area at a temperature typically above 200° C. Although the bond temperatures used to cure ACA materials are significantly lower than the temperatures necessary for conventional flip chip bonding, even for moderately large LCD cells, residual stresses caused by the difference in thermal coefficient of expansion (TCE) between the cell and the substrate will distort the cell after the bonding process is completed. The distortion caused by the residual stresses will manifest itself as non-uniform color fidelity and other undesirable optical distortions across the surface of the display cell.
Current flip chip on glass (FCOG) processes utilize an ACA material that is initiated by ultraviolet light transmitted through the glass substrate. The UV curable ACA does not require exposure to high temperatures and therefore results in little or no residual stress between the chip and its glass substrate. Unfortunately, UV curable ACA materials must be used with glass substrates which are transparent to the ultraviolet light. They are not compatible with flex circuitry because the polyimide material used in flex circuitry is almost totally opaque to UV light. Although fiber push connection would be a feasible alternative for producing a low stress attachment that is compatible with polyimide flex circuit substrates, as discussed hereinbefore, the FPC method requires a costly and complex feedback loop to prevent overheating the circuit traces with the high laser power utilized in the FPC method. Moreover, FPC requires a special blackening treatment to be applied to the copper traces to cause them to absorb the energy from the Nd: YAG laser. An untreated copper trace reflects about 97% of the energy from the Nd: YAG laser, and therefore would not heat quickly enough to melt the solder bumps before the polyimide backing was damaged. The special blackening treatment also adds to the cost of the FPC method. Accordingly, what is needed is a method for bonding a microelectronic element to a flexible substrate using a low temperature ACA material that can be photo initiated by a wavelength of light that can pass through the polyimide flex circuit substrate.
SUMMARY OF THE INVENTION
The present invention comprises a very low stress attachment method in which a microelectronic device is attached to a flexible substrate by a photo initiated anisotropic conductive adhesive. In a preferred embodiment of a method of attaching a microelectronic device to a substrate in accordance with the present invention, an infrared photo initiated anisotropic paste is applied to the bonding terminals of a microelectronic device. The flex circuit is then aligned with the corresponding pads on the device. The flex circuit is then biased against the microelectronic device and the infrared photo initiated anisotropic adhesive is exposed to infrared light by means of a dual purpose light guide that presses against the back side of the polyimide flex circuit and simultaneously receives and guides light energy from an Nd: YAG laser to the back side of the polyimide flex circuit. Light energy from the laser passes through the polyimide substrate and between the circuit tracks of the flex circuit to excite the photo initiator of the photo initiated ACA material, thereby causing it to cure. Preferably the ACA material is a dual cure epoxy such that the photo initiator is required only to begin the polymerization process and a secondary (low-temperature thermal or preferably cationic) polymerization process will carry out through the bulk of the material.
REFERENCES:
patent: 4811081 (1989-03-01), Lyden
patent: 5744557 (1998-04-01), McCormick et al.
pa
Cardellino Terri
Richards Michael
Ball Michael W.
Gallagher & Kennedy P.A.
Three - Five Systems, Inc.
Titus John D.
Tolin Michael A.
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