Electricity: conductors and insulators – Conduits – cables or conductors – Preformed panel circuit arrangement
Reexamination Certificate
1999-03-12
2002-01-08
Cuneo, Kamand (Department: 2841)
Electricity: conductors and insulators
Conduits, cables or conductors
Preformed panel circuit arrangement
C361S768000, C228S180220, C174S257000
Reexamination Certificate
active
06337445
ABSTRACT:
This invention relates generally to the field of integrated circuit interconnections, and more particularly to a bumped connection structure used on integrated circuits, and on other assemblies, such as integrated circuit packages.
BRIEF DESCRIPTION OF PRIOR ART
A number of forces have influenced the selection o structures which provide connection of an integrateded circuit to the next level of interconnection. Lead density has been a major factor in focusing designers toward connections which are contained within the area of the integrated circuit or of first level packaging, such as flip-chip, ball grid array (BGA) and chip size packages (CSP). These interconnections are generally referred to as area array interconnections and are formed by a bump type of structure as opposed to a flexible lead type of structure which extends out from the perimeter of the integrated circuit element.
Connections for these assemblies are designed as a finely pitched matrix of conductive contacts on the surface with a corresponding matrix of contact pads on the next level of interconnection. Typically the connecting structures have included solder balls or bumps of various lead (Pb) and tin (Sn)compositions, which are reflowed to form the electrical and mechanical connection.
Flip-chip is the oldest of these technologies wherein the input/output contact pads on the surface of the integrated circuit device are soldered directly to the corresponding contact pads of a substrate. The original flip-chip concept employed small solder coated copper balls sandwiched between the chip termination lands and the contact pads of the ceramic substrate. Owing to manufacturing difficulty in handling and placement of the balls, this procedure was replaced by forming solder connection structures or bumps on the chip terminals while still in wafer form. This consisted of depositing thin films of metal on the wafer and patterning the contact pads by photolithography and etch processing and forming bumps on said contact pads by evaporating solder through apertures in metal masks. The wafers were diced, the chips aligned to the substrate and solder reflowed. Owing to the short, rigid connecting structures, fatigue at the solder joints is a concern. Waste treatment for clean-up of lead containing solder from said metal masks is both expensive and a growing environmental issue. Alternately, solder bumps and/or solder over copper bumps have been formed on the contact pads by combinations of sputtered and electroplating.
More recently, techniques have been described for forming flip-chip connecting structures by capturing prefabricated spheres of solder in a matrix of tacky areas defined by photosensitive polymers, aligning the matrix which corresponds to the contact pads on the chips in wafer form and releasing by reflowing the solder.
Flip-chip interconnections have continued to evolve because the technology provides advantages for maximum lead density, and very low inductance. However, the connecting structures have been largely restricted to the use of lead/tin solders of different compositions which are subject to fatigue failures as a function of thermally induced stresses. Further, lead (Pb) containing solders on the surface of soft error sensitive devices has caused concern due to alpha particle emission from lead (Pb), as well as environmental issues with the use of lead (Pb).
Ball grid array (BGA) packages have gained acceptance as low cost, high yielding packages which offer the maximum in board space efficiency as a result of direct connections under the package to the printed wiring board. Both plastic and ceramic BGA packages have been commercialized. Connections between the integrated circuit and the top surface of the BGA package can be by wire bonding or by flip-chip connection. The connecting structures on the bottom surface of the BGA package are typically solder bumps configured in an area array. Typically the structures are formed by partially reflowing eutectic or other lead/tin solders to metal contact pads on the package and these in turn are attached to the printed wiring board by reflow using a solder paste which has been screened onto contact pads on the board. Inspection of the joints has been a concern for BGA packages assembled to boards. The reliability concerns are somewhat similar to those for flip-chip in that thermally induced stresses in the solder joints result from mismatches CTE and Young's modulus and which may vary within the package and board area due to the large size and underlying construction. In addition, stand-off height must be controlled, both to minimize stresses on the solder joints and to allow cleaning. Package weight can contribute to the difficulty in controlling stand off distance of solder bumped packages.
Chip scale (CSP) packaging provides the minimum size at no more than 1.2 times that of the integrated circuit, and is a directly surface mountable package which facilitates testing and ease of handling. There are a number of package styles available, both with leads and bump connection structures. The bumped structures, typically Sn/Pb solder bumps are attached by reflow of prefabricated spheres onto a contact pad on the CSP. As with BGA and flip-chip, the presence of leaded compounds presents an environmental issue to some users. And in particular, thermal mismatch must be compensated in order to avoid solder fatigue failures because the package is dominated by rigid, low CTE silicon in close proximity to a printed wiring board. Clean-up of fluxes presents reliability concerns with the small stand-off height of CSP.
For some specific applications where assembly temperature is restricted by the circuit being assembled, such as liquid crystal displays, raised polymeric structures have been formed on contact pads. The exposed surface of the structure is coated with a metallic film, and attached to the corresponding pads by solder reflow or by conducting adhesive materials. This process of fabricating polymeric bumps is extremely expensive, is limited by processing capabilities and in some cases will not be compatible with thermal testing routinely required of integrated circuits.
Similarly, composite bump structures of metal bumps with tin or other metallic coatings have been formed on wafers or on polymeric flex films for tape automated bonding (TAB) connections. The fabrication process is not unlike that described above for polymeric bumps and is expensive.
SUMMARY OF THE INVENTION
The primary object of this present invention is to provide an integrated circuit, or circuit assemblies with bump connections which allow attachment to the next level of interconnection by using prefabricated structures coated with solder-compatible metals, and which are attached to the circuit or circuit package by solder. The integrated circuit structures compatible with the present invention are those which employ bump or ball connections typical of area array assembly, namely but not limited to ball grid array packages, flip-chip assembly and chip scale packages.
The composite bump structures comprise two or more thin films of solder compatible metal or metals coated onto a prefabricated structure and connecting said structure to the contact pads of an integrated circuit element by solder. The prefabricated core element is comprised of metal, ceramic or polymer and is of the approximate size and uniformity as specified by the integrated circuit element. The connecting structure will be compatible with known manufacturing techniques for attachment to the next level of interconnection. This device provides an economical and reliable connection and does not have the disadvantages of devices assembled by the techniques described above.
Another object of the invention is to provide a solderable composite prefabricated connecting structure whose thermal coefficients of expansion and Young's modulus are selected to improve reliability.
Still another object of the invention is to provide a solderable composite prefabricated bump structure whose thermal conductivity is optimize
Abbott Donald C.
Romm Douglas W.
Cuneo Kamand
Honeycutt Gary C.
Navarro Arthur I.
Telecky Fred
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