Semiconductor device manufacturing: process – Coating with electrically or thermally conductive material – To form ohmic contact to semiconductive material
Reexamination Certificate
1999-02-19
2001-05-29
Bowers, Charles (Department: 2813)
Semiconductor device manufacturing: process
Coating with electrically or thermally conductive material
To form ohmic contact to semiconductive material
C438S612000, C438S613000, C438S615000, C257S737000, C257S738000, C257S780000, C257S781000
Reexamination Certificate
active
06239013
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates in general to the field of electronic device packages and more particularly to a method and apparatus for attaching electrically conductive particles to a substrate.
BACKGROUND OF THE INVENTION
Without limiting the scope of the invention, its background is described in connection with electronic device packaging, as an example.
Modern electronic components utilize numerous integrated circuits. These integrated circuits are electrically connected to each other or to other electronic components. One method for connecting integrated circuits to electronic components utilizes an area array electronic package, such as a ball-grid array (BGA) package or a flip-chip package. The electrical connections between an integrated circuit packaged in an area array package design and a printed circuit board (PCB) are typically composed of solder.
With ball grid array packages, various input and output ports of an integrated circuit are typically connected via wire bonds to contact pads of the ball grid array electronic package. Solder balls formed on the contact pads of the ball grid array electronic package are used to complete the connection to another electronic component, such as a printed circuit board (PCB).
Integrated circuits are also connected to electronic components through a flip-chip electronic package design. The flip-chip electronic package is similar to the ball grid array electronic package in that solder balls are used to make a connection with other electronic components, such as a PCB. Solder balls are also used in a flip-chip design to attach the input and output ports of the substrate to the contact pads of the integrated circuit. As such, flip-chip packages do not require wire bonds. These solder balls or bumps may be formed on the face of integrated circuits as they reside on semiconductor wafers before being sawed into individual dies.
Therefore, an important step in the interconnection of many electronic components is the formation and attachment of solder balls.
Heretofore, in this field, solder bumps or balls have been typically formed utilizing one of four methods: (1) printing of solder paste through a stencil or mask; (2) electroplating; (3) evaporation; or (4) mechanical transfer of preformed solder spheres. While electroplating, printing of solder paste through a stencil or mask, and evaporation techniques have been typically utilized for forming solder bumps on wafers and integrated circuits, BGA and chip-scale packages (CSP) have commonly utilized printing of solder paste and mechanical transfer of solder ball techniques.
Transfer of solder balls has been customarily achieved by means of vacuum chucks or machined templates. Another method for transferring preformed solder balls utilizes formation of a pattern of dots onto a photoimageable coating laminated to an organic film. Typically the organic film is composed of a material having a high melting temperature that is capable of being exposed to temperatures exceeding 200 C. with very little degradation, such as polyimide.
The pattern is formed by placing a photomask on the coating and then exposing the coating to a dose of ultraviolet radiation. For example, for an area array package design, the photomask contains a mirror image of the contact pads design. The areas protected by the photomask design retain their adhesiveness while the unprotected areas exposed to the ultraviolet radiation lose their adhesiveness. The array of adhesive areas corresponds to the pattern of contact pads found on the substrate, wafer or die to receive the solder connections.
After the adhesive areas are formed, solder balls are loaded onto the surface of the film and attach to the adhesive areas. The excess solder balls that lie on non-adhesive areas are removed. The populated film is then aligned and brought into contact with contact pads, which may be fluxed. A solder reflow is performed to transfer the solder balls from the adhesive areas to the contact pads of the substrate. Following the reflow cycle, the film is removed from the solder balls.
SUMMARY OF THE INVENTION
It has been discovered that leaving the organic film or polyimide carrier in contact with the solder balls during reflow results in several problems. For example, flux residues generated during the reflow process have been trapped underneath the polyimide. The residue has often formed large deposits on the solder balls and substrate. These large deposits have made removal by cleaning more difficult.
In addition to flux residue, adhesive residue from the organic film has been left on the solder balls. Removal of the organic film prior to reflow may decrease the adhesive residue left on the solder balls.
Moreover, the polyimide sheet has pressed against the molten solder balls, creating flat tops which have required a second reflow to correct and conform to customers' specifications. An additional reflow process increases the cycle time as well as the costs of manufacturing. A second reflow may not be necessary in the absence of the polyimide sheet.
In addition, the organic film is composed of a material that has a high melting temperature and is capable of being exposed to temperatures exceeding 200 C. with very little degradation. One such material is polyimide; however, polyimide is significantly more expensive than other organic films such as Mylar® which are less capable of withstanding exposure to solder reflow temperatures. Removing the organic film before reflow may allow the substitution of cheaper organic films, thereby reducing manufacturing costs.
Also, the polyimide sheet has been apt to stretch or grow with increasing temperature according to its coefficient of thermal expansion. The thermal expansion may be non-uniform, resulting in wrinkling or sagging of the polyimide sheet. The wrinkling has flattened solder balls and has caused them to agglomerate together in clusters. The wrinkling has also caused the solder balls to push other solder balls off the contact pads, which may result in a non-functional device.
Furthermore, the photopolymer has reacted adversely to certain chemicals contained in the solder flux. The trapping of the fluxes by the polyimide sheet may exacerbate the situation, resulting in more unwanted reactions among the flux, photopolymer, and solder balls.
Therefore, a need has arisen for a method of attaching metal or solder balls to a substrate that overcomes the problems in the prior art. A need has also arisen for an apparatus for manufacturing an integrated circuit package that overcomes the problems in the prior art.
The present invention comprises a method for transferring metal or solder particles from an adhesive sheet to a substrate comprising the steps of obtaining an adhesive sheet having a plurality of adhesive areas, loading the particles onto the adhesive sheet, removing the particles not adhered to the adhesive areas, aligning the particles on the adhesive sheet with contact pads of the substrate, transferring thermal energy to the adhesive sheet, removing the adhesive sheet from the particles prior to reflow, and securely attaching the particles to the contact pads using reflow techniques, for example. In one embodiment of the present invention, the step of transferring thermal energy comprises maintaining a temperature less than the melting point of the particles.
The present invention also comprises an apparatus for manufacturing an integrated circuit package, comprising an adhesive sheet heated to a temperature less than the melting point of the particles and a plurality of particles connected to the adhesive sheet.
REFERENCES:
patent: 3719981 (1973-03-01), Steitz
patent: 5356751 (1994-10-01), Cairncross et al.
patent: 5909634 (1999-06-01), Hotchkiss et al.
patent: 9-82719 (1997-03-01), None
patent: 9-326412 (1997-12-01), None
Bowers Charles
Honeycutt Gary C.
Kielin Erik J
Navarro Arthur I.
Telecky Fred
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