Semiconductor device manufacturing: process – Packaging or treatment of packaged semiconductor – With vibration step
Reexamination Certificate
1999-02-02
2001-03-20
Bowers, Charles (Department: 2813)
Semiconductor device manufacturing: process
Packaging or treatment of packaged semiconductor
With vibration step
C438S108000, C228S245000
Reexamination Certificate
active
06204094
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates in general to the field of electronic device packaging 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 must often be 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
In order to produce a functional electronic device, it is preferable that the loading of the solder balls onto an organic film having discrete adhesive areas thereon results in 100% population of the adhesive areas. It has been discovered, however, that loading of solder balls is adversely affected by.many factors. For example, generation of electrostatic charges through the effects of tribocharging has often resulted in increased numbers of excess solder balls residing on the surface and clinging together in groups. Tribocharging is the ionic charging of particles resulting from moving them in the air. Without extensive ionizing equipment, it has been difficult to remove the excess solder balls or to break up the pairs and triplets. Tribocharging is also affected by the relative humidity of the surrounding air.
Moreover, the surface texture and contamination of solder balls have had an adverse effect on the attachment of solder balls to the adhesive areas. For example, solder balls with surface oxides have been more apt to collect electrostatic charges than oxide-free solder balls.
Furthermore, the adhesiveness of the adhesive areas has affected the ability of the adhesive areas to capture and retain solder balls. Areas with insufficient adhesiveness have often failed to capture or retain solder balls, even following repeated attempts. Such a failure to attach a solder ball to each adhesive area may result in the failure of the electronic device.
In addition to the difficulties associated with loading the solder balls, it has also been discovered that problems exist with respect to the removing of excess solder balls. Removal of the extra solder balls that are not intended to be attached to the adhesive areas has generally been accomplished by two methods. In one method, a gas stream is passed over the film to remove any excess solder balls. In particular, the gas stream may be at a slight angle to the surface of the film. The forces created by the stream, however, may not exceed the adhesiveness of the adhesive areas. Oftentimes increasing the adhesiveness of the adhesive areas also increases the adhesiveness of the background which confounds the removal process. The removal is more effective if the gas stream is integrated with a ionizer such as an AC corona discharge to prevent tribocharging of the organic film and solder balls.
In another method, mechanical means for removing solder balls has been utilized, albeit less successfully than gas removal, prior to the present invention. Strictly mechanical removal has also been thought to be more difficult to automate. For example, vibrating tables, manual gyrations or shaking, and pulsating the solder balls have been attempted and have sometimes been used in conjunction with a gas stream.
Although the gas stream may remove extra solder balls, it is not an effective way of increasing the chances that an adhesive area will contact and capture a solder ball during loading. The gas stream only moves the spheres sideways, whereas the preferred motion is a force acting normal to the surface of the adhesive film. It is preferable if the spheres bounce on top of the adhesive film. This type of motion may be effectively accomplished mechanically.
Therefore, a need has arisen for an automated method of attaching solder balls to, and removing excess solder balls from, an adhesive film. A need has also arisen for an apparatus for attaching solder balls to a substrate.
The present invention disclosed herein may comprise a method for attaching solder particles to, and removing excess solder particles from an adhesive sheet, comprising the steps of obtaining an adhesive sheet having a plurality of adhesive areas, loading the solder particles to the adhesive sheet, transferring kinetic energy to the solder particles for distributing the solder particles on the adhesive sheet, transferring kinetic energy to the solder particles for removing the solder p
Hotchkiss Gregory B.
Lessard Robert J.
Berezny Nema
Bowers Charles
Honeycutt Gary C.
Navarro Arthur I.
Telecky Fred
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