Method for forming a flip chip on leadframe semiconductor...

Details Method for forming a flip chip on leadframe semiconductor... Method for forming a flip chip on leadframe semiconductor...

2001-08-21

2003-04-22

Elve, M. Alexandra (Department: 1725)

Metal fusion bonding

Process

Plural joints

C228S207000, C228S215000, C228S223000, C228S254000

Reexamination Certificate

active

06550666

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to forming a flip chip semiconductor package on a leadframe, and more particularly to forming a flip chip semiconductor package with a bumped semiconductor die on a leadframe.
BACKGROUND OF THE INVENTION
In semiconductor packaging, a relatively sensitive and difficult to handle semiconductor die is encapsulated in a package with external connections. Packaging allows the semiconductor die to be more conveniently handled, and it also allows external circuitry to be easily coupled thereto.
A known method of forming a flip chip on leadframe (FCOL) semiconductor package employs a plated leadframe. A leadframe is a patterned sheet of metal, typically copper, that has been plated, usually with silver, nickel or palladium. Conventionally, a leadframe is plated to prevent the copper from oxidizing, and to provide a surface to which solder will adhere. The pattern of the sheet of metal provides a leadframe for forming a semiconductor package.
Currently, leadframes for forming FCOL semiconductor packages have leads with inner lead portions and outer lead portions. The inner lead portions are arranged in a pattern with interconnect locations on the inner lead portions matching the pattern of pads on a semiconductor die. During the packaging process, typically eutectic solder is deposited on the interconnect locations. In addition, the pads on the semiconductor die are bumped. Bumping can comprise metal posts extending from the pads of the semiconductor die with solder balls on free ends of the metal posts. Typically, the solder balls are made of high lead solder.
U.S. patent application Ser. No. 09/564,382 by Francisca Tung, filed on Apr. 27, 2000, titled “Improved Pillar Connections For Semiconductor Chip”, and Continuation-In-Part U.S. patent application Ser. No. 09/843,248 by Francisca Tung, filed on Apr. 27, 2001 titled “Pillar Connections For Semiconductor Chips and Method Of Manufacture”, and assigned to a common assignee as this patent application, teaches forming pillar bump structures as described herein. These patent applications are incorporated herein by reference thereto.
Subsequently, the solder balls on the die, and the semiconductor die is flipped over, and placed on the leadframe, with the solder balls abutting the solder paste deposits on the interconnect locations. The assembly is then reflowed using an appropriate reflow profile.
Under the elevated reflow temperatures, the solder paste deposits melt, and with the assistance of the flux, the eutectic solder adheres to the interconnect locations on the leadframe and the high lead solder balls on the copper posts, thus forming solder interconnects between the high lead solder balls on the free ends of the metal posts and the interconnect locations on the leadframe. After reflow, when normal flux is used the assembly is cleaned to remove residual flux and encapsulated in mold compound. However, when no-clean flux is employed, the cleaning step is not necessarily required. The resultant package is known as a FCOL semiconductor package.
A disadvantage of this process is, when the solder paste melts, the molten solder tends to flows across the surface of the lead portions. This flow of solder is often referred to as overrun, and results in a variety of adverse effects in FCOL semiconductor packages.
A first concern is, when the solder flows away from an interconnect location, the respective solder interconnect constitutes less solder than required to provide a reliable electrical connection between the solder balls and the interconnect locations. A second concern is, solder interconnects formed with the reduced amount of solder do not support the semiconductor die evenly on the leadframe. Consequently, the planarity of the semiconductor die on the leadframe is adversely affected, and a non-planar die can give rise to shorting between the metal posts on the die. This condition is sometimes referred to as a collapsed die.
A third concern is the overrun results in solder flowing over the edges and onto the opposite surface of the lead portions. Later, during molding mold compound will not adhere well to the affected surfaces. A fourth concern is wicking, which occurs when a lead portion on a leadframe is shaped such that there is a small gap between the side of a downset die and the lead portion, and where there is an interconnect location close to the edge of the die. In this arrangement, the solder from the interconnect location can flow along the lead portion and, through capillary action, flow upwards through the small gap.
Further, eutectic solder paste is disposed on the interconnect locations using a printing process. This process of dispensing solder is known to suffer wide process variations. Consequently, the amount of solder paste dispensed on a leadframe can vary considerably. Such variations in the amount of solder paste dispensed for a particular semiconductor package leads to variations in the resultant solder joints, adversely affecting the reliability of the semiconductor package.
In an effort to reduce costs of producing FCOL semiconductor packages, un-plated or bare copper leadframes, simply referred to as copper leadframes, have been tried. However, to a large extent, the copper leadframes suffer the same disadvantages discussed hereinabove as the plated leadframe, and in some instances to a greater degree.
BRIEF SUMMARY OF THE INVENTION
The present invention seeks to provide a method for forming a flip chip on leadframe semiconductor package, which overcomes or at least reduces the abovementioned problems of the prior art.
Accordingly, in one aspect, the present invention provides a method for forming a flip chip semiconductor package, the method comprising the steps of:
a) providing a patterned layer of metal conductors having a first surface for providing a pattern of interconnect locations thereon;
b) providing a semiconductor die having a first surface with a corresponding pattern of pads thereon, the pads having non-reflowable material thereon:
c) disposing a predetermined amount of reflowable conductive material on the non-reflowable material;
d) placing the semiconductor die on the patterned layer of metal conductors, wherein the reflowable conductive material abuts the interconnect locations; and
e) reflowing the reflowable conductive material, wherein a substantial portion of the reflowable conductive material remains substantially at the interconnect locations to form conductive interconnects between the non-reflowable material and the interconnect locations.
In another aspect the present invention provides a method for determining the amount of reflowable material to be disposed on non-reflowable bumps on a semiconductor die to mount the semiconductor die on a leadframe having interconnect locations thereon, the method comprising the steps of:
a) determining surface area of one of the non-reflowable bumps;
b) defining a corresponding interconnect location on the leadframe having substantially the same area as the surface area of the one of the non-reflowable bumps; and
c) selecting an amount of reflowable material such that a substantial portion of the selected amount of reflowable material remains at the interconnect location during reflow.


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