The present invention relates in general to integrated circuit packaging and more particularly to a process for fabricating an improved integrated circuit package.
According to well known prior art IC (integrated circuit) packaging methodologies, semiconductor dice are singulated and mounted using epoxy or other conventional means onto respective die attach pads (attach paddles) of a leadframe strip. Traditional QFP (Quad Flat Pack) packages incorporate inner leads which function as lands for wire bonding the semiconductor die bond pads. These inner leads typically require mold locking features to ensure proper positioning of the leadframe strip during subsequent molding to encapsulate the package. The inner leads terminate in outer leads that are bent down to contact a mother board, thereby limiting the packaging density of such prior art devices.
In order to overcome these and other disadvantages of the prior art, the Applicants previously developed a Leadless Plastic Chip Carrier (LPCC). According to Applicants' LPCC methodology, a leadframe strip is provided for supporting several hundred devices. Singulated IC dice are placed on the strip die attach pads using conventional die mount and epoxy techniques. After curing of the epoxy, the dice are wire bonded to the peripheral internal leads by gold (Au), copper (Cu), aluminum (Al) or doped aluminum wire bonding. The leadframe strip is then molded in plastic or resin using a modified mold wherein the bottom cavity is a flat plate. In the resulting molded package, the die pad and leadframe inner leads are exposed. By exposing the bottom of the die attach pad, mold delamination at the bottom of the die pad is eliminated, thereby increasing the moisture sensitivity performance. Also, thermal performance of the IC package is improved by providing a direct thermal path from the exposed die attach pad to the motherboard. By exposing the leadframe inner leads, the requirement for mold locking features is eliminated and no external lead standoff is necessary, thereby increasing device density and reducing package thickness over prior art methodologies. The exposed inner leadframe leads function as solder pads for motherboard assembly such that less gold wire bonding is required as compared to prior art methodologies, thereby improving electrical performance in terms of board level parasitics and enhancing package design flexibility over prior art packages (i.e. custom trim tools and form tools are not required). These and several other advantages of Applicants' own prior art LPCC process are discussed in Applicants' U.S. Pat. No. 6,229,200, the contents of which are incorporated herein by reference.
In applicant's own U.S. Pat. No. 6,585,905, issued Jul. 1, 2003 and incorporated herein by reference, a process is provided for fabricating a leadless plastic chip carrier in which the semiconductor die sits in a portion of the die attach pad that is reduced in thickness. The leadframe strip is selectively etched to define a pattern for the die attach pad and the contact pads such that a portion of the die attach pad has reduced thickness compared to the thickness of the contact pads. The semiconductor die is mounted in the portion of the die attach pad with reduced thickness, followed by wire bonding and overmolding. This structure provides for several advantages. For example, the package profile can be reduced and the length of the wire bonds from the semiconductor die to the die attach pad and to the contact pads is reduced, thereby reducing electrical impedance in the package. Also, the three-dimensional nature of the die attach pad provides additional exposed metal for adhering to the molding compound, thereby providing a more robust package.
Still further improvements in high performance integrated circuit (IC) packages are driven by industry demands for increased thermal and electrical performance, decreased size and cost of manufacture.
According to one aspect of the present invention, there is provided a process for fabricating an integrated circuit package. The process includes: selectively etching a leadframe strip to define a die attach pad and at least one row of contact pads; mounting a semiconductor die to one side of the leadframe strip, on the die attach pad; wire bonding the semiconductor die to ones of the contact pads; releasably clamping the leadframe strip in a mold by releasably clamping the contact pads; molding in a molding compound to cover the semiconductor die, the wire bonds and a portion of the contact pads not covered by the clamping; releasing the leadframe strip from the mold; depositing a plurality of external contacts in the form of solder ball contacts, on the one side of the leadframe strip, on the contact pads, such that the external contacts protrude from the molding compound; and singulating to provide the integrated circuit package.
According to another aspect of the present invention, there is provided a integrated circuit package including a semiconductor die mounted to a die attach pad and at least one row of contact pads circumscribing the die attach pad. Wire bonds connect the semiconductor die to ones of the contact pads. A molding compound covers one side of the semiconductor die, the wire bonds and a portion of one side of the contact pads such that a remaining portion of the one side of the contact pads is uncovered and an opposing side of the contact pads is uncovered. External contacts are disposed on the one side of the contact pads, at the uncovered remaining portion, such that the external contacts protrude from the molding compound.
Advantageously, the integrated circuit package of the present invention provides for connection to a motherboard through the external contacts that protrude from the molding compound. The contact pads and the die attach pad are exposed on an opposing side of the package to allow heat dissipation from the package, from a side of the package that is opposite the side that faces the motherboard after assembly. Thus, heat is dissipated away from the motherboard. Also, air flow over the top of the package provides additional heat dissipation from the exposed side of the die attach pad.
In a particular aspect, the external contacts are solder balls that provide package standoff from the motherboard after assembly. This improves thermal characteristics of the package as thermal cycling of the package is less likely to cause fatigue failure of solder joints to the motherboard. Also, the solder balls facilitize reworking of the motherboard assembly by allowing for package removal and replacement or re-attachment to the motherboard without the addition of further solder.
During fabrication, the leadframe strip is releasably clamped in a mold by releasably clamping the contact pads, followed by molding in the molding compound. Thus, during molding, the contact pads are protected from mold flash contamination, a significant problem in prior art packages. This obviates the need for the use of deflash tape that is placed on the strip before molding to reduce the occurrence of mold flash contamination, thereby reducing manufacturing cost. Also, the need for deflash abrasion of the package is obviated.
In a particular embodiment, the die sits in a portion of the die attach pad that is reduced in thickness and therefore the length of the wire bonds can be reduced. Also, the wire bonds are lower in the profile of the package, which allows for reduction of the integrated circuit package profile.
Also, the profile of the mold used in molding the package varies. Thus, the thickness of the molding compound is not uniform in the resulting package. Also, the molding compound is not continuous. The molding compound is broken up by the contact pads, where molding compound flows between but does not cover portions of the contact pads. This reduces the stress caused by mismatch in the coefficients of thermal expansion between the molding compound and the leadframe, in comparison with prior art packages.
The invention will be better understood with reference to the drawings and to the following detailed description in which like numerals denote like parts, and in which:
Reference is made to the Figures to describe a process for fabricating an integrated circuit package according to an embodiment of the present invention. The integrated circuit package is indicated generally by the numeral 18 and includes a semiconductor die 22 mounted to a die attach pad 24 and at least one row of contact pads 26 circumscribing the die attach pad 24. Wire bonds 28 are then bonded to connect the semiconductor die 22 to ones of the contact pads 26. A molding compound 30 covers one side of the semiconductor die 22, the wire bonds 28 and a portion 32 of one side of the contact pads 26 such that a remaining portion 34 of the one side of the contact pads 26 is uncovered and an opposing side of the contact pads 26 is uncovered. External contacts 36 are disposed on the one side of the contact pads 26, at the uncovered remaining portion 34, such that the external contacts 36 protrude from the molding compound 30.
The integrated circuit package 18 will now be described in more detail with reference to
Referring first to
Referring to
As shown in
The contact pads 26 are also etched to provide portions of different thickness. As shown in
As shown in
The leadframe strip 20 is then releasably clamped in a modified mold by releasably clamping the remaining portion 34 of the contact pads (
After curing the molding compound 30, the leadframe strip is released from the mold (
Next, a plurality of external contacts 36 in the form of solder ball contacts are deposited on the exposed remaining portion 34 on one side of the contact pads 26 (
Singulation of the individual units from the full leadframe strip 20 is then performed by die punching, resulting in the integrated circuit package shown in
Specific embodiments of the present invention have been shown and described herein. Modifications and variations to these embodiments may occur to those skilled in the art. For example, the size and shape of many of the elements may differ while still performing the same function. In one alternative embodiment, rather than tin/lead eutectic solder as described above, the external contacts 36 can be substantially lead-free solder.
Still other modifications and variations may occur to those skilled in the art. All such modifications and variations are believed to be within the sphere and scope of the present invention.
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