Flexible preformed planar structures for interposing between a chip and a substrate

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TECHNICAL FIELD OF THE INVENTION

The invention relates to semiconductor "flip-chip" manfacturing techniques and, more particularly, to the fluxing and soldering steps employed in flip-chip manufacture.

BACKGOUND OF THE INVENTION

"Flip-chip" manufacturing techniques involve soldering one or more semiconductor (silicon) chips (one is discussed), in face-to-face relationship, to another semiconductor chip termed a "substrate". Typically, solder balls (otherwise known as pads or bumps) are formed (raised above the planar surface of the chip and substrate) on facing surfaces of both the chip and the substrate at intended points of contact between the two, liquid flux (rosin) is often applied to the face of the chip and/or substrate, the chip is mechanically held in register with the substrate, and the chip and the substrate are subjected to elevated temperature to effect soldering, or fusion of the solder balls on the chip and the corresponding solder balls on the substrate.

The "solder balls" on either the chip or substrate, typically those on the substrate, may be solderable metallized surfaces. The soldering process may be carried out in a reducing atmosphere. A typical flip-chip structure is shown in FIG. 1, and is discussed in greater detail hereinafter.

Previous systems of rigid attachment of chips to chucks have been used for chip alignment, but they must allow some degree of compliance because the chips tend to change relative alignment during soldering by surface tension between the solder balls. The addition of liquid flux to the chip/substrate (flip-chip) assembly creates capillary attraction between the chip and the substrate, which serves to mis-align the chip with respect to the substrate. This is illustrated in FIG. 2, and is discussed in greater detail hereinafter. Further, much of the flux that is applied to the flip-chip assembly is wasted. Still further, the dimension of the remaining gap between the chip and the substrate and the mechanical properties of the solder joints formed by the solder balls and corresponding solder balls tends to be indeterminate.

The present invention is more broadly (i.e., than chip-to-chip) directed to connecting one or more semiconductor "chips" (dies) to one or more "substrates" such as other semiconductor dies, printed circuit (or wiring) boards, and the like. The resulting assembly is termed a "flip-chip structure".

DISCLOSURE OF THE INVENTION

It is therefore an object of the invention to provide a flip-chip manufacturing technique which reduces capillary action, and hence misalignment, between a chip and a substrate.

It is a further object of the invention to provide a flip-chip manufacturing technique which requires the use of less flux, and which controls the position of the flux between the chip and the substrate.

It is a further object of the invention to provide a flip-chip manufacturing technique which provides a controlled spacing between chips and the substrate.

It is a further object of the invention to provide a flip-chip manufacturing technique which provides solder joints having predictable and tailorable mechanical characteristics.

It is a further object of the invention to provide a flip-chip manufacturing technique that simplifies the face-to-face joining of the chip and substrate.

According to the invention, a preformed planar structure is interposed between the chip(s) and the substrate in a flip-chip structure. The preformed planar structure establishes a minimum gap between the chip(s) and the substrate.

According to a feature of the invention, liquid flux is applied to the preformed planar structure in order that flux is selectively applied to the solder balls (pads) on the chip and the substrate.

In an embodiment of the invention, the preformed planar structure is provided with through holes in registration with the solder balls (pads) on the chip(s) and the substrate. In this embodiment, liquid flux selectively fills the through holes for delivery to the solder balls during soldering. The through holes also aid in maintaining registration of the chip(s) and the substrate.

According to an aspect of the invention, the through holes are sized to establish a predetermined mechanical structure of solder joints formed by the solder balls when fused together.

According to an aspect of the invention, the preformed planar structure has a planar core and opposing planar faces. The core is formed of thermosetting organic resin, such as polyimide, or non-organic material such as alumina, polished sapphire, beryllium oxide, aluminum or aluminum nitride. The planar faces of the preformed planar structure are formed of thermoplastic resin or thermosetting material, such as polyacetal, epoxy (epoxy resins) or polystyrene. The preformed planar structure tends to draw the chip(s) together to the substrate, establishing a flip-chip structure or mechanical integrity.

According to an aspect of the invention, the preformed planar structure has a thickness of 5-50 microns, preferably on the order of 20-30 microns.

Additional and further embodiments and variations of a preformed planar structure are set forth below, with respect to the descriptions of FIGS. 6-15.

Other objects, features and advantages of the invention will become apparent in light of the following description thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a typical, prior art flip-chip structure.

FIG. 2 is a cross-sectional view of a prior art flip-chip assembly illustrating capillary action caused by liquid flux, resulting in the misalignment of a chip with respect to a substrate.

FIG. 3 is an exploded cross-sectional view of a flip-chip assembly, prior to soldering, according to a technique of the invention.

FIG. 4 is a perspective view of a plastic standoff element (preformed planar structure) employed in the technique of FIG. 3.

FIG. 5 is a perspective view of an alternate embodiment of a standoff element suitable to be employed in the technique of FIG. 3.

In the description of further embodiments of a preformed planar structure that follows, the preformed planar structure may alternately be referred to as an "interposer".

FIG. 6 is an exploded cross-sectional view of a flip-chip assembly incorporating a preformed planar structure (interposer), according to the invention.

FIG. 6a is a cross-sectional view of another embodiment of a preformed planar structure, similar to that shown in FIG. 6, according to the invention.

FIG. 6b is a cross-sectional view of another embodiment of a preformed planar structure, similar to that shown in FIG. 6a, according to the invention.

FIG. 6c is a cross-sectional view of an encapsulated semiconductor die assembled to a preformed planar structure, according to the invention.

FIG. 7 is a cutaway view of a preformed planar structure including a ring-shaped array of angled through holes, according to the invention.

FIG. 7a is a top view of the preformed planar structure of FIG. 7.

FIG. 7b is a top view of a preformed planar structure with a rectangular array of angled through holes, according to the invention.

FIG. 8 is a cross-sectional view of a semiconductor device assembly employing a preformed planar structure as a connection "pitch adapter", according to the invention.

FIG. 8a is a cross-sectional view of a semiconductor device assembly employing a multi-layer preformed planar structure as a connection pitch adapter, according to the invention.

FIG. 9a is a view of a semiconductor device assembly employing dissolvable preformed planar structures, according to the invention.

FIG. 9b is a view of a the semiconductor device assembly of FIG. 9a, after dissolving the preformed planar structures.

FIG. 10a is a view of a ring-shaped preformed planar structure, according to the invention.

FIG. 10b is a view of a gapped ring-shaped preformed planar structure, according to the invention.

FIG. 10c is a close-up top view of a portion of a leg of a kerfed ring-shaped preformed planar structure (interposer), according to the invention.

FIG. 10d is a side view of the kerfed interposer of FIG. 10c.