System and method for forming uniform rigid interconnect structures

Disclosed herein is a system and method for mounting semiconductor packages by forming one or more interconnects, optionally, with a wirebonder, and mounting the interconnects to a mounting pad on a target package. Mounting the interconnect may comprise ultrasonically welding the interconnects to the mounting pads, and the interconnect may be mounted via a mounting node on the end of the interconnect, wherein the mounting node may be formed by an electric flame off process. The interconnects may be trimmed to one or more substantially uniform heights, optionally using a laser or contact-type trimming system, and the tails of the interconnects may be supported during trimming. A top package may be bonded on the trimmed ends of the interconnects. During mounting, a support plate may be used to support the package, and a mask maybe used during interconnect mounting.

BACKGROUND

Generally, one of the driving factors in the design of modern electronics is the amount of computing power and storage that can be shoehorned into a given space. The well-known Moore's law states that the number of transistors on a given device will roughly double every eighteen months. In order to compress more processing power into ever smaller packages, transistor sizes have been reduced to the point where the ability to further shrink transistor sizes has been limited by the physical properties of the materials and processes. Designers have attempted to overcome the limits of transistor size by packaging ever larger subsystems into one chip (systems on chip), or by reducing the distance between ships, and subsequent interconnect distance.

One method used to reduce the distance between various chips forming a system is to stack chips, with electrical interconnects running vertically. This can involve multiple substrate layers, with chips on the upper and lower surfaces of a substrate. One method for applying chips to the upper and lower side of a substrate is called “flip-chip” packaging, where a substrate has conductive vias disposed through the substrate to provide an electrical connection between the upper and lower surfaces. These interposer substrates for flip chips are commonly silicon, glass or some other insulator with copper, gold or other conductors disposed in the vias through the interposer.

Through silicon vias (TSVs) are also used to create 3D integrated circuits, and are advantageous over wire bonding or other connection techniques because the technique permits a substantially higher density vias in a given amount of space, and because the length of the connections is shorter. A 3D package such as System in Package, Chip Stack Multi-Chip Module (MCM), etc. contains two or more chips (integrated circuits, ICs) stacked vertically so that they occupy less space and/or have greater connectivity. The different dies in the stack may be heterogeneous, e.g. combining CMOS logic, DRAM and III-V materials into a single IC. An alternate type of 3D package is Silicon Carrier Packaging Technology, where ICs are not stacked but a carrier substrate containing TSVs is used to connect multiple ICs together in a package. In most 3D packages, the stacked chips are wired together along their edges and this edge wiring slightly increases the length and width of the package and usually requires an interposer layer between the chips. In some 3D packages, through-silicon vias replace edge wiring by creating vertical connections through the body of the chips. The resulting package has no added length or width. Because no interposer is required, a TSV 3D package can also be flatter than an edge-wired 3D package. This TSV technique is sometimes also referred to as TSS (Through-Silicon Stacking or Thru-Silicon Stacking). A 3D integrated circuit (3D IC) is a single integrated circuit built by stacking silicon wafers and/or dies and interconnecting them vertically so that they behave as a single device. By using TSV technology, 3D ICs can pack a great deal of functionality into a small footprint.

Frequently, packages are joined using wire bonding, where a conductive wire is spot welded or soldered to a pad, and then cut and welded to a second pad. Gold is frequently used for both the bonding pads and wires in such a case, primarily due to gold's resistance to oxidation and relatively low welding temperature. Solder ball grid arrays are also a technique sometimes used to joining packages, with an array of solder balls deposited on the bonding pads of a first package, and with a second package joined at its own bonding pad sites to the first pad via the solder balls. The environment with the solder ball grid array is heated to melt the solder balls and the packages compressed to cause the solder balls to contact the pads on both packages.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the present embodiments are discussed in detail below. It should be appreciated, however, that the present disclosure provides many applicable concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the disclosed subject matter, and do not limit the scope of the different embodiments.

Embodiments will be described with respect to a specific context, namely making and using uniform rigid interconnects useful in, for example, package-on-package assemblies. Other embodiments may also be applied, however, to other electrically connected components, including, but not limited to, bare chips without packaging, displays, input components, board mounting, die or component mounting, or connection packaging or mounting of combinations of any type of integrated circuit or electrical component.

The present concepts are directed to providing a system and method for creating solid interconnects of uniform height to separate, support and electrically connect one or more electrical components. Solid interconnect systems may provide a higher density of interconnects than alternative methods of packaging, and reduce the failure rate of interconnected assemblies. Solid interconnects may be used to attach, or stack multiple packages vertically, connecting the stacked packaged via redirection layer (RDL) contacts, electrical traces, mounting pads or the like.

The solid interconnects may be referred to as studs, stud bumps or nails. The nails may be mounted between two packages and be used as stand-offs for package-on-package (PoP) assemblies, where the solid nail supports an upper package when stacked on a lower, carrier package. Component packages may be one or more components mounted onto a carrier board, or substrate. While the embodiments of the solid interconnects are described herein as solid, in particularly useful embodiments, the interconnects are solidly constructed, rigid or otherwise capable of securely separating opposing packages. Additionally, while the interconnects are also described as uniform, the interconnects need not be perfectly uniform, but are ideally substantially uniform in height, or at least substantially uniform across a desired plane. For example, interconnects may be mounted on a lower package, extending upward to accept or support an upper package mounted on the interconnects. The lower package may have mounting pads arranged in virtually any topography, however, trimming the tops of each interconnect to a generally uniform height may permit mounting a top package such that each interconnect contacts a mounting pad of other connection point on the top package. Disparate groups of interconnects may also each be trimmed to a predetermined, substantially uniform height, so that packages having a non-planar mounting pad topography, or multiple packages mounted at different heights, may be advantageously mounted on the interconnects.

In particularly useful embodiments of the presented principles, a package on package assembly may be created, or a package may be mounted, using interconnects that may optionally be pre-formed, created with bulk material or created in place by extrusion or via wire from a wirebonder system. The interconnects may be optionally formed with a mounting node on the end which may be created by, for example, an electric flame off process. The interconnects may be mounted to mounting pads on a target package, via ultrasonic welding, solder paste, solder, conductive adhesive, or the like, and may be cut to a rough length.

In this manner, multiple interconnects may be mounted and then trimmed, with groups of interconnects trimmed to one or more predetermined, substantially uniform heights. Interconnects may also be mounted and trimmed one at a time, or in groups. The trimming may be done with a laser, with another non-contact-type trimming system, or with a contact-type trimming system such as a blade, abrasive or hot wire. A laser trimming system may have a cutting beam wide enough to vaporize any excess material when trimming, and trimming may take place in a single pass, or in multiple passes. The interconnects may be supported at the tails, or the non-mounted ends, during trimming. After trimming, one or more top packages may be mounted on the interconnects, with the mounting pads on the top package bonded to the trimmed ends of the interconnects. The mounting material for the interconnects, target package and bottom package may be optionally reflowed during the mounting and trimming process to permanently bond the interconnects to the mounting pads of the target package and top package.

During mounting and trimming, a support plate may support the back side of the target package, particularly to minimize flexion or distortion of the target package when mounting the interconnects using a wirebonder or pressure inducing mounting technique such as ultrasonic welding. A mask may also be used to mask the interconnect side of the target package. The mask may have openings permitting the mounting and trimming of interconnects, and the mask may support and protect the target package during the interconnect processing steps.

With reference now toFIG. 1A, a first embodiment of method for applying and trimming a rigid interconnect100is depicted. A rigid interconnect may, in some embodiments, be formed from a wire104or another substantially solid material. In particularly useful embodiments, a interconnect placement system102may be a wirebonder system, a pick-and-place system, or the like. When using a wirebonder as the interconnect placement system102, the wirebonder may extend some length of wire104and a mounting node106may be formed on the wire104. One useful embodiment of the presented principles may be where an electric spark, or electric flame off, is applied to the end of the wire104, to melt the tip of the wire104forming the mounting node106. Such free air ball formation may be employed to form the mounting node106in situ, however the mounting node may be formed using any suitable method, including performing the interconnect116through extrusion, deposition, casting, milling or the like.

FIG. 1Billustrates application of the mounting node106to a mounting pad110. A target package109may have a substrate108with one or more RDL layers applied thereon, and may include one or more mounting pads110separated by insulating layers112or the like. The interconnect116may be attached to the mounting pad110through any suitable system, including, but not limited to, ultrasonic welding, soldering, arc welding, adhesives, solder paste and solder ball reflow or the like. In a deformation type attachment system such as ultrasonic welding, the interconnect placement system102may be used to apply pressure and ultrasonic or electrical energy to attach the interconnect116to the mounting pad110. A support plate and mask114may optionally be applied to the face of the target package109to support or protect the target package during the interconnect116processing.

FIG. 1Cillustrates an interconnect116attached to the mounting pad110, with the mounting node106generally flattened or deformed to more completely bond with the mounting pad110. In particular, ultrasonic welding or the like may be used in combination with pressure from the interconnect placement system102to deform the mounting node106to increase the area of contact between the mounting pad110and the interconnect116. Alternatively, a preformed nterconnect116may be advantageously used, and that pre-formed interconnect may have a sufficiently broadened mounting node106such that pressure causing deformation on mounting may not be required.

FIG. 1Dillustrates an interconnect116prepared for trimming. In some embodiments, a tool, die, clamp or other supporting system will hold the top, or tail, end of an interconnect116prior to trimming to ensure the interconnect116does not deform or shift during trimming. In an embodiment where the interconnect placement system102is a wirebonder, the wirebonder may act as the supporting system, and the wirebonding head may clamp the tail end of the interconnect116prior to trimming. Alternatively, a wirebonder102may cut the interconnect116to rough size, and then a separate clamp or reinforcing system may hold the interconnect116for trimming.

FIG. 1Eillustrates trimming of an interconnect116. In particularly useful embodiments, the trimming system120may be used to trim one or more interconnects116to separate a trimmed interconnect122from the excess interconnect tail118. The trimming system120may advantageously employ a laser, trimming wire, blade, saw, or any other system useful for accurately trimming an interconnect122.

FIG. 1Fillustrates a trimmed interconnect122separated from the interconnect tail118. In systems where a wirebonder is used to form the interconnect and to clamp the interconnect122for trimming, the interconnect122may be formed from a continuous wire so that the wirebonder may place numerous interconnects without reloading. The interconnect tail118may be the remaining wirebonder wire. In embodiments where the interconnect122is preformed, or where the interconnect122is cut from another bulk material before trimming, the interconnect122tail may be held or clamped during trimming, and any remaining material discarded.

In some embodiments the mask114may be a plate having openings arranged to permit application of interconnects122through the mask114openings, with the areas of a substrate or package109covered by the mask114to protect from residue or excess interconnect tail118material resulting from interconnect122trimming.

While the foregoing figures illustrate a single interconnect122being trimmed, the presented principles may be advantageously used to trim multiple interconnects122at once. Such gang trimming may, in one embodiment, be used to bring a plurality of interconnects122to a substantially uniform level for mounting an upper package. In such an embodiment, an upper package may have an array of planar mounting pads, and the interconnects122may be attached to the mounting pads using solder paste, conductive adhesive, or solder reflow techniques. Additionally, trimming the interconnects122may advantageously permit the interconnects122to be applied to a non-planar target package and still result in a uniform level of the interconnect122top surfaces. By extension, trimming interconnects122may also compensate for interconnects that are not orthogonal to the target package, that is, interconnects that may be mounted crookedly, or not mounted perfectly flat to the target package109mounting pad110. Thus, a target package109having mounting pads110at different RDL levels, or interconnects122applied unevenly can be compensated for prior to mounting an upper or top package.

FIG. 2Aillustrates an embodiment of method for trimming multiple interconnects. In this embodiment, a target package109may have a plurality of mounting plates110, each with an untrimmed interconnect116bonded thereon. Generally, the target package may have a substrate202with an optional insulating layer112disposed between the mounting pads110. A mask114may be applied to the target package109so that the interconnects116protrude through openings in the mask114.

The interconnects116may each protrude higher than a trimming plane204. The interconnects116may then be advantageously trimmed via a trimming system120, which may include, but is not limited to, a laser, trim wire, hot wire, abrasive, saw blade or the like. In this embodiment, with multiple untrimmed interconnects116, it may be impractical to support the tails of each untrimmed interconnect116during trimming, and thus, a non-contact trimming method such as a laser may be advantageous. Alternatively, a cutting tool that does not bend or deform the interconnects116, such as a hotwire operated well over the melting point of the interconnect material may be employed.

FIG. 2Billustrates a target package with trimmed interconnects122, andFIG. 2Cillustrates mounting of a top package218to the trimmed interconnects122. Any mask114may preferably be removed after trimming the interconnects122, but prior to mounting the top package218. The top package218may have a one or more mounting pads222with solder balls220or another adhesive that can effectively hold the top package in place when placed on top of the interconnects122.

FIG. 3illustrates a mask302and support plate304that may be advantageously used when applying or trimming solid interconnects122. The mask302may be placed on the bottom package, and may have one or more openings306through which the target package109mounting pads110are exposed. Additionally, a support plate304may be used to support the target package109when the interconnects116are applied. In embodiments where a deformation type attachment system is used to apply interconnects122to the target package109, a support plate304may assist in resisting flex in the target package when the interconnects122are applied.

FIG. 4is a flow diagram illustrating an embodiment of a method for applying and trimming rigid interconnects400in a package-on-package arrangement. Initially, a target package109may be prepared in block402by creating mounting pads110, applying solder paste or an organic solder preservative or the like. In one embodiment, the target package109may be a completed package, with housings or covers, passivation layers and the like in place before application of the mask104, and interconnects122to avoid the applied interconnects122interfering with later packaging steps.

Interconnects122may also be formed in block404before placing and bonding to a target package109in block406. The interconnects122may be formed as they are applied to the target package, as in a wirebonder application system. Alternatively, the interconnects122may be pre-formed in bulk through any suitable fabrication process.

After the target package109has been prepared, interconnects122may be placed and bonded on the target package109. In some embodiments, the interconnects122may be permanently bonded, through, for example, soldering, ultrasonic welding or arc welding. Alternatively, the interconnects122may be placed and bonded to the target package109using a non-permanent bonding technique, including, but not limited to solder paste, temporary soldering, or the like.

Once placed in block406, the interconnects may optionally be separated in block408from any bulk material if made in place. Thus, for example, where the interconnects122are formed from a continuous wire using a wirebonder, or the interconnects122are extruded in place, the interconnect122may be roughly cut, or separated from the material remaining in the interconnect placement system102.

The interconnects122are trimmed in block410. The trimming may be accomplished by trimming one, or more than one, interconnect at a time. For example, where a contact-type trimming system120, such as a hot wire or blade, is used, a clamping or interconnect122tail supporting system may grasp individual interconnect122tails, followed by the trimming system trimming the interconnect tail supported by the tail supporting system. Skilled artisans will recognize that greater throughput maybe achieved where a tail supporting system may clamp multiple interconnect122tails, permitting the trimming system120to trim multiple tails at once. Additionally, a non-contact trimming system120may also trim multiple interconnect122tails at once. For example, a laser trimming system120may be configured to extend across one or more interconnects, and the laser or cutting edge may be moved across the target package109to trim the interconnects122. Thus, the laser may trim multiple interconnect122tails in a single pass. In particularly useful embodiments of a laser trimming system120, the laser cutting area width may be thick or wide enough to vaporize all of the interconnect122tail extending above a predetermined height, leaving no excess material. Alternatively, a laser will at least vaporize part of the excess interconnect122material, and permit the remaining excess material to melt and flow onto the interconnect122, leaving the interconnect122at the desired height.

The placement and trimming, or trimming alone, may also be performed in stages. Thus, a trimming system120may trim one or more interconnects to be uniform on a first plane, and then trim other interconnects to be uniform across a different plane at a different height. Such an embodiment may be useful where multiple packages are mounted to a single target package109, or there a single top package218has non-planar mounting pads. In a bulk trimming system, such as a laser trimming system120, interconnects122maybe placed and bonded as in block406and then trimmed as in block410. These block406and410may be repeated one or more times, trimming the interconnects122at the same height as the first group already trimmed, or at a different height for each trimming pass. Additionally, another embodiment may have the trimming system cutting interconnects122at different heights during the same trim pass, as in block410, permitting placement of all the interconnects122, and trimming to multiple heights using a single trim pass.

Once the interconnects122are trimmed in block410, one or more top packages218may be mounted on the interconnects122. One embodiment of the presented principles may be where the interconnects122have substantially flat ends to provide superior contact with the mounting pads222.

Any final interconnect122processing may also be performed at this stage as well. For example, in one embodiment, any solder attaching the interconnects122to the target package109or top package218may be reflowed to permanently bond the interconnects122. This particular embodiment may be useful where laser trimming is used, as the laser applies little force to the interconnects122as they are trimmed, and the interconnects may be placed and initially bonded using a non-permanent bonding method that causes less pad deterioration or oxidation. Furthermore, another embodiment may further comprise applying an underfill between the target package109and top package218.

Although the present embodiments and their advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the disclosure as defined by the appended claims. It will be readily understood by those skilled in the art that many of the features and functions discussed above can be implemented using a variety of materials and orders to the processing steps. For example, interconnects may be virtually any shape, as long as they are rigid or otherwise capable of securely separating opposing packages. Interconnects may also be any conductive material, or even a semiconductor material where such material is called for. As another example, it will be readily understood by those skilled in the art that many of the steps for creating a package-on-package structure may be performed in any advantageous order while remaining within the scope of the present disclosure.