System and method for multi-chip module die extraction and replacement

A system and method are provided in which a first chip in a stacked multi-chip module configuration is affixed via one or more adhesion layers to a first portion of a partitioned interposer unit. Planar partitions of the interposer are physically bonded via multiple solder “bumps,” which possess high tensile strength but low resistance to horizontal shear force or torque. A second chip is affixed via one or more adhesion layers to the second portion of the partitioned interposer. The chips may thus be separated by horizontally and oppositely shearing or twisting the first and second portions of the partitioned interposer away from one another.

TECHNICAL FIELD

Embodiments of the present invention relate generally to integrated circuit device packaging, and more specifically to multichip module packages that may have stacked chip arrangements.

BACKGROUND

When it comes to chip or integrated circuit device packaging, it is often desirable and sometimes imperative to have a relatively high device packaging density. Device packaging density can be defined as the number of devices per unit package volume. To such end, multichip module (MCM) packages are increasingly attractive for a variety of reasons. For example, MCM packages, which contain more than one chip per package, provide increased functionality of a given package, and decrease the interconnection length among chips in the package, thereby reducing signal delays and access times among chips.

One common MCM package is the three-dimensional “stacked” MCM package, in which one chip is disposed on a substrate and one or more other chips are stacked successively on top of one another and the first chip. Interconnections among chips and conductive traces on the common substrate are electrically made via bond wires.

FIG. 1shows a cross-sectional view of a stacked multichip module (MCM) package110according to a known configuration. As shown, the MCM package110includes a substrate111, a first chip112and a second chip113. First chip112includes a bondable surface121and an active surface122. Bondable surface121is adhered to substrate111by means of an adhesive, such as an epoxy, thermoplastic material, tape, tapes coated with thermoplastic materials, etc. Active surface122includes an active circuit area (not shown) typically in the center of first chip112, and multiple bonding pads112alocated peripheral to the active circuit area. Similarly, second chip113includes a bondable surface123and an active surface124. Active surface124also includes an active circuit area (not shown) typically in the center of second chip113, and multiple bonding pads113alocated peripheral to the active circuit area.

The active circuit area of first chip112is covered by a passivation layer125. An adhesive layer126is interposed between and connects passivation layer125and an interposer127. Interposer127is often made of a material similar in properties to first chip112and second chip113in order to avoid thermal expansion mismatch over temperature variations. For example, if first chip112and second chip113are made of bulk silicon, interposer127should also be made of silicon. Interposer127has a thickness sufficient to allow clearance and access to the bond pads112aalong the edges of first chip112. Interposer127also serves as a pedestal for supporting second chip113. An adhesive layer128is disposed between and connects interposer127and bondable surface123of second chip113.

Several bond wires114are bonded to and between respective bonding pads112aon first chip112and substrate111. Similarly, several bond wires116are bonded to and between respective bonding pads113aon second chip113and substrate111.

One application in which stacked MCM packages are commonly used is space applications, or applications in other environments wherein physical space is limited and tolerance to high levels of radiation required. Packages with such tolerance to high levels of radiation are referred to as “hardened” packages. The chips in such hardened packages are also typically “hardened” through the addition of redundant circuitry and/or error detection and correction circuitry so that the chips function properly in high radiation environments like space. Due to the hardened nature of the chips used in such environments, manufacturing costs for these chips can be inordinately expensive—often tens or even hundreds of times more expensive than counterparts of equivalent complexity used in consumer applications. For example, a hardened microprocessor could cost $10,000.

Unfortunately, due to current methods for manufacturing an MCM package in a stacked configuration, each chip or die in an MCM is so securely affixed to those above and below it that separation of that chip from the body of the MCM requires processes that are expensive, require high amounts of heat, or both. Thus, reworking or replacing a chip or die that has failed within a stacked MCM package often results in the destruction of one or more chips immediately above or below it. This naturally multiplies the cost of the original chip failure.

Consequently, it is desirable to provide an improved system and method for both manufacturing stacked MCM packages, and for reworking specific failed chips or dies within such packages.

SUMMARY

A need still exists, therefore, for providing an MCM package that allows the removal and/or replacement of one or more failed chips or dies without inflicting concomitant damage or destruction to nearby chips that would otherwise still be functional. In particular, there is a need to provide such functionality that is low in cost and allows for the removal or replacement to occur at room temperature.

According to certain embodiments of the present invention, a system and method are provided in which a first chip in a stacked MCM configuration is affixed via one or more adhesion layers to a first portion of a partitioned interposer unit. Planar partitions of the interposer are physically bonded via multiple solder balls or “bumps,” which possess high tensile strength but low resistance to horizontal shear force or torque. A second chip is affixed via one or more adhesion layers to the second portion of the partitioned interposer. The chips may thus be separated by horizontally and oppositely shearing or twisting the first and second portions of the partitioned interposer away from one another.

DETAILED DESCRIPTION

In the following description, certain details are set forth in conjunction with the described embodiments of the present invention to provide a sufficient understanding of the invention. One skilled in the art will appreciate, however, that the invention may be practiced without these particular details. Furthermore, one skilled in the art will appreciate that the example embodiments described below do not limit the scope of the present invention, and will also understand that various modifications, equivalents, and combinations of the disclosed embodiments and components of such embodiments are within the scope of the present invention. Embodiments including fewer than all the components of any of the respective described embodiments may also be within the scope of the present invention although not expressly described in detail below. Finally, the operation of well-known components and/or processes has not been shown or described in detail below to avoid unnecessarily obscuring the present invention.

FIG. 2is a cross-sectional schematic representation of a two-part interposer unit200in accordance with an embodiment of the present invention. The interposer unit200comprises a top partition250and a bottom partition255. Because these interposer partitions are to be affixed to integrated circuit dies or chips, they are ideally constructed of material similar in properties to those chips in order to avoid thermal expansion mismatch over temperature variations. For example, if the integrated circuit chips are made of bulk silicon, interposer partitions250and255should also typically be made of silicon. Solder bumps260are disposed between partitions250and255and possess a high tensile strength, enabling the partitions250and255to be firmly affixed to one another along the vertical dimension indicated by arrow256. It may require over ten pounds of force to break the bonds created by solder bumps260in the vertical dimension256. However, the bumps are susceptible to relatively low horizontal shear force254or rotational torque257about the vertical dimension256as illustrated inFIG. 2A. This rotational torque257is often less than a pound of such force. Thus, by applying rotational torque257to rotate the top partition250relative to the bottom partition255, the partitions can be separated. This enables interposer partitions250and255to be separated without excessive force and without the use of a high-heat environment, either of which could cause the chips in the MCM package to become damaged or destroyed.

FIG. 3is a cross-sectional view of an MCM package310including the partitioned interposer unit200ofFIG. 2in accordance with an embodiment of the present invention. MCM package310includes a substrate111, a first chip112and a second chip113. Substrate111is adhered to first chip112, which includes an active surface122on its top side that is covered by a passivation layer125and adhered to that passivation layer via adhesive layer126. Similarly, second chip113includes a bondable surface123along its lower side and an active surface124on its upper side. An adhesive layer128is bonded to bondable surface123. First and second chips112and113are respectively wire-bonded to substrate111by bond wires114and116.

The upper interposer partition250is connected to second chip113via adhesive layer128and also soldered to lower interposer partition255via solder bumps260. Solder bumps260, as discussed above with respect toFIG. 2, provide a high tensile strength for vertically bonding the upper and lower interposer partitions250and255, but are easily broken via horizontal shear force254or rotational torque257. Lower interposer partition255is, in turn, adhered to passivation layer125via adhesive layer126.

FIG. 4shows a cross-sectional view of another embodiment, presenting a possible torque vector for separating upper and lower interposer partitions450,455of an interposer449of MCM package400. Substrate411is affixed below first chip412via adhesive layer425. The top of first chip412is in turn affixed to lower interposer partition455via adhesive layer430, and lower interposer partition455is affixed to upper interposer partition450via a plurality of solder bumps460. The upper interposer partition450is affixed to second chip413via adhesive layer440. No bond wires appear inFIG. 4, although in practice both chips in the MCM package are still wire-bonded to substrate411.

To examine the MCM package400ofFIG. 4in operation of the interposer449, assume that second chip413has been shown to be nonoperational. This type of failure can occur at the time of the chip manufacture or through the course of operation over time. In either case, it is advantageous to be able to remove (and possibly replace) the non-operational chip.

Force line AA shows a line substantially bisecting the solder bumps460. The vertical placement of line AA is arbitrary with respect to the vertical extension of the bumps. To separate upper and lower interposer partitions350and355, a technician or end-user collectively creates torsion in solder bumps460by applying equal but opposing shear forces to upper and lower interposer partitions350,355respectively, parallel to line AA and the plane of substrate411. The shear force thus provided to each solder bump460causes the bonds formed by those solder bumps to break and allows the separation of MCM package400along line AA. Because the force required to effectuate the removal of second chip413is so low, it can be accomplished without damaging first chip412. Furthermore, because the chip removal can be done at room temperature, it may be performed in the same area where modules are tested, allowing immediate confirmation that the first chip has not been damaged.

FIG. 5shows the MCM package400ofFIG. 4after separation. Solder bumps460have each been sheared away at line AA, leaving the lower interposer partition455separated from upper interposer partition450and second chip413. If desired, the remainder of solder bumps460may be mechanically or chemo-mechanically removed so that a new chip and interposer unit may replace the non-operational (and now removed) chip413(not shown), as discussed in more detail below.

FIG. 6shows an embodiment of the present invention in which a replacement chip613has been affixed (via adhesion layer640) to replacement upper interposer partition650. Replacement upper interposer partition650is affixed to replacement lower interposer partition655via solder bumps660. Finally, replacement lower interposer partition655is affixed to the existing lower interposer partition455via new adhesive layer630. In this way, the lower interposer partition does not need to be removed from the existing first chip412; a new interposer unit (comprising replacement upper interposer partition650, replacement lower interposer partition655, and solder bumps660) is simply adhered to the lower partition of the pre-existing interposer unit fromFIG. 4.

In certain embodiments, the surface of the dies or of the interposer partitions are constructed with such shape or footprint as may easily integrate with a rotational tool, such as a wrench, to enable faster and more precise breaking of the bonds between the upper and lower interposer partitions. In other embodiments, a technician or other user may simply separate the interposer partitions by applying the needed horizontal shear force or torque by hand.

FIG. 7shows a cross-sectional partial view of a stacked MCM package700according to another embodiment of the present invention, in which an encapsulant702has been additionally disposed between the upper and lower interposer partitions450and455. In some circumstances, the weak horizontal shear resilience of the solder bumps may not be sufficient to provide the strength or stability desired in the MCM package as a whole. By forming an encapsulant such as an epoxy compound around the solder bumps460, the MCM package700is strengthened and stabilized while retaining the ability to cheaply and safely remove a particular nonoperational chip. Typically, the encapsulant702is disposed around the solder bumps460and between upper and lower interposer partitions450and455after the chips412and413have already been operationally tested.

FIG. 8is a block diagram of an electronic system800, an exemplary instance of which may be a satellite system, including electronic circuitry810and the multi-chip module package (MCM)300ofFIG. 3. Typically, the electronic circuitry810and MCM package300are coupled to a memory802. Also typically, the electronic circuitry810is coupled through address, data, and control buses to the MCM package300to provide for writing data to and reading data from the MCM package. The electronic circuitry810includes circuitry for performing various computing functions, such as executing specific software to perform specific calculations or tasks. In addition, the electronic system800includes one or more input devices804, such as a keyboard or mouse for local input or receivers for receiving input from remote or ground locations, coupled to the electronic circuitry810to allow an operator to interface with the electronic system. Typically, the electronic system800also includes one or more output devices806coupled to the electronic circuitry810, such output devices typically including transmitters (for relaying information to remote or ground locations) and display devices. One or more data storage devices808are also typically coupled to the electronic circuitry810to store data or retrieve data from external storage media (not shown). Examples of typical storage devices808include hard and floppy disks, tape cassettes, compact disc read-only (CD-ROMs) and compact disc read-write (CD-RW) memories, and digital video discs (DVDs).

It is to be understood that even though various embodiments and advantages of the present invention have been set forth in the foregoing description, the above disclosure is illustrative only, and changes may be made in detail, and yet remain within the broad principles of the invention. For example, variations on certain embodiments described above or depicted in the drawings may include three or more chips in a single MCM package, many or all of which may be separated by interposer units in accord with the present invention. As another example, certain embodiments may include additional structures affixed within the MCM package between one or more chips and a respective interposer unit. Therefore, the present invention is to be limited only by the appended claims.