This invention relates to a microelectronic assembly that includes an integrated circuit die attached to a substrate by solder bump interconnections. More particularly, this invention relates to such assembly that includes an overmolded polymeric encapsulant that extends within a gap between the integrated circuit die and the substrate to protect the solder bump interconnections and further that defines an optical window to allow the integrated circuit device to receive and emit optical signals through the substrate.
A typical flip chip microelectronic assembly comprises an integrated circuit die, also commonly referred to as a chip, mounted on a substrate, such as a printed circuit board, by solder bump interconnections that physically attach the chip to the substrate and also form electrical connections for conducting electrical signals to and from the chip for processing. To form the assembly, solder bumps are affixed to bond pads disposed on the active face of the chip. The chip is arranged on the substrate, and the arrangement is heated and cooled to reflow the solder and thereby form the interconnections. In the assembly, the chip is spaced apart from the substrate by a gap. The space about the solder bump interconnections within the gap is typically filled with a polymeric encapsulant. The encapsulant protects the solder bump interconnections from corrosion, and also reinforces the interconnections to withstand vibration and other mechanical forces to which the assembly is subjected during use.
Also, because of thermal cycling experienced by the assembly during operation, it is an important role of the encapsulant to reduce stresses in the interconnections due to differences in expansion and contraction of the die relative to the substrate during operation. This thermally induced stress, if not for the encapsulant, would cause fatigue in the solder and lead to fracture of the interconnections and failure of the assembly. The difference in expansion and contraction are indicated by a mismatch in the coefficients of thermal expansion, referred to as CTE. In general, it is desired to use an encapsulant having a CTE similar to the adjacent materials. For example, for a printed circuit board formed of a common polymer glass laminate, it is desired to adjust the encapsulant CTE to between about 12 and 17 parts per million (ppm) per degree Centigrade (° C.), whereas a CTE between about 6 and 10 ppm per ° C. is preferred for glass substrates. The encapsulant is composed of a polymeric matrix and contains an addition, typically between about 75% and 90% percent by weight, of particulate silica or other inorganic filler to reduce the CTE to within the desired range. The high filler content produces an encapsulant that is opaque.
It is known to fabricate an integrated circuit die that includes an optical element. For example, a digital camera comprises a die having an array of light sensors that receive and process light to produce an image. Other known dies include optical emitter or detector for sending or receiving optical signals for processing. The optical element is formed as on the active face of the die and receives or emits signals generally perpendicular to the face. When incorporated in a flip chip assembly, in which the active face faces the substrate, the optical signal is received through or emitted toward the substrate. Opaque encapsulant within the gap blocks the optical signal and thus interferes with useful operation of the assembly.
Therefore, a need exists for a microelectronic assembly in which the integrated circuit chip, is mounted onto a substrate by solder bump interconnections, and wherein the solder bump interconnections are protected by a polymeric encapsulant disposed within the gap between the chip and the substrate, and further wherein the encapsulant defines an optical window for transmitting optical signals to or from the chip through the substrate.
In accordance with this invention, a microelectronic assembly comprises an integrated circuit die mounted on a substrate by a plurality of bump interconnections. The die includes an active face having a central region surrounded by a perimeter region, and is arranged relative to the substrate such that the active face faces the substrate and is spaced apart by a gap. The bump interconnections are bonded to the perimeter region of the die and to the substrate, to thereby attach the die to the substrate. The assembly also includes a polymeric encapsulant about the die on the substrate and extending into the gap to encapsulate the interconnections. The encapsulant defines an optical window within the gap underlying the central region. It is an advantage of this invention that the window allows optical access to the active face of the die, including the optical sensors thereon. Moreover, the assembly may be formed using an encapsulant having a desired CTE that is adjusted for protecting the bump interconnections from thermally induced stress.
In one aspect of this invention a method is provided for forming a microelectronic assembly having an overmolded polymeric encapsulant that defines an underchip optical window. The method comprises attaching an integrated circuit device to a substrate by a plurality of bump interconnections, such that the active face of the die faces the substrate spaced apart by a gap. The bump interconnections are bonded to the die at a perimeter region surrounding a central region. The method further comprises molding or otherwise disposing a polymeric encapsulant about said integrated circuit device on said substrate such that the polymeric encapsulant extends within the gap to encapsulate the bump interconnections, but not within the central region. In a preferred embodiment, this is accomplished by forming a molding cavity about the die on the substrate, injecting a polymeric material into the cavity at a first pressure effective to initiate flow into the gap about the bump interconnections, reducing the applied pressure to prevent flow of the polymeric material into the gap adjacent the central region, and thereafter curing the polymeric material to form the encapsulant. In this manner, injection of the polymeric material is controlled to assure protection of the bump interconnections without blocking optical access to the central region of the die.
This invention will be further described with reference to the accompanying drawings in which:
In accordance with a preferred embodiment of this invention, referring to
In the preferred embodiment, die 12 is attached to substrate 14 by a plurality of solder bump interconnections 24. For this purpose, die 12 comprises bond pads 26 distributed about the perimeter region in a perimeter array. Substrate 14 includes bond pads 28 in a corresponding arrangement to register with bond pads 26. Connections 24 are formed of near-eutectic tin-lead solder alloy or other suitable solder alloy and bond to pads 26 and 28 to attach die 12 to substrate 14. Connections 24 are also adapted for transmitting electrical signals to and from die 12 for processing. Although solder bump interconnections are used in the preferred embodiment, the assembly may suitably comprise stud bump connections wherein a bump, typically formed of gold, is affixed to the bond pad on the die and attached to the substrate pad by solder or conductive adhesive. As used herein, bump interconnections refers to solder bump interconnections, stud bump interconnections or other suitable interconnections formed within the gap to mechanically and electrically attach the die to the substrate. In the preferred embodiment, it is a significant feature that die 12 is attached such that optical feature 22 faces substrate 14 and is spaced apart by a gap 30.
In accordance with this invention, assembly 10 further comprises an overmolded polymeric encapsulant 32 to protect die 12 on substrate 14. Preferably, encapsulant 32 forms a continuous body that overlies rear face 34 of die 12 opposite active face 16 and bonds to the surface of substrate 14 about the die. Alternately, the encapsulant 32 may be disposed about the die without covering the rear face. Significantly, encapsulant 32 extends within gap 30 to encapsulate interconnections 24. The encapsulant composition suitably comprises particulate inorganic filler, such as silica particles, dispersed in a thermoset polymeric matrix, which is preferably an epoxy polymer. In general, it is desired to formulate the encapsulant to contain particulate silica or other filler in an amount, typically between about 75% and 90%, to adjust the CTE to within a desired range. The desired CTE is dependent upon the nature of the substrate, and is between about 6 and 10 ppm per C for a preferred glass substrate. The high filler content renders the encapsulant opaque. In addition, the encapsulant commonly includes a carbon powder addition, typically less than 1%, that imparts a black color. By way of example, a suitable encapsulant material is commercially available from Cookson Semiconductor Inc. under the trade designation 200.302B. It is a significant feature that the encapsulant encloses the interconnections 24 within gap 30 and preferably exhibits a CTE comparable to the substrate material. During use, die 12 and substrate 14 are subjected to cyclic heating and cooling caused by ambient temperature fluctuations, or as the result of heat generated by the die during operation. Encapsulant 32 surrounds the interconnections to reduce stress that would otherwise result from differences in the expansion or contraction of the die and substrate. In addition, encapsulant 32 also forms a barrier to protect the interconnections from the atmosphere that would otherwise tend to cause corrosion of the solder alloy, and reinforces the interconnections against damage due to vibration or other mechanical forces.
In accordance with this invention, encapsulant 32 within gap 30 is limited to the perimeter region 18 and does not extend within the central region 18. In this manner, encapsulant 32 defines an optical window 36 adjacent optical feature 22 on die 12. During use, light, indicated by arrow 38, propagates through substrate 14 and through optical window 36 and is received by optical feature 22 for detection and processing. In an alternate embodiment wherein feature 22 emits light, the emitted light is transmitted through window 36 and substrate 14. In any event, the absence of encapsulant material allows the light to be transmitted through the optical window without interference.
Referring now
Following attachment, a mold 50 is positioned about the die on the substrate. Mold 50 engages the substrate about the die attach region, so that the mold and substrate cooperate to form a molding cavity 52. A charge of an encapsulant precursor material 56 is injected into cavity 52 through an opening 54 in mold 50. In a preferred embodiment, the material comprises particulate silica filler dispersed in a liquid phase that contains curable epoxy polymer compound. Preferably, cavity 52 is evacuated prior to injecting the polymeric material to facilitate charging and minimize gas bubbles in the product encapsulant. During injection, pressure is applied to the material to increase flow into the cavity. The applied pressure initiates flow of the material into the gap about the interconnections. In accordance with this invention, following initial flow of the material within the cavity, the applied pressure is reduced to a second value that limits flow of the material into the gap. As a result, flow of the material is confined to the perimeter region, and the window is formed adjacent the central region. The material is cured within the mold at about 165° C., thereby forming the encapsulant, whereafter the mold is removed.
Referring to
Therefore, this invention provides a flip chip microelectronic assembly having an overmolded encapsulant that defines an optical window for transmitting light to and from the active face of the die. Despite the window, the encapsulant extends within the gap sufficient to protect the solder bump interconnections. It is a particular advantage of this invention that the encapsulant material may be formulated to exhibit a coefficient of thermal expansion comparable to the die and substrate. As a result, the encapsulant is effective to reinforce the interconnections to withstand thermally induced stresses due to cyclic heating during use. In commercial materials, adjustment of the coefficient of thermal expansion is accomplished by addition of a filler that renders the encapsulant opaque. Nevertheless, this invention permits use of conventional materials assuring sufficient flow of the encapsulant about the interconnection to provide adequate protection, while forming a widow to assure optical access to the die active face.
While this invention has been described in terms of the preferred embodiments thereof, it is not intended to be so limited, but rather only to the extent set forth in the claims that follow.