Molded leadframe substrate semiconductor package

Description

RELATED APPLICATIONS

This patent application is a Divisional application which claims priority under 35 U.S.C. section 121 of the co-pending U.S. patent application Ser. No. 12/002,054, filed Dec. 14, 2007, entitled MOLDED LEADFRAME SUBSTRATE SEMICONDUCTOR PACKAGE which claims benefit of priority under 35 U.S.C. section 119(e) of U.S. Provisional Patent Application 60/875,162 filed Dec. 14, 2006, entitled MOLDED-LEADFRAME SUBSTRATE SEMICONDUCTOR PACKAGE and U.S. Provisional Patent Application 60/877,274 filed Dec. 26, 2006, entitled MOLDED-LEADFRAME SUBSTRATE SEMICONDUCTOR PACKAGE, which are all incorporated herein by reference.

FIELD OF THE INVENTION

The present invention is in the field of semiconductor packaging and is more specifically directed to package with heat transfer.

BACKGROUND

The increasing demand for computer performance has led to higher chip internal clock frequencies and parallelism, and has increased the need for higher bandwidth and lower latencies. Processor frequencies are predicted to reach 29 GHz by 2018, and off-chip signaling interface speeds are expected to exceed 56 Gb/s. Optimization of bandwidth, power, pin count, or number of wires and cost are the goals for high-speed interconnect design. The electrical performance of interconnects is restricted by noise and timing limitations of the silicon, package, board and cable. To that end, semiconductor packages must be made smaller, conforming more and more closely to the size of the die encapsulated within. However, as the size of the package shrinks to the size of the die itself, the size of the package becomes insufficient to support the number of leads generally required by current applications.

Chip Scale Packages (CSP) have emerged as the dominant package for such applications. FIG. 1 shows an example of a CSP in current practice. More specifically, the package in FIG. 1 is a Wafer Level Chip Scale Package 10 (WLCSP), commonly marketed by companies such as National Semiconductor Corporation as the Micro SMD and Maxim Integrated Products as the UCSP. Generally, solder bumps 11 are formed on processed and completed semiconductor wafers 12 before the wafers are sawn to form an individual semiconductor device 13. Although this has dramatically reduced package size and can be useful in some instances, it suffers from drawbacks which remove it from consideration for certain applications. First, the pitch between the solder bumps 11 must be made wide enough to effectuate assembly of the device onto a printed circuit board in application. This requirement can force manufacturers to artificially grow die sizes to meet the minimum pitch, thereby increasing cost. Second, the total I/O count of the device is generally constrained due to the decreased reliability at the high bump counts. At bump counts higher than 49, or a 7×7 array, reliability becomes critical and applications such as hand held devices, which require a high degree of reliability, no long become a possible marketplace.

To overcome the issues mentioned above, the semiconductor industry has moved toward Ball Grid Array (BGA) packages. The BGA is descended from the pin grid array (PGA), which is a package with one face covered (or partly covered) with pins in a grid pattern. These pins are used to conduct electrical signals from the integrated circuit (IC) to the printed circuit board (PCB) it is placed on. In a BGA, the pins are replaced by balls of solder stuck to the bottom of the package. The device is placed on a PCB that carries copper pads in a pattern that matches the solder balls. The assembly is then heated, either in a reflow oven or by an infrared heater, causing the solder balls to melt. Surface tension causes the molten solder to hold the package in alignment with the circuit board, at the correct separation distance, while the solder cools and solidifies. The BGA is a solution to the problem of producing a miniature package for an IC with many hundreds of I/O. As pin grid arrays and dual-in-line (DIP) surface mount (SOIC) packages are produced with more and more pins, and with decreasing spacing between the pins, difficulties arose in the soldering process. As package pins got closer together, the danger of accidentally bridging adjacent pins with solder grew. BGAs do not have this problem, because the solder is factory-applied to the package in exactly the right amount. Alternatively, solder balls can be replaced by solder landing pads, forming a Land Grid Array (LGA) package.

FIG. 2 shows a cutaway image of a generic BGA package 20. Generally, an IC 21 has bondpads 22 to which bondwires 23 are affixed. The IC 21 is mounted on a substrate 24. In current practice, the substrate 24 is a laminate, such as polyimide. Generally, the substrate 24 is of a similar construction to a PCB. The substrate 24 has copper patterns 25 formed thereon. The bondwires 23 effectuate electrical contact between the IC 21 and the copper patterns 25. The copper patterns 25 are electrically connected to solder balls 26 through via holes 27 in the substrate 24. In most embodiments of BGA packages, the IC 21 is encapsulated by a mold compound 28. Although BGA packages effectuate large I/O count devices in small areas, they are susceptible to moisture. Generally, moisture seeps into packages while awaiting assembly into a finished product, such as a computer. When the package is heated to solder the device into its end application, moisture trapped within the device turns into vapor and cannot escape quickly enough, causing the package to burst open. This phenomenon is known as the “popcorn” effect. What is needed is a semiconductor package that is robust to both structural stressors and moisture.

SUMMARY OF THE DISCLOSURE

In one aspect of the invention, a process for forming a land grid array package comprises at least partially encasing a first leadframe strip in a first mold compound thereby forming a molded leadframe strip, mounting at least one semiconductor device on the molded leadframe strip, mounting bondwires on the at least one semiconductor device to effectuate electrical contact between the semiconductor device and the molded leadframe, at least partially encasing the molded leadframe strip, the semiconductor device, and bondwires, and singulating the molded leadframe strip to form discrete land grid array packages. In some embodiments, The process further comprises embossing at least one step cavity into the molded leadframe strip for encapsulating the at least one semiconductor device. Optionally, a cap is mounted thereby forming a full cavity into the molded leadframe strip for encapsulating the semiconductor device. The cap comprises at least one of the following materials: glass, silicon, ceramic, metal, epoxy, and plastic. In some embodiments, a second leadframe strip is coupled to the first leadframe strip to form a dual leadframe strip. The first leadframe strip and the second leadframe strip are able to be coupled by a soft metal which comprises at least one of the following materials: gold, silver, lead, and tin. The first and second mold compounds can be identical or differing materials.

In another aspect of the invention, an apparatus for forming a land grid array package comprises means for at least partially encasing a first leadframe strip in a first mold compound thereby forming a molded leadframe strip, means for mounting at least one semiconductor device on the molded leadframe strip, means for mounting bondwires on the at least one semiconductor device to effectuate electrical contact between the at least one semiconductor device and the molded leadframe, means for at least partially encasing the molded leadframe strip, the at least one semiconductor device, and bondwires in a second mold compound, and means for singulating the molded leadframe strip to form discrete land grid array packages. In some embodiments, the apparatus further comprises an embossing surface for forming a step cavity into the molded leadframe strip for encapsulating the at least one semiconductor device. Optionally, the apparatus further comprises means for mounting a cap thereby forming a full cavity into the molded leadframe strip for encapsulating the at least one semiconductor device. The cap comprises at least one of the following materials: glass, silicon, ceramic, metal, epoxy, and plastic. In some embodiments, the apparatus comprises means to couple the first leadframe to a second leadframe by a soft metal. The soft metal comprises at least one of the following materials: gold, silver, lead, and tin. The first and second mold compounds can be identical or differing materials.

In another aspect of the invention, a land grid array package comprises a first leadframe, a substrate for supporting the first leadframe, at least one semiconductor die mounted on the first leadframe, a plurality of bondwires to effectuate electrical contact between the at least one leadframe and the at least one semiconductor die. In some embodiments, the substrate comprises a first mold compound. Furthermore, the semiconductor device can comprise a second mold compound for at least partially encasing the at least one leadframe, the substrate, the at least one semiconductor device and the plurality of wirebonds. In some embodiments, the package further comprises a step cavity. Alternatively, the package comprises a cap for forming a full cavity. Optionally, package comprises a second leadframe coupled to the first leadframe by a soft metal. The soft metal is comprised of at least one of the following materials: gold, silver, lead and tin.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features of the invention are set forth in the appended claims. However, for purpose of explanation, several embodiments of the invention are set forth in the following figures.

FIG. 1 is a prior art Chip Scale Package.

FIG. 2 is a prior art Ball Grid Array package in cross section.

FIG. 3 is a process for forming a molded leadframe per an embodiment of the current invention.

FIG. 4A is a process for forming a molded leadframe per an embodiment of the current invention.

FIG. 4B is a process for forming a molded leadframe per an embodiment of the current invention.

FIG. 4C illustrates two exemplary processes for forming a molded leadframe of the current invention.

FIG. 5 is a process for forming individual packages per an embodiment of the current invention.

FIG. 6A is a semiconductor package per an embodiment of the current invention.

FIG. 6B is apparatus for realizing the package depicted in FIG. 6A.

FIG. 6C is an alternate process for forming a package in FIG. 6A.

FIG. 6D is the remainder of the process for forming the package FIG. 6A.

FIG. 6E is an alternate apparatus for realizing the package depicted in FIG. 6A.

FIG. 7 is a process for forming a Land Grid Array (LGA) package.

FIG. 8 is a process for forming a Step Cavity LGA package.

FIG. 9 is a process for forming a Cavity LGA package.

FIG. 10 is a process for forming a Dual Leadframe LGA package.

DETAILED DESCRIPTION

In the following description, numerous details and alternatives are set forth for purpose of explanation. However, one of ordinary skill in the art will realize that the invention can be practiced without the use of these specific details. In other instances, well-known structures and devices are shown in block diagram form in order not to obscure the description of the invention with unnecessary detail. For example, it is commonly known in the art of semiconductor device assembly that assembly is generally done on a matrix array of leadframes, often referred to as leadframe strips, each strip having a plurality of individual positions that will be processed in various ways to form individual packaged semiconductor devices. A position can have one or more semiconductor die within.

In a first aspect of the invention, a process 300 for forming semiconductor packages is detailed in FIG. 3. A leadframe strip 301 is shown in cross section. In some embodiments, a top mold 302 and a bottom mold 303 are placed to effectuate the injection therein of a mold compound 304. The top and bottom molds 302, 303 can be metal, ceramic, or any material having an appropriate thermal characteristic to withstand the temperatures of the mold compound 304 in its liquid state. It is commonly known by those of ordinary skill in the art of semiconductor device manufacturing that a wide variety of mold compounds 304 are able to be used, each having advantages, disadvantages, and characteristics appropriate for a given application. By way of example, in high temperature applications such as microprocessors which generate a significant amount of heat, a high thermal conductivity mold compound 304 is able to be used. What is formed is a molded lead frame 305. Advantageously, the molded leadframe strip 305 will display enhanced rigidity and robust reliability characteristics. The use of a mold compound 304 further enhances encapsulation and protection from external moisture that standard PCB substrates such as polyimide or FR4 cannot provide.

For more predictable molding results, carrier tape is able to be used effectuate the molding process as shown in FIG. 4A. A process 400 includes applying tape 405 on its adhesive side to a leadframe strip 401. The leadframe strip 401 is then placed in a top mold 412 by the top surface of the leadframe 401. On the opposite side of the leadframe strip 401, non-adhesive tape 406 is prepared in a tape loader 407 at the bottom mold 413. Once the leadframe strip 401 is in place between the top mold 412 and the bottom bold 413, mold compound 404 is injected and fills all empty cavities. When removed from the mold, a molded leadframe strip 410 is formed. Optionally, a de-gate/de-runner step removes excess mold compound 411.

FIG. 4B shows an alternate embodiments for the process detailed in FIG. 4A. In some embodiments, the leadframe strip 401 is able to be placed between the top mold 412 and bottom mold 413 with adhesive tape 405 applied to the bottom. FIG. 4C shows embodiments wherein the leadframe strip 401 is able to be placed between the top mold 412 and bottom mold 413 without the use of adhesive tape. In an exemplary embodiment, non adhesive tape 406 is able to be provided by a tape loader 407 on the bottom surface of the leadframe strip 401. In another exemplary embodiment, two tape loaders 407 are provided to effectuate the molding of the leadframe strip 401. It will be appreciated by those of ordinary skill in the art of semiconductor manufacturing that several embodiments exist to place a leadframe strip 401 between a top mold 412 and a bottom mold 413 and the embodiments discussed herein are written solely to be exemplary and non limiting.

FIG. 5 shows a process 500 for the completion of the semiconductor packaging process. Semiconductor devices 501 are mounted on the molded leadframe strip 502. In some embodiments, multiple semiconductor devices 501 are mounted in each individual position on the molded leadframe strip 502. Such devices are known as multi chip modules (MCM). Bondwires 503 are mounted on the semiconductor devices 501 to effectuate electrical contact between the molded leadframe strip 502 and the semiconductor devices 501. In some embodiments where multiple semiconductor devices 501 are placed in each position, bondwires 503 can be placed to effectuate electrical contact between them as applications require. Next, a second mold compound 505 is applied to the molded leadframe strip 502. The second mold 505 encases the semiconductor devices 501 and bondwires 503 to protect them from harsh outer environments. In some embodiments, the second mold compound 505 and the first mold compound described in FIGS. 3, 4A, 4B and 4C are the same. Alternatively, the first and second mold compound 505 are able to be different to meet the demands of particular applications. By way of example, the semiconductor device 501 and the leadframe 401 in FIGS. 4A, 4B and 4C can have different coefficients of expansion in response to heat, and different mold compounds having different thermal characteristics such as thermal resistivity and thermal expansion are able to offset such effects. The molded leadframe strip 502 are then singulated by saw blades 515 to form singulated semiconductor packages 520, 530 and 540. The singulated devices 520, 530 and 540 are generally tested, subjected to stress, and tested again to ensure reliability and to filter out non passing or non standard units.

In some applications, it is advantageous for greater height clearance within the semiconductor package. FIG. 6A shows a singulated semiconductor package 600 in cross section. Within the package, a step cavity 601 is capable of receiving a thicker semiconductor die 602, larger bondwires 603 or in certain embodiments multiple stacked die. FIG. 6B shows an exemplary surface 610 of the mold 412 or 413 shown in FIG. 4B. Elevated protrusions 611 are placed to coincide with a leadframe strip to emboss a recessed area 601 into the leadframe. In an exemplary embodiment, adhesive tape 621 is applied to the back surface of the leadframe strip 622 as shown in FIG. 6C. The leadframe is flipped over such that its top surface is embossed by the surface 610 of the mold 412 or 413 having the protrusions 611.

FIG. 6D shows the leadframe strip 622 with a first mold compound 623 to form a molded leadframe 630 having recessed areas 601. To form singulated packages, semiconductor devices 602 and bondwires 603 are affixed onto the molded leadframe 630. The devices 602, bondwires 603 and molded leadframe 630 are encased in a second mold compound 650. The second mold compound 650 and the first mold compound 623 are able to be the same compound or different compounds depending on the application. Saw blades 655 then singulate the molded leadframe strip 630 into individual semiconductor packages 690.

An alternative surface is shown in FIG. 6E. In certain applications, such as high temperature applications, thick leadframes are advantageous. To accommodate thick leadframes, the non adhesive tape 610 provided by a tape loader is able to have pre-formed holes 660 configured to receive protrusions 670 on a mold surface 675. The mold surface 675 can be the surface of the top mold 412 or the bottom mold 413 as shown on FIGS. 4A, 4B and 4C. The mold is able to be formed of metal, ceramic, hard impact rubber, or any other suitable material.

FIG. 7 details a process 700 for forming singulated Land Grid Array (LGA) packaged devices 790. A leadframe strip 701 is mounted to adhesive tape 702. In some embodiments, the leadframe strip 701 is a half etched leadframe. The leadframe strip 701 is molded by a first mold compound 703 by any of the processes detailed in FIGS. 4A, 4B, 4C and 5. The tape 702 is removed forming a molded leadframe strip 705. Next, semiconductor devices 706 are affixed onto the molded leadframe strip onto each individual position. In some embodiments, multiple devices 706 can be placed in each position as applications require. Bondwires 707 are affixed to effectuate electrical contact between the molded leadframe strip 705 and the devices 706. The molded leadframe strip 705, devices 706 and bondwires 707 are encased in a second mold compound 710. The second 710 and the first 703 are able to be identical mold compounds or different mold compounds as applications require. The double molded leadframe strip 705 is singulated by saw blades 712 forming individual LGA package devices 790. These individual devices are then able to be tested, marked and bulk packaged for shipping and assembly. It will be apparent to those of ordinary skill in the art of semiconductor device assembly that although few leads 720 are shown, many dozens to hundreds of leads are able to be realized using the process described herein.

In another aspect of the invention, a step cavity LGA and a process for producing the same 800 are disclosed in FIG. 8. A leadframe strip 801 is mounted to adhesive tape 802. In some embodiments, the leadframe 801 is a half etched leadframe. The leadframe strip 801 is molded with a first mold compound 803. By way of example, the first mold compound is able to be a thermoset compound or a thermoplastic compound. Preferably, step cavities 804 are formed by the embossing procedure described in FIGS. 6A-6D. The adhesive tape 802 is removed forming a molded step cavity leadframe strip 805. At least one semiconductor device 806 is mounted within each cavity 804. Wirebonds 807 effectuate electrical contact between the semiconductor device and molded step cavity leadframe strip 805. In some embodiments where multiple semiconductor devices 806 are mounted in each step cavity 804, wirebonds 807 are able to effectuate electrical contact between the multiple devices 806 as applications require. A second mold compound 810 is formed over the molded step cavity leadframe strip 805, semiconductor devices 806 and wirebonds 807. The second mold compound 810 is able to be identical to or different from the first mold compound 803 as applications require. Saw blades 815 singulate the molded step cavity leadframe strip 805 into individual step cavity LGA packaged devices 820. The devices 820 are then able to be marked, tested and shipped to customers.

In another aspect of the invention, a cavity LGA and a process for making the same 900 are disclosed. A leadframe strip 901 is mounted to adhesive tape 902. In some embodiments, the leadframe 901 is a half etched leadframe. The leadframe strip 901 is molded with a first mold compound 903. By way of example, the first mold compound is able to be a thermoset compound or a thermoplastic compound. In some embodiments, step cavities 904 are formed by the embossing procedure described in FIGS. 6A-6E. The adhesive tape 902 is removed forming a molded step cavity leadframe strip 905. At least one semiconductor device 906 is mounted within each cavity 904. Wirebonds 907 effectuate electrical contact between the semiconductor device and molded step cavity leadframe strip 905. In some embodiments where multiple semiconductor devices 906 are mounted in each step cavity 904, wirebonds 907 are able to effectuate electrical contact between the multiple devices 906 as applications require. A cap 908 is affixed to the molded cavity leadframe strip forming a full cavity 909. The cap 908 is able to be comprised of silicon, glass, metal, ceramic, or any other convenient material or combination of materials as particular applications require. A second mold compound 910 is formed over the molded step cavity leadframe strip 905, semiconductor devices 906 and wirebonds 907. The second mold compound 910 is able to be identical to or different from the first mold compound 903 as applications require. Saw blades 915 singulate the molded step cavity leadframe strip 905 into individual cavity LGA packaged devices 920. The devices 920 are then able to be marked, tested and shipped to customers.

In some applications, multiple hundreds of I/O are required, and more than one leadframe is required to effectuate contact between a semiconductor device and its application. Furthermore, flexibility in routing I/O is advantageous, since end users can have specific demands as to the locations of I/O on a package landing pattern. To those ends, a dual molded leadframe LGA package and a process for making the same 1000 are disclosed in FIGS. 10A and 10B. Referring first to FIG. 10A, a first leadframe strip 1001 and a second leadframe strip 1002 are coupled to each other. In some embodiments, the first 1001 and second 1002 leadframe strips are held by adhesive tape 1003. The two leadframe strips are clamped together to effectuate adhesion between them in preparation for a later molding step. In some embodiments, a soft metal 1004 is able to be used to enhance electrical contact between the two leadframe strips. The soft metal 1004 is able to be applied to the top leadframe 1001 or the bottom leadframe 1002. Referring to FIG. 10B, the top leadframe 1001 and bottom leadframe 1002 are molded with a first mold compound 1005. The tape is removed forming a stacked molded leadframe strip 1006. Semiconductor devices 1010 are mounted and bondwires 1020 effectuate electrical contact between the semiconductor devices 1010 and the stacked molded leadframe strip 1006. At least one semiconductor device 1010 is mounted in every position and electrically coupled to the stacked molded leadframe strip 1006 via bondwires 1020. A second mold compound 1030 encases the stacked molded leadframe strip 1006, semiconductor devices 1010 and bondwires 1020. The second mold 1030 is able to be identical to or different than the first mold compound 1005. Saw blades 1035 singulate the stacked molded leadframe strip 1006 forming discrete semiconductor devices 1050.

While the invention has been described with reference to numerous specific details, one of ordinary skill in the art will recognize that the invention can be embodied in other specific forms without departing from the spirit of the invention. Thus, one of ordinary skill in the art will understand that the invention is not to be limited by the foregoing illustrative details, but rather is to be defined by the appended claims.