Semiconductor device with nested rows of contacts

A method of making semiconductor devices includes producing an array of first lead frames having rows of first electrical contact elements on respective sides. Sub-assemblies are produced by applying a first molding compound peripherally to provide support between the first electrical contact elements of each of the first lead frames, and singulating the sub-assemblies. An array of assemblies is produced, each of which includes a second lead frame having rows of second electrical contact elements on respective sides, a respective one of the sub-assemblies disposed in the second lead frame with the rows of first electrical contact elements nested adjacent to and inside the rows of second electrical contact elements, and a semiconductor die mounted on the sub-assembly. The assemblies are encapsulated using a second molding compound with the rows of first and second electrical contact elements exposed on adjacent sides of an active face of the respective assembly.

BACKGROUND OF THE INVENTION

The present invention is directed to semiconductor devices with nested rows of contacts and to a method of making such semiconductor devices.

Semiconductor devices, such as integrated circuits, comprise a semiconductor die (or chip) in a package with exposed electrical contact surfaces. The completed devices may be mounted on a support with electrical connections, such as a printed circuit board (PCB), for example. Using surface mount technology the electrical contact surfaces of the package can be soldered directly to corresponding pads on the support, providing mechanical attachment as well as electrical connections.

A completed surface mount device typically includes an electrically insulating molding material that covers the semiconductor die such that the device presents a top face and a bottom, active face, which are generally rectangular or square, and transversely extending edges. The molding compound may encapsulate the semiconductor die completely, or may define an air cavity that is then sealed with a ceramic or plastic lid. Typically, the device has a pair of sets of electrical contact surfaces on opposite sides of the device (‘dual in-line package’) or two orthogonal pairs of sets of electrical leads on respective sides of the device (‘quad package’).

In one type of package, the electrical contact surfaces are positioned in the bottom active face of the device. Each set of electrical contact surfaces includes discrete elements disposed side by side at intervals in rows in the active face of the device for soldering to the electrical connections of the support. In order to increase the number of contact surfaces available, more than one row of electrical contact surfaces may be provided in each set on the respective side of the device. The adjacent rows at each respective side of the device are nested, extending parallel to each other and to the adjacent side of the device, one row being further from the adjacent side of the device than the other row is.

The semiconductor die may be mounted in the device on a pad or flag of the same material as the electrical contact surfaces, which is usually a metal, such as copper, which may be plated. The die pad may be exposed at the bottom face of the device, to assist cooling the die, known as an exposed-pad package. Alternatively, the die pad may be omitted, known as a non-exposed pad package. In a non-exposed pad package the die may be mounted on the discrete electrical contact elements. In each case, the die and electrical contact elements and any die pad are held together mechanically by the encapsulating molding material. The electrical contact elements of the device may be connected electrically to electrical contact pads on the die itself by bonded wires, of gold, copper or aluminum for example, accommodating differential thermal expansion of the die and the package materials.

A prevalent technique used in manufacturing such a surface mount device includes forming an array of lead frames in a strip or sheet of electrically conductive material, usually metal, by etching and/or stamping for example. Each lead frame comprises a frame structure common to adjacent lead frames and supporting in the array the sets of discrete electrical contact portions which will form the sets of electrical contacts of the completed device after singulation and any die pad for mounting the die. The array of lead frames could comprise a single strip but typically comprises a two-dimensional array, with the supporting frame structure of the complete array comprising surrounding bars on the outer edges of the array and intersecting intermediate bars common to adjacent lead frames.

In a typical surface mount semiconductor device packaging process using lead frames, the semiconductor dies are mounted on and connected electrically to respective ones of the lead frames. The encapsulation material is then molded over and around the lead frame strip or sheet, possibly with a lid in the case of an air cavity package, so as to encapsulate the integrated circuit dies, the electrical contact surface elements and the bonded connection wires of each of the lead frames. The individual devices are then separated by a singulation process, in which the lead frame strip or sheet is cut apart. The singulation may be a saw operation. If desired, saw singulation enables the molding compound to be applied over the entire array, being cut subsequently during the singulation process. During saw singulation, a saw blade is advanced along ‘saw streets’ which extend between the electrical contact surface elements of adjacent lead frames, so as to cut off the supporting frame structures of the lead frames from the electrical contact surface portions of the lead frames and separate the individual devices from each other.

A high level of quality control of production process, including the singulation process, is desirable.

In one embodiment, the present invention provides a method of making semiconductor devices. The method includes providing an array of first lead frames, each of which comprises a plurality of rows of first electrical contact elements on respective sides thereof, applying a first molding compound to said array of first lead frames to provide support between said first electrical contact elements of each of said first lead frames, and singulating said first lead frames to produce a plurality of sub-assemblies. A plurality of second lead frames each of which comprises a plurality of rows of second electrical contact elements on respective sides thereof also is provided. Respective ones of said sub-assemblies are placed inside ones of said second lead frames such that the rows of first electrical contact elements are nested adjacent to and inside said rows of second electrical contact elements. A semiconductor die is attached on said respective ones of said sub-assemblies and then pads on the die are electrically connected to said first and second electrical contact elements of the respective first and second lead frames, thereby forming a plurality of assemblies. The assemblies are then encapsulated with a second molding compound, wherein said rows of first and second electrical contact elements are exposed on adjacent sides of an active face of the respective semiconductor device. Finally the assemblies are singulated.

In another embodiment, the present invention provides a semiconductor device including a first lead frame having on respective sides thereof a plurality of rows of first electrical contact elements and a first molding compound that provides support between said first electrical contact elements. The first lead frame and said first mold compound form a sub-assembly. The device also has a second lead frame including on respective sides thereof a plurality of rows of second electrical contact elements. The said sub-assembly is disposed between said rows of second electrical contact elements with said rows of first electrical contact elements nested adjacent to and inside said rows of second electrical contact elements. A semiconductor die is mounted on said sub-assembly, electrical connections are made between said semiconductor die and said first and second electrical contact elements; and a second molding compound is disposed on said rows of first and second electrical contact elements, wherein said first and second electrical contact elements present exposed electrical contact surfaces on adjacent sides of an active face of the semiconductor die.

FIG. 1shows a single lead frame100in a two-dimensional array of lead frames used in a known method of making quad package semiconductor devices. The lead frame100comprises a rectangular (in this case square) frame structure102surrounding the lead frame100. Adjacent to each side of the device, the lead frame100includes a first row of electrical contact elements104and a second row of electrical contact elements106. The first row of electrical contact elements104is an inner row, positioned further from the adjacent side of the device than the second row, which is an outer row. The electrical contact elements104of the first, inner row are initially supported by an inner structure107, which may be part of a die pad, by the intermediary of connection bars108. The electrical contact elements106of the second, outer row are directly supported by the frame structure102. The connection bars108and the frame structure102connect the electrical contact elements104and106electrically as well as mechanically. These electrical connections of the electrical contact elements104and106must be cut once the electrical contact elements104and106are supported by further structure of the device, notably molding compound. The frame structure102may be cut during normal saw singulation of the devices, after molding and de-taping, by sawing along saw streets110and the molding compound in the saw street is cut at the same time. However, when cutting the connection bars108along saw streets112, the saw must cut from the bottom active face of the device only through the metal of the lead frame and penetrate as little as possible the molding compound. Otherwise, if the molding compound were cut through as well as the metal, the inner structure of the device, including the electrical contact elements104of the first, inner row (and any die pad), would be separated from the outer structure of the device, including the electrical contact elements106of the second, outer row and the frame structure102.

Sawing part way through the thickness of a device during manufacture in this way is difficult to control to a sufficient high level of quality specification.

FIGS. 2 to 12illustrate a method of making semiconductor devices, and a semiconductor device, in accordance with an example of an embodiment of the present invention. In this example, a method of making semiconductor devices comprises producing an array of first lead frames200having rows of first electrical contact elements202on respective sides. Production of sub-assemblies500includes applying a first molding compound204to the array of first lead frames to provide support between the first electrical contact elements202of each of the first lead frames200, and singulating the sub-assemblies500. An array of assemblies800is produced, each of which includes a second lead frame600having rows of second electrical contact elements602on respective sides, a respective one of the sub-assemblies500disposed with the rows of first electrical contact elements nested adjacent to and inside the rows of second electrical contact elements, and a semiconductor die502mounted on the sub-assembly500. The semiconductor dies502are connected electrically to the first and second electrical contact elements202and602. The assemblies800are encapsulated using a second molding compound902with the rows of first and second electrical contact elements202and602exposed adjacent sides of an active face904of the respective semiconductor device900, and the assemblies800are singulated. An example of this method is summarized in a flow chart inFIG. 12.

FIGS. 2 to 11illustrate by way of example a method of producing a quad package device having first and second rows of electrical contact elements at each of the four sides of the device. It will be appreciated that the method can be adapted to producing an in-line package with first and second rows of electrical contact elements at only two opposite sides of the device.FIGS. 2 to 11illustrate by way of example a method of producing an exposed die pad semiconductor device. It will be appreciated that the method can be adapted to producing a non-exposed die pad semiconductor device.

In more detail,FIGS. 2 to 4show a single lead frame of a two-dimensional array of first lead frames200formed by stamping and/or etching, for example. Each of the first lead frames200comprises two orthogonal pairs of rows of first electrical contact elements202, the rows being disposed on respective sides of the first lead frame. As seen inFIGS. 3 and 4, in the array of first lead frames200, the contact elements202are supported by orthogonal bars206which are common to adjacent lead frames of the array of first lead frames and which form a supporting frame structure surrounding the contact elements202of each of the first lead frames. Each of the first lead frames200comprises a respective die pad208, disposed between the rows of first electrical contact elements202and the bars206, and on which the semiconductor die502will be mounted, the die pad208also being supported by corner connections208connecting the die pad208to the bars206in the lead frame array.

When the lead frames200are singulated to form the sub-assemblies500, the bars206will be cut away. In order to support and hold together the first electrical contact elements202and the die pad208during subsequent operations, molding compound is applied selectively between the first electrical contact elements202, and between the first electrical contact elements and the die pad208, forming a first molding compound204, so that the sub-assemblies500can be manipulated. However, the first molding compound204extends only peripherally around each sub-assembly500and does not cover the die pad208, so that the semiconductor die502can be mounted subsequently on the die pad208and electrical connections can be made subsequently to the semiconductor die502. As shown inFIG. 2, the array of lead frames200is mounted on a first sheet of adhesive tape205and the first molding compound204is applied and cured. The tape205is then removed, the array of lead frames200then appearing as seen in the top and bottom views ofFIGS. 3 and 4. The sub-assemblies500are then singulated by sawing along saw streets210in row and column directions, as illustrated at210inFIGS. 4 and 5. One of the resulting sub-assemblies500is shown inFIG. 5. The array200of lead frames has the corner connections208etched partly through their thickness from the underside, so that in the sub-assemblies500the first molding compound204penetrates under the corner connections208to support the die pads after singulation, as seen inFIGS. 3 and 4.

In this example of an embodiment of the invention, the semiconductor dies502are mounted on the sub-assemblies500by bonding to the die pad208after singulation of the sub-assemblies, as shown inFIG. 5, but before incorporating the sub-assemblies500in the assemblies800(FIG. 8). It will be appreciated that otherwise the semiconductor dies502can be mounted on the sub-assemblies500before singulation of the sub-assemblies. Alternatively, the semiconductor dies502can be mounted on the sub-assemblies500after their incorporation in the assemblies800. If no die pads such as502are provided in the sub-assemblies500, the semiconductor dies502can be mounted on the first electrical contact elements202, for example.

FIG. 6shows a single lead frame of a two-dimensional array of second lead frames600formed by stamping and/or etching, for example. Each of the second lead frames600comprises two orthogonal pairs of rows of second electrical contact elements602on respective sides of the second lead frame. The contact elements602are supported by orthogonal bars604which are common to adjacent lead frames of the array of second lead frames and which form a rectangular (in this example square) supporting frame surrounding the contact elements602. The second lead frames600do not have die pads, and each of the second lead frames600presents a central aperture606between the rows of second electrical contact elements602. The aperture606is wider than the sub-assembly500, so that the sub-assembly can fit in the aperture606inside the second lead frames600between the rows of second electrical contact elements602with sufficient clearance between the first and second electrical contact elements202and602.

In the next step, the array of second lead frames600is mounted on a second sheet of adhesive tape608, as shown inFIGS. 7 and 8. An array of assemblies800is then produced by mounting the sub-assemblies500on the adhesive tape608in the apertures606of respective ones of the second lead frames600. In this example of an embodiment of the invention, the semiconductor dies502are mounted on the sub-assemblies500before the sub-assemblies500are mounted in the second lead frames600but alternatively, the semiconductor dies502can be mounted on the sub-assemblies500afterwards.FIG. 10is a top view of an assembly800mounted on the adhesive tape608at this stage of the process.

In each assembly800, the rows of first electrical contact elements202are nested adjacent to and inside the rows of second electrical contact elements602. In the example illustrated of a quad package, each of the first lead frames comprises two orthogonal pairs of rows of first electrical contact elements on respective sides of the first lead frame and each of the second lead frames comprises two orthogonal pairs of rows of second electrical contact elements on respective sides of the second lead frame, the assembly800having first and second rows of electrical contact elements202and602at each of the four sides of the assembly. In the case of an in-line package, each of the first lead frames comprises a single pair of rows of first electrical contact elements on respective sides of the first lead frame and each of the second lead frames comprises a single pair of rows of second electrical contact elements on respective sides of the second lead frame, the assembly800having first and second rows of electrical contact elements202and602aligned at each of two opposite sides of the assembly.

Each of the semiconductor dies502is then connected electrically to the first and second electrical contact elements202and602of the corresponding assembly. In this example, the electrical connections are established using individual wires802each bonded to a pad on the semiconductor dies502and an electrical contact element202or602, as shown inFIG. 8.

In the next step, the assemblies800are encapsulated using a molding compound to form a second molding compound902. Encapsulating the assemblies includes applying the second molding compound902to the sub-assemblies500and the second lead frames on the second sheet of adhesive tape608. The second sheet of adhesive tape608is then removed.FIG. 11is a bottom view of an encapsulated assembly800at this stage of the process.

The encapsulated assemblies800are then singulated by sawing along column and row saw streets indicated by dashed lines610to produce the semiconductor devices900. The orthogonal supporting bars604are separated from the rows of second electrical contact elements602and removed by the singulation process. The molding compound902leaves the rows of first and second electrical contact elements202and602exposed adjacent sides of an active face904of the respective semiconductor device900.

Each of the resulting semiconductor devices900comprises a sub-assembly500including on respective sides thereof a plurality of rows of first electrical contact elements202from a first lead frame200, and a first molding compound204providing support between the first electrical contact elements. The semiconductor device900also comprises an assembly800including on respective sides thereof a plurality of rows of second electrical contact elements602from a second lead frame600, the sub-assembly500disposed between the rows of second electrical contact elements with the rows of first electrical contact elements202nested adjacent to and inside the rows of second electrical contact elements602, a semiconductor die502mounted on the sub-assembly500, and electrical connections802between the semiconductor die and the first and second electrical contact elements. The assembly800is encapsulated using a second molding compound902with the rows of first and second electrical contact elements202and602presenting electrical contact surfaces exposed adjacent sides of an active face904of the semiconductor device.

FIG. 12is a flow chart of a method1200of making semiconductor devices, as described above with reference toFIGS. 2 to 11. The method1200starts at1202by producing an array of first lead frames200having rows of first electrical contact elements202on respective sides. At1204, the array of first lead frames200is mounted on a first sheet of adhesive tape. Sub-assemblies500are produced at1206by applying a first molding compound204peripherally of each first lead frame200to provide support between the first electrical contact elements202of each of the first lead frames200. The first sheet of tape is then removed from the array of first lead frames200at1208, and the sub-assemblies500are saw singulated at1210. At1212semiconductor dies502are bonded on each of the sub-assemblies500.

An array of assemblies800is then produced. Producing the assemblies800includes at1214producing an array of non-exposed die pad second lead frames600each having rows of second electrical contact elements602on respective sides. At1216, the array of second lead frames600is mounted on a second sheet of adhesive tape608and, at1218, a respective one of the sub-assemblies500is disposed in each second lead frame600with the rows of first electrical contact elements202nested adjacent to and inside the rows of second electrical contact elements602, and a semiconductor die502is mounted on the sub-assembly500.

At1220, the semiconductor dies502are connected electrically to the first and second electrical contact elements202and602by wire bonding. The assemblies800are then encapsulated at1222using a second molding compound902. At1224, the adhesive tape608is removed from the encapsulated assemblies800are the assemblies800are then saw singulated at1226.

For example, the semiconductor device described herein can comprise any semiconductor material or combinations of materials, such as gallium arsenide, silicon germanium, silicon-on-insulator (SOI), silicon, monocrystalline silicon, the like, and combinations of the above.

Also for example, in one embodiment, the illustrated examples may be implemented as circuitry located on a single integrated circuit within a single complete package of the semiconductor device. Alternatively, the examples may be implemented as more than one separate integrated circuits or separate devices interconnected with each other in a suitable manner within a single complete package of the semiconductor device