Leadless Chip Carrier for RF Power Transistors or the Like

The present invention generally relates to transistor chip carriers, and more specifically, to leadless chip carriers for radio frequency (RF) power transistors.

Transistors find use in a wide range of electronic applications. For example, transistors find use in low power applications such as oscillator circuits, and high power applications such as power amplifiers. To facilitate the use of transistors in such applications, the transistors, which are usually in the form of semiconductor dies or chips, are usually housed in a carrier or package suitable for either a low power, high density application, such as a chip carrier, or for a high power application, such as a stripline-opposed-emitter (SOE) package having low lead inductance. Such packages are usually formed from an electrically insulating material having thermal properties and mounting configurations appropriate for the amount of heat dissipation expected to be encountered from the transistor devices. In the case of an RF device, the packages not only provide a housing for the transistor devices, but they also provide room for impedance matching circuitry which is necessary for proper operation in a given circuit application. The carriers provide encapsulation of the transistor device dies, or chips, to protect them from the external environment as well as from rough handling encountered during manufacturing, testing, or assembling them into a final product.

One problem associated with prior art carriers for RF power devices is that they do not afford automated testing of the packaged transistor devices before final interconnection into a circuit. This is due to the fact that prior art carriers, especially the SOE package, require special circuit board techniques to accommodate the relatively wide, external metal straps or leads which electrically connect to the base, emitter, and collector terminals of the internal individual devices. The fact that these packages require special circuit boards for the metal straps makes automated testing very awkward. The leads themselves are fragile, easily bent or broken, and they greatly affect the overall manufacturing cost. As a result, these packages preclude automated placement, requiring individual placement and soldering into a circuit board in a final product. Such prior art carriers for RF power devices, therefore, cannot take advantage of more efficient and economical manufacturing processes, such as automated placement and reflow soldering.

It is therefore an object of the present invention to provide a new and improved leadless chip carrier suitable for RF power transistors or the like which overcomes the foregoing deficiencies.

It is a further object of the present invention to provide a leadless chip carrier of the foregoing type which affords testing of the completed chip carrier including the transistor devices and any included matching circuitry prior to interconnection into a circuit.

It is a still further object of the present invention to provide a leadless chip carrier of the foregoing type which permits electrical connection of the terminals beneath the completed chip carrier to accompanying circuitry by automated placement and reflow soldering techniques.

It is a still further object of the present invention to provide a method of making a new and improved leadless chip carrier suitable for RF power transistors or the like.

The present invention provides various assemblies and a method of making a leadless chip carrier for power transistors, such as radio frequency (RF) power transistors, which facilitate testing of the completed chip carrier with transistor devices prior to their interconnection into a circuit. The invention includes a body of insulating material known as a substrate having a pair of major, substantially planar surfaces and a thickness made up of four side walls joining these major surfaces. A pattern of conductive material is deposited on one of the major surfaces to form separate conductive pads for connection to respective terminals of a semiconductor device die such as a transistor.

A plurality of separated contact areas of similar conductive material are affixed or deposited on the other major surface. Interconnecting portions of conductive material are deposited on or within the substrate to connect each conductive pad on one of the major surfaces to its respective corresponding contact area on the other of the major surfaces.

One of the pads of the chip carrier is arranged to receive a semiconductor device die, such as a transistor die, and make direct contact to one of the device terminals. In the several exemplary embodiments which follow, the leadless chip carrier provides one pad which connects to the collector terminal, another pad to the base terminal, and a third pad to the emitter terminal of the semiconductor device.

The leadless chip carrier of the present invention may also include at least one "via" comprising a hole or slot in the substrate body of the chip carrier. The interconnecting portion of conductive material, which extends through the substrate, connects a particular pad on the top surface with the corresponding contact area on the bottom surface of the leadless chip carrier. These "vias" may be utilized in conjunction with edge-wrapped interconnecting portions or may be utilized without any edge-wrapping around the side walls of the body of the chip carrier.

These and other objects of the present invention will become apparent to those skilled in the art upon consideration of the accompanying specification, claims, and drawings.

Referring to the drawings, wherein like reference numerals identify like elements in the several figures and in which:

FIG. 1 is a front view of a leadless chip carrier for RF power transistors or the like in accordance with one embodiment of the present invention.

FIG. 2 is a back view of the leadless chip carrier of FIG. 1.

FIG. 3 is a front view of a leadless chip carrier for RF power transistors or the like in accordance with a further embodiment of the present invention.

FIG. 4 is a back view of the leadless chip carrier of FIG. 3.

FIG. 5 is a circuit diagram illustrating the inductances and capacitance to ground provided by the structure in the leadless chip carrier of FIGS. 3 and 4.

FIG. 6 is a front view of another leadless chip carrier for RF power transistors or the like in accordance with an alternate embodiment of the present invention.

FIG. 7 is a back view corresponding to the leadless chip carrier of FIG. 6.

FIG. 8 is a perspective view of still another embodiment of the present invention, which evolved from the embodiment shown in FIGS. 6 and 7.

With respect to the drawings, FIGS. 1 and 2 respectively illustrate front and back views of a first embodiment of the leadless chip carrier 10 for transistors, such as radio frequency (RF) power transistors, in accordance with the present invention. Chip carrier 10 is particularly suitable for low RF output power transistor applications where the transistor must provide an output power on the order of three (3) watts. Chip carrier 10 includes a substrate 12 which is preferably formed of insulative material such as alumina, or other suitable material chosen for its electrical insulating as well as heat dissipating characteristics. Substrate 12 has two major surfaces 16 and 18, respectively, serving as a top surface, shown in FIG. 1, and a bottom surface, shown in FIG. 2. Major surfaces 16 and 18 are separated by a finite thickness and joined by side walls 20, 21, 22, and 23.

Chip carrier 10 also includes on its top surface 16 a pattern of conductive material deposited by any one of a number of well known methods. The pattern of conductive material includes a plurality of separated conductive pads, such as pads 24, 26, and 28 shown in FIG. 1. On the bottom surface 18, a pattern of similar conductive material is deposited by the same method suitable for the top surface, and includes a plurality of separated conductive contact areas, such as areas 30, 32, and 34 shown in FIG. 2. Note in this figure that there are two areas labelled 32 on opposing ends of the surface which are connected together across the top surface, as depicted in FIG. 1.

Substrate 12 is also configured to have a semiconductor device die 36, such as a transistor die, mounted generally on surface 16. When the device die 36 is a transistor die having collector, emitter, and base terminals, they are connected respectively to pads 24, 26, and 28, as shown. Such connections are represented in FIG. 1 by base bond sites 38, which are connected to base pad 28 by leads 40, and emitter bond sites 42, which are connected to the emitter pad 26 via leads 44. The collector of the transistor die here is specifically connected to collector pad 24 to which it is adjacent. Of course, the collector of the transistor die could also be directly connected to the collector pad 24, if pad 24 extended out to encompass the area beneath transistor die 36.

The chip carrier 10 also includes on its top major surface 16, a crossover 46, known as an emitter bridge, connecting opposite extremes of emitter pad portion 26 to one another. Crossover 46 serves to equalize the emitter currents which flow out of the device towards both sides 52, and thus provides more uniform power output over the device frequency range.

To complete the chip carrier 10, interconnecting portions 24a, 26a, and 28a of conductive material are respectively deposited onto and wrapped around the sides 20, 21, 22, and 23 of substrate 12. The interconnecting portion 24a serves to interconnect collector pad 24 on top surface 16 with the corresponding contact area 30 on bottom surface 18. The two interconnecting portions 26a serve to interconnect the opposite extremes 52 of emitter pad 26 with the respective contact areas 32. And, the interconnecting portion 28a serves to interconnect the base pad 28 with its respective contact area 34.

As can be seen from the description above with reference to FIGS. 1 and 2, chip carrier 10 allows device die 36, such as a transistor, to be readily mounted onto substrate 12 of the leadless chip carrier. The device die has its collector, emitter, and base electrically connected to collector pad 24, emitter pad 26, and base pad 28, respectively. Through the use of the previously mentioned interconnecting portions, these pads connect to the bottom surface areas to effectively connect the collector, emitter, and base of transistor 36 to collector contact area 30, emitter contact area 32, and bsae contact area 34. To finish the assembly, an encapsulation or installation of a cover (not shown) may be attached to surface 16 utilizing an adhesive such as thermal epoxy which is readily available for this purpose. Clearly, it is then a simple matter to test transistor 36 at this early stage in the manufacture of this device configuration utilizing the now mounted transistor 36 before this leadless chip carrier is permanently attached in a given application, such as to a finished printed circuit board. This configuration, therefore, electrically communicates the collector, emitter, and base leads to the bottom surface 18 of the leadless chip carrier primarily by virtue of its "wrap-around" leads. It should now also be clear that the invention facilitates the attachment of the transistor to an installation such as a finished printed circuit board, whether it be for temporary testing or for permanent mounting utilizing reflow soldering.

Therefore, the present invention provides a semiconductor device which functions as a stand-alone unit which can be mechanically and electrically tested, stored for further use without significant damage to its leads and internal connections, or readily reflow soldered directly to a hybrid of finished printed circuit board. In addition, this leadless chip carrier assembly is well suited to automated robot handling by virtue of its shape and relative ruggedness.

A further leadless chip carrier 60 embodying the present invention is illustrated in FIGS. 3 and 4. This leadless chip carrier 60 is particularly suited to high power output applications, having power outputs on the order of eight (8) watts. As previously noted, where like structural elements exist among the various figures, they are identified by like reference numerals. An initial departure of the chip carrier 60 from the chip carrier 10 which is primarily responsible for making carrier 60 more suitable to higher output applications is the specific choice of insulating material for substrate 12. Chip carrier 60 utilizes a material such as beryllia for substrate 12 because of its greater heat dissipating capability than a material such as alumina, which was the preferred material for substrate 12 of chip carrier 10. In chip carrier 60, the same configuration of pads 24, 26, and 28 are wrapped around the top, bottom, and sides of substrate 12 to provide distributed interconnecting portions 24a, 26a, and 28a so as to be contiguous with contact areas 30, 32, and 34, respectively. Thus, chip carrier 60 achieves the same advantages as noted above with respect to chip carrier 10.

FIG. 3 illustrates an arrangement different from that of FIG. 1. The chip carrier 60 depicted in FIG. 3 includes an additional component near the semiconductor device die 36. This additional component is a passive component in the form of a metal oxide semiconductor (MOS) capacitor 62. Capacitor 62 is mounted on pad 28, which functions in this embodiment as the emitter pad. Capacitor 62 is also connected by wire bond leads 66 to pad 26, which functions in this embodiment as the input pad. In addition, capacitor 62 is connected to base bond sites 38 by wire bond leads 64.

FIG. 4 illustrates the bottom side of chip carrier 60, including distributed interconnecting portions 24a, 26a, and 28a, with each of the opposite extremes of bottom surface 18 connected via contact area 34. This structure provides an extremely low loss connection from the emitter terminals 42 of device 36 by minimizing resistive power loss (due to I.sup.2 R) as well as reactive power loss (due to inductance from the top surface of the emitter pad 28 around to the bottom emitter contact area 34). Note that the emitter terminals 42 are connected together in zig-zag fashion (stitch bonding) to also provide multiple parallel paths for the emitter currents flowing through them.

FIG. 5 illustrates a schematic circuit diagram representing the inductances and capacitances to ground as provided by the elements within the structure of FIGS. 3 and 4. L.sub.1 represents the wire bond leads 66 between the input pad 26 and the MOS capacitor 62. L.sub.2 represents the wire bond leads 64 between the MOS capacitor 62 and the base bond sites 38. C.sub.2 represents the MOS capacitor 62. This schematic circuit diagram is a simplification of the actual circuitry employed by leadless chip carrier 60 in that, although one transistor device is shown, in fact multiple transistor devices are usually paralleled up to provide the desired amount of RF power output from single or multiple transistor dies. FIG. 3 depicts by way of example six transistor devices on one semiconductor die which are connected in parallel to pads 24, 26, and 28. Also as shown, capacitor 62 is actually six separate capacitors configured on one MOS capacitor die. Here chip carrier 60 employs direct contact between MOS capacitor 62 and emitter pad 28. Thus, it is apparent that the wire bond leads 64 and 66 provide series inductances and capacitor 62 provides capacitance to ground to form a "T" configuration matching circuit to the semiconductor device which reduces power loss and provides broader bandwidth.

FIGS. 6 and 7 illustrate an alternate chip carrier 70 embodying the present invention. Chip carrier 70 includes the same configuration of pads 26 and 28 wrapping around the side walls of substrate 12 as distributed interconnecting portions 26a and 28a to effectively be contiguous with contact areas 32 and 34 on bottom surface 18 to achieve the same advantages. Although not illustrated explicitly in detail in FIG. 6, a semiconductor device die 36 is mountable in a region designated for its placement, as shown in FIG. 6 and as illustrated in phantom in FIG. 7. However, chip carrier 70 departs from the chip carriers 10 and 60 primarily in that it contains at least one open slot 72 through substrate 12 and within the bounds of collector pad 24. This is outlined on top surface 16 and within the bounds of collector contact area 30 on the bottom surface 18. Such a configuration allows the distributed interconnecting portion 72a to be routed through slot 72 rather than around a side wall of substrate 12, as was done in FIGS. 3 and 4. Here again, distributed interconnecting portion 72a may be deposited by utilizing conductive material on an inside wall of slot 72. It provides a distributed interconnecting portion near the center of a carrier substrate having a slot near the center of the substrate, and certainly multiple separate slots could be utilized to interconnect one or more pads on the top surface with their respective contact areas on the bottom surface of the substrate. It should also be noted that the essence of the present invention is that all of the semiconductor device leads are electrically communicated from the top surface 16 to the bottom surface 18 of the leadless chip carrier to facilitate surface-mount connections suitable for automated placement, testing, and automated assembly of a packaged power device onto a finished printed circuit board or hybrid.

FIG. 8 illustrates a perspective view of still another embodiment of the present invention, which evolved from the embodiment shown in FIGS. 6 and 7, utilizing multiple slots or holes to provide the distributed interconnecting portions referred to above. In FIG. 8, a leadless chip carrier 80 includes a substrate 12 which has a top surface for mounting or affixing conductive pads 24, 26, and 28. Substrate 12 also has a bottom major surface to which is mounted or affixed conductive areas 30, 32, and 34 as shown. A semiconductor device 36, such as a transistor device die having a multiplicity of transistor devices, is mounted to contact pad 24 as shown by the dashed region on the surface of pad 24. This transistor device die 36 includes base terminals 38, three in this example, and also emitter terminals 42, as shown. The emitter terminals 42 include wire bonds 44 which connect to emitter pad 28 in two different regions as shown by solid dots on emitter pad 28. Also included in MOS capacitor 62, which is an optional element that has wire bond leads 64 connecting to transistor device base terminals 38 as well as input pad 26 via wire bond leads 66 as shown. Each of the sets of wire bond leads 44 and 66 connect to contact pad 28 and 26, respectively. Then, collector contact pad 24 is electrically coupled to contact area 30 via three distributed interconnecting portions 84 directly inbetween as shown. These distributed interconnecting portions are sealed holes or "vias" which are filled with tungsten to be electrically conductive. Furthermore, input pad 26 couples to input contact area 32 via another set of three distributed interconnecting portions 84 directly inbetween as shown. And finally, contact pad 28 which represents the emitter, couples to contact area 34 via distributed interconnecting portions represented by a group of metallized through holes 84 along the outside edge of substrate 12 so that the outer ends of "H" shaped contact area 34 connect to contact pad 28. Thus, chip carrier 80 utilizes a multiplicity of tungsten filled through holes 84 through substrate 12 to achieve low resistive and reactive power loss between the contact pads on the top surface with the contact areas on the bottom surface of substrate 12 to achieve a leadless chip carrier suitable for RF power transistors or the like. An encapsulation 82, such as a cover affixed utilizing thermal epoxy, can completely seal the leadless chip carrier 80 and the transistor device die 36 with its delicate matching circuit elements included. And, the smaller holes are easily manufactured and filled in substrate 12 when compared with the difficulty in making a slot 72 and adding a distributed interconnecting portion 72a in a configuration such as leadless chip carrier 70 of FIGS. 6 and 7. Therefore, leadless chip carrier 80 provides a very rugged and virtually hermetically sealed package capable of automatic testing, placement, and reflow soldering assembly procedures. Such a configuration would be desirable for certain stringent applications and clearly does not depart from the spirit or scope of the present invention.

While the present invention has been described in terms of common emitter configurations shown in several specific embodiments, it will be understood that the broader aspects of the present invention include common base configurations as well as the use of alternate conventional components, as would be obvious to those of skill in the art.