GB2034520A

GB2034520A - Semiconductor assembly

Info

Publication number: GB2034520A
Application number: GB7936380A
Authority: GB
Original assignee: Ford Motor Co Ltd; Ford Motor Co
Current assignee: Ford Motor Co Ltd; Ford Motor Co
Priority date: 1978-10-23
Filing date: 1979-10-19
Publication date: 1980-06-04
Also published as: US4218694A; JPS5559752A; DE2939732A1; GB2034520B; DE2939732C2; CA1142585A

Description

1 GB 2 034 520 A 1

SPECIFICATION

Semiconductor assembly Description

This invention relates to semiconductor assemblies, full wave rectifiers incorporating such assemblies and the use of such assemblies, particularly in alternators for motor vehicles.

Alternators for automobiles usually incorporate a wave rectifier which typically takes up a substantial portion of the volume of the alternator. This is both because of the structure of the components of the rectifiers and the need for having a sufficiently large cooling surface. Cooling is a sufficiently important problem that automobile alternators typically have fans for forcing air through the alternator.

Atypical rectifier makes use of individually mounted and packaged semiconductor diodes which are then wired to form the full wave rectifier. A typical mounting for each diode is a "can" which is a generally cup-shaped metal housing wherein is positioned a piece of preformed solder, a semiconductor diode chip, another piece of preformed solder and an external connection. The open end of the can is sealed so thatthe external connection extends out of the can. There is sufficient internal space in the can to allow for expansion and contraction of the lead. Typically, the cost of the functional element, the semiconductor chip, is a relatively small fraction of the total cost of the can. That is, the packaging accounts for a substantial portion of the total cost of each can.

Another method of packaging a semiconductor rectifier chip is in the "button" form. A pair of conductive leads having flattened ends each abut preformed solder connections which contain therebetween the semiconductor chip. By heating this construction, the solder flows and connects the semiconductor chip to the leads. The entire assembly is then coated in an epoxy resin to seal the interior. With the "button" construction as with the aforementioned "can" construction, the package contains only one semiconductor rectifier chip.

Thus, further fabrication is necessary to assemble these chips into a rectifier suitable for use with an alternator.

Various other prior art structures are known for mounting semiconductor rectifier chips. For exam- ple, it is known to start with a ceramic base, mount the semiconductor chip on the ceramic base and then cap the semiconductor chip so that is an air space on one side of the semiconductor chip. The structure has a disadvantage in that air is a poor heat conductor and most of the heat generated within the 120 semiconductor chip must be dissipated through the ceramic base. Typically, in an automobile environment, it is difficult for such an arrangementto provide sufficient heat dissipation.

It would be desirable to have a simple full wave rectifier structure which is easily fabricated and readily cooled.

This invention provides a semiconductor assem bly comprising a semiconductor device sandwiched between an upper and lower ceramic base which 130 have a sufficiently high heat transfer capability to cool the semiconductor device and provide for transfer of heat from the semiconductor device through the upper and lower ceramic bases.

In a particular embodiment of this invention, six semi-conductor wafers are mounted between two selectively metallized ceramic bases to provide a full wave rectifier for an alternator. This structure is particularly advantageous in that not only is its fabrication relatively simple but heat transfer is greatly improved.

In a preferred embodiment of the invention a semiconductor full wave rectifier assembly includes a generally planar first lower ceramic base having two opposing major surfaces. The first lower ceramic base includes a first metallization covering a portion of both of the major surfaces. A first, second and third semiconductor device are connected to the first metallization and a fourth, fifth and sixth semiconductor device are connected to the second metallization. A second, third and fourth generally planar upper ceramic bases each have two opposing major surfaces and each have metallization on at least one of the two major surfaces. The second upper ceramic base is positioned so its associated metallization contacts the first and second diodes, the third upper ceramic base is positioned so it is contacting the third and fourth diodes and the fourth ceramic base is positioned so that it contacts the fifth and sixth diodes. The metallizations on the second, third and fourth ceramic bases are adapted to receive the three phases of the alternator output and the first and second metallizations on the first ceramic base are adapted to provide a DC output from the rectifier. The positioning of ceramic bases on both sides of each of the diodes provides for heat transfer in two opposing directions.

The use of ceramic in conjunction with silicon is particularly advantageous because the thermal ex- pansion rates are similarthus reducing the failure rate. That is, similar rates of expansion eliminate stress which would otherwise be present when there are different rates of expansion which is particularly advantageous in an automobile environment where there are substantial temperature changes. In the particular embodiment discussed above, wherein there are six semiconductor rectifier chips used in conjunction with the ceramic wafers, encapsulation of all six diodes can be done at the same time instead of individually as in the prior art. However, because one of the diodes in the assembled six diode package may be defective, it is particularly advantageous that one of the upper ceramic bases can be removed without disturbing the other two upper ceramic bases to replace the defective diode. If the upper ceramic base were a single piece and had to be removed, thus once again exposing all six diodes to heat, there is a greater chance that the repaired rectifier would have a new fault than when only one of three upper ceramic bases is removed.

A preferred embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:Figure 1 is a perspective view of a partly exploded alternator with a front portion of the housing 2 GB 2 034 520 A 2 removed to expose a rectifier and regulator mounted on the rear housing in accordance with an embodiment of this invention; Figure 2 is an exploded perspective view of a rectifier in accordance with an embodiment of this invention with two different upper ceramic base embodiments for external connection; Figure 3 is a top plan view of an assembled full wave rectifier in accordance with an embodiment of this invention; Figure 4 is a side elevation view of the embodiment shown in Figure 3; Figure 5 is a schematic diagram of the full wave rectifier shown in Figures 2 and 3; and Figure 6 is a partial sectional view taken substan- tially along line VINI of Figure 1.

Referring to Figures 1 and 6, an alternator 10 includes a rectifier 20 which is sufficiently small that a voltage regulator 11 can also be positioned within a rear housing 12. Advantageously, there is a removable access port so that rectifier 20 can be easily reached from outside alternator 10. By posi tioning voltage regulator 11 within alternator 10, it is possible to eliminate external wiring harnesses between the rectifier 20 and the regulator 11. The 90 assembly of the alternator 10 as shown has an improved cooling, reduced assembly labor, and a weight reduction which is advantageous for increas ing fuel mileage.

Referring to Figure 2, the exploded view shows a ceramic base layer 21 with metallizations 22 and 23 and top ceramic layers 41, 43 and 45. Ceramic layer 45'is an alternative embodiment of a ceramic layer 45, shown in Figures 3 and 4, which is similarto ceramic layers 41 and 43. Similar features of ceramic 100 layer 45 and 45' use the same numbering with the addition of a prime ('). Six rectifier chips or semicon ductor diodes 31, 32,33,34, 35 and 36 are shown positioned for mounting in a chess-board pattern between the ceramic base layer 21 and the top ceramic layers 41, 43 and 45'. To facilitate assembly as later discussed, semiconductor diodes 31 to 36 have an associated top solder piece 31 a to 36a, respectively, and a bottom solder piece 31b to 36b, respectively.

Referring to Figures 2, 3 and 4, ceramic base layer 21 is a generally rectangular planarwafer having a major surface 24facing the rectifier chips and a major surface 25 facing away from the rectifier chips.

Ceramic base layer 21 also includes two truncated corners 26 and 27 which facilitate the fit of rectifier within rear housing portion 12 of alternator 10.

Wrapped metallization 22 extends across about one-third of the area of major surface 24, down a forward edge 28 of ceramic base layer 21 connecting 120 major surfaces 24 and 25, and across the entire major surface 25. Metallization 23 is spaced from metallization 22 on major surface 24 and extends across the face of major surface 24 between corners 26 and 27.

Top ceramic layer 41 is a rectangular wafer which has a major surface 41a facing away from rectifier chips 31 and 32 and a major surface 41b facing toward rectifier chips 31 and 32. A metallization 42 extends in a continuous belt around ceramic layer 41130 and goes across major surface 41a down a front edge 41c, across major surface 41b and up a rear edge 41 d, opposite edge 41 c, back to major surface 41 a. Metallization 42 includes a contact 42a which is generally L-shaped having one leg attached to metallization 42 and another leg extending away from metallization 42 for connection to an external lead. The top ceramic layer 43 is similar to top ceramic layer 41 and includes surfaces 43a, 43b, edges 43c and 43d. A metallization 44 corresponds to metallization 42 and includes a contact 44a.

Figure 2 also shows an alternative embodiment of a top ceramic layer 45'which is generally rectangular and planar, with a major top surface 45'a and a major bottom surface 45'b. A metallization 46'a extends along major surface 45'b but does not extend up the edges of top ceramic layer 45'. Top major surface 45'a has a metallization 46'b. An opening 47'extends through metallization 46'b, and top ceramic layer 45'so that a lead can be extended through ceramic layer 45'to contact metallization 46'a. A metallization such as 46'may be desirable when fabricating a belt-like metallization such as 42 is difficult.

The use of a metallization such as 46 (Figure 3) instead of a metallization such as 46'a and 46'b is advantageous because the contact to metallization 46 has two conductive parts to rectifier chips 35 and 36. That is, current flowing through contact 46a can go round metallization 46 at edge 45c and 45d. In contrast, all current flowing to metallization 46'a must pass through the lead going to opening 47'. Such reduced resistance is particularly desirable because it reduces the power loss, defined by the square of the current times the resistance, and thus reduces heating which is known to be a significant problem in rectifiers for use in alternators in automobiles.

Referring to Figure 3, the longitudinal length of ceramic layers 41, 43 and 45 is sufficiently short to fit within the boundary of ceramic base layer 21. Similarly, the combined widths of ceramic layers 41, 43 and 45 is also suff iciently narrow to fit within the boundary of ceramic base layer 21.

Rectifier chips 31 and 32 are spaced and posi- tioned to fit between top ceramic layer 41 and ceramic base layer 21. Top solder piece 31a of rectifier chip 31 contacts metallization 42 and bottom solder piece 31 b of rectifier chip 31 contacts metallization 22. Top solder piece 32a of rectifier chip 32 contacts metallization 23. Similarly, rectifier chips 33 and 34 are associated with top ceramic layer 43. Top solder piece 33a of rectifier chip 33 is in contact with metallization 44 and bottom solder piece 33b is in contact with metallization 22. Top solder piece 34a of rectifier chip 34 is in contact with metallization 23. Diodes 35 and 36 are associated with top ceramic layer 45; diode 35 has top solder piece 35a associated with metallization 46 and bottom solder piece 36b with metallization 22; and diode 36 has top solder piece 36a associated with metallization 46 and bottom solder piece 36b with metallization 23. External connection to both metallizations 22 and 23 is made by leads attached to those metallizations.

3 GB 2 034 520 A 3 Referring to Figure 5, the electrical circuitry formed by the embodiment shown in Figures 2,3 and 4 is a three phase bridge circuit for use with a three phase alternator. Three input terminals 51, 52 and 53 are provided for application of a three phase alternating current source and two output terminals 54 and 55, negative and positive terminals respec tively, are provided for supplying direct current to a load such as the electrical load of an automobile. The bridge circuit includes three pairs of rectifier diodes (61 and 62, 63 and 64 and 65 and 66). Each diode of a pair of rectifier diodes is connected in series with the other of the pair between the negative and positive direct current terminals 54 and 55 in such a manner as to pass conventional current in the direction from the negative terminal 54 to the diode, through the alternating current source, and to positive terminal 55. Each of the alternating current terminals 51, 52 and 53 are electrically connected at a point between the two diodes of their respective pairs. That is, alternating current terminal 51 is electrically con nected between rectifier diodes 61 and 62, alternat ing current of terminal 52 is connected between rectifier diodes 63 and 64 and alternating current terminal 53 is connected between rectifier diodes 65 and 66. Thus, a three phase rectifier bridge circuit is formed.

In fabricating rectifier 20, ceramic layers 21, 41, 43 and 45 are formed of a material such as beryllia.

Beryllia is particularly desirable because it is electric- 95 ally insulating and has a high heat conductivity.

Another material which also can be used but has a somewhat decreased heat transfer capability is alumina. A molybdenum-manganese paste is ap plied to the ceramic and fired on atthe positions where metallizations are desired. Copper is electro plated onto the molybdenum manganese and serves to provide an improved conductive layer. That is, the molybdenum-manganese is less conductive than the copper but acts as an expansion buffer to accommo- 105 date the difference in expansion rates between the copper and the ceramic layer and the semiconductor chip. Typically, the circuit configuration of the metal lization on a ceramic layer is formed by selectively masking the copper and molybdenum manganese 110 and then etching off the exposed metallization so that metallization remains only where it is desired.

Solder pieces 31a to 36a and 31b to 36b are positioned between the metallization and associated rectifier chips 31 through 36 and the entire assembly is heated to electrically connect all the components as desired. Typically, a seal is then formed around the assembled package using a material such as an epoxy base resin. For example, metallized ceramic base layer 21 can be epoxied to an aluminum housing with high thermal conductivity epoxy resin. Typically, when rectifier 20 is mounted in alternator 10, one ceramic plate, such as ceramic base layer 21, is mounted to rear housing portion 12 and cooled by conduction and the top ceramic layers 41,43 and 45 are cooled by convection.

Typical specifications for a rectifier 20 include 0.001 minimum metallization including the copper plating thickness; a ceramic material composed of

99.5% beryllia; a 2.0 OC/watt maximum thermal resistance junction through ceramic base layer; a 0.150" to 0.180" square silicon pellet for each rectifier chip; and the use of 10% tin, 90% lead solder or equivalent. Typical dimensions include ceramic base layer 21 having outside dimension of 1.2" by 1.625"; a thickness including metallization of about 0.1 "; top ceramic layer 40 having a width of 0.5"; a length of 1.0" and a thickness of 0.05". The width of metallization 42 around top ceramic layer 41 can be aboutO.4"..0- The embodiment of the invention described above provides a particularly compact, robust, and lowweight rectifier assembly which can be installed in a motor vehicle alternator whilst leaving suff icient space to permit the installation of a voltage regulator in the same alternator housing. This is particularly advantageous in that not only is the cost of fabricating the alternator reduced, but also the weight of the alternator. Moreover, the need for separate installa- tion of the voltage regulator, with a separate wiring harness connecting it to the alternator, is also eliminated. Additionally, the use of circuit boards is eliminated from the construction, and the rectifier can be installed in a part of the alternator housing which protects itfrom exposure to dirt and moisture.

Various modifications and variations will no doubt occurto those skilled in the various arts to which this invention pertains. For example, the particular dimensions may be varied from that disclosed herein. Also, variations in the shape of the ceramic layers and the metallizations may be made from that disclosed herein. These and all other variations which basically rely on the teachings through which this disclosure has advanced the art are properly considered within the scope of this invention.

Claims

1. A semiconductor assembly comprising one or more semiconductor devices sandwiched between upper and lower bases composed of an insulating material having a sufficiently high heat transfer capabilityto cool the or each semiconductor device during operation.

2. A semiconductor assembly according to Claim 1 wherein said upper and lower plates carry contact layers of electrically conductive material coextensive with at least a portion of their surfaces.

3. A semiconductor assembly according to Claim 1 or Claim 2 including at least two of said semiconductor devices and at least two of said upper bases, each of said semiconductor devices being associated with said lower base and at least one of said semiconductor devices being associated with each of the upper bases so that the semiconductor devices can be removed from the lower base without disturbing the said semiconductor devices associated with the or each other upper base.

4. A semiconductor assembly according to Claim 3 including six semiconductor devices and three upper ceramic bases, all six of said semiconductor devices being associated with said lower base and two semiconductor devices being associated with each of said upper bases.

5. A semiconductor assembly according to Claim 4 GB 2 034 520 A 4 4 wherein said lower base includes a first layer of electrically conductive material electrically common with a first group of three of said semiconductor devices and a second layer of electrically conductive material electrically common with a second group of three of said semiconductor devices, different from said first group, each of said upper bases having a layer of electrically conductive material electrically associated with a group of two of said semiconduc- tor devices, one chosertfrom said first group of three, each of said three of said upper bases being adapted to be electrically connected to a different one of the three phases of the output of an alternator.

6. A semiconductor assembly according to any one of Claims 1 to 5 wherein said lower base is generally planar, has two opposing major surfaces, and carries a layer of electrically conductive material extending around an edge of said lower base and covering at least a portion of each of said two major surfaces, only one of said two opposing major surfaces being adjacent said semiconductor devices.

7. A semiconductor assembly according to any one of Claims 1 to 6 wherein the or each of said upper bases is generally planar, has two major surfaces, and carries a layer of electrically conductive material covering an edge of said upper ceramic base and at least a portion of each of said two major surfaces of one of said upper ceramic bases, one such portion being adjacent to a semiconductor device and the other such portion being adapted for connection to an electrical circuit.

8. A semiconductor assembly according to any one of Claims 1 to 6 wherein the or each of said upper bases is generally planar, has two major surfaces, carries a layer of electrically conductive material which covers a portion of one of said two major surfaces, and includes an opening effecting communication between said two major surfaces and adapted to receive a lead extending therethrough for electrically coupling said layer to the other side of said upper base.

9. A semiconductor assembly according to any one of Claims 1 to 8 wherein the upper and lower bases are composed of a ceramic material having a high thermal conductivity.

10. An alternator incorporating a full wave rectifier assembly comprising a semiconductor assembly according to any one of Claims 1 to 9.

11. A semiconductor full wave rectifier assembly for mounting in the alternator of an automobile including:

a generally planar first lower ceramic base having two opposing major surfaces, a first metallization covering a portion of one of said major surfaces and a second metallization covering a portion of one of said major surfaces; a second, third and fourth generally planar upper ceramic bases, each having two opposing major surfaces, the combined area of the major surfaces of said second, third and fourth ceramic bases being about equal to or less than the area, the major surfaces of said first lower ceramic base; a third, fourth and fifth metallization covering at least a portion of one major surface of said second, third and fourth ceramic bases, respectively; a first and second diode having opposing terminals connected to said third metallization so that forward current flow through said first and second diodes is in opposite directions with respect to said third metallization; a third and fourth diode having opposing terminals connected to said fourth metallization so that forward current flow through said third and fourth diodes is in opposite directions with respect to said fourth metallization; a fifth and sixth diode having opposing terminals connected to said fifth metallization so that forward current flow through said fifth and sixth diodes is in opposite directions with respect to said fifth metallization; said first, third and fifth diodes having terminals connected to said first metallization; said second, fourth and sixth diodes having ter- minals connected to said second metallization; said third, fourth and fifth metallizations each being adapted to be connected to one of the three output phases of the alternator; said first and second metallizations being adapted to provide a direct current output from said rectifier; and said first, second, third and fourth ceramic bases being positioned adjacent said diodes so that there is substantial heat dissipation from said diodes.

12. A semiconductor assembly according to Claim 11 wherein:

said metallization is molybdenum coated with copper so that the stress between the copper and the ceramic base due to thermal expansion is reduced; and said diodes are semiconductor wafers.

13. A semiconductor assembly according to Claim 11 or Claim 12 wherein the area of a major surface of said lower ceramic base is about 1.2 inches by about 1.625 inches.

14. A method of rectifying the three phases of the output signal generated by an alternator including the steps of:

applying a first phase of the generated signal to a first metallization in contact with a first and a second diode and with an electrically insulating, thermally conductive first ceramic base; applying a second phase of generated signal to a second metallization in contact with a third and a fourth diode and with an electrically insulating, thermally conductive second ceramic base; applying a third phase of the generated signal to a third metallization in contact with a fifth and a sixth diode and with an electrically insulating thermally conductive third ceramic base; detecting a rectifier output between a first metallized contact adjacent a fourth ceramic base and adjacent a terminal of the first, third, and fifth diodes, and a second metallized contact adjacent the fourth ceramic base and adjacent a terminal of the second, fourth and sixth diodes; and conducting heat from the diodes in a first direction through the first, second and third ceramic bases and in a second direction through the fourth ceramic base.

4 GB 2 034 520 A 5

15. A method of rectifying as recited in Claim 11 wherein said step of applying the first phase ineludes the steps of:

conducting the first phase of the generated signal by means of two current paths to each of the first and second diodes, the two current paths being formed by the first metallization forming a belt around the first ceramic base.

16. A semiconductor assembly substantially as hereinbefore described with reference to the drawings.

Printed for Her Majesty's Stationery Office by Croydon Printing Company Limited, Croydon Surrey, 1980. Published by the Patent Office, 25 Southampton Buildings, London, WC2A lAY, from which copies may be obtained.

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