HEATSINK-MOUNTABLE POWER RESISTOR HAVING IMPROVED HEAT-TRANSFER INTERFACE WITH THE HEATSINK

Abstract: A power resistor (46) having improved heat dissipation ability to an underlying heatsink (43) because a step or protuberance (40) is provided that cooperates with a mounting bolt or screw (54) to largely nullify the effects of molding-caused camber or curvature (42).

Description: HEATSINK-MOUNTABLE POWER RESISTOR HAVING IMPROVED HEAT-TRANSFER INTERFACE WITH THE HEATSINK BACKGROUND OF THE INVENTION For years, a certain type of power package--having synthetic resin (plastic) surrounding a mounting hole--has been popular for electronic components such as power semi- conductors and power resistors . Examples include the Motorola "Full Pak" power semiconductors and Caddock MP816, MP850, and MP930 power resistors. In this type of package, the active element and its support structure are often located in close heat-dissipating relationship to the bottom (mounting surface) of the device and thus to an external heatsink on which the device is mounted. Mounting to the external heatsink is typically effected by a single bolt (screw) . There is always a need for increased power ratings in electronic or electrical devices, especially where the in¬ creased power ratings may be achieved at substantially no cost. Accordingly, it would be a distinct benefit to the industry if power resistor devices of the type indicated could--at little or no cost--be so constructed as to have higher power ratings. SUMMARY OF THE INVENTION When the active element of the power resistor is located well below center (well below the medial plane of the device) , such as at the bottom of the device or bonded to a metal piece that is located at the bottom of the de¬ vice, the package develops, inherently, a slight camber or bend. (The camber or bend is not necessarily a smooth cur¬ vature.) This is true even though the bottom surface of the mold for the package (body) is flat and the synthetic resin forming the package is a rigid epoxy (or equivalent) . The indicated camber or bend is convex at the mounting (bottom) surface of the device (the surface adjacent the external heatsink) , and is oriented longitudinally of the device. (There is also a side-to-side camber which is less, due to the geometry of the part . ) The cause of the camber is the higher thermal coeffi- cient of expansion (TCE) of the plastic (molding material) relative to the active element or active element with bonded metal piece. During the cooling down of the device follow¬ ing the high temperature molding process, the higher TCE of the plastic and the relatively thick section above the active element cause the bending force and result in the bend or camber. In addition, the shrinkage resulting from the post-molding cure of the molding plastic adds further to the force or forces that cause the camber. The molding process indicated in the specification is typically transfer molding; it may also be (for example) injection-molding. The indicated camber is typically in the range of about 0.0005 inch to 0.0040 inch for parts having common sizes, depending upon the type of plastic (that is to say, depending upon the TCE and cure shrinkage of the particular plastic) . The convex bend (camber) at the bottom surface of the device reduces the power handling capability or performance of the indicated power package. When the single bolt is used to mount the device to the heatsink, the device end remote from the bolt (this being typically the end having the terminals) is lifted off the heatsink by what may be termed rocker action. Such lifting creates a space between the heatsink and the mounting (bottom) surface of the part, thus reducing the power handling capability. Such space has a high thermal impedance even when it is filled with thermal grease. Stated otherwise, when compared to a much smaller space or no space, the indicated space resulting from the rocker action has a thermal impedance that is sufficiently higher to substantially affect the power rating of the electronic device. The present invention involves power resistors wherein the application of mounting force is not at the center but instead relatively adjacent one end. Such application of mounting force is typically by means of a single bolt (screw) extending through a hole in the synthetic resin. In accordance with another aspect of the invention, a molded step or protuberance is provided on the package-- namely, on the underside of the synthetic resin--at the end that is relatively adjacent the hole for the bolt. Preferably, the step is located at the extreme edge region of the mounting surface at the end remote from the terminals . In accordance with another aspect of the invention, the protuberance or step is caused to have a height (relative to the mounting surface) that is selected to compensate for the indicated camber. Stated otherwise, the step is so con¬ structed, intentionally, as to cause the space between the mounting surface and the heatsink--at the portion remote from the bolt hole--to be reduced substantially or eliminated. Such substantial reduction or elimination of the space causes better thermal contact between the heatsink and the heat-generating active element region of the resistor. The results are higher power capability and/or more reliable performance. In the preferred form, the indicated step or protuber¬ ance extends along the edge and is spaced from the bolt hole, that is to say, set back from the hole in a direction away from the terminals, to thereby increase the rotational movement that occurs when the bolt is tightened. The result is a greater downward force at the heat-dissipating portion of the device that is remote (toward the terminals) from the hole. The indicated downward force pushes with greater force the heat-dissipating portion of the package, towards the heatsink, and acting against the viscous thermal grease that is used as a thermal interface material between the heatsink and the mounting surface of the device. Such greater downward force improves the thermal interface between device and heatsink. The relationship between the step, the bolt and the heat-dissipating portion is in the nature of a third-class lever. The invention is applicable to "TO-220" power packages, and also other power packages, that have plastic surrounding the single bolt hole and where the hole is offset from the center of the package. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an enlarged isometric view, as viewed from below, of a power resistor incorporating the present invention; FIG. 2 is a side elevational view of the resistor of FIG. 1; FIG. 3 is a further enlarged view of the right portion of FIG. 2; FIG. 4 is a schematic, exaggerated, primarily side elevational view, showing prior art, and illustrating the rocker effect that occurs in the absence of the present invention, the terminals being unshown; FIG. 5 is a schematic view, also exaggerated, showing the present invention, the terminals being unshown; FIG. 6 is a longitudinal sectional view, showing prior art, of one type of power resistor in which the present invention is incorporated; and FIG. 7 is a longitudinal sectional view, showing prior art, illustrating another type of resistor in which the present invention is incorporated. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS United States Patents 5,291,178, issued March 1, 1994, and 5,304,977, issued April 19, 1994, are hereby incor¬ porated by reference herein, except that FIG. 8 of 5,304,977 is not incorporated herein. FIG. 6 hereof is from Patent 5,291,178, and FIG. 7 hereof is from Patent 5,304,977 (except that each number in FIG. 7 hereof is followed by the letter "a") . FIG. 1 shows a power resistor (or molded package) that is identical to the resistor (or package) of FIG. 7 hereof, or to the resistor (or package) of FIG. 6 hereof, with one major exception. The exception is that there is a step or protuberance 40 on the lower side of the resistor (package) , at the end of the bottom (mounting) surface that is nearest the off-center bolt hole (screw hole) 41. Step 40 is spaced from hole 41 by a space 42. Step 40, hole 41, and space 42 are on the portion of the resistor that is remote from the primary heat-dissi¬ pating surface 43 of the resistor. Surface 43 is the lower surface of the ceramic substrate of the active element, or is the lower surface of a metal piece bonded to the active element, reference being made to the cited patents. Leads (terminals) 44 are normally on the same portion of the resistor as is the surface 43, being therefore remote from step 40, hole 41, and space 42. The illustrated preferred power resistor has a molded body (package) that is substantially rectangular and elon¬ gate in the direction of terminals 44. It has a top surface 45; mirror-image side surfaces 46; end surfaces 47, 48; and a bottom surface 49. Bottom surface 49 is the mounting surface adapted to engage the upper flat surface of a heatsink such as a chassis, metal plate, etc. Bottom surface 49 is flat, except for the above-stated slight camber or bend. Hole 41 is substantially perpendicular to bottom surface 49. The heat-dissipating surface 43 is flush with the remainder of bottom surface (mounting surface) 49. All of the surfaces of the resistor (resistor package) are synthetic resin (plastic) , except the surface 43 which is ceramic or metal. The schematic, exaggerated views of FIGS. 4 and 5, each represent FIG. 7 hereof. As a second embodiment, each of FIGS. 4 and 5 hereof represents FIG. 6 hereof. In each instance, FIG. 4 does not incorporate the present invention whereas FIG. 5 does. A copper (or aluminum, etc.) heatsink plate 52 having a flat upper surface 53 is shown in FIGS. 4 and 5, having a bolt hole adapted to receive the indicated bolt 54 (screw) that is perpendicular thereto. A bolt 54 (screw) extends through bolt hole 41 and is tightened down relative to the heatsink 52. Referring to FIG. 4, this shows how--absent the inven¬ tion--tightening of bolt 54 results in a rocking action by which the portion of the resistor adjacent bolt hole 41 is held down, but the portion remote from the bolt hole is lifted up so as to form a gap 56. Even though gap 56 is filled with thermal grease or other thermal conduction- enhancing substance (not shown) , the condition of FIG. 4 is such as to markedly retard transmission of heat from the ceramic substrate (which is numbered 13a in FIG. 7 hereof from Patent 5,304,977) to heatsink 52. Referring to FIG. 5, when the present invention is employed, the step 40 operates upon tightening of bolt 54 to cause the region of the bottom surface 49 remote from the step to come close to or engage upper surface 53 of the heatsink over a wide area. Thus, the gap 56 is greatly reduced and the amount of thermal grease interposed between the somewhat curved or cambered bottom surface 49 and the flat upper surface 53 is greatly reduced. The improvement in power rating, compared to the construction of FIG. 4, is substantial. Proceeding to a description of the preferred configur¬ ation of step or protuberance 40, and referring especially to FIGS. 1, 3 and 5, it is pointed out that there is a sub- stantially flat-bottomed ridge 57 that is adjacent end 48, and that is parallel to the region of surface 49 at space 42, and that preferably extends all the way across the resistor body. On its side relatively adjacent hole 41, the step 40 is inclined towards the surface 49 at space 42. Thus, the step merges towards surface 49 at a region 58 that 5 is preferably spaced from bolt hole 41. The vertical distance between the bottom of ridge 57 and the adjacent portion of surface 49 (namely, at space 42) is in the range 0.0005 inch to 0.0040 inch for the most common sizes of resistors. The thickness of the resistor 10 body, that is to say the distance between surfaces 49 and 45 of the resistors, is typically about 0.125 inch. Typically, the bottom and top surfaces of the resistor (package) are about 0.6 inch long and about 0.4 inch wide, the surface area of the bottom surface being less than 1 15 square inch. Typically, the length of the resistor (package) is in the range 0.40 inch to 1.10 inches. Typically, the width of the resistor (package) is in the range 0.250 inch to 0.700 inch. Typically, the thickness of the resistor 20 (package) is in the range 0.080 inch to 0.300 inch. In practicing the invention, it is merely necessary to convert existing molds by deepening the bottoms thereof ad¬ jacent the bolt-hole end thereof in a manner complementary to the step 40. Then, the resistor is merely mounted on a 25 heatsink, and a bolt (screw) is inserted and cranked down to the same extent as is conventional in the art. The result, especially with the resistor of FIG. 7 hereof having the ceramic heat-dissipating surface 14a, is a striking increase in power-handling capability. 30 The relationship between step 40, bolt 54, and the resistor portion remote from hole 41 (namely, at heat- dissipating surface 43) is that of a third-class lever, with the resistor (package) body being the actual lever. Step 40 is the fulcrum, belt 54 applies the force, and surface 35 53 and grease thereon are the resistance or load. TABLE I. Table of Numbers and Components from Patent 5.304,977. Except "a" Is Added to Each Number, and Which Appear in FIG. 7 (Prior Art) of this Specification 10a synthetic resin body (molded) 11a flat upper surface 12a lower surface 13a substrate (ceramic) 14a bottom surface of substrate 16a bolt hole 22a lead or terminal 23a end section 24a tab 26a riser portion 27a section 28a shoulder 29a prong 31a notch in riser 32a notch or recess TABLE II. Table of Numbers and Components from Patent 5,291.178. and Which Appear in FIG. 6 (Prior Art) of This Specification 10 substrate 11 internal heatsink (metal) 12 metalization traces 13 resistive film 14 coating (overglaze) 15 leads or pins 17 body of synthetic resin (molded) 31 top surface of molded body 35 end surface 36 end surface 38 hole In the preferred embodiments of the invention, the resistive element is flat, being a ceramic substrate on which resistive film is deposited--preferably by screen printing. In the embodiment of FIG. 7, as described in the related patent, there is a film of resistive material on the upper surface of substrate 13a. As shown by both FIG. 6 and FIG. 7, the amount of synthetic resin above the horizontal medial plane is distinctly greater than the amount of resin beneath such plane. The foregoing detailed description is to be clearly understood as given by way of illustration and example only, the spirit and scope of this invention being limited solely by the appended claims.

Claims: WHAT IS CLAIMED IS: 1. A power resistor, which comprises: (a) a resistive element, (b) a generally rectangular body of synthetic resin molded around said resistive element, the relationship between said resistive element and said body being such that said resistive element is located near the bottom surface of said body, and is spaced from the top surface of said body, said bottom surface of said body being downwardly somewhat convex due to forces inherent in the molding material and in said location of said resistive element in said body, the convexity being longitudinal to said body, (c) a bolt hole extending through said body and located relatively near one end of said body, and (d) protuberant means provided on said bottom surface of said body near said one end of said body, said protuberant means being so shaped, sized and located that when a bolt is passed through said bolt hole and tightened relative to an underlying heatsink having a flat upper sur¬ face, said protuberant means and said convexity cause the region of said bottom surface at said resistive element to be substantially closer to said flat upper surface of said heatsink than would be the case if said protuberant means were not present. 2. The invention as claimed in claim 1, in which said power resistor is combined with a bolt and with a heatsink having a flat upper surface, with said bolt extending through said bolt hole into said heatsink. 3. The invention as claimed in claim 2. in which thermal grease is provided between said bottom surface and said flat upper surface. 4. The invention as claimed in claim 1, in which said resistive element includes a ceramic substrate having a bottom side that is flush with said bottom surface, said bottom side being exposed and being adapted to be close to said flat upper surface when said bolt is tightened. 5. The invention as claimed in claim 1, in which said protuberant means has a lower side that is spaced below said bottom surface of said resistor a distance in the range of 0.0005 inch to 0.0040 inch. 6. The invention as claimed in claim 1, in which the length of said body is in the range of 0.400 inch to 1.10 inches . 7. The invention as claimed in claim 1, in which said resistive element is close to a metal plate having a bottom side that is flush with said bottom surface, said bottom side being exposed and being adapted to be close to said flat upper surface when said bolt is tightened. 8. The invention as claimed in claim 7, in which said resistive element is bonded to said metal plate. 9. A highly thermally efficient, heatsink-mountable heat-transfer package for an electrical device that gener¬ ates substantial heat when operating, which comprises: (a) a generally rectangular body of synthetic resin, said body having been molded around an electrical device in such relationship that the amount of resin above the medial plane of said body is greater than the amount of resin below said plane, said body having a bottom or mounting sur¬ face adapted to be mounted on a flat surface of a heatsink, said bottom surface having been intended to be flat but having become downwardly convex longitudinally of the package, said body having a mounting hole there¬ through for a bolt, and which is not centered in said body but instead relatively near one end thereof, and (b) protuberant means intentionally provided on the bottom of said body, on the same portion of said body as said mounting hole, to engage said heatsink and increase heat-transfer efficiency when a bolt is inserted through said hole and tightened to said heatsink at said flat surface of said heatsink. 10. The invention as claimed in claim 9, in which said electrical device is a resistive element. 11. The invention as claimed in claim 10, in which said resistive element comprises a thin ceramic substrate having a resistive film on the upper surface thereof. 12. The invention as claimed in claim 11, in which the bottom of said substrate is exposed, there being no metal piece or resin therebeneath. 13. The invention as claimed in claim 10, in which said resistive element comprises a thin ceramic substrate having a resistive film on the upper surface thereof, and further comprises a metal piece bonded to the underside of said substrate, the bottom of said metal piece being exposed. 14. The invention as claimed in claim 9, in which said package is disposed on a flat surface of a heatsink having a hole therein, and in which a bolt is inserted through said mounting hole in said body and into said hole in said heatsink and is tightened sufficiently to cooper¬ ate with said protuberant means in moving towards said heatsink the portion of said body remote from said mounting hole. 15. The invention as claimed in claim 9, in which said protuberant means is closer to said one end of said body than is said mounting hole. 16. The invention as claimed in claim 15, in which said protuberant means is spaced from said mounting hole. 17. The invention as claimed in claim 15, in which said protuberant means is a step molded on said bottom or mounting surface. 18. The invention as claimed in claim 15, in which said protuberant means is a ridge the height of which, relative to said mounting surface, is in the range of 0.0005 inch to 0.0040 inch. 19. The invention as claimed in claim 18, in which the length of said package is in the range 0.400 inch to 1.10 inches, the width of said package is in the range 0.250 inch to 0.700 inch, and the thickness of said package is in the range 0.080 inch to 0.300 inch. 20. A package for the heat-generating active element of an electrical component, which comprises: (a) a synthetic resin body molded around the heat-generating active element, in such manner that: i) the bottom of said body is nominally flat, and ii) there is much more of the synthetic resin above the medial plane of said body than below it, whereby said bottom surface of said body is externally convex or cambered in a direction longitudinal to said body, (b) a hole through said body, generally perpen¬ dicular to said bottom surface, and spaced away from the center of said body towards one end thereof, and (c) a step provided on said bottom surface between said hole and said one end, characterized in that when a bolt is passed through said hole and tightened relative to the flat surface of a heatsink, a third-class lever action results to increase heat transfer from said body to said heatsink.

Inventors: CADDOCK RICHARD E, CADDOCK RICHARD E JR, CADDOCK, RICHARD, E., CADDOCK, RICHARD, E., JR.
Applicants: CADDOCK ELECTRONICS INC, CADDOCK ELECTRONICS, INC.
Application year: 1996
Main classification: H01C1
IPC codes: H01C1