Method of making a mounting base pad for semiconductor devices

A mounting pad means for use in combination with solid-state semiconductive translating devices and including a base pad with flange walls extending from said base pad, and with device mounting pad means being secured to the surface of the base pad and comprising a thin layer of silicone base rubber. The base pad and flange walls are formed of a generally rigid laminate with a core having outer metal foil layers disposed on opposite surfaces thereof, and wherein the core is composed of a thin layer of silicone base rubber.

BACKGROUND OF THE INVENTION 
The present invention relates generally to an improved mounting means for 
use in combination with solid-state semiconductive translating devices 
such as transistors, diodes, or the like, and wherein the mounting means 
provides a heat sink or other means of dissipating heat, as well as a 
mechanical mounting device. The arrangement of the invention is such that 
the mounting means which may be fabricated from a formable metal base, may 
be either a solid metallic pad or a pad formed as a laminate with outer 
metal foil layers disposed on opposite surfaces of a thermally conductive 
pliable pad or layer such as a silicone rubber core. The arrangement 
facilitates and permits the use of relatively thin layers of metal foil 
such as aluminum or copper, which is stiffened or rendered rigid by the 
forming of the structure. 
It has long been recognized that the proper utilization of solid-state 
semiconductive translating devices requires a system for dissipating the 
heat generated in the normal use and operation of these devices. 
Specifically, as the power requirements of the semiconductive translating 
devices increases, the need for heat dissipation correspondingly 
increases. While metallic layers, that is layers of aluminum or copper, 
may be readily employed for heat-dissipating mounting means, the cost 
involved in fabricating, working and mounting such devices renders it 
desirable to employ alternate materials of construction for the mounting 
means. However, as alternate materials are proposed for ease of handling 
and fabrication, these alternate means have normally suffered from an 
inability to dissipate thermal energy, and thereby impose an unreasonable 
constraint or limitation upon the operational characteristics of the 
solid-state semiconductive translating device mounted thereon. For most 
purposes, however, metals such as aluminum or copper are the material of 
choice for these applications, particularly in view of the electrical and 
thermal properties of these materials. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, therefore, a mounting means is 
provided for use in combination with solid-state semiconductive 
translating devices which is highly thermally conductive, and which 
employs a formable metallic base pad in the form of a solid metal sheet or 
a pair of outer metal foil layers disposed on opposite surfaces of a 
thermally conductive adherent core layer such as a silicone rubber core. 
The structure is rendered generally self-supporting and rigid so as to 
facilitate handling and working. In the laminated structure, the 
utilization of the pair of metallic foil layers on opposite sides of the 
silicone base rubber core provide a relatively highly thermally conductive 
system for heat dissipation. 
In the preparation of the base heat sink, various forms may be employed 
such as an extrudedor stamped metal part, a machined metal component, a 
metallic foil, or a laminated metallic-nonmetallic system. The common 
element between each of these forms is, as indicated, that the thermally 
conductive material be deformable as a base pad. 
The mounting arrangement also includes a thermally conductive insulator 
which may be integral with the base heat sink element or semiconductor 
base. The thermally conductive insulator material, normally silicone 
rubber, is applied to the material as a coating, composite, or an 
adhesively applied material such as may be continuously applied through a 
coating operation onto the surface of the metallic component. While the 
thermally conductive material is preferably applied directly to the heat 
sink surface, in certain instances and depending upon the design of the 
transistor or other semiconductor assembly, it may alternatively be 
applied directly to the semiconductor device. 
When the thermally conductive insulator material is applied to a surface as 
a coating, various opertions may be employed such as spray, dip, brush, 
roller coating, or silk-screen technique. When the insulator is 
pre-applied to the surface of the metallic heat sink element, the ultimate 
bonding of the semiconductor device to the pre-formed composite may also 
be achieved through conventional adhesive bonding operations or 
vulcanization. 
The system of the present invention is highly useful in combination with 
semiconductive translating devices which are destined for mounting upon 
printed circuit boards. This system further provides a means for securing 
the semiconductive device to the mounting means so that direct or adhesive 
assembly operations can be undertaken so that direct assembly operations 
can be undertaken either on a strip-basis or, in certain instances, on a 
reel-to-reel basis. Also, the mounting means may include use of a 
conductive adhesive for electrical coupling to the solid-state 
semiconductive device, if desired. 
The mounting means of the present invention may be conveniently fabricated 
in an elongated strip form. In this connection, the fabrication operation 
may include the initial preparation of an elongated strip of a generally 
rigid but deformable metal such as a metallic strip or sheet or as a 
laminate with outer metal foil layers. Score lines may be formed 
longitudinally along the elongated strip to provide fold lines for the 
devices. Also, deep score lines may be formed transversely at regularly 
spaced intervals to form a break-off line for the separation of individual 
semiconductor assemblies or mounting pads from an elongated strip 
containing a large number of repeating units. 
Therefore, it is a primary object of the present invention to provide an 
improved mounting means for use in combination with solid-state 
semiconductive translating devices which comprises a base pad having 
flange walls coupled to opposed side edges thereof, and with the base pad 
and flange walls comprising a generally rigid but deformable base with a 
thermally conductive layer being secured or applied to the surface 
thereof. 
It is yet a further object of the present invention to provide an improved 
method for the fabrication of mounting means for use in combination with 
solid-state semiconductive translating devices wherein there is initially 
prepared an elongated strip of a generally rigid but deformable and 
self-supporting metal pad or metallic laminate pad with a core having 
outer metal foil layers disposed on opposite surfaces thereof. Parallelly 
disposed score lines are formed along the strip to faciliate formation of 
the mounting means, and wherein transversely disposed score lines are 
formed along the elongated strip to form break-off lines defining or 
delineating individual assemblies or individual mounting pad structures. 
Other and further objects of the present invention will become apparent to 
those skilled in the art upon a study of the following specification, 
appended claims and accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
In accordance with the preferred embodiment of the present invention, and 
with particular attention being directed to FIG. 1 of the drawings, the 
elongated strip member generally designated 10 includes a repeating 
structure in the form of a plurality of base pad members 11--11. The strip 
10 is provided with a pair of generally parallelly disposed score lines 15 
and 16 along said elongated strip at a point spaced inwardly from the 
outer edge surfaces 17 and 18 thereof. Furthermore, spaced-apart 
transversely arranged score lines as at 20 and 21 are formed along the 
elongated strip 10 to facilitate a breaking-off of individual mounting pad 
means 11--11 such as the individual pad means 11--11 encompassed by 
brackets 22 and 23 for example. 
In order to facilitate mounting of a solid-state semiconductive translating 
device on the surface of the laminate structure, device mounting 
electrically insulating-thermally conducting pad means such as 25--25 may 
be secured or bonded to the surface of the central base pad portion 26 of 
the elongated strip 10. Base pad central portion 26 is flanked by flange 
walls 27 and 28, with flange walls 27 and 28 being coupled and integral 
with base pad portion 26. In other words, flange walls 27 and 28 are 
extensions of base pad portion 26, and are coupled to the opposed side 
edges thereof. 
As indicated above, the metallic member 11 is preferably highly thermally 
conductive. Copper and aluminum are, of course, the most desirable 
materials, since each are widely commercially available in a wide variety 
of thickness dimensions. For most purposes, copper and aluminum foil with 
a thickness of between about 2 and up to about 200 mils is useful. In 
connection with these materials, it will be noted that in general, 
metallic foil thicknesses ranging up to about 125 mils may be 
advantageously employed. 
The elongated strip illustrated in FIG. 2 shows a modified structure. 
Specifically, the base portion of the modification shown in FIG. 2 
includes a pair of outer metal foil layers 12 and 13 disposed on opposite 
surfaces of a thermally conductive core 14 consisting of silicone rubber. 
The metallic layers, preferably metallic foil layers 12 and 13, are highly 
thermally conductive, and copper and aluminum are the materials of choice. 
For most purposes, the copper and/or aluminum foil is formed with a 
thickness of between about 2 and 5 mils each. In connection with these 
materials, it will be noted that metallic foil thicknesses ranging up to 
in excess of 5 mils each may be satisfactorily employed. 
The core material 14 is preferably silicone base rubber, and silicone base 
rubbers are, of course, widely commercially available. Such silicone base 
rubbers have adhesive properties, and hence may be readily employed as the 
core material for securing and bonding layers of metal foil to opposite 
surfaces thereof. Preferably, the silicone base rubber material forming 
the core has a thickness of between about 8 mils and 12 mils. 
The combination of metallic foil layers on opposite surfaces of a core 
provide a generally rigid laminate with desirable mechanical and thermal 
properties. The material is sufficiently durable so as to withstand 
exposure to the normal events occurring during manufacture and use of 
conventional solid-state semiconductive translating devices. 
The structure of FIG. 2 includes a plurality of openings or bores formed as 
at 30--30 and 31--31. Specifically, bores 30--30 and 31--31 are regularly 
spaced elliptical bores formed along the flange portion of the elongated 
strip. These bores are formed generally laterally outwardly of the 
parallelly disposed score lines 33 and 34, and with the major axes of each 
of the bores being generally normal to the longitudinal axis of the 
elongated strip member. In the illustration of FIG. 2, the axis of 
elongated strip member generally designated 35 is, of course, normal to 
the major axis of the individual elliptically shaped bores. Also, the 
transverse score lines for the break-off zones are formed generally 
coincidentally with the major axes of one laterally disposed pair of 
elliptical bores, such as the laterally disposed pair of partial 
elliptical bores shown partially as at 37 and 38. It will be appreciated, 
of course, that the configuration of the individual bores formed in the 
pad material is not critical. Circular, rectangular, or other regularly 
shaped bores may, of course, be appropriately utilized. The purpose of the 
bores is to assist in the flow of cooling air around and through the 
device. 
Attention is now directed to FIG. 3 of the drawings wherein mounting pad 40 
is illustrated formed as a fragmentary portion of strip 35 of FIG. 2. In 
this view, the edge portions 41 and 42 have been folded inwardly to create 
a channel-like member to receive a semiconductor device along the base pad 
central portion 43. For affecting a bond between the bonding pad assembly 
and a suitable substrate, a second thermally conductive insulator pad is 
formed in opposed relationship to the base pad 43. 
Attention is now directed to FIG. 4 of the drawings wherein a modified form 
of elongated strip 50 from which individual mounting means may be formed 
is illustrated. Specifically, the elongated strip means 50 of FIG. 4 
provides the same ingredients or elements as are available from that shown 
in FIGS. 1 and 2, with the exception being the utilization of an 
accordion-fold arrangement as illustrated in FIG. 5, wherein the mutually 
adjacent flange portions 51--51 and 52--52 are folded in accordion-pleat 
fashion on opposite sides of a center base pad member as at 54. Score 
lines are formed at the juncture point between individual members 51--51 
and 52--52 as at 53 and 54 so as to separate the strip into individually 
useful mounting pad means. 
With attention being directed to FIG. 6, a mounting means having 
perforations formed therein similar to those shown in FIG. 2 is 
illustrated. In this connection, the pad means 60 is formed in a fashion 
similar to that of FIG. 5, with the exception being the formation of the 
perforations as at 61--61 between what becomes individual pads upon 
severing or breaking along the score lines 62 and 63. 
Attention is now directed to FIG. 7 wherein a single mounting means 70 is 
shown. Pad 70 includes the flange elements 71 and 72 disposed on opposite 
sides of the base pad 73. Score lines are formed as at 74 and 75 to permit 
inward and upward folding of flanges 71 and 72 in accordance with the 
arcuate arrows as at 76 and 77. In addition, silicone rubber pads are 
provided as at 78 and 79. Pad 78 facilitates direct mounting of the member 
70 onto a chassis surface and/or printed circuit surface, while pad 79 
permits direct mounting of the solid-state semiconductive translating 
device. One significant advantage derived from pre-assembly of these 
devices is the virtual elimination of air entrapment, thereby enhancing 
the thermal conductivity capability of the overall assembly. 
Attention is now directed to FIG. 8 wherein mounting member 70 is equipped 
with a solid-state semiconductive translating device such as device 81. 
Device 81 is in the form of an encapsulated solid-state semiconductive 
translating device, the back of which consists of a metallic mounting 
bracket or member 82. A bore may be formed in bracket member 82 as at 83 
to accept a screw for mounting onto a chassis member, if desired for the 
specific application. 
Attention is now directed to FIG. 9 of the drawings wherein the assembly of 
FIG. 8 is subjected to a further operation wherein the individual tine 
leads 84, 85 and 86 are bent downwardly at right angles along the length 
thereof so as to permit the direct mounting of the device onto a printed 
circuit board. 
THE FORMABLE METAL BASE 
The formable metal base is utilized as the primary thermal conducting 
material. As has been indicated, the metallic base is preferably copper or 
aluminum, because of its desirable thermal, electrical, and mechanical 
properties, and is available and usable as an extruded metal part, 
extruded on an elongated basis, a machined metal part, a metal foil, 
either solid metal or a laminated metal structure with a thermally 
conductive core. In each of these instances, the structure is such that it 
can be readily formed with simple hand tools and without need for heavy 
metal presses, metal bending equipment, or the like. Also, as indicated 
above, the thickness of individual layers in a laminate should be selected 
in the area of about 5 mils, with certain composite structures up to a 
thickness of about 125 mils being usable. As suggested hereinabove, the 
base pad assembly may be formed in either strip-form, or alternatively on 
a reel-to-reel basis. Such fabrication techniques enhance the rate of 
production of the devices. 
THE THERMAL CONDUCTING INSULATOR 
As indicated above, this material is preferably fabricated from silicone 
rubber, and may be applied as a coating, a composite, or as an adhesive 
pad. The substance of the present invention eliminates the need for a 
loose insulator element, such as a mica-pad or the like. 
The thermally conducting insulator is preferably applied to the surface of 
the metallic foil, as is indicated in the assemblies of FIGS. 1 and 2, for 
example. In certain applications, the thermally conducting insulator may 
be applied to the base of the semiconductor device undergoing 
consideration. Also, the application technique is selected and undertaken 
in such a way that air entrapment is either avoided or substantially 
reduced so as to increase the overall thermal conductivity properties of 
the device. 
FORMING THE THERMALLY CONDUCTING INSULATOR 
The thermally conducting insulator may be applied and/or formed by 
conventional techniques including spraying, dipping, brushing, screening, 
roller coating, or transfer coating techniques. These applicating 
techniques are commonly used with materials including the desired material 
of the present invention, silicone rubber. 
In these application techniques, the thermally conducting insulator is 
secured to the base of the transistor (or other semiconductor translating 
device) either directly to the base or to the assembly. Adhesive bonding 
or vulcanization are usable application techniques. 
It will be appreciated, therefore, that the mounting means of the present 
invention facilitates and provides effective use in combination with 
solid-state semiconductive translating devices, and wherein the base pad 
is formed of a generally rigid, but both durable and deformable. The 
structure may be formed as a metallic plate or foil structure, or as a 
laminate with a silicone rubber core having outer metal layer foil layers 
secured to opposite surfaces thereof. The combination of the metal foil 
layers and the silicone rubber core provides a finished product having 
desirable mechanical properties, along with desired thermal properties 
including a high degree of thermal conductivity. 
SURFACE COATING 
The thermally conductive insulative material is formed as a stripe on the 
overall heat sink structure. The coating, while preferably applied to the 
heat sink device, may alternatively be applied to the metallic base 
element of the semiconductor device. As such, a strong adherent and 
coherent coating is provided in precisely defined and delineated areas, 
thereby contributing to the overall heat dissipating qualities of the 
device. Also, it will be appreciated that the surface coating may be 
employed as a bonding agent for the assembly for bonding or otherwise 
securing the assembly to a suitable substrate, and/or bonding the 
semiconductor device to the heat dissipating assembly. As indicated above, 
the utilization of such a surface coating, for practical purposes, 
eliminates the formation of any air barrier which impedes or otherwise 
reduces the thermal performance. 
The surface coating may be either bonded through a vulcanizing operation, 
or otherwise. The stripe may be applied either through transfer coating, 
or other operations including, for example, silk screening. Because of its 
excellent release properties, polytetrafluoroethylene film may be employed 
as a temporary base for silicone rubber coatings. Thus, silicone rubber 
may be applied to a transistor base through transfer coating techniques 
utilizing a polytetrafluoroethylene film base pad. 
The method of fabricating the mounting means of the present invention is 
also advantageous from the standpoint of the preparation of elongated 
strips of material which may be broken-away into individual mounting pad 
means. Furthermore, the utilization of silicone rubber adhesive pads 
faciliates direct mounting of a solid-state semiconductive translating 
device to the surface of the base pad. Such direct-mounting of a 
solid-state semiconductive translating device may be accomplished with 
either strip-form assemblies or rolls on a reel-to-reel basis. Such 
assembly techniques expedite production rates and provide for versatility 
in manufacture and assembly.