Mounting assembly for electronic power devices

An assembly for mounting electronic power devices to a flexible circuit board and in an enclosure. A slotted plastic button is mounted to the built-in heat sink portion of the power device and a spring member is mounted between two such power devices. The spring forces the heat sink surfaces of the power devices against opposite walls of the enclosure and transmits insertion forces to the flexible board during assembly.

FIELD OF THE INVENTION 
This invention relates generally to modular electronic circuit packaging 
and more particularly concerns a device for positively mounting power 
devices in an enclosure and combining such power devices with a flexible 
circuit board. 
DISCUSSION OF THE PRIOR ART 
Electronic power devices, normally semiconductors, are normally formed with 
an insulating package and a surface on one side which functions as a heat 
sink. This surface may be electrically or thermally conductive or both. 
The heat sink portion normally has a hole therethrough by which the power 
device is secured to a larger surface, such as a heat sink or the wall of 
an enclosure, by means of screws or bolts. Alternatively, such power 
devices could be secured by some type of adhesive or by soldering. 
An example of such a device and the mounting thereof to a larger surface is 
shown in U.S. Pat. No. 3,641,474. Another means for mounting a heat sink 
to a power device is shown in U.S. Pat. No. 3,566,958 wherein two 
radiating type heat sink elements are clamped by means of spring devices 
to either side of the power electronic component. The former patent is a 
good example of a conventional mounting means in this particular technical 
area and it may be noted that the power device could be replaced by 
removing the bolt and disconnecting the leads which may be inserted into a 
socket or soldered to circuitry as appropriate. This prior art structure 
involves the semipermanent mounting of the power device to the heat sink 
or wall of an enclosure. 
SUMMARY OF THE INVENTION 
It is an object of this invention to connect electronic power devices to 
circuit sub-assemblies which are then mounted in an enclosure wherein 
positive heat sink surface contact is made to the enclosure walls but no 
permanent connection is made thereto. 
Broadly speaking, this invention is concerned with a spring member 
extending between two oppositely disposed power devices wherein the 
coupling between the spring member and each of the devices comprises a 
slotted plastic button which positively holds the spring and the power 
device together and is so configured as to absorb significant tolerances 
in the sizes of the holes in the power device and in the size of the 
spring member. The plastic button is formed with a cylindrical projection 
having an axial blind hole extending from the opposite side into the 
cylinder, and a radial slot through the button extending from the axis of 
the hole. Viewed from the side opposite the cylinder, the hole forms a 
key-shaped opening in conjunction with the slot. The cylindrical 
projection extends into the hole in the heat sink portion of the power 
device and the slot absorbs variations in the hole diameter. The end of 
the spring device extends into the hole in the opposite side of the button 
and the slot absorbs variations in diameter of the spring device. The 
connection between the spring device and the hole in the button is a force 
fit, as is the cylindrical projection mounted in the hole in the heat sink 
.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
With reference now to the drawing and more particularly to FIGS. 1-3 
thereof, there is shown a flexible circuit board 11 having various 
electronic components 12 mounted thereto. When in use the sides of the 
flexible board are substantially parallel but are shown somewhat open in 
FIG. 1 for purposes of clarity. At the bottom central portion of the 
U-shaped flexible circuit board adjacent opposite edges are mounted 
electronic power devices 13 and 14. These are normally semiconductors such 
as power transistors and have a plurality of leads 15 which are soldered 
to the circuit board in normal fashion. It should be noted that leads 15 
are S-shaped as shown in the drawing, the function of which will be 
discussed in greater detail below. A plastic button 16 is shown mounted to 
power device 14 with a cylindrical projection 31 completely surrounded by 
heat sink area 17 (FIG. 3). Heat sink 18 is mounted flush with and extends 
beyond one edge of the power device. The other side of plastic button 16 
is visible on power device 13 in FIG. 1. 
Extending between power devices 13 and 14 is spring member 21 having a 
flared U-shaped configuration. The bottom portion 22 of the spring member 
rests against the top surface of flexible board 11 and bends 23 are 
typically located between leads 15 of the power devices which are 
connected to the circuit board. Outwardly flared end portions 24 of the 
spring extend into buttons 16 and are thereby secured to power devices 13 
and 14. 
The entire assembly shown in FIG. 1 is mounted in an enclosure having walls 
25 as shown in FIG. 2. The walls of the enclosure are normally heat 
conductive but are often formed with an anodized coating such that they 
are electrically insulative. However, the particular surface 
characteristics of the enclosure do not form any part of the present 
invention. Spring member 21 is in compression as shown in FIG. 2, that is, 
end portions 24 are forced toward each other, thereby holding the heat 
sink surfaces 17 and 18 of respective power devices 13 and 14 flush 
against the walls of the enclosure. In order to insure that the surfaces 
of heat sink elements 17 and 18 remains flat against walls 25, the 
S-shaped bends of leads 15 are formed so that flexibility is provided 
between the bottom of power devices 13 and 14 and the flexible board 11. 
Leads 15 themselves act as springs and, with the S-shaped bend, force the 
bottom portions of the power devices against walls 25 so that the force of 
spring member 21 does not tend to orient the power devices at an angle 
against the walls of the enclosure. Thus the S-shaped leads allow for 
adjustability in mounting the device to the flexible circuit and provide 
spring pressure to assist in making good thermal contact between the 
device heat sink surface and the wall of the enclosure. An insulative 
sleeve is shown on leads 15 and is typically used to prevent accidental 
electrical shorting. 
Normally a hard or rigid printed circuit board 26 extends across the tops 
of power devices 13 and 14 making physical contact therewith. When the 
assembly of FIG. 1 is mounted in the enclosure of FIG. 2, downward 
pressure is applied to the circuit board 26. This force is transmitted 
through power devices 13 and 14 and spring member 21 to flexible board 11 
such that the entire assembly can be inserted between walls 25 of the 
enclosure. Note that with this structure, there is no stress upon leads 15 
during assembly but that the entire downward force exerted upon board 26 
is transmitted through spring 21 wherein central portion 22 contacts the 
flexible circuit board. Upstanding terminal pins 27 are electrically and 
physically connected to flexible board 11 and extend through rigid board 
26 to which they may be electrically connected or not, as desired. These 
terminals provide for external electrical connection of the assembly shown 
in the drawing. The combination of spring 21, power devices 13, 14 and 
plastic buttons 16 may be referred to as a power subassembly. Note that 
the electronic components 12 mounted to circuit board 11 are interleaved 
with spring 21, contact pins 27, power devices 13, 14 and themselves, but 
do not interfere with any of the components of the power sub-assembly. A 
thin insulating sheet 28 is normally provided between the printed 
circuitry surface of flexible board 11 and side walls 25 and the bottom 29 
of the enclosure. 
Plastic button 16 is shown in detail in FIGS. 3 and 4. This button may be 
made from any suitable plastic such as an acetal resin having relatively 
high temperature characteristics but which allows plastic flow as 
necessary, typical material examples being sold under the trademarks 
Delrin and Celcon. Button 16 is oval-shaped and is formed with a 
cylindrical projection 31 extending through hole 32 in heat sink 18 of 
power device 14. The other side of button 16 which projects outwardly from 
the surface of heat sink 17 as shown in FIGS. 1 and 2 is formed with a 
blind hole 33 through a substantial portion of the thickness of button 16. 
This hole is shown extending into and is axially aligned with cylindrical 
projection 31. An oval slot 34 centered with respect to the oval exterior 
shape of button 16 extends entirely through the thickness of the button 
and projects radially from the axis of hole 33. Thus as viewed from the 
side of power device 13 as shown in FIG. 1, the combination hole 33 and 
slot 34 assume that appearance of a keyhole. 
The specific dimensions are not critical but size relationships are 
important and typical dimensions are set forth to assist in visualizing 
these relationships. Hole 32 in heat sink 18 passes entirely through the 
heat sink and has a diameter typically 0.139 to 0.147 inch (3.53 to 3.73 
mm). The diameter of projection 31 of button 16 would in such case be 
0.151 inch (3.84 mm), such that even with the relatively large tolerance 
of hole 32, there will be a press fit of projection 31 into that hole. The 
slot 34 passing through projection 31 accommodates the press fit and any 
variations in tolerance of the hole in the heat sink. Thus, as shown in 
FIG. 3, slot 34 assumes a narrow pie shape when projection 31 is forced 
into the hole in the heat sink. 
The spring member 21 is made of appropriate material such as music wire 
having a nominal diameter of 0.067 inch (1.702 mm) and having an 
electrically insulative coating which may be made of a plastic or an 
epoxy, a typical example being parylene, and adds 0.002 to 0.008 inch 
(0.051 to 0.203 mm) to the diameter of the wire. Blind hole 33 in button 
16 has a nominal diameter of 0.066 inch (1.68 mm) so that there is a 
substantial press fit between the end 24 of the wire and hole 33. Slot 34 
accommodates this press fit together with any tolerances in the diameter 
of the wire and coating size so that there is a positive connection 
between the end of the spring wire and button 16. Because of slot 34, the 
button can expand as necessary to receive the wire and is not ruptured in 
any way. Thus the slot which passes entirely through button 16 
accommodates an external inwardly directed compression force acting on 
cylindrical projection 31 and accommodates an outwardly directed expansion 
force acting upon blind hole 33 while maintaining both hole 33 and 
cylindrical projection 31 in substantially cylindrical configuration, that 
is, without causing a cone shape to be formed resulting from the 
combination of press fits. The thickness of heat sink 17 ranges from 0.045 
to 0.055 inch (1.143 to 1.397 mm) and the cylindrical projection 31 
extends from the body of button 16 by approximately 0.045 inch (1.143 mm). 
Thus this projection will not extend beyond the flat surface of heat sink 
17 which makes contact with the wall 25 of the enclosure as shown in FIG. 
2. 
The semiconductor 41 as shown in FIG. 5 has a slightly different 
configuration wherein the heat sink is mounted or embedded in one side of 
the power device as opposed to extending beyond one end of the power 
device as is the case with the semiconductor of FIG. 3. The plastic button 
42 has basically the same configuration and operates in the same manner as 
button 16 but because the hole in the semiconductor and the heat sink is 
larger than the power device of FIG. 3, there is no need for an oval 
configuration to accommodate the slot. Thus button 42 is round and both 
cylindrical projection 43 and blind hole 44 are axially located with 
respect to the external diameter of the button. Heat sink 45 as shown in 
FIG. 6, is normally approximately 0.031 inch (0.787 mm) thick but has a 
countersink or chamfer on the outwardly facing side of hole 53 which 
extends approximately half way through the thickness of the heat sink. 
Because less than the full circumferential surface area of projection 43 
makes frictional engagement with heat sink 45, it has been found 
advantageous to add a ring stake 46 to the outwardly facing circular 
surface 47 of the projection, thereby causing plastic material 51 to flow 
partially into the chamfered area. By adding the ring stake, the 
connection between button 42 and heat sink 45 is quite positive. The slot 
52 in button 42 acts in the same manner as slot 34 in button 16 and 
absorbs the tolerance variations in the hole 53 in heat sink 45 and in the 
outside diameter of the coated spring member 21 within hole 44. 
As an alternative configuration, the cylindrical projection 31 of buttons 
16 may be ganged together on a connecting strip 54 both for ease of 
manufacture and for registration of several power devices 13 as shown in 
FIG. 7. These interconnected buttons have the same configuration and 
function in the same manner as the button previously discussed. 
The present invention provides several significant advantages in the 
component packaging art to which it pertains. By providing a positive 
connection between spring member 21 and oppositely disposed power devices 
such as 13 and 14 by means of the plastic buttons such as 16, the power 
devices can be easily aligned and maintained in opposite facing 
relationship so as to maintain the maximum amount of force exerted by 
spring 21 between the two power devices. This, of course, insures that the 
power devices are forced against the enclosure walls 25 making excellent 
thermal contact therewith. Because of the positive interconnection of the 
spring member and the power devices through the plastic buttons, assembly 
of the electronic components is facilitated. The spring is not likely to 
pop out of the button because of the force fit. If the spring were loosely 
coupled by some means not in accordance with this invention, it could 
become separated and possibly damage or short other electronic components 
in the sub-assembly. Furthermore, if it became disconnected, it would not 
provide the force transmittal from the rigid board 26 to flexible circuit 
11 and could result in crushing the leads 15 of the power devices or 
causing other component damage or failure. Furthermore, spring 21 
maintains positive contact with the power devices through the plastic 
buttons and insures that the power devices remain upright and properly 
oriented with respect to the flexible circuit which remains flat prior to 
and during wave or dip soldering. Because the component leads are normally 
substantially in line and are originally only loosely mounted in holes in 
the flexible circuit, they would be relatively difficult to control 
without the spring member 21 connecting them together and aligning them 
with respect to the flexible printed circuit board prior to soldering. It 
is also possible that with this positive interconnection between the 
spring and the power devices, a power sub-assembly of these parts could be 
made and shipped or handled together and later mounted to a flexible 
board. This would also permit matched components to be more easily 
handled, packaged and shipped. 
It is likely that in view of the above description, modifications and 
improvements will occur to those skilled in the art which are within the 
scope of this invention.