Surface mount power supply device

The present invention relates to a method and apparatus for manufacturing a surface mount power supply device having effective thermal management. The surface mount power supply device comprises a printed circuit board mounted to a thermal plastic lead frame attach by means of vertically-extending aluminum pins embedded in the lead frame attach. A cylindrical member is centered within the lead frame attach by means of inwardly protruding arms transversely connected to the lead frame attach to allow for a pick-and-place operation.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to surface mount device ("SMD") 
technology and more particularly to a surface mount power supply device 
having high thermal performance. 
2. Description of the Related Art 
For many years, the fabrication of an electronic circuit board was 
accomplished by the "through-hole" method. This method involved 
interconnecting electronic components on a planar printed circuit board 
("PCB") typically comprised of an epoxy/fiberglass laminate substrate clad 
with an etched sheet of copper to delineate conductive paths. Holes were 
drilled at the ends of the conductive paths to receive leads of components 
("through-hole devices") which were subsequently soldered thereto. 
Recently, SMD technology has emerged to allow for more efficient automatic 
mass production of PCB's with higher component densities. Package 
components are automatically placed at preselected locations on top of a 
PCB in such a manner that their leads are aligned with and lie on top of 
corresponding solder pads. The components are then soldered to the PCB by 
a soldering process, such as vapor phase soldering, reflow soldering or 
infrared soldering, to thereby establish a permanent electrical connection 
between the leads and their corresponding conductive paths. 
SMD technology has many advantages over through-hole technology. Most 
importantly, it avoids the necessity for drilled holes and allows 
components to be attached to both sides of a PCB. Other important 
advantages of SMD technology include speed of assembly, greater 
reliability of components and the availability of large-scale soldering 
methods. Despite these advantages, the exclusive use of surface mount 
technology at the PCB assembly level has been stymied by a limited 
accessibility to certain components that are not yet readily available as 
SMD's. 
Five to thirty watt dc-to-dc converter power supplies are components that 
are presently not available in surface mount packages. Currently, dc-to-dc 
converter power supplies are through-hole devices that require different 
and more costly assembly methods from conventional SMD methods. These 
devices are commonly used in telecommunication switches, network devices, 
mainframe computers and minicomputers. Lack of availability of SMD 
dc-to-dc converters ultimately has prevented manufacturers from realizing 
the full potential of cost savings from automatic SMD operations. 
Surface-mountable dc-to-dc converters have been presently available only 
as custom-built components or standard components in a limited set of 
inputs and outputs. Switching to surface-mount for dc-to-dc converters 
would be especially desirable where SMD is already widely used, as in 
telecommunications, computers, wireless cellular base stations, computer 
mainframes or work stations. However, weight has been the chief obstacle 
in constructing these devices to be surface-mountable. An SMD needs to be 
light enough to be handled by surface mount pick-and-place equipment, but 
encapsulation and other techniques necessary for effective thermal 
dissipation have not allowed adequate weight reductions. 
It is therefore an object of the present invention to provide a 
surface-mountable power supply device by making the necessary reductions 
in component weight for pick-and-place operations while still allowing for 
effective thermal dissipation. 
SUMMARY OF THE INVENTION 
According to the present invention, the foregoing and other objects are 
attained by a surface mount power supply device having effective thermal 
management. 
The surface mount power supply device generally includes a PCB mounted to a 
lead frame attach by means of a plurality of vertically extending 
"L"-shaped pins. Rectangular surfaces on the heads of the pins come into 
contact with a customer PCB to form electrical connections. 
The PCB is comprised of a thermally conductive material referred to as 
T-LAM.TM. and the lead frame attach is comprised of thermal plastic. With 
this structure, heat is dissipated mostly upward and away from the PCB and 
the sides via the pins. The result is effective thermal dissipation with 
minimal and even heat rise on the PCB. 
For pick-and-place operation, a cylindrical member is disposed within the 
lead frame attach to simulate a bottom surface, and is connected by 
protruding arms to side members of the lead frame attach.

DETAILED DESCRIPTION 
Referring to FIGS. 1 and 2, a surface mount power supply device 10 
according to the present invention is shown. The power supply device 10 
generally includes a PCB 30 mounted to a lead frame attach 16 by means of 
a plurality of metallic pins 12a-12x. 
The PCB 30 has a top surface 14 and a bottom surface 32 and is comprised of 
a thermally conductive material referred to as the T-LAM.TM. system by 
Thermagon, Inc., of Cleveland, Ohio. The T-Lam.TM. system includes an 
Insulated Metal Printed Circuit Board (IMPCB.TM.) laminate, T-preg.TM. 
dielectric, and Double-Sided Laminate (DSL.TM.) materials for multilayer 
boards. 
The IMPCB.TM. laminate, which provides the raw material for single-sided 
surface mount PCBs as with the present invention, contains a metal base 
plate layer, a thermally conductive T-preg.TM. layer, and a copper foil 
circuitry layer which comprises the top surface 14 of the PCB 30. The 
IMPCB.TM. laminate is capable of dissipating many times more heat than its 
conventional FR-4 counterpart used in prior art power supply devices. In 
many configurations, the IMPCB.TM. can also outperform alumina in heat 
dissipating capacity. Moreover, the pure copper traces provide better 
current carrying capability than alumina thick film circuits. The copper 
foil circuitry layer is high ductility, electrodeposited copper. The metal 
base plate of the IMPCB.TM. laminate may be made of various materials 
including aluminum, copper or copper-molybdenum-copper. In the present 
embodiment, the metal base plate is a copper heat spreader on the bottom 
surface 32 of the PCB 30. 
The T-preg.TM. layer serves as the bonding agent for laminating the copper 
circuitry layer to the heat dissipating metal base plate, and possesses 
high dielectric strength and low capacitance. The T-preg.TM. layer serves 
three major functions: (1) heat conduction; (2) electrical isolation; and 
(3) adhesion. 
T-LAM.TM. has a very good thermal conductivity allowing for a single-sided 
PCB design. More importantly, thermal resistance of T-LAM.TM. is 
approximately 0.04-0.06.degree. C.-in.sup.2 /W, vastly superior to that of 
the conventional FR-4 of approximately 8.3.degree. C.-in.sup.2 /W. The 
copper heat spreader will dissipate thermal losses into the localized 
environment through natural convection. Superior thermal performance is 
also due to the additional surface area of the pins. This aids in better 
distribution of thermal losses to the localized environment and away from 
the customer's PCB. 
The lead frame attach 16 is a rectangular structure having parallel side 
members 50 and 52 connected transversely at their ends to parallel side 
members 54 and 56. The lead frame attach 16 is a custom molded-injected 
pin carrier comprised of thermal plastic and having pins 12a-12l embedded 
on side member 56 of the structure and pins 12m-12x embedded on side 
member 54 of the structure. The lead frame attach 16 further comprises a 
thermal plastic cylindrical member 18 at its center having protruding arms 
20, 58, 60 and 62. Arms 60 and 62 extend from a vertical midpoint of the 
cylindrical structure 18 and connect transversely to the side members 54 
and 56, respectively. Arms 20 and 58 extend perpendicularly planar from 
the top surface 64 of the cylindrical member 18 and connect at points 
overlapping side members 52 and 50, respectively. 
The cylindrical member 18 is essential for a pick-and-place operation with 
conventional pick-and-place machinery. Conventional pick-and-place 
machinery will not pick-and-place a component unless it detects a bottom 
element, such as the bottom surface of a prior art encapsulated component 
having a "box"-like structure. The cylindrical member 18 is configured to 
be just large enough for the pick-and-place machinery to detect a bottom 
surface without adding unnecessary weight to the power supply device 10. 
Each pin 12a-12x comprises an "L"-shaped structure having a vertically 
extending shaft terminating in a pin head 24 with elevated contact members 
26 having rectangular surface areas which come into contact with a 
customer's PCB to form electrical connections. As will be discussed in 
more detail below, the ends of the pins 12a-12x opposite the pin heads 24 
are soldered into pin holes 28 drilled into the PCB 30. 
In one example of the invention, the device 10 is 1.4 inches long by 1.4 
inches wide by 0.35 inches thick, produces a nominal output power of 10 
Watts and a power density of 14.58 in.sup.3 and has a % Delta of 
-2.0/+2.4% in.sup.2 /in.sup.3. In a second example, the power supply 
device 10 is 1.9 inches long by 1.9 inches wide by 0.35 inches thick, 
produces a nominal output power of 20 Watts and a power density of 15.83 
W/in.sup.3 and has a % Delta of +12.8/17.9% in.sup.2 /in.sup.3. In a third 
example, the power supply device 10 is 2.5 inches long by 2.5 inches wide 
by 0.35 inches thick, produces a nominal output power of 30 Watts and a 
power density of 13.71 W/in.sup.3, and has a % Delta of -16.7/-12.9% 
in.sup.2 /in.sup.3. 
The process for manufacturing the power supply device 10 is as follows. A 
manufacturer starts with a T-LAM.TM. PCB 30 and surface mounts components 
onto the top surface 14 of the PCB 30 with high speed placement and 
reflow. The lead frame attach 16 is mounted via alignment pins (not shown) 
by inserting pins 12a-12x into holes 28 drilled into the PCB 30 while 
providing a consistent height for the pins 12a-12x to be reflowed with the 
rest of the components. As a final step, an optional conformal coating, 
spray or dip is automatically deposited on both sides of the PCB 30. 
FIGS. 3-5 illustrate the superior thermal management of the power supply 
device 10 over the prior art. 
Referring to FIG. 4, the power supply device 10 is shown having a pair of 
DPAK devices 36 and 38 and a device 66 mounted on the top surface 14 of 
the PCB 30. The rectangular surfaces of pin heads 24 of pins 12a and 12m 
are illustrated in contact with a customer PCB 34. 
As shown in FIGS. 3 and 4, thermal management of the power supply device 10 
is due to direct transfer of thermal losses from the two mounted DPAK 
devices 36 and 38 into the component copper layer, through the T-LAM.TM. 
dielectric material referred to as T-preg.TM., into the copper clad heat 
spreader on the opposite side of the board and dissipated out the 24 pins 
12a-12x. Even distribution of thermal energy throughout the 24 pins 
12a-12x cause minimal temperature rise at the customer's board 34. As 
shown in FIG. 4, at an ambient temperature of 22.degree. C., the 
temperature at the customer's PCB 34 is 48.degree. C. 
Referring now to FIG. 5, a prior art encapsulated through-hole power supply 
device is shown. The through-hole power supply device comprises 
through-hole pins 42a and 42b connected to a PCB 70 comprised of FR-4 
material and extending through the customer's PCB 34. DPAK devices 44 and 
46 and device 68 are mounted on the surface of PCB 70. The PCB 70 is 
surrounded by box attach 40. As compared to the present invention, at an 
ambient temperature of 22.degree. C., the temperature at the customer's 
PCB 34 is 61.degree. C., i.e., an increase of 13.degree. C. 
There are many benefits to the power supply device 10 of the present 
invention. First, it allows the copper heat spreader on the bottom surface 
32 of the PCB 30 to be facing upward or away from a customer's PCB 34, 
thereby minimizing the transfer of thermal energy back into the customer's 
PCB. Second, a box attach is not required due to the components facing 
downward and being somewhat protected by the lead frame attach 16. Third 
is the potential ability for the lead frame attach 16 to act as an EMI 
shield in conjunction with the copper heat spreader of the PCB 30. 
Finally, noncoplanarity (defined as the maximum distance between the 
lowest and highest pin when the finished package rests on a perfectly flat 
surface) is kept to a minimum in this configuration where the stable lead 
frame attach 16 can be used to isolate the PCB 30 from potential warpage 
during reflow. 
Although illustrative preferred embodiments have been described herein in 
detail, it should be noted and will be appreciated by those skilled in the 
art that numerous variations may be made within the scope of this 
invention and without sacrificing its advantages. The terms and 
expressions have been used herein as terms of description and not terms of 
limitation. There is no intention to use the terms or expressions to 
exclude any equivalents of features shown and described or portions 
thereof and this invention should be defined in accordance with the claims 
which follow.