Patent Grant
8395058
This application relates to circuit boards and, more particularly, to a metal core circuit board modified with conductive pins.
Light emitting diodes (LEDs) are rapidly replacing conventional sources of illumination such as incandescent bulbs. Because an LED is typically a discrete circuit, it is common to mount LEDs on circuit boards so that the LED may receive the appropriate circuit leads. Although LEDs are more efficient than conventional illumination sources, they still emit an appreciable amount of heat during operation. Metal core printed circuit boards are thus used to provide thermal management for mounted LEDs. Such a board would include a conductive core such as aluminum that is coated with a one or more dielectric layers. A printed or lithographed foil layer, such as copper, overlays the dielectric layer. The foil layer forms the electrical leads to couple to the LED. The dielectric layer(s) act to insulate the foil layer and the coupled circuits from the conductive core. Although the core is thus electrically isolated, it is still thermally connected to the LED such that it acts as an adequate heat sink.
But conventional metal core board technology is problematic for applications that must pass electrical leads through the board. For example, such a need is present in LED flashlight applications. In that regard, consider the construction of a conventional flashlight—there is an elongated cylindrical battery housing that holds the batteries and allows a user to handle the device. The battery housing connects to a flashlight head that includes a lens or transparent cover held by a bezel. At the base of the bezel is the circuit board holding the LED(s).
The circuit board is mounted within the flashlight orthogonally to the optical axis of the lens. However, such a circuit board arrangement then forms a natural barrier to the necessary electrical leads for coupling between the batteries and the board's printed foil layer (and ultimately to the LED). Conductive pins passing through the circuit board to couple to the printed foil layer need insulation from the metal core in the board to prevent the batteries from shorting out through the resulting conduction in the metal core. But an insulated pin then requires an extra soldering step to couple to the printed foil layer, which increases manufacturing costs. Alternatively, wires can be passed through a gap between the edge of the board and the bezel, which still requires an extra soldering step and requires a bigger installation space.
Accordingly, there is a need in the art for metal core circuit board configurations that enable efficient construction and soldering of leads such as conductive pins to pass through the board.
In accordance with a first embodiment of the invention, a circuit board assembly is provided that includes: a metal core circuit board having a principal surface for mounted circuits and at least one through hole extending between the principal surface and a backside surface for the metal core circuit board; and an at least one conductive pin, wherein each conductive pin includes a shaft extending through a corresponding through hole and a pin cap abutting the principal surface adjacent the corresponding through hole such that an undercutting for the pin cap circumferentially surrounds the corresponding through hole.
In accordance with a second embodiment of the invention, a method of manufacturing a circuit board assembly is provided that includes: providing a circuit board having a through hole at least partially surrounded by solder; inserting a shaft for a conductive pin into the through hole such that a cap for the conductive pin abuts the solder wherein the through hole has a diameter sufficiently exceeding a diameter for shaft such that the shaft is electrically isolated from a metal core for the circuit board, and wherein the cap has an inner undercut portion surrounding the through hole and an outer remaining portion abutting the solder; and heating the solder such that it reflows and electrically couples the pin to a metal foil layer on the circuit board.
In accordance with a third embodiment of the invention, a flashlight is provided that includes a flashlight head including a lens held by a bezel; a metal core circuit board secured to the bezel, the circuit board including a light emitting diode (LED) for illumination through the lens; and a battery housing for holding batteries for powering the LED through a conductive path that includes a first conductive pin having a shaft extending through a first through hole in the circuit board, the first conductive pin including a pin cap abutting a principal surface of the circuit board adjacent the first through hole such that an undercutting for the pin cap circumferentially surrounds the first through hole.
The scope of the invention is defined by the claims, which are incorporated into this section by reference. A more complete understanding of embodiments of the present invention will be afforded to those skilled in the art, as well as a realization of additional advantages thereof, by a consideration of the following detailed description of one or more embodiments. Reference will be made to the appended sheets of drawings that will first be described briefly.
a is a cross-sectional view of a conductive pin transecting a metal core board without any undercutting in accordance with an embodiment of the invention.
b is a close-up cross-sectional view of the through hole and pin junction for
c is a close-up cross-sectional view of the through hole and pin junction for
Embodiments of the present invention and their advantages are best understood by referring to the detailed description that follows. It should be appreciated that like reference numerals are used to identify like elements illustrated in one or more of the figures.
The following metal core circuit board configuration will be discussed with regard to an example flashlight embodiment. However, it will be appreciated that such a configuration can be widely applied to other applications besides flashlights where there is a need to pass conductors through the metal core circuit board to electrically couple to the printed foil layer. Turning now to the drawings,
A flashlight head 115 includes a bezel 120 holding a lens 125 and forming a backing plate 130. A metal core circuit board 135 mounts to backing plate 130 so that heat from an LED 140 may dissipate as discussed previously. To turn the flashlight on and off, a user activates a switch 145. With the switch on, current from the batteries flows through one or more conductive pins 150 to the LED.
In some embodiments, a single conductive pin 150 is sufficient in that a ground connection to LED 140 is available through an appropriate coupling through the metal core circuit board 135 to bezel 120 and from bezel 120 to battery housing 105 back to batteries 110. However, a ground connection through a second conductive pin as shown in
Additional conductive pins may be provided as necessary for coupling control signals or other desired signals to circuits on metal core board 135. For example,
Pin cap 200 is circumferentially undercut around pin shaft 205 such that an annular portion 215 on an underside of cap 200 is also electrically isolated from metal core board 135. To provide an electrical coupling, some portion of cap 200 connects through solder to a foil layer (discussed further with regard to
Note the advantages of such a coupling to LED 140—no insulation layer or sleeve is necessary for through holes 210. Although an insulating sleeve is not necessary, metal core circuit board 135 can include such sleeves if desired. For example, consider
As seen in the close-up view of
For example, there may be minor defects along edge 325 that allow metal-to-metal contact between cap 320 and metal core 306. Alternatively, the voltage between cap 320 and core 306 may cause arcing across such short distances. Thus, the mere presence of an insulating sleeve 310 does not provide adequate isolation between cap 320 and core 306 at edge 325 of dielectric layer 315 as compared to embodiments with a circumferential undercutting on the pin cap as discussed with regard to
In that regard, consider the cross-sectional view of undercut cap 200 at the junction with dielectric layer edge 325 as seen in
In one embodiment, annular undercut region 215 is undercut to a depth of 5 mil. Foil thickness may range from 3 to 5 mil such that the cap lower surface in annular undercut region 215 is displaced from dielectric layer 315 by 8 to 10 mil in such an embodiment. In contrast, as seen in
A method for manufacturing circuit board 135 with pins 150 will now be discussed. As seen in
Solder/flux paste is also laid down in an appropriate pattern for coupling to LED 140. The leads within printed foil layer 330 that couple from pins 150 to LED 140 are underneath solder mask 400 in
The solder/flux paste will not only promote fusion but also is adhesive. Thus, when pins 150 and LED 140 are placed onto the solder/flux paste layers, these components will tend to adhere to metal core circuit board 135 before the solder is reflowed in a reflow oven. For illustration purposes, a pin 150 is left unmounted in
To assist robotic placement of pins 150 into through holes 210, solder mask 400 may include one or more fiducials 405. Despite the presence of fiducials 405, there is some tolerance with regard to an exact centering of each pin in a corresponding through hole. Thus, it is desirable that the difference between the pin shaft diameter and the through hole diameter accommodate this tolerance.
For example, suppose the tolerance is plus or minus 5 mil. If one desires at least a 5 mil separation between the pin shaft and the through hole wall, the through hole diameter should be at least 20 mil greater than the pin shaft diameter to satisfy the desired separation. In general, the diameter difference between the pin shaft and the through hole will depend upon the tolerance provided by the placement method for centering the pins within each through hole upon insertion.
After the pins and LED have been placed upon the circuit board, the resulting assembly may be heated in a reflow oven such that the pins and LED are soldered to the foil layer. In that regard, note that the solder ring surrounding through holes 210 in
It will be appreciated however that although an automated assembly lowers manufacturing costs, the pins could also be placed manually in their corresponding through holes. Referring back to
Referring again to
Embodiments described above illustrate but do not limit the invention. For example, a manufacturing method was discussed with regard to a reflow soldering process but it will be appreciated that other soldering techniques could be used to connect the pin cap to the board's printed foil layer. Thus, it should also be understood that numerous modifications and variations are possible in accordance with the principles of the present invention. Accordingly, the scope of the invention is defined only by the following claims.
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| Number | Date | Country |
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| Number | Date | Country | |
|---|---|---|---|
| 20110261557 A1 | Oct 2011 | US |
