Printed circuit board assembly with improved thermal performance

A printed circuit board assembly comprises a printed circuit board and a heat sink that has two surface planes. One surface plane of the heat sink is raised, and provides a surface area for contact with the printed circuit board. The other surface plane is lower than the raised surface plane. The printed circuit board extends over the lower surface plane. The distance between the raised surface plane and the lower surface plane provides space for through-hole pins that extend beneath the printed circuit board, because the underside of the printed circuit board lies on the raised surface plane. The multi-plane heat sink can be formed by an extrusion process, as well as a stamping or other process that allows for more complex patterns of surfaces.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 This invention relates to the field of printed circuit board assemblies,
 and in particular to printed circuit board assemblies that include a heat
 sink.
 2. Description of Related Art
 Heat sinks are often used to dissipate heat from electronic components.
 Often, a heat sink is provided for the particular component that generates
 substantial heat. In many cases, however, the accumulated heat of many
 components necessitates the heat dissipation. In such cases, it is often
 more efficient and cost effective to provide a heat sink that dissipates
 heat from a region of a circuit board, rather than individual component
 heat sinks. Heat sinks are commonly required in ballast circuits that
 provide high voltage or high current signals to operate lamps.
 U.S. Pat. No. 5,798,908, issued Aug. 25, 1998 to Herzberger et al,
 discloses a printed circuit board assembly comprising a printed circuit
 board and an integral heat sink, and is incorporated by reference herein.
 The heat sink of the '908 patent comprises an aluminum plate that is
 adhered to the printed circuit board via a hot-press process. To
 accommodate both through-hole components as well as surface mount
 components, slots are cut into the aluminum plate to create voids under
 select areas of the printed circuit board, to accommodate the pins of the
 through hole components. This arrangement is well suited for use in a
 ballast assembly, wherein one or more transformers include pins that serve
 to mechanically hold the transformer in place on the printed circuit
 board, as well as providing the electrical contact between the printed
 circuit board and the coils of the transformer. After assembly, the voids
 are filled, or covered by another plate, to avoid inadvertent contact with
 the high voltages that are typically associated with a lamp ballast.
 BRIEF SUMMARY OF THE INVENTION
 It is an object of this invention to provide a heat sink for a printed
 circuit board that allows for through-hole components, but does not
 require cutting voids in the heat sink. It is a further object of this
 invention to provide a printed board assembly with integral heat sink that
 is particularly well suited for ballast applications.
 These objects and others are achieved by providing a printed circuit board
 assembly that comprises a printed circuit board and a heat sink having two
 surface planes. One surface plane of the heat sink is raised, and provides
 a surface area for contact with the printed circuit board. The other
 surface plane is lower than the raised surface plane. The printed circuit
 board extends over the lower surface plane. The distance between the
 raised surface plane and the lower surface plane provides space for
 through-hole pins that extend beneath the printed circuit board, because
 the underside of the printed circuit board lies on the raised surface
 plane. The multi-plane heat sink can be formed by an extrusion process, as
 well as a stamping or other process that allows for more complex patterns
 of surfaces.

DETAILED DESCRIPTION OF THE INVENTION
 FIGS. 1 and 2 illustrate two views of an example ballast assembly 100 that
 includes a printed circuit board assembly that includes a printed circuit
 board 120, and a heat sink 110. In accordance with this invention, the
 heat sink 110 includes sections 110a, 110b at different planes. As
 illustrated, section 110a is at a higher horizontal plane than section
 110b. For ease of reference, section 110a is hereinafter referred to as
 the "raised" section, and section 110b as the "recessed" section. As
 illustrated in FIG. 1, the recessed section 110b of the heat sink 110
 includes two sections, one to each side of the raised section 110a. The
 singular form "section" is used herein for ease of reference, and not
 intended to limit the scope of this invention. The recessed section 110b,
 for example, includes all portions of the heat sink that are at a plane
 that is below the plane of all the other portions of the heat sink that
 form the raised section 110a.
 The printed circuit board 120 comprises two types of regions. One region
 120b is configured to allow for "feed-through" holes that facilitate
 connection from one side of the printed circuit board 120 to the other. As
 is known in the art, feed-through components 150, 160, 170 typically use
 "pins" 151 that pass through the circuit board 120, and extend beyond the
 circuit board 120. The extension of the pins 151 beyond the circuit board
 120 facilitates soldering and other manufacturing processes. For example,
 the pins and holes may be used to mechanically fasten the component to the
 printed circuit board, in addition to, or independent of, an electrical
 connection. The other region 120a is primarily intended for components
 190, such as surfacemount devices, that do not require a "feed-through"
 hole in the printed circuit board 120. The term "region" is used herein in
 the same manner as "section" discussed above. That is, the "feed-through
 region" 120b of the circuit board 120, as used herein, may include
 multiple discontinuous areas of the circuit board 120, each area being
 configured to allow for feed-through holes. The region 120a, hereinafter
 termed the "surface-mount" region, may similarly include multiple
 discontinuous areas that do not require feed-through holes.
 In accordance with this invention, the printed circuit board assembly
 includes the printed circuit board 120 and heat-sink 110 being arranged so
 as to provide a coincidence between the surface mount region 120a of the
 circuit board 120 and the raised section 110a of the heat sink 110, and a
 correspondence between the feed-through region 120b and the recessed
 section 110b. Because the recessed section 110b is at a lower plane than
 the raised section 110a, this arrangement allows for space between the
 feed-through region 120b and the heat sink 110 in the area of the recessed
 section 110b, to allow for feed-through protrusions, such as the pins 151.
 Because the heat sink 110 is in contact with a region 120a of the printed
 circuit board 120, it serves to dissipate heat from the components 150,
 160, 170, and 190, as required. As is common in the art, heat sinks
 dissipate thermal energy by a variety of techniques. The increased mass of
 a heat sink allows for a slower rise in temperature for the same thermal
 energy output from the components; an increased surface area of a heat
 sink allows for a faster dissipation of the thermal energy into the
 surrounding space; the thermal conductivity of a heat sink allows for
 thermal energy to be conducted to other devices that can serve as further
 heat sinks. In a preferred embodiment, the heat sink 100 comprises
 aluminum, steel, or other inexpensive material having a high thermal
 conductivity.
 The principles of this invention are particularly well suited to ballast
 assemblies that are used to provide high voltage or high current driving
 signals to lamps. The reliability of a ballast assembly is highly
 dependent upon the peak operating temperature of the components in the
 ballast assembly. FIGS. 1 and 2 illustrate an example of a typical ballast
 assembly 100 that includes a transformer 150, circuit devices 160, 190,
 and a connector 170 for connection to a power source and to a lamp.
 Electronic ballasts are often subject to a variety of safety regulations,
 most of which are designed to prevent inadvertent contact with high
 voltage or high current terminals within the ballast. It is significant to
 note that the heat sink 110 in accordance with this invention facilitates
 compliance with such safety regulations by preventing access to the
 circuit board 120 from beneath the heat sink 110. An enclosure (not shown)
 can be configured to envelop the ballast assembly on each of the sides and
 the top, and can use the heat sink as the remaining enveloping member,
 thereby saving the cost of an enclosure base. Alternatively, a
 conventional six-sided (top, bottom, 4 sides) enclosure can be used,
 wherein the bottom of the enclosure is in contact with the heat sink 110.
 In this embodiment, assuming that the enclosure is a thermal conductive
 material, such as a sheet metal enclosure, the heat sink 110 conducts heat
 to the enclosure to further reduce the operational temperature of the
 ballast 100. Illustrated in FIG. 1 is an optional thermal coupling device
 115 that can be used to further facilitate a conduction of thermal energy
 from the heat sink 110 to a sheet of thermal conductive material placed
 beneath the heat sink 110, such as the bottom plate of an enclosure that
 envelops the ballast 110, or to the light fixture that contains this
 ballast 100. Note also that the relative mass and structural integrity of
 the heat sink 110 provides an integral base and mounting surface for
 attaching the ballast 100 to the light fixture.
 The shape and orientation of the heat sink 110 will be dependent upon the
 intended arrangement of the components 150-190, and the intended
 integration with the light fixture. The uniform-thickness shape
 illustrated in the figures is easily manufactured using a stamping
 process. A stamping process can also be used to provide a relatively low
 cost means of providing a more complex arrangement of recessed and raised
 sections, as required. The shape illustrated in the figures also allows
 for an extrusion of the heat sink, which can be expected to be less
 expensive than other techniques in high volume applications. As would be
 evident to one or ordinary skill in the art, if an extrusion process is
 used, the thickness of the heat sink 110 in the raised area 110a can be
 increased so as to provide a substantially flat lower surface area below
 the areas 110a and 110b, thereby filling the area of, and eliminating the
 need for, the optional thermal coupling device 115. These and other metal
 forming techniques are common to one of ordinary skill in the art. Also
 evident to one of ordinary skill in the art, a variety of techniques are
 available to incorporate the principles of this invention within a variety
 of constraints. For example, if the printed circuit board contains
 conductors on the lower surface, an insulating layer of tape can be
 affixed to either the printed circuit board or the heat sink to prevent an
 electrical short. In another example, a thermal conductive paste can be
 applied to the heat sink 110 to increase the thermal transfer efficiency
 between the printed circuit board 120 and the heat sink 110, or between
 the heat sink 110 and an enclosure.
 The foregoing merely illustrates the principles of the invention. It will
 thus be appreciated that those skilled in the art will be able to devise
 various arrangements which, although not explicitly described or shown
 herein, embody the principles of the invention and are thus within the
 spirit and scope of the following claims.