Patent Application
20080084671
Referring now to the drawings, and more particularly to
ECM 10 generally includes a housing 12, a control board 14 and an electrical circuit assembly 16. Housing 12 may be of any suitable configuration, and may be formed from any suitable material such as plastic or metal. Housing 12 carries control board 14, and provides external access to an input/output (I/O) connector 18 which is electrically connected with control board 14. Housing 12 also carries and provides access to a pair of input power terminals 20 which are electrically coupled with electrical circuit assembly 16. A flexible jumper circuit 22 interconnects control board 14 with electrical circuit assembly 16. Alternatively, control board 14 may be coupled with electrical circuit assembly 16 using suitable electrical connectors, such as a single inline or dual inline type connector.
Electrical circuit assembly 16 generally includes a flexible electrical circuit 24, heat sink 26, and an adhesive 28. Flexible electrical circuit 24 includes a first side 30 and a second side 32. First side 30 is adhered to heat sink 26 using adhesive 28, as will be described below. Second side 32 carries a plurality of electrical components, such as input power terminals 20, power components 34, capacitors 36 and output power connectors 38. In the embodiment shown, power components 34 are in the form of field effect transistors (FETs) which typically dissipate an appreciable amount of heat during operation. Capacitors 36 may be of any suitable configuration, depending upon the application, and in the embodiment shown are configured as 22 mm diameter by 41 mm length capacitors which are electrically coupled with flexible electrical circuit 24. Output power connectors 38 may also be suitably configured depending upon the application, and are configured as threaded studs in the embodiment shown in
Flexible electrical circuit 24 (
Power components 34, capacitors 36 and power connectors 38 are preferably each configured as surface mount components, providing quick and easy soldering with corresponding pads (not numbered) associated with copper traces 42 using a “pick-and-place” machine.
Flexible electrical circuit 24 may optionally also include one or more thermal vias 46 extending through flexible electrical circuit 24 from first side 30 to second side 32. Each thermal via 46 is in the form of a plated hole (i.e., a metal filled hole) positioned under a corresponding power component 34 for better conducting heat away from flexible electrical circuit 24.
Additionally, flexible electrical circuit 24 may optionally include a solder mask 48 (
Heat sink 26 includes a metal base plate 50 having a first side 52 and a second side 54. A plurality of heat conducting fins 56 extend from second side 54. Fins 56 can be coupled with base plate 50 in a number of suitable ways, such as welding, bending, etc. Fins 56 are preferably formed as an integral unit with base plate 50, such that heat sink 26 is of monolithic construction. Heat sink 26, including base plate 50 and fins 56, is also preferably formed from aluminum with a sufficient heat conducting coefficient, but may be formed from a different type of material depending upon the application.
Adhesive 28 is a thermally conductive and electrically insulating adhesive which directly interconnects at least a portion of first side 30 of flexible electrical circuit 24 with first side 52 of base plate 50. In one embodiment, adhesive 28 is a pressure sensitive adhesive (PSA) which thermally couples and electrically isolates flexible electrical circuit 24 and base plate 50. For example, adhesive 28 may be in the form of a 2-5 mm thick ceramic based PSA which is used to couple flexible electrical circuit 24 with base plate 50. Other types of adhesives may also be used, such as a prepreg material which is die cut to size (a prepreg material is basically a fiberglass cloth impregnated with a resin which may be cut, placed and cured for adhesive bonding). An example of a prepreg material is Isola 1060 no-flow prepreg.
In the embodiment shown in
As another option, heat sink 26 may be formed with a pocket (not shown) in base plate 50 beneath a portion of flexible electrical circuit 24 carrying through-hole components, and the leads from the through-hole components may be received within the pocket.
As a further option, flexible electrical circuit 24 can be configured as a rigid board for some applications, which is still nonetheless adhesively bonded directly to heat sink 26 using an appropriate thermally conductive and electrically insulating adhesive 28.
During manufacture, flexible electrical circuit 24 is formed with a suitable trace configuration, and placed on heat sink 26. Locating pins or the like can optionally be used for accurate placement of flexible electrical circuit 24 on heat sink 26. Flexible electrical circuit 24 is adhered to heat sink 26 using a PSA or other suitable adhesive material or technology. The electrical components, including FETs 34, capacitors 36 and power connectors 38, are accurately placed onto flexible electrical circuit 24, preferably using an automated process such as a pick-and-place machine. The assembly is then passed through a solder reflow stage to electrically and mechanically couple the electrical components with flexible electrical circuit 24.
Referring now to
According to the present invention described above, a flexible electrical circuit is used to connect the power devices, heat sink, bus capacitors, bus structure, external power connectors, signal interconnect, and enclosure. The flexible electrical circuit is bonded directly to the flat side of a large, finned metal heat sink using a PSA or other adhesion method. The PSA acts as a thermal conductor (to help draw heat out of the circuit toward the heat sink) and is also an electrical insulator, effectively isolating the flex circuit from the metal heat sink. The PSA does not require heat curing, as does the dielectric layer in IMST.
The present invention maximizes heat transfer out of the module and therefore allows for the use of smaller, less expensive, surface mount components that can be placed by automated manufacturing pick-and-place machines. (Even though a larger number of these smaller, surface mount devices are needed for high-power applications, in comparison to the larger through-hole versions, they are considerably cheaper and easier to manufacture than the larger versions.) Traditional solutions require larger components, some of which need to be manually inserted or placed through a separate machine or process.
The electrical circuit assembly of the present invention provides two major benefits, namely, 1) simplification of the manufacturing process, and 2) improved conduction of heat away from the high-power circuitry. To reduce the complexity of the design and automate the process, the structure of the module (including the high-power electronics) is interconnected with a flexible electrical circuit. This allows the entire unit to be manufactured on a conventional, high-throughput manufacturing line, and eliminates processes needed for traditional circuits.
Because the flexible electrical circuit is bonded directly to a single-piece finned heat sink, several mechanical components (separate heat sink, screws, clips, etc.) found in traditional heat sink designs can be eliminated. The flexible circuit substrate is directly bonded to a single-piece finned aluminum (or other metal) heat sink using a PSA or other bonding technology. Conventional designs require that the circuit layer be bonded to a flat metal plate, which is in turn connected to a separate finned heat sink to maximize heat conduction. The present invention eliminates the flat metal plate and bonds the circuit directly to a flat side of the finned heat sink. This elimination of an additional external interface increases the thermal conductivity (i.e., improves heat dissipation) for the ECM.
Traditional solutions, such as the IMST technology described above, require a dielectric material or other thin material to be placed between the circuit and the metal surface to which it is to be connected. This dielectric material is a ceramic and must be heat cured, adding an additional process to the manufacture of the module. The present invention eliminates this intermediate layer and bonds the flexible circuit directly to the finned heat sink with a PSA (or other adhesion material or technique).
The present invention eliminates the need for a solder mask material to be used on the end product. A solder mask is used in traditional circuits to keep solder from flowing into sensitive areas of the circuit and causing unwanted electrical connections between traces. However, a solder mask can impede the flow of heat energy out of the circuit. Solder masking is eliminated from the present invention since the flexible circuit does not contain components on the bottom side, which is bonded directly to the finned heat sink. A solder mask is not required on this side and the elimination of the solder mask provides better thermal conduction.
Instead of a solder mask on the topside of the circuit, ink dams (that is, lines placed on and across the circuit traces via a silk screen method) are used to keep solder from flowing into areas on the circuit where it is not wanted. The solder dams are formed with a silkscreen process to “paint” lines on the flexible circuit to prevent solder from flowing into areas where it is not wanted. Silk-screening is a much less expensive process than the application of a solder mask, which reduces product cost and complexity.
Thermal vias (plated holes that pass through the entire flexible circuit to conduct heat to the heat sink) are also used near and under high-power electronics to further improve heat conduction.
The use of a flexible electrical circuit allows for an intrinsic low-inductance bus structure. By its nature, a flexible circuit uses thin copper traces and thin board layers. This arrangement minimizes the amount of inductance present on the circuit traces. The lower the inductance present in the circuit, the better the circuit is able to handle voltage spikes and supply the in-rush current needed in start-up situations.
Having described the preferred embodiment, it will become apparent that various modifications can be made without departing from the scope of the invention as defined in the accompanying claims.
