Patent Application
20150207287
The present invention is directed to high current electrical connections and, more particularly, to structure and methods for attaching a conductor to a power terminal.
Metal-oxide-semiconductor field-effect transistors (MOSFETs) are often utilized in automotive electronics subassemblies such as those designed for implementing generating and motoring functions of an alternator-starter. Power MOSFETs may be packaged in power electronic modules as part of a rectifier/inverter circuit, the MOSFETs typically being arranged in a bridge configuration for rectifying an alternating current (AC) in generating mode in order to provide a direct current (DC) to charge a battery, and forming an inverter for transforming DC voltage into multiple-phase AC voltage in motoring mode to provide starting motor torque. For example, motoring currents in a cranking operation may be 1200 Amperes or greater.
The terminals of power MOSFETs and/or power electronics modules have been connected to one another and to other electrical components using bus conductors that are attached to these power terminals by welding, brazing, soldering, terminal devices, crimp connectors, lugs, wire, and by other structure. Such connection methods and structure are not optimized for high current capacity, reliability, or for efficient manufacturing.
It is therefore desirable to obviate the above-mentioned disadvantages by providing a method of bonding a high-current conductor to a terminal of a power electronic module.
According to an exemplary embodiment, a method includes resistance brazing using high-phosphorous electroless nickel (EN) plating both as a resistance path and as a flux.
According to another exemplary embodiment, a method for fixing an electrical terminal to a pair of copper conductor bus strips includes the steps of plating the electrical terminal with high-phosphorous electroless nickel (EN), placing the EN-plated terminal between the bus strips, and resistance brazing the terminal to the bus strips using the EN plating both as a resistance path and as a flux.
According to a further exemplary embodiment, a method of providing a high current electrical connection includes the steps of extending a terminal from a bottom surface of an electrical module, plating the terminal extension with high-phosphorous electroless nickel, placing a bus conductor into abutment with the plated terminal extension, and resistance brazing the bus conductor to the plated terminal extension using the high-phosphorous electroless nickel as a resistance path.
The foregoing summary does not limit the invention, which is defined by the attached claims. Similarly, neither the Title nor the Abstract is to be taken as limiting in any way the scope of the claimed invention.
The above-mentioned aspects of exemplary embodiments will become more apparent and will be better understood by reference to the following description of the embodiments taken in conjunction with the accompanying drawings, wherein:
Corresponding reference characters indicate corresponding or similar parts throughout the several views.
The embodiments described below are not intended to be exhaustive or to limit the invention to the precise forms disclosed. Rather, the embodiments are chosen and described so that others skilled in the art may appreciate and understand the principles and practices of these teachings.
Module 1 may include a mounting portion 15 having a mounting hole 16 and an insert 17. As described further by example below, module 1 may be secured to the housing of an alternator-starter with a threaded fastener that passes through hole 16 and mates with a corresponding threaded housing receptacle. Insert 17 may act as a washer/spacer, provide structural support, provide an electrical conductor, and may be formed with or without threads.
Each terminal extension 12 is plated using high-phosphorous Electroless Nickel (EN). Terminal extensions 12 are cleaned and dried before being plated, whereby excellent adhesion of the nickel-phosphorous plating to terminal extensions 12 is obtained. The EN plating operation is an auto-catalytic process that forms a layer of nickel-phosphorous on each terminal extension 12. The electroless process does not require passing an electric current through a solution but, instead, utilizes a reducing agent that reacts with metal ions to thereby deposit metal on terminal extension 12. For example, a reducing agent may include sodium hypophosphite, dimethyl amino borane, sodium borohydride, formaldehyde, or another compound such as potassium hypophosphite or ammonium hypophosphite. The EN plating bath may variously contain silicon carbide, silicon nitride, ammonia, and other materials such as a metal ion complexing agent, a pH buffer, a stabilizer, and/or a surfactant, in relatively small quantities. Terminal extensions 12 are placed into the EN plating bath for a period of time, until the thickness of the resultant plating is approximately 2.5 microns. Generally, the thickness of the plating increases with time in the bath.
Various EN plating methods may be used to achieve fifteen percent phosphorous in the plating deposited on terminal extension 12, the EN plating operation usually requiring tight tolerances for bath composition and process parameters. Sodium hydroxide may be used to maintain a constant pH of the plating solution. Electroless plating occurs by two simultaneous half reactions involving electron generation and electron reduction. The metal ions in the solution accept electrons at the deposition surface, become reduced, and are deposited as metal on the surface of the workpiece.
The metal surfaces of terminal extensions 12 are prepared by thorough cleaning with a mild acid or etch. An exemplary electroless nickel plating bath contains approximately six grams per liter of nickel and uses sodium hypophosphite as the reducing agent. The temperature is maintained at 82-92 degrees Celsius, and the pH is maintained between about 4.6 to 5.0. The resultant phosphorous content may be up to about fifteen percent by mass. Generally, a faster plating rate results in a higher percentage of phosphorous. A higher plating rate may be obtained, for example, by adjusting chelate and stablizer mixtures. It is possible to raise the plating rate to about 0.7 mil/hour, but an increased rate may cause deposit properties to change. When polyphosphate salts or polyphosphoric acid is added to the bath, more phosphorous may be deposited, whereby the plating has a smaller granularity and is more amorphous. During the plating process, terminal extensions 12 are secured into an immersion fixture (not shown), immersed in the plating bath, and agitated slightly. Alternatively, electrolytic plating, vapor deposition and/or sputtering may be utilized for depositing a nickel-phosphorous composition onto terminal extensions 12. The resultant very-high phosphorous EN plating is typically brittle and may be subject to flaking. These physical properties at a high phosphorous content may be detrimental when an EN plating is a coating on a finished product, but such properties may actually enhance and improve a subsequent resistance brazing of EN-plated terminal extensions 12, described further below.
Electrodes 52, 54 are moved toward one another with force, and the electric current of the brazing machine is turned on for a duration of approximately 0.25 seconds. Typically, the electric current is at least 1000 Amperes, for example 10,000 Amps for a ⅛ square inch area (e.g., 0.5×0.25). In the resultant resistance welding/brazing, the locations having the greatest electrical resistance along the current path create heat. In particular, the heat at the high resistance locations is sufficient to melt EN plating 51, whereby terminal extension surface 14 is bonded to copper conductor inside surface 49 and terminal extension surface 13 is bonded to copper conductor inside surface 50. Electrodes 52, 54 may remain biased toward one another after the electrical current has been turned off, so that the bonding of surfaces and the flow of EN plating material 51 stops and a resultant alloy has cooled into a stable solid. Electrodes 52, 54 may contain coolant passages (not shown) for active removal of heat after the brazing process. Ancillary structure (not shown) may be provided for biasing and/or securing conductor strip portions 25, 26 before, during, or after the flow of brazing current.
Since electrodes 52, 54 only contact the outside copper conductor surfaces 56, 57, the melted EN material flows out of the interfaces between terminal 9 and inner copper conductor surfaces 49, 50. The force of electrodes 52, 54 is sufficient to squish the brazing material out of the contact area between surface 14 and surface 49 and out of the contact area between surface 13 and surface 50, whereby this displaced material forms fillets 60 around the joints. A thin layer of the brazing material, having a thickness of about one to three atoms, remains in the two conductive interfaces, filling up the volumes created by any surface imperfections. The melting range for electroless nickel coatings varies depending upon the phosphorus content of the deposit. The present inventors have determined that when the deposited EN plating contains approximately fifteen percent phosphorous by mass, the melting point of such EN plating is around 1190 degrees Fahrenheit. By comparison, the melting point of copper is 1981 degrees Fahrenheit. As a result of this difference in melting points, all or almost all of the copper of conductor strip portions 25, 26 remains unmelted during the brazing operation. Typically, a small amount of the copper in the vicinity of terminal 9 alloys with the nickel. Most of the phosphorous burns away during the brazing, although some remains, and the resultant fillets may include a copper-nickel-phosphorous alloy. The finished color of these EN fillets is typically brownish in color and the fillets may lack the shine and attractiveness of a traditional EN surface. The phosphorous within EN plating 51 acts as a flux for the brazing, and no separate brazing flux is required.
The EN plating process and the brazing process may each minimize creation of alloys. For example, non-eutectoid compositions of nickel-phosphorous may be electrolessly plated onto terminal extensions 12, whereby phosphorous content may be increased to 15-18 percent. These very-high-phosphorous deposits have a reduced amorphous condition, and may contain a mixture of microcrystalline and amorphous phases. During brazing, as electroless nickel deposits are heated to temperatures above a threshold range of 420° to 500° F., structural changes begin to occur when coherent and then distinct particles of nickel phosphite (Ni 3P) may begin to form within the deposit. When temperatures become greater than about 600° F., the deposit begins to crystallize and begins to lose its amorphous character. When continued heating is performed relatively slowly, the nickel phosphite particles conglomerate and a two phase alloy forms. By comparison, when resistance brazing/welding is performed quickly and with a very-high phosphorous content, the alloying is minimized and/or may be controlled. At about 1620° F., the eutectic temperature of EN alloys, significant melting of the EN coating occurs, but the resultant alloying is localized at the faying surfaces and does not cause any significant deformation of the copper. By adjusting the force of electrodes 52, 54 as they press the layers (e.g., copper strip 25, terminal 9, copper strip 26) together, and by adjusting the modulation profile of electric current flowing through the mid-frequency resistance brazing machine, the time at which junction temperatures are above 1620° F. may be minimized and/or controlled. Process parameters may be adjusted to also minimize the thickness of nickel interface layers, whereby electrical resistivity at copper surfaces 49, 50 is not significant. The conductivity at the connection of terminal 9 and copper strips 25, 26 is further significantly increased by vaporization and/or migration of phosphorous during the brazing operation.
When alternator-starter 21 (e.g.,
While various embodiments incorporating the present invention have been described in detail, further modifications and adaptations of the invention may occur to those skilled in the art. However, it is to be expressly understood that such modifications and adaptations are within the spirit and scope of the present invention.
