Method for attaching discrete power modules to substrate

Abstract

The present disclosure provides a method for attaching a discrete power module to a substrate. A method of attaching a plurality of discrete power modules (DPMs) to a substrate includes positioning the substrate such that a bottom surface of the substrate is opposite a top surface of a backplane. The method includes placing solder preforms on a top side of the substrate, placing each DPM on a top surface of a corresponding solder preform, and performing a reflow process. The reflow process includes melting the solder preform by providing heat to the substrate via the base plate. The method further includes measuring, via at least one sensor, measured temperatures of a plurality of temperature zones of the DPM during the reflow process. The method includes determining, via a controller, whether the measured temperatures of the plurality of temperature zones are below a threshold temperature during the reflow process, and maintaining the measured temperatures of the temperature zones below the threshold temperature during the reflow process.

Description

Technical Field

[0001] This disclosure relates to attaching discrete power modules to a substrate. Background Art

[0002] The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.

[0003] Electric vehicles require an inverter to manage the electric drive system. The inverter utilizes discrete power modules (DPMs) or non-discrete power module assemblies to perform pulse width modulation (PWM) to convert direct current (DC) electricity from a battery or energy storage system into alternating current (AC) electricity for vehicle systems such as an electric motor that propels the vehicle. In some applications, a silver sintering process is used to attach the DPMs to a substrate in the inverter. The silver sintering process is an alternative to traditional soldering, which involves heating a silver paste to transform the silver powder into a solid without going through a liquid phase. Traditional soldering involves applying a flux to the joint where the components are to be attached, then heating the components while applying solder to the joint such that the solder melts and flows into the joint. In other soldering applications, a solder paste is used, where the solder paste contains solder balls and a flux. Then, the solder paste melts, and a sticky flux residue is left behind, and the sticky flux residue is managed through a flux management system and a cleaning plan. The flux left behind in the solder paste application may increase voids in the solder joint. Some DPMs are molded in an epoxy resin or polymer that has a peak temperature (e.g., a peak temperature of 240°C to 260°C), above which the molded part may experience excessive delamination. This excessive delamination may render the DPM defective or shorten its lifespan. To form a more robust thermal interface and a secure mechanical joint at the DPM and substrate interface when soldering the DPM, a soldering material with a higher liquidus temperature is needed. Unfortunately, the liquidus temperature of a suitable soldering material may be close to the peak temperature at which this excessive epoxy molding delamination may occur.

[0004] This disclosure addresses these and other problems of attaching DPMs to a substrate of a power inverter. Summary of the Invention

[0005] This section provides a general overview of the present disclosure and is not a full disclosure of its entire scope or all of its features.

[0006] In one form, the present disclosure provides a method of attaching a plurality of discrete power modules (DPMs) to a substrate. The method includes positioning the substrate such that a bottom surface of the substrate faces a top surface of a base plate. The method includes: placing a plurality of solder preforms on a top side of the substrate; placing each of the plurality of DPMs on a top surface of a corresponding one of the plurality of solder preforms; and performing a reflow process. The reflow process includes melting the solder preforms by providing heat to the substrate via the base plate. The method further includes measuring, via at least one sensor, measured temperatures of a plurality of temperature zones of the DPMs during the reflow process. The method includes determining, via a controller, whether the measured temperatures of the plurality of temperature zones are below a threshold temperature during the reflow process. The method further includes maintaining the measured temperatures of the plurality of temperature zones below the threshold temperature during the reflow process.

[0007] In variations of the method of the above paragraphs that may be implemented individually or in any combination: maintaining the measured temperature below the threshold includes dynamically adjusting the power supplied to the substrate to reduce the base plate temperature of the base plate; maintaining the measured temperature below the threshold further includes dynamically increasing the hold time of the base plate temperature; maintaining the measured temperature below the threshold includes dynamically reducing the hold time of the base plate temperature of the base plate; the method further includes dispensing a tackifier at: (a) between the top side of the substrate and the plurality of solder preforms to hold the plurality of solder preforms on the substrate, or (b) between each DPM and the top surface of the corresponding one of the plurality of solder preforms to hold the DPM on the plurality of solder preforms, or (c) (a) and (b); the method further includes placing a tray between the substrate and a plurality of busbars electrically connected to each of the DPMs of the plurality of DPMs, wherein the tray provides at least one of: electrical insulation between the busbars and the substrate and a reaction support for a clamping force applied to the busbars; the method further includes placing an alignment fixture such that the plurality of DPMs are between the alignment fixture and the substrate, and applying a force toward the substrate to the alignment fixture such that the alignment fixture clamps the plurality of DPMs to the substrate during the reflow process; the substrate is a material having a first coefficient of thermal expansion, and the alignment fixture is a material having a second coefficient of thermal expansion, the second coefficient of thermal expansion differing from the first coefficient of thermal expansion by ±7.0*10 -6within m / (m*℃); the alignment fixture defines a plurality of holes, each of the plurality of holes being aligned with a corresponding DPM of the plurality of DPMs and opening through the alignment fixture such that the DPMs are visible through the holes, wherein the alignment fixture contacts a plurality of peripheral regions of each DPM; the plurality of peripheral regions include four corners of each DPM; during the reflow process, the bottom surface of the substrate contacts the top surface of the bottom plate; during the reflow process, the bottom surface of the substrate is disposed a predefined distance above the top surface of the bottom plate; the at least one sensor includes at least one non-contact sensor; the at least one sensor includes three sensors, and the plurality of temperature zones includes three temperature zones, wherein each of the three sensors is positioned to measure a corresponding one of the three temperature zones; the reflow process occurs in at least one chamber, and the reflow process further includes filling the at least one chamber with a gaseous substance including HCOOH, increasing the pressure of the gaseous substance in the at least one chamber to between 750 mbar and 1050 mbar, and heating the bottom plate while the pressure is between 750 mbar and 1050 mbar to raise the measured temperature of the plurality of temperature zones to between 215 degrees Celsius and 265 degrees Celsius for a first holding time.

[0008] In another form, the present disclosure provides a method of attaching a plurality of discrete power modules (DPMs) to a substrate, the method including positioning the substrate such that the bottom surface of the substrate faces the top surface of a bottom plate. The method includes: placing a plurality of solder preforms on the top side of the substrate; placing each of the plurality of DPMs on the top surface of a corresponding one of the plurality of solder preforms; and performing a reflow process. The reflow process includes melting the solder preforms by providing heat to the substrate via the bottom plate. The method further includes measuring, via at least one sensor, the measured temperature of a plurality of temperature zones of the DPMs during the reflow process. The method includes determining, via a controller, whether the measured temperature of the plurality of temperature zones is below a threshold temperature during the reflow process. The method includes maintaining, during the reflow process, the measured temperature of the plurality of temperature zones below the threshold temperature by at least one of: dynamically adjusting the power supplied to the bottom plate to reduce the bottom plate temperature, and dynamically adjusting the holding time of the power supplied to the bottom plate.

[0009] In variations of the methods of the above paragraphs that may be implemented individually or in any combination: the method further includes placing a tray between the substrate and a plurality of bus bars electrically coupled to each of the DPMs of the plurality of DPMs, wherein the tray provides at least one of: electrical insulation between the bus bars and the substrate and a reaction support for the clamping force applied to the bus bars; the method further includes placing an alignment fixture such that the plurality of DPMs are between the alignment fixture and the substrate, and applying a force towards the substrate to the alignment fixture such that the alignment fixture clamps the plurality of DPMs to the substrate during the reflow process; the at least one sensor includes three sensors, and the plurality of temperature zones includes three temperature zones, wherein each of the three sensors is positioned to measure a corresponding one of the three temperature zones.

[0010] In yet another form, the present disclosure provides a system that includes a substrate, a plurality of solder preforms, a plurality of discrete power modules (DPMs), an insulating tray, and an alignment fixture. The plurality of solder preforms are disposed on a top side of the substrate. Each DPM of the plurality of DPMs is disposed on a corresponding solder preform of the plurality of solder preforms, and wherein each DPM includes a plurality of electrical contacts. The insulating tray is disposed between the electrical contacts and the substrate and electrically insulates the electrical contacts from the substrate. The alignment fixture is disposed on the substrate such that the plurality of DPMs are between the alignment fixture and the substrate, and the alignment fixture is configured to clamp the DPMs to the substrate.

[0011] Based on the description provided herein, additional applicable fields will become apparent. It should be understood that the description and specific examples are for illustrative purposes only and are not intended to limit the scope of the present disclosure. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] For a better understanding of the present disclosure, various forms of the present disclosure will now be described by way of example with reference to the drawings, in which:

[0013] Figure 1 is a perspective view of an electrical power component according to the present disclosure;

[0014] Figure 2 is Figure 1 an exploded perspective view of the electrical power component of

[0015] Figure 3 is a flow chart of a method for assembling an electrical power component according to the present disclosure Figure 1 of

[0016] Figure 4 is a diagram showing according to the present disclosure Figure 1Perspective view of an electric power component in a pre-assembled state;

[0017] Figure 5 is in accordance with the present disclosure Figure 1 Perspective view of an electric power component in another pre-assembled state;

[0018] Figure 6 is in accordance with the present disclosure Figure 1 Perspective view of an electric power component in yet another pre-assembled state, showing an alignment fixture for assembling the electric power component;

[0019] Figure 7 is Figure 6 Bottom perspective view of a part of the alignment fixture;

[0020] Figure 8 is Figure 6 Cross-sectional view of the electric power component and the alignment fixture;

[0021] Figure 9 is Figure 6 Different cross-sectional view of the electric power component and the alignment fixture;

[0022] Figure 10 is a flowchart showing Figure 3 more details of the steps of the method;

[0023] Figure 11 is a schematic diagram of a system for melting Figure 1 a solder preform of an electric power component in accordance with the present disclosure; and

[0024] Figure 12 is a schematic diagram of another form of a system for melting Figure 1 a solder preform of an electric power component in accordance with the present disclosure.

[0025] The accompanying drawings described herein are for illustrative purposes only and are not intended to limit the scope of the present disclosure in any way. Detailed Description

[0026] The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. It should be understood that throughout the drawings, corresponding reference numerals indicate the same or corresponding parts and features.

[0027] Referring to Figure 1 and Figure 2, the electric power component 110 includes a substrate 114, an insulating tray 118, and a plurality of discrete power modules (DPMs) 122. In the provided example, the electric power component 110 is a component of an inverter of an electric vehicle (not shown), but the electric power component can be used in other types of electronic devices. The inverter manages the electric drive system of the electric vehicle by converting direct current (DC) power from a battery (not shown) into alternating current (AC) power for propelling an electric motor (not shown) of the electric vehicle.

[0028] Although not specifically shown, the inverter may also include a housing that encapsulates the entire electric power component 110, or may include a cover that can be coupled to the substrate 114 to encapsulate the insulating tray 118 and the discrete power modules 122 between the substrate 114 and the cover.

[0029] The discrete power modules 122 function as electronic switching devices within the inverter. Direct current from the battery flows into the inverter, and the discrete power modules 122 are configured to control the direction of current flow to generate alternating current for the electric motor of the vehicle. In another form, alternating current from the motor (e.g., during the regenerative mode) may flow into the inverter, and the discrete power modules 122 may be configured to generate direct current to recharge the battery. The discrete power modules 122 include a housing 210, a bus bar 212, and connection pins 214. The housing 210 houses electrical components such as semiconductor chips (not shown). The bus bar 212 extends through the housing 210 and is electrically coupled to the electrical components therein. The connection pins 214 extend through the housing 210 and are electrically coupled to the electrical components therein.

[0030] The insulating tray 118 is located between the bus bar 212 of the discrete power modules 122 and the substrate 114. The insulating tray 118 includes a plurality of longitudinal partitions 215 that electrically isolate the bus bar 212 of the discrete power modules 122 from the substrate 114. Thus, the longitudinal partitions 215 are formed of a non-conductive material having a high thermal resistance, such as but not limited to plastic or polymer (e.g., PA6T / BT-GF35 or PEEK). In the provided example, the entire insulating tray 118 is formed of a non-conductive material, but other configurations may be used. The insulating tray 118 may also optionally include a plurality of transverse partitions 216 to assist in holding the discrete power modules 122 during the reflow process. The transverse partitions 216 extend laterally between the longitudinal partitions 215.

[0031] Additionally, the insulating tray 118 provides support for the discrete power module 122 during the laser welding process. To laser weld the bus bar 212 of the discrete power module 122, a clamping force is applied to the bus bar 212. The clamping force clamps the bus bar 212 towards the substrate 114 such that the bus bar 212 is clamped to the longitudinal separator 215. When the laser welding clamping tool (not shown) contacts the top side of the bus bar 212, the insulating tray 118 provides support for the underside of the bus bar 212.

[0032] The substrate 114 of the electrical power component 110 serves as a heat sink within the inverter. The substrate is formed of a thermally conductive material such as, but not limited to, copper, aluminum, silver, gold. In one form, as Figure 2 shown, the substrate 114 includes a plurality of pads 218 corresponding to the plurality of discrete power modules 122. The plurality of pads 218 extend upwardly from the top surface 220 of the substrate 114. In the example provided, each pad 218 is discrete from every other pad 218 such that the top surface 220 of the substrate 114 defines a gap between each pad 218, but other configurations may be used. Each of the plurality of discrete power modules 122 is aligned with and disposed on a corresponding pad 218 such that the underside of each discrete power module 122 is in thermal conductive relation with its corresponding pad 218. The substrate 114 is configured to dissipate heat generated from the plurality of discrete power modules 122 over a larger surface area. The substrate 114 may be configured to dissipate heat into the atmosphere, a cooling medium, or a subsequent component (e.g., a heat exchanger) to which the substrate is attached. Additionally, during the reflow process, the substrate 114 serves as a thermal conductor, as described in more detail below.

[0033] The substrate 114 further defines a plurality of apertures 222 disposed around the perimeter of the substrate 114. In the example provided, the substrate 114 defines a plurality of fingers 224 extending outwardly from the perimeter of the substrate 114 and the apertures 222 are defined through the fingers 224, but other configurations may be used. The apertures 222 may be outside the perimeter of the insulating tray 118 such that the apertures are accessible when the insulating tray 118 is disposed on the substrate 114. The apertures 222 may be configured to receive fasteners (not shown) to mount the substrate to another component (not shown; e.g., another component of a vehicle or an inverter).

[0034] In the provided example, the discrete power modules 122 are arranged in two rows, each row extending in the longitudinal direction, but other configurations may be used. In another configuration, only a single row of discrete power modules 122 is used. In yet another configuration, three or more rows are used. Although the provided example includes nine discrete power modules 122 in each row, other configurations may be used. In one configuration, there are two or more but less than nine discrete power modules 122 in each row. In another configuration, there are more than nine discrete power modules 122 in each row.

[0035] Reference Figure 3 , a flowchart of an example method 300 for assembling the electrical power component 110 is provided. In step 302, the substrate 114 is positioned relative to Figure 11 the shown base plate 1110 such that the base plate 1110 can provide conductive heating to the underside of the substrate 114.

[0036] Reference Figure 11 , in the provided example, a carrier plate 1112 may optionally be disposed between the base plate 1110 and the substrate 114 such that the bottom surface of the substrate 114 faces and contacts the top surface of the carrier plate 1112, and the bottom surface of the carrier plate 1112 faces and contacts the top surface of the base plate 1110. In one form, the base plate 1110 includes one or more heating elements (not shown, e.g., resistive heating elements) that provide heating. In another form, the base plate 1110 is heated by one or more infrared heating elements (not shown, e.g., electromagnetic radiation) that provide heating. In another form, the base plate 1110 includes an induction heating element that heats the carrier plate 1112 via induction heating. Thus, a gap may optionally be provided between the base plate 1110 and the carrier plate 1112. In an alternative form not specifically shown, the carrier plate 1112 is omitted such that the base plate 1110 provides heating to the substrate 114 directly via conduction, infrared, or induction.

[0037] Reference Figure 12 , in an alternative form, the substrate 114 may be spaced apart from the carrier plate 1112 by a spacer 1210. In this configuration, the base plate 1110 heats the substrate 114 via radiative heating. In the provided example, the base plate 1110 heats the carrier plate 1112 via conduction or induction as described above Figure 11 but the heat then radiates from the carrier plate 1112 to rather than directly conducts to the substrate 114. In this form, the spacer 1210 may optionally be a thermally insulating material to prevent direct conduction. In another form not specifically shown, the carrier plate 1112 is omitted and the spacer 1210 spaces the substrate 114 from the base plate 1110 such that the base plate 1110 directly heats the substrate 114 via radiation rather than conduction.

[0038] Returning to Figure 3, after step 302, method 300 may proceed to step 304. In step 304, Figure 4 A plurality of solder preforms 410 as shown are placed on the substrate 114. In the example provided, each solder preform 410 is placed on the top surface 226 of a corresponding pad in the pads 218. A tackifier may optionally be dispensed between the substrate 114 and the solder preforms 410 to hold the solder preforms 410 in place. In one form, the tackifier is a no-clean flux tackifier that evaporates at a temperature above 210 degrees Celsius. Other forms of tackifiers with different evaporation temperatures may also be used.

[0039] In the example provided, each solder preform 410 is discrete from each other solder preform 410. Each solder preform 410 is a brazing material in solid form and in a predetermined flat shape that is substantially the shape and size of the bottom surface of each discrete power module 122. Thus, each solder preform 410 also substantially corresponds to the size and shape of the top surface 226 of each pad 218. In one form, the solder preform 410 has a liquidus temperature of 220 degrees Celsius, but brazing materials with other liquidus temperatures may be used.

[0040] A robot 412 may place the solder preforms 410 on the substrate 114. Although the robot 412 is shown as a multi-axis robotic arm, the robot 412 can be any suitable robotic device, including but not limited to a pick-and-place gantry system. In the example provided, the robot 412 individually picks and places each solder preform 410. In another form, the robot 412 may be configured to pick and place more than one solder preform 410 at a time.

[0041] In the example provided, the solder preforms 410 are picked up by the robot 412 from a reel 414 that unwinds when solder preforms 410 are needed, but other configurations may be used, such as an array or a tray or other dispenser of solder preforms 410.

[0042] Returning to Figure 3 , method 300 may proceed to step 306. In step 306, an insulating tray 118 is placed on the top surface 220 of the substrate 114. In another form, step 306 may be before step 304 such that the insulating tray 118 can be placed on the top surface 220 of the substrate 114 before placing the plurality of solder preforms 410 on the substrate 114.

[0043] Method 300 may then proceed to step 308. In step 308, each of the plurality of discrete power modules 122 is placed on the top surface of a corresponding solder preform 410 among the plurality of solder preforms 410. In one form, the discrete power modules 122 may be placed by different robots (not shown; e.g., similar to robot 412), while robot 412 places the solder preforms 410 such that once the individual solder preforms 410 are placed on the substrate 114, the corresponding discrete power modules 122 can be placed on the solder preforms 410 while robot 412 places the next solder preform 410. In another form, the discrete power modules 122 may be placed after all the solder preforms 410 are placed on the substrate. A tackifier may optionally be provided between each discrete power module 122 and its corresponding solder preform 410. In one form, the tackifier is a no-clean tackifier that evaporates at a temperature above 210 degrees Celsius. Other forms of tackifiers with different evaporation temperatures may also be used. As Figure 5 shown, the bus bar 212 of the discrete power module 122 is positioned above the longitudinal separator 215 of the insulating tray 118.

[0044] Returning to Figure 3 , method 300 may proceed to step 310. In step 310, Figure 6 the alignment fixture 610 shown is placed above the plurality of discrete power modules 122 and contacts the substrate 114 and the discrete power modules 122. In this position, the alignment fixture 610 does not restrict or contact the insulating tray 118. As Figure 11 shown, the fixture 614 is positioned above the alignment fixture 610 and applies a force towards the substrate 114 to the alignment fixture 610 such that the alignment fixture 610 clamps the plurality of discrete power modules 122 during the reflow process, as described in detail below.

[0045] In one form, the alignment fixture 610 applies up to 100 psi to the discrete power modules 122. In another variant, the alignment fixture 610 applies up to 25 psi to the discrete power modules 122. The amount of pressure provided by the alignment fixture 610 can be adjusted based on specific application requirements. The alignment fixture is described in more detail below.

[0046] Referring to Figure 6 and Figure 7, the alignment fixture 610 is configured to hold the discrete power module 122 in its position before and during reflow. The alignment fixture 610 includes a plurality of holes 712 corresponding to the discrete power module 122. Each hole 712 is aligned with a corresponding discrete power module in the discrete power module 122. Each hole 712 opens through the top and bottom of the alignment fixture 610 such that at least a portion of the top surface of the housing 210 of each discrete power module 122 is visible through the orifice 712. The alignment fixture 610 is configured to overlap and contact at least a portion of the top surface of each housing 210 near the perimeter of each housing 210. In the example provided, each hole 712 is defined by a pair of longitudinal walls 711 and a pair of transverse walls 713 such that the hole 712 can be generally square or rectangular in shape, but other configurations may be used.

[0047] In the example provided, the alignment fixture 610 may include a plurality of protrusions 714 that extend from the longitudinal walls 711 and / or the transverse walls 713 into the holes 712 to extend above the top surface of the housing 210 of the discrete power module 122. In the example provided, four protrusions 714 are provided at the four corners of each hole 712 to correspond to and overlap the four corners of the housing 210 of the corresponding discrete power module 122. Accordingly, at least a portion of the peripheral region 811 ( Figure 8 ) of the top surface of the housing 210 overlaps with the protrusions 714. In an alternative configuration (not shown), the entire peripheral region 811 ( Figure 8 ) of the top surface of each housing 210 may overlap with the alignment fixture 610.

[0048] The alignment fixture 610 may optionally include a plurality of fingers 810. Each finger 810 may extend from a corresponding one of the protrusions 714 in a downward direction to contact the top surface of the housing 210 in the peripheral region 811 ( Figure 8 ). In the example provided, the fingers 810 are located at the corners of each protrusion 714 that are distal to the longitudinal walls 711 and the transverse walls 713, but other configurations may be used. The fingers 810 provide a smaller contact area than the protrusions 714. The reduced contact area of the fingers 810 may reduce heat transfer between the alignment fixture 610 and the discrete power module 122.

[0049] Reference Figure 8 and Figure 9, the alignment fixture 610 includes a plurality of alignment pins 812 that extend downwardly from the lower side 813 of the alignment fixture 610 toward the substrate 114. Each alignment pin 812 is received in a corresponding aperture in the aperture 222 to couple the alignment fixture 610 to the substrate 114 and ensure the correct positioning of the alignment fixture 610 on the substrate 114. In the example provided, there are three alignment pins 812, but other configurations may be used. The alignment pins 812 space the remainder of the alignment fixture 610 from the substrate 114 to limit heat conduction between the alignment fixture 610 and the substrate 114. The alignment pins 812 may optionally be formed of a heat insulating material. In one form, the entire alignment fixture 610 is formed of a heat insulating material. In another form, the alignment pins 812 may be a heat insulating material and the remainder of the alignment fixture 610 may be a heat conducting material.

[0050] The alignment pins 812 also space the remainder of the alignment fixture 610 from the insulating tray 118. The spacing between the alignment fixture 610 and the insulating tray 118 is configured to allow the insulating tray 118 to thermally expand during the reflow process described below with reference to step 312.

[0051] Returning to Figure 3 , the method may proceed to step 312, where the electrical power component 110 then undergoes a reflow process. During the reflow process of step 312, power is provided to the base plate 1110 to heat the substrate 114. The substrate 114 acts as a heat conductor to melt the solder preform 410. During the reflow process of step 312, Figure 11 at least one of the illustrated sensors 1114 measures the temperature of a plurality of temperature zones 1116 of the discrete power module 122. In the example provided, two sensors 1114 are shown, but in alternative forms, one sensor 1114 or more than two sensors 1114 may be used. The sensors 1114 detect the temperature at the top surface of the housing 210 of the discrete power module 122. In the example provided, the sensors 1114 are non-contact sensors, such as, for example, optical sensors (e.g., laser, infrared). In another configuration, the sensors 1114 may be sensors that contact the top surface of the housing 210 (e.g., thermocouples). In the example provided, two temperature zones 1116 are shown, but more temperature zones 1116 may be used. Each temperature zone 1116 may be limited to a single power module in the power module 122, or may cover more than one power module 122.

[0052] Continuing to refer to Figure 11, the sensor 1114 communicates with the controller 1120. During the reflow process of step 312, the controller 1120 determines whether the measured temperatures of the plurality of temperature zones 1116 are lower than a threshold temperature. The controller 1120 adjusts the power supplied to the base plate 1110 to maintain the measured temperature of each of the plurality of temperature zones 1116 below the threshold temperature throughout the reflow process of step 312. In other words, if one of the temperature zones 1116 begins to approach the threshold temperature, the controller 1120 can reduce the power to the base plate 1110 to lower the temperature of the entire base plate 1110 to maintain all the temperature zones 1116 below the threshold temperature. In an alternative form, the base plate 1110 can have separate heating zones, and the controller 1120 can reduce the power to the heating zone aligned with the temperature zone 1116 approaching the threshold, such that the temperature zone 1116 is maintained below the threshold temperature.

[0053] In one form, the threshold temperature is 260 degrees Celsius. The threshold temperature can be determined based on the maximum operating temperature of the discrete power module 122 and / or the epoxy molding of the discrete power module 122.

[0054] Since the insulating tray 118 is formed of a non-conductive material and the substrate 114 is formed of a metallic material, the thermal expansion coefficients between the insulating tray 118 and the substrate 114 are not the same. Thus, the space between the insulating tray 118 and the alignment fixture 610 allows the insulating tray 118 to expand, while the contact between the alignment fixture 610 (e.g., at the fingers 810) and the top surface of the housing 210 of the discrete power module 122 prevents the discrete power module 122 from separating from the top surface 226 of the pad 218. Accordingly, the solder preform 410 is maintained in contact with the pad 218 and the discrete power module 122, and package tilt and bond line planarity issues are prevented from occurring.

[0055] In one form, the alignment fixture 610 is formed of a material having a thermal expansion coefficient similar to that of the substrate 114. In one form, the difference in the thermal expansion coefficient of the material of the alignment fixture 610 and the thermal expansion coefficient of the material of the substrate 114 is within ±7*10 -6 m / (m°C). Additionally, the material of the alignment fixture 610 can have a low thermal conductivity to prevent heat transfer to the discrete power module 122. In one form, the alignment fixture 610 is formed of stainless steel. However, other materials having similar thermal expansion coefficients and low thermal conductivities can be used, such as but not limited to, for example, aluminum graphite.

[0056] Reference Figure 10 and Figure 11 , the reflow process at step 312 occurs within at least one chamber 1100 and also includes the steps provided in Figure 10 . In other words, Figure 10 the steps provided inFigure 3 Further details of step 312. At step 1002, chamber 1100 is evacuated to atmosphere and filled with a gaseous substance (e.g., an inert gas such as nitrogen), and then exchanged for a second gaseous substance. In one form, the second gaseous substance can be a reducing agent such as, but not limited to, formic acid (HCOOH). Formic acid can be activated at temperatures as low as 180 degrees Celsius to perform reduction in a chemical redox reaction and can provide oxidation removal on the solderable surface of the barrier for bonding the discrete power module 122 to the substrate 114. Other gases such as forming gas, e.g., hydrogen, can also be used.

[0057] After step 1002, method 300 can proceed to step 1004. In step 1004, the pressure of the gaseous substance is increased. In one form, the gaseous substance in chamber 1100 is increased to between 750 mbar and 1050 mbar.

[0058] In step 1006, the base plate 1110 is powered to heat up to raise the temperature of the substrate 114 as discussed above. The substrate 114 acts as a heat conductor to melt the solder preform 410 as discussed above. As discussed above, the sensor 1114 measures the temperature of the plurality of temperature zones 1116 of the discrete power module 122. The sensor 1114 measures the temperature through the hole 712.

[0059] By raising the temperature of the substrate 114, the measured temperature of the plurality of temperature zones 1116 increases. In one form, the base plate 1110 is heated to raise the measured temperature of the plurality of temperature zones 1116 to between 215 degrees Celsius and 260 degrees Celsius.

[0060] Step 1006 can occur before or simultaneously with step 1004. Once the temperature zones 1116 are between 215 degrees Celsius and 260 degrees Celsius and the pressure is between 750 mbar and 1050 mbar, the measured temperature and pressure are maintained within the desired bounds for a first hold time. In one form, the first hold time can be between one minute and four minutes. Then, a vacuum pump (not shown) evacuates the gas from chamber 1100 to create a vacuum environment in the chamber. This vacuum environment is maintained between 15 seconds and 90 seconds while the temperature is maintained between 215 degrees Celsius and 260 degrees Celsius to significantly reduce voids at the solder joint. Then, the vacuum is released (e.g., allowing a gas such as nitrogen or air to return to the chamber until atmospheric pressure), and the electrical power component 110 is cooled to between 20 degrees Celsius and 51 degrees Celsius to allow the solder to re-solidify.

[0061] The controller 1120 continuously monitors the temperature of the temperature zones 1116 and determines whether the measured temperatures of the plurality of temperature zones 1116 are below a threshold temperature. As discussed above, the controller 1120 adjusts the power supplied to the base plate 1110 to maintain the measured temperatures of the plurality of temperature zones 1116 below the threshold temperature throughout the reflow process of step 312.

[0062] In one form, maintaining the measured temperature below the threshold includes dynamically adjusting the power supplied to the base plate 1110 to reduce the base plate temperature of the base plate 1110. In one variant, the holding time of the base plate temperature can also be dynamically adjusted.

[0063] In another form, maintaining the measured temperature below the threshold includes dynamically reducing the holding time of the base plate temperature of the base plate. For example, the controller 1120 can maintain the power level to the base plate 1110, but reduce the holding time at that power level. Alternatively, the controller 1120 can reduce the power level and reduce the holding time.

[0064] In one form, the threshold temperature is 260 degrees Celsius. The threshold temperature can be determined based on the maximum operating temperature of the discrete power module 122 and / or the epoxy molding of the discrete power module 122.

[0065] Return reference Figure 3 , in an alternative configuration, step 302 can be performed at any point prior to step 312. For example, step 302 can be performed between steps 310 and 312.

[0066] Unless expressly indicated otherwise herein, all numerical values indicating mechanical / thermal properties, percentage compositions, dimensions, and / or tolerances or other characteristics should be understood to be modified by the word "about" or "approximately" when describing the scope of the present disclosure. This type of modification is desired for various reasons, including: industrial practice; material, manufacturing, and assembly tolerances; and test capabilities.

[0067] As used herein, the phrase "at least one of A, B, and C" should be construed to represent the logic (A or B or C) using non-exclusive logic "or", and should not be construed to mean "at least one of A, at least one of B, and at least one of C".

[0068] In the present application, the terms "controller" and / or "module" may refer to, be part of, or include the following: application specific integrated circuit (ASIC); digital, analog, or mixed analog / digital discrete circuits; digital, analog, or mixed analog / digital integrated circuits; combinational logic circuits; field programmable gate arrays (FPGA); processor circuits that execute code (shared, dedicated, or group); memory circuits that store code executed by the processor circuits (shared, dedicated, or group); other suitable hardware components that provide the described functionality (e.g., operational amplifier circuit integrators as part of a heat flux data module); or combinations of some or all of the above, such as in a system on a chip.

[0069] The term memory is a subset of the term computer-readable medium. As used herein, the term computer-readable medium does not cover transitory electrical or electromagnetic signals propagated through a medium (such as on a carrier wave); thus, the term computer-readable medium can be considered tangible and non-transitory. Non-limiting examples of non-transitory tangible computer-readable media are non-volatile memory circuits (such as flash memory circuits, erasable programmable read-only memory circuits, or mask read-only circuits), volatile memory circuits (such as static random access memory circuits or dynamic random access memory circuits), magnetic storage media (such as analog or digital magnetic tape or hard disk drives), and optical storage media (such as CDs, DVDs, or Blu-ray discs).

[0070] The devices and methods described in the present application may be implemented in part or in whole by a special-purpose computer created by configuring a general-purpose computer to execute one or more specific functions embodied in a computer program. Functional blocks, flowchart components, and other elements described above serve as software specifications that can be translated into a computer program by routine work of a technician or programmer.

[0071] The description of the present disclosure is exemplary in nature and, thus, variations that do not depart from the essence of the present disclosure are intended to be within the scope of the present disclosure. Such variations should not be regarded as departing from the spirit and scope of the present disclosure.

[0072] In one aspect of the present invention, the plurality of peripheral regions include the four corners of each DPM.

[0073] According to the present invention, a method of attaching a plurality of discrete power modules (DPMs) to a substrate includes: positioning the substrate such that a bottom surface of the substrate faces a top surface of a base plate; placing a plurality of solder preforms on a top side of the substrate; placing each of the plurality of DPMs on a top surface of a corresponding one of the plurality of solder preforms; performing a reflow process that includes: melting the solder preforms by providing heat to the substrate via the base plate; measuring, during the reflow process, measured temperatures of a plurality of temperature zones of the DPM via at least one sensor; determining, via a controller, whether the measured temperatures of the plurality of temperature zones are below a threshold temperature during the reflow process; and maintaining, during the reflow process, the measured temperatures of the plurality of temperature zones below the threshold temperature by at least one of: dynamically adjusting power supplied to the base plate to reduce a base plate temperature of the base plate, and dynamically adjusting a hold time of the power supplied to the base plate.

[0074] In one aspect of the present invention, the method includes placing a tray between the substrate and a plurality of bus bars electrically coupled to each of the DPMs of the plurality of DPMs, wherein the tray provides at least one of: electrical insulation between the bus bars and the substrate; and a reaction support for a clamping force applied to the bus bars.

[0075] In one aspect of the present invention, the method includes: placing an alignment fixture such that the plurality of DPMs are between the alignment fixture and the substrate; and applying a force to the alignment fixture toward the substrate such that the alignment fixture clamps the plurality of DPMs to the substrate during the reflow process.

[0076] In one aspect of the present invention, the at least one sensor includes three sensors, and the plurality of temperature zones includes three temperature zones, wherein each of the three sensors is positioned to measure a corresponding one of the three temperature zones.

Claims

1. A method of attaching a plurality of discrete power modules (DPMs) to a substrate, the method comprising: Positioning the substrate such that a bottom surface of the substrate faces a top surface of a base plate; Placing a plurality of solder preforms on a top side of the substrate; Placing each of the plurality of DPMs on a top surface of a corresponding one of the plurality of solder preforms; Performing a reflow process, the reflow process comprising: Melting the solder preforms by providing heat to the substrate via the base plate; Measuring temperatures of a plurality of temperature zones of the DPM via at least one sensor during the reflow process; Determining via a controller whether the measured temperatures of the plurality of temperature zones are below a threshold temperature during the reflow process; and Maintaining the measured temperatures of the plurality of temperature zones below the threshold temperature during the reflow process.

2. The method of claim 1, wherein maintaining the measured temperature below the threshold includes dynamically adjusting power supplied to the base plate to reduce a base plate temperature of the base plate.

3. The method of claim 2, wherein maintaining the measured temperature below the threshold further includes dynamically increasing a holding time of the base plate temperature.

4. The method of claim 1, wherein maintaining the measured temperature below the threshold includes dynamically reducing a holding time of the base plate temperature of the base plate.

5. The method of claim 1, further comprising: Dispensing a tackifier at: (a) between the top side of the substrate and the plurality of solder preforms to hold the plurality of solder preforms on the substrate, or (b) between each DPM and a top surface of a corresponding one of the plurality of solder preforms to hold the DPM on the plurality of solder preforms, or (c) both (a) and (b).

6. The method of claim 1, further comprising placing a tray between the substrate and a plurality of bus bars electrically connected to each of the DPMs in the plurality of DPMs, wherein the tray provides at least one of: Electrical insulation between the bus bars and the substrate; and A reaction support for a clamping force applied to the bus bars.

7. The method of claim 1, wherein during the reflow process, the bottom surface of the substrate contacts the top surface of the base plate.

8. The method of claim 1, wherein during the reflow process, the bottom surface of the substrate is disposed at a predefined distance above the top surface of the base plate.

9. The method of claim 1, wherein the at least one sensor includes at least one non-contact sensor.

10. The method of claim 1, wherein the at least one sensor includes three sensors, and the plurality of temperature zones includes three temperature zones, wherein each of the three sensors is positioned to measure a corresponding one of the three temperature zones.

11. The method according to claim 1, wherein the reflow process occurs in at least one chamber, and the reflow process further comprises: filling the at least one chamber with a gaseous substance comprising HCOOH; increasing the pressure of the gaseous substance in the at least one chamber to between 750 mbar and 1050 mbar; and heating the base plate while the pressure is between 750 mbar and 1050 mbar to raise the measured temperature of the plurality of temperature zones to between 215 °C and 265 °C for a first holding time.

12. The method according to any one of claims 1 to 11, further comprising: placing an alignment fixture such that the plurality of DPMs are between the alignment fixture and the substrate; and applying a force to the alignment fixture towards the substrate such that the alignment fixture clamps the plurality of DPMs to the substrate during the reflow process.

13. The method according to claim 12, wherein the substrate is a material having a first coefficient of thermal expansion, and the alignment fixture is a material having a second coefficient of thermal expansion, and the difference between the second coefficient of thermal expansion and the first coefficient of thermal expansion is within ±7.0×10 -6 m / (m·°C).

14. The method according to claim 12, wherein the alignment fixture defines a plurality of holes, each of the plurality of holes being aligned with a corresponding DPM of the plurality of DPMs and opening through the alignment fixture such that the DPMs are visible through the holes, wherein the alignment fixture contacts a plurality of peripheral regions of each DPM.

15. A system, comprising: a substrate; a plurality of solder preforms disposed on a top side of the substrate; a plurality of discrete power modules (DPMs), wherein each DPM of the plurality of DPMs is disposed on a corresponding solder preform of the plurality of solder preforms, and wherein each DPM comprises a plurality of electrical contacts; an insulating tray disposed between the electrical contacts and the substrate and electrically insulating the electrical contacts from the substrate; and an alignment fixture disposed on the substrate such that the plurality of DPMs are between the alignment fixture and the substrate, the alignment fixture being configured to clamp the DPMs to the substrate.

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