Low inductance bond-wireless co-package for high power density devices, especially for IGBTs and diodes

A power semiconductor package that includes at least two semiconductor devices electrically coupled to one another through a common metallic web.

BACKGROUND OF THE INVENTION

Modern applications increasingly demand the capability to manage high currents in a very small space and often in harsh environments, e.g. large temperature changes during the lifetime of the package. Particularly, in the automotive sector, due to the increasing electrification of functions the high current demand has increased enormously; e.g. inverter and E-motor drives for hybrid car applications, starter-generator applications, high power DC-DC converter or x-by-wire applications like electric power steering or electric braking. These applications need high current carrying capabilities on a minimum space challenging the state-of-the-art power modules in terms of achievable power density. To respond to the demand for high current applications, new technologies need to be developed to overcome the thermal and electrical limitations of the state-of-the-art power switch packages and power modules.

Progress in semiconductor processes and device design have extended the performance limits of semiconductor devices beyond the capabilities of the state-of-the-art packages. Therefore, newer state-of-the-art packaging technologies for power devices try to achieve low inductivity and better thermal connectivity to a heatsink through bond wireless connection techniques to maximize the thermal contact area of the power device to a heatsink and/or maximize the electrical connection of the device to a power terminal/leadframe.

U.S. Pat. No. 6,624,522, and U.S. patent application Ser. No. 11/641,270 both assigned to the assignee of the present application, disclose bond-wireless packaging techniques.

The concepts disclosed in the above follow a similar principle: By connecting the topside of the power device (especially the source or the emitter contact) to a larger metal area the package gains a higher current carrying capability, better thermal properties and a lower inductivity at the same time to achieve high power densities, good thermal performance, improved low inductivity and higher reliability.

The packaging concept disclosed in U.S. patent application Ser. No. 11/641,270 solves the main problems regarding thermal mismatch, bond wire inductance, high current carrying capability and package inductance.

The inductance of the packaged device is an important contributor to the final performance of the power device. State-of-the-art power modules normally try to reduce package resistance by minimizing Cu-leads and distances between various devices. It is especially important in applications which use IGBT devices as power switches that the so called free wheeling diode is connected to the corresponding IGBT using a low inductance connection. The diode is a “partner device” for the IGBT and carries the inductive current when the IGBT is switched off preventing the IGBT from break through due to inductive current flow. The typical combination of IGBT and diode is shown inFIG. 1.

It is a conventional packaging technique to solder and wire-bond the diode as close as possible to the IGBT on a substrate like a DBC, PCB or IMS in order to minimize parasitic inductance between the diode and the IGBT.

It is also a conventional packaging technique to co-package an IGBT and a diode in one package by soldering both devices on a shared leadframe, attach wire bonds and finally passivate this package with e.g. a mold plastic.

BRIEF DESCRIPTION OF THE INVENTION

It is an object of the present invention to provide a co-packaging concept that includes all the advantages of the stress reduced horseshoe-shaped DBC-can described in U.S. patent application Ser. No. 11/641,270.

It is another object of the present invention to improve the existing state-of-the-art by introducing a stress reduced and bond wireless co-package for two discrete power devices especially for a combination of an IGBT and an anti-parallel diode or free-wheeling diode.

A power semiconductor package according to the present invention includes a metallic body having a web portion configured for electrical and mechanical coupling using a conductive (e.g. solder or a conductive epoxy) adhesive to active electrodes of at least two semiconductor devices, and a connector portion extending from an edge of the web portion to connect electrically the active electrodes of the semiconductor devices, a first semiconductor die and a second semiconductor die electrically and mechanically coupled to said web portion, a ceramic insulation body directly bonded at one surface thereof to a surface of said metallic body, and another metallic body directly bonded to another opposing surface of said ceramic insulation body.

As will become apparent from the disclosure set forth below a package according to the present invention offers the following advantages:

a) improved mechanical properties:i) stress-reduced two-sided cooling of power devices;ii) a housing that is thermally compatible with Si (i.e. matched thermal expansion coefficient);iii) increased reliability due to the matching thermal expansion coefficients;

b) improved electrical and thermal properties:i) low inductance due to a shared contact pad for the diode and the IGBT (minimum distance between the devices and no inductive bond wires);ii) low inductance of the overall co-package due to the use of a large soldered contact area for all pads of the two devices;iii) increased current/power capability compared to the state-of-the-art technologies due to low electrical and thermal resistance using solder die attach and large contact areas;iv) electrical isolation (HV and automotive suitable);v) efficient usage of the available package space and therefore optimized power;vi) smaller temperature changes during power cycling due to the fact that either the IGBT or the diode are generating power losses inside of the co-package keeping the overall temperature relatively constant (If only one device is inside the DBC-can a temperature change is generated between switch-on and switch-off state of the device leading to a faster solder wear out due to the fast changing temperatures at the solder joint (typically at switching frequencies of 10-100 kHz). This can be avoided or minimized by the co-packaging of the diode and the IGBT where power loss are more homogeneously generated since the diode is generating conduction losses when the IGBT is switched off and vice versa when the IGBT is switched on. Since both devices share a contact pad the temperature during the switching operation does not change too much compared to a single-die package);

c) improved manufacturing and handling properties:i) pre-assembled co-packaged components is suitable for easy handling and integration into power modules;

d) low manufacturing tests and costs:i) a high volume production possibility without application specific customization which will be done by the end-user;ii) die attach to the DBC-can can be done on a DBC-card instead of handling and assembling discrete co-packages;iii) electrical/parametric end-tests after or during assembly can be done at the DBC-card level before separating the card into discrete co-packages;iv) transportation from the fab to the end-user can be done by using the DBC-card assembly as a whole which offers protection without the need for a sophisticated additional transport package;

e) unique properties for end users:i) pre-assembled discrete component co-package is matching the thermal expansion coefficient of the state-of-the-art power substrates and therefore attractive for a large variety of applications;ii) application-flexibility of the sub-assembly which can easily be combined into an application specific circuit at the end-customer using the co-pack of an IGBT and a diode as basic “construction-kit”;iii) application-flexibility due to various die attach possibilities inside of the DBC-can like up-side down or bottom up, providing optimum low and high side driver or half/full-bridge configurations just by combining several DBC-can packaged die on a power substrate or in a power module;iv) cost-effective material choice by matching the ceramic type of the DBC-can to the application requirements (e.g. Al2O3, AlN, SiN, . . . ceramics);

f) unique properties related to the implementation of optional features:i) an additional EMI screening function can be implemented using the top-Cu layer of the DBC-can;ii) an additional heat-spreader can be mounted on top of the DBC-can while the bottom of the die is soldered to the cooled power substrate of the application giving highly efficient double sided cooling for highest power densities;iii) easy integration of smartness like a gate-driver by mounting on top of the co-pack DBC-can.

The main application field for a package according to the present invention will be in high power circuits and modules switching high currents or high voltages and requiring low inductance and EMI-screening. A package according to the present invention is also well-suited for high voltage applications using a combination of IGBTs and diodes, and applications under harsh environmental conditions or tough temperature cycling requirements like automotive or safety critical functions with high reliability demand.

DETAILED DESCRIPTION OF THE FIGURES

Referring toFIG. 2, a package according to the present invention includes a basic building block10, which has been described in co-pending U.S. application Ser. No. 11/641,270, entitled Package for High Power Density Devices, assigned to the assignee of the present application, the entire disclosure of which incorporated by reference. Basic building block10(sometimes referred to herein as DBC-can) includes a dielectric ceramic body12(e.g. Al2O3), a metallic body14directly bonded to one surface of ceramic body12, and a metallic connector16directly bonded to another, opposite surface of ceramic body12. Metallic body14may be a flat metallic web that is formed from copper, aluminum or the like material. Metallic connector16may also be formed from copper, aluminum, or the like and includes a rectangular or square web portion18, and a lead portion20that extends from (preferably in a vertical direction) web portion18. In the first embodiment of the present invention, lead portion20is horseshoe-shaped, and extends from three edges of web portion18, leaving one edge open. Preferably, web portion18includes a die connection area22for electrical connection to an electrode of a semiconductor die, and a plurality of spaced circular dimples24surrounding the die connection area22. Each dimple24is a depression formed (by etching or the like method) in web portion18.

In the first embodiment of the present invention, an IGBT die26and diode28(e.g. Schottky diode, PN diode, or the like diode) are assembled onto die connection area22of web portion18. Specifically, the collector electrode (not shown) of IGBT26and the cathode electrode (not shown) of diode28are electrically and mechanically connected to connection area22using a conductive adhesive body30such as solder or a conductive epoxy. That is, a conductive adhesive body30is disposed between the collector electrode of IGBT die26and the cathode electrode of diode28and connection area22, whereby the collector electrode and the cathode electrode are electrically and mechanically connected to web portion18and electrically connected to one another whereby low inductance is achieved. Note that dimples24serve as a barrier against conductive adhesive30(e.g. during reflow where solder is used) whereby IGBT die26and diode28can be kept spaced from lead20and the combination of IGBT26, and diode28can be preserved. Dimples24can also be located between diode28and IGBT26to keep space between the two dice (not shown in the figures). Emitter electrode32and gate electrode34of IGBT26which are disposed on a surface opposite to the collector electrode, and anode electrode36of diode28, which is disposed on a surface opposite the cathode electrode, are preferably readied for flip-mounting onto a conductive pad of a circuit board or the like. Flip-mounting or flip-mounted as used herein refers to electrical and mechanical mounting using a conductive adhesive body (e.g. solder, conductive epoxy or the like) in which a conductive adhesive body is disposed between an electrode of the die and a conductive pad on a circuit board, or a die receiving area of a leadframe or the like. Note that lead20includes an external connection surface60which is preferably coplanar with the free surfaces of the electrodes of die26,28, and serves to connect the collector electrode of IGBT26and the cathode electrode of diode28to a conductive pad of a circuit board through flip-mounting.

A package according to the second embodiment has two open sides which advantageously allows for filling the space underneath the DBC-can with an under filler or an isolating gel after soldering the DBC-can on a substrate.

Referring toFIG. 3, in which like numerals identify like features, in a package according to the second embodiment of the present invention, the horseshoe-shaped lead20is replaced with two spaced and oppositely disposed leads20each extending vertically from a respective edge of web18of connector16. In all other respects, a package according to the second embodiment is identical to a package according to the first embodiment.

Referring toFIG. 4, in which like numerals identify like features, a package according to the third embodiment includes two spaced and electrically isolated connectors40,42. Connector40includes a die connection area22for electrical connection (using a conductive adhesive such as solder or conductive epoxy) to the gate electrode (not shown) of an IGBT die26, while connector42includes a die connection area22for electrical connection (using a conductive adhesive such as solder or conductive epoxy) to the emitter electrode (not shown) of IGBT die26. Note that the electrodes of the die may be bumped for connection to surfaces22. Note further that collector electrode44of IGBT die26is preferably coplanar with the connection surface46of lead42and readied for flip-mounting onto a conductive pad of a circuit board or the like substrate. Note also that in a package according to the third embodiment, connector42includes a horseshoe-shaped lead20similar to the lead20in the first embodiment, while connector40includes two opposing leads20extending from opposite edges of a web, similar to leads20of the second embodiment. Furthermore, note that in a package according to the third embodiment diode28, although not shown, may be electrically mounted on metallic body14, and then connected to form the circuit shown inFIG. 1during the assembly of the package in a module or the like integrated device. A clip, or wirebonds (not preferred can be used to accomplish such a connection. Note that, alternatively, the anode electrode of a diode28may be coupled to connection area22of connector42, whereby the anode electrode and the emitter electrode of IGBT26can be electrically coupled through connector42.

Referring toFIG. 5, in which like numerals identify like features, in a package according to the fourth embodiment, web18is electrically connected to metallic body14through a conductive filled via48that extends through ceramic body12; for example, a copper filled via which connects web18to metallic body14. The cathode electrode of diode28is then electrically and mechanically connected to metallic body14using a conductive adhesive30, and the collector electrode of IGBT die26is electrically and mechanically coupled to web18through a conductive adhesive body30. The emitter electrode of IGBT die26can then be connected to the anode electrode of diode28when the package is assembled on a circuit board or the like substrate in a module or an integrated device. Cu-stripes/clips, a leadframe or even bond wires (but this is not a preferred option) could be used to form that contact. Note that the emitter electrode and the gate electrode each includes a respective solder bump50to facilitate flip-mounting of the package.

Referring toFIG. 6, in which like numerals identify like features, in a package according to the fifth embodiment, the anode electrode of diode28is electrically connected to a web18′ using a conductive adhesive body30. Web30includes leads20′, and is bonded to a ceramic body12, which is bonded to metallic body14. Copper, aluminum or the like material can be used to form web18′ and leads20′. Leads20′ are electrically and mechanically connected to pads49using a conductive adhesive body30. Each pad49is electrically connected through a conductive filled via48to a respective conductive (e.g. copper) lead52. Each conductive lead52preferably includes a surface for external connection60through flip-mounting which is coplanar with connection surfaces of leads20, whereby the entire assembly is rendered flip-mountable. Note that in a package according to the fifth embodiment only one lead52is required, and the second lead is optional and provided for symmetry.

Referring now toFIG. 7, in which like numerals identify like features, in a package according to the sixth embodiment of the present invention, the anode electrode of diode28is electrically and mechanically coupled to the interior surface of a metallic clip54using a conductive adhesive30. Metallic clip54is formed from copper or the like material and includes legs56each extending from an edge or a side of a web portion58. Each leg includes a surface60for external connection which is coplanar with external connection surfaces60of leads20. Note that clip54can be copper, an example of which can be found in U.S. Pat. No. 6,624,522, assigned to the assignee of the present application. If the application does not have severe temperature requirements it might be a cost-effective solution to use an outer metal-can as an outer package and as an electric contact from the diode to the front side of the co-package.

Referring now toFIG. 8, in which like numerals identify like features, in a package according to the seventh embodiment, the anode electrode of diode28is electrically and mechanically connected to web portion18, while the cathode electrode thereof is electrically and mechanically connected to the back surface of a copper clip54. Copper clip54includes web portion58, and two legs extending from opposite edges of web58. The collector electrode of IGBT die26is electrically and mechanically coupled to the interior surface of web58, and connection surfaces60of leads20and legs56are coplanar, whereby the entire package is rendered flip-mountable. If the application does not have severe temperature requirements it might be a cost-effective solution to use a metal-can similar to a can disclosed in U.S. Pat. No. 6,624,522 as an inner package and as an electric contact from the IGBT to the diode soldered to the backside of the metal-can. A DBC-can10is used as outer package covering the stack.

Referring now toFIG. 9, in which like numerals identify like features, in a package according to the eighth embodiment, two connectors62,64are provided. First connector62includes a web portion66which is bonded to ceramic body12, and at least one lead20extending from an edge thereof. Web portion30is electrically and mechanically connected to the anode electrodes of diode28, while the cathode electrode of diode28is electrically and mechanically coupled to a portion of the collector electrode of IGBT die26. Second connector64also includes a web portion68that is bonded to ceramic body12and is electrically and mechanically connected to another portion of the collector electrode of IGBT die26. Note that web portion68is separated and spaced from web portion66and is thicker than web portion66(at least thicker than the combination of diode28and web66). Connector64also includes a lead20that extends from an edge of web68, and includes an external connection surface that is coplanar with external connection surface60of lead20of connector62.

In order to minimize package costs, the packages according to the present invention can be formed simultaneously on a DBC card and then singulated from the card. Thus, a DBC card90is shown inFIG. 10. Such cards are produced in sizes such as 5″×7″ or 4″×6″ and have a continuous central ceramic layer41with top and bottom copper layers. These layers can be simultaneously masked and etched to define the individual building blocks (DBC-cans)10. Specifically, the copper layers are patterned to obtain connectors16and metallic bodies14spaced by streets95and thereafter the die can be loaded onto connections surfaces22of each DBC-can10. Note that connectors16can be tested before singulation.

It is very desirable to test connectors16before the die are mounted to reduce yield loss. After the tests are carried out and the die are assembled in place, DBC-cans10can be singulated by sawing, dicing or physically breaking at the streets95.

Advantageously, cards90can be protected by a suitable foil for shipment and can be pre-scribed for easy break-off or singulation of packages by the end user.

Instead of separating DBC-cans10right after production and assembling Si-devices in a single DBC-cans the whole DBC card can be used to perform the Si-die attach. That way placing the die, the soldering and also the end test/parameter testing of the packaged dice can be done more cost-effectively on the DBC card which is faster, easier and more precise than handling discrete devices.

To optimize package performance solder is preferred as high current carrying and low thermal resistance conductive adhesive.

The soldering of the devices into the cans is an important step for the reliability of the packages device. A flat and homogeneous solder layer of exactly defined thickness would be important for the reliability of the solder joint. The solder thickness is the main parameter determining the solder wear out and the occurrence of fatigue, wear out and delamination. Further the solder thickness needs to be controlled in order to level the die surface with the Cu-frame.

During the soldering process the die can move and make contact with connector16. Such a contact can be avoided.by putting an isolating lacquer or a solder stop on the inside of the connector;by providing an isolating edge-termination/passivation on the die which should be sufficient to provide edge isolation towards the lead20. (This technique on a wafer process level is normally more cost effective than mechanical isolation means using a laquer);by using a “smooth solder process”, e.g. by means of a fluxfree solder (e.g. solder pre-forms instead of solder paste with flux) and by using a vacuum solder process under formic gas atmosphere which will avoid strong movements of the dice inside the DBC-cans and supports keeping the correct die orientation;by designing a dimple structure inside of the DBC-can which will work as a fixation of the die during soldering. Besides the dimples will work as a stress release inside of the can for the bond force between Cu and ceramics.

All dice in DBC-cans10on DBC card90can be tested very efficiently in parallel since the cans are electrically isolated from each other. For example, a special probe-card of a parametric tester can test all dice on the DBC-card90at the same time reducing test time enormously and establishing a high production rate to reduce manufacturing costs.

Besides the electric testing after each die attach on a card level also automated visual inspection of the whole card can easily determine if dice are too close to the DBC-frame (leakage currents). So potential miss-assembled dice can be found and inked out without the risk of going to the field.