METHOD TO CONNECT POWER TERMINAL TO SUBSTRATE WITHIN SEMICONDUCTOR PACKAGE

A method of manufacturing a power semiconductor device in accordance with an embodiment of the present disclosure may include providing a substrate disposed atop a heatsink, electrically connecting a semiconductor die to a top surface of the substrate, disposing a thin metallic layer atop the substrate, disposing a terminal atop the thin metallic layer, and performing a welding operation wherein a laser beam is directed at a top surface of the terminal to produce a plurality of weld connections connecting the terminal to the substrate, wherein the weld connections are separated by gaps, and wherein heat generated during the welding operation melts the thin metallic layer and molten material of the thin metallic flows into the gaps.

FIELD OF THE DISCLOSURE

Embodiments of the present disclosure relate to power semiconductors and, more particularly, to a technique for connecting a power terminal to a substrate within a power module semiconductor package.

BACKGROUND

Power semiconductors are components used to convert energy from one form to another at various stages between the points of energy generation and energy consumption. A power semiconductor component can take the form of a discrete transistor, thyristor, diode, insulated gate bipolar transistor (IGBT), or metal oxide semiconductor field effect transistor (MOSFET). Or, if a higher level of current or integration is required, the component can take the form of a multi-chip module, which contains more than one of these chips or dies in a desired configuration or topology. Power semiconductors may be packaged in a variety of discrete and multi-chip module formats.

Power semiconductor devices include power terminals extending from the semiconductor packaging for connection to printed circuit boards and other circuit elements. The power terminals may be connected to the semiconductor packaging using conventional technologies such as soldering, sintering, and welding, e.g., high-current pulse welding, and ultrasonic welding. However, ultrasonic welding is messy, as particles, whiskers, or debris-like particles are generated during the welding process. Further, the generated debris is electrically conductive and able to disturb the function of the power semiconductor unit and/or the substrate of the semiconductor packaging may be cracked or otherwise damaged during the ultrasonic welding process.

To address the shortcomings described above, U.S. Pat. No. 10,720,376 discloses a method to connect terminals inside discrete packages using laser welding to attach a leadframe to a Direct Copper Bonded (DCB) substrate. While laser welding is cleaner and generally provides stronger bonds than conventional bonding techniques (e.g., ultrasonic welding), it is associated with certain disadvantages. For example, depending on the laser energy and the particular weld pattern used, laser welding may leave certain areas of a desired bonding region unbonded or weakly bonded. Such unbonded or weakly bonded regions may be detrimental to the conductivity and mechanical strength of the interface, which may compromise the reliability of a power semiconductor device.

It is with respect to these and other considerations that the present improvements may be useful.

SUMMARY

A power seimcodnuctor device in accordance with an embodiment of the present disclosure may include a heatsink, a substrate disposed atop the heatsink, a semiconductor die disposed atop, and electrically connected to, the substrate, and a terminal disposed atop, and electrically connected to, the substrate by weld connections separated by gaps, wherein the gaps are filled by a thin metallic layer.

A method of manufacturing a power semiconductor device in accordance with an embodiment of the present disclosure may include providing a substrate disposed atop a heatsink, electrically connecting a semiconductor die to a top surface of the substrate, disposing a thin metallic layer atop the substrate, disposing a terminal atop the thin metallic layer, and performing a welding operation wherein a laser beam is directed at a top surface of the terminal to produce a plurality of weld connections connecting the terminal to the substrate, wherein the weld connections are separated by gaps, and wherein heat generated during the welding operation melts the thin metallic layer and molten material of the thin metallic flows into the gaps.

DETAILED DESCRIPTION

Embodiments of a laser bonding method and an associated power semiconductor device in accordance with the present disclosure will now be described more fully with reference to the accompanying drawings, in which preferred embodiments of the present disclosure are presented. The method and device of the present disclosure may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the method and device to those skilled in the art. In the drawings, like numbers refer to like elements throughout unless otherwise noted.

FIG.1is a representative cross-sectional view of a power semiconductor device100(hereinafter “the device100”) according to exemplary embodiments of the present disclosure. The device100may include a metal heatsink102, which pulls heat away from other components of the device and forms the baseplate of the device100. A multilayer substrate104may be positioned atop the heatsink102and may include first and second conductive layers106A and106B (collectively, “the conductive layers106”) disposed on top and bottom surfaces of an insulating layer110. In various embodiments, the conductive layers106may be formed of copper and the insulating layer110may be formed of ceramic (e.g., aluminum oxide, aluminum nitride, etc.). The present disclosure is not limited in this regard.

The substrate104may be soldered or sintered to the heatsink102by a solder or sinter layer112. Silicon or silicon-based semiconductor chips or dies114may be connected to a top surface of the first conductive layer106A of the substrate104by a solder or sinter layer116. Metal wires, ribbons, clips, or the like (hereinafter “the metal wires118”) may connect top surfaces of the semiconductor dies114to portions of first conductive layer106A, thus providing electrical interconnections within the package. Contact elements/terminal leads120(hereinafter referred to as “the terminals120”) may also be connected to the top surface of the first conductive layer106A of the substrate104as further described below. In exemplary embodiments, the terminals120may be formed of highly electrically conductive metals, such as copper, copper alloy, aluminum, aluminum alloy, silver, or silver alloy. Additionally, the terminals120may be plated with nickel, silver, or gold, which may be physically or chemically applied to the surfaces of the terminals120. The present disclosure is not limited in this regard.

An encapsulation layer122may encase the circuit components above the heatsink102, and a cover124formed of a durable, dielectric material (e.g., plastic) may be disposed over the circuit components and may protect the circuit components from external elements, with portions of the terminals120protruding from the cover124for facilitating electrical connection of the device100within a circuit. In various embodiments, the encapsulation layer122may be formed of silicone gel, epoxy molding compound (EMC), or mixtures thereof. Additionally, embodiments of the device100are contemplated in which the cover124is entirely omitted, such as if the device100includes module-type semiconductor packing with EMC used for the encapsulation layer122. The present disclosure is not limited in this regard.

Referring toFIG.2, there is shown a cross-sectional view of the above-described device100during an assembly step thereof, wherein one of the terminals120is being bonded to the first conductive layer106A of the substrate104using a laser bonding technique in accordance with embodiments of the present disclosure. During this assembly step, a thin metallic layer202may be disposed atop the first conductive layer106A, and the terminal120may then be placed atop the thin metallic layer202. The thin metallic layer202may be formed of tin or other metals and/or metal alloys having good electrical conductivity and a low melting point (e.g., a melting point below 450 degrees Celsius). More generally, the thin metallic layer202may have a melting point lower than that of the material from which the terminal120is formed. In various embodiments, the thin metallic layer202may be a thin sheet of metal or foil. In other embodiments, the thin metallic layer202may be formed of solder paste, sinter paste, etc. The thin metallic layer202may have a thickness in a range of 20-100 μm. The present disclosure is not limited in this regard.

Using a laser device203, the laser bonding technique of the present disclosure employs laser treatment to affix or bond the terminal120to the first conductive layer106A in a manner sufficient to allow electrical current to flow from the terminal120to the first conductive layer106A and vice-versa. Particularly, a laser beam204may be directed at a top surface of the terminal120above an area where the terminal120is to be bonded to the first conductive layer106A, hereinafter referred to as “the bonding region206.” The energy of the laser beam204causes the solid materials of the terminal120and the first conductive layer106A to be transformed (melted with consecutive rapid solidification), resulting in weld connections208therebetween as shown in the detailed view ofFIG.3.

The weld connections208may be separated by spaces or gaps209(as dictated by the particular weld pattern employed) where the terminal120and the first conductive layer106A are not welded together. However, heat generated by the laser treatment may be sufficient to melt the thin metallic layer202, whereafter the molten material of the thin metallic layer202may flow into the gaps209as motivated by capillary forces and wettability of the surrounding surfaces of the terminal120and the first conductive layer106A. After filing the gaps209between/adjacent the weld connections208, the molten material of the thin metallic layer202may cool and solidify and, in combination with the weld connections208, may provide robust electrical and mechanical connections between the terminal120and the first conductive layer106A. The laser bonding technique of the present disclosure thereby reduces the likelihood of mechanical failure (e.g., cracking) in the bonding region206and improves the conductivity and reliability of the bond relative to traditional laser bonding techniques.

Referring toFIG.4, a flow diagram illustrating an embodiment of the laser bonding technique of the present disclosure is shown. The method will now be described in conjunction with the illustrations of the device100shown inFIGS.2and3.

At block300of the exemplary method, the substrate104may be provided, wherein the substrate104includes the insulating layer110and first and second conductive layers106A,106B disposed on top and bottom surfaces of the insulating layer110, respectively. The substrate104may be positioned atop a heatsink102with the second conductive layer106B soldered or sintered to the heatsink102. At block310of the method, the thin metallic layer202formed of a low melting point metal may be disposed atop the first conductive layer106A. At block320of the method, the terminal120may be placed atop the thin metallic layer202.

At block330of the exemplary method, the laser device203may be used to direct the laser beam204at the top surface of the terminal120above an area where the terminal120is to be bonded to the first conductive layer106A. The energy of the laser beam204may melt the solid materials of the terminal120and the first conductive layer106A to be transformed (melted with consecutive rapid solidification), resulting in weld connections208therebetween. At block340of the method, which may happen simultaneously with the action of block330, heat generated by the laser treatment may melt the thin metallic layer202, whereafter the molten material of the thin metallic layer202may flow into the gaps209as motivated by capillary forces and wettability of the surrounding surfaces of the terminal120and the first conductive layer106A. After filling the gaps209between/adjacent the weld connections208, the molten material of the thin metallic layer202may cool and solidify and, in combination with the weld connections208, may provide robust electrical and mechanical connections between the terminal120and the first conductive layer106A.

While the present disclosure makes reference to certain embodiments, numerous modifications, alterations and changes to the described embodiments are possible without departing from the sphere and scope of the present disclosure, as defined in the appended claim(s). Accordingly, it is intended that the present disclosure not be limited to the described embodiments, but that it has the full scope defined by the language of the following claims, and equivalents thereof.