Patent ID: 12230551

DESCRIPTION

This disclosure, its aspects and implementations, are not limited to the specific components, assembly procedures or method elements disclosed herein. Many additional components, assembly procedures and/or method elements known in the art consistent with the intended immersion direct cooling modules will become apparent for use with particular implementations from this disclosure. Accordingly, for example, although particular implementations are disclosed, such implementations and implementing components may comprise any shape, size, style, type, model, version, measurement, concentration, material, quantity, method element, step, and/or the like as is known in the art for such immersion direct cooling modules, and implementing components and methods, consistent with the intended operation and methods.

Referring toFIG.1, three implementations of semiconductor packages2,4,6are illustrated placed within an immersion cooling enclosure8that includes a coolant10therein. As illustrated, the immersion cooling enclosure8includes both liquid phase coolant12and vapor phase coolant14in this implementation. However, in other implementations, the coolant10may be only liquid phase in the immersion cooling enclosure8. As used herein, “immersion cooling” includes where at least a portion of the semiconductor packages are immersed in a liquid phase of the coolant and thus includes situations where single phase or two phase coolant is employed in an immersion cooling enclosure. In various implementations, the immersion cooling enclosure8is included in a dual side direct cooling module, where coolant, whether liquid or vapor, is able to contact both sides of a semiconductor package mounted in the module. The immersion cooling enclosure8may be self-contained and a closed system without an outlet relying on ambient or external forced convection cooling of the outer surface of the enclosure to remove heat transferred from the coolant10(for both two phase and single phase coolant situations). In other implementations, the immersion cooling enclosure8may be coupled with a cooling exchanger which receives heated coolant10from the interior of the enclosure8, removes heat therefrom, and then cycles the cooled coolant back to the immersion cooling enclosure. As illustrated inFIG.1, a cooling exchanger16is illustrated that receives vaporized coolant14and then condenses it using heat exchanger18to return liquid coolant12to the enclosure8. In other implementations, however, the cooling exchanger may only receive and cool liquid coolant from the immersion cooling enclosure even where two phases of the coolant are present in the enclosure. A wide variety of enclosure types and cooling exchange types may be constructed using the principles disclosed in this document. In particular implementations, the coolant used may be a chemically inert coolant. In other implementations, the coolant used may be a dielectric fluid. In some implementations, the coolant may be one of the inert and dielectric coolants marketed under the tradename FLUORINERT by 3M of St. Paul, Minnesota.

InFIG.1, each semiconductor package2,4,6is mounted separately into the enclosure8and, for example using package2, leads20,22extend through openings in the enclosure8to allow electrical connections to be made with the semiconductor die in the package2. In various implementations, the leads20,22work both as a mechanical support for the semiconductor package2and as an electrical connection. In other implementations, however, additional mechanical support for the semiconductor package2may be provided by fixtures or other fastening/supporting devices provided in, on, and/or through the material of the immersion cooling enclosure8. Mold compound molded around portions of the leads20,22forms seals24,26which act to prevent any flow or leakage of the coolant10out from around the leads20,22during operation. Because of the use of the mold compound seals24,26, the various semiconductor package implementations disclosed herein may not include the use of mold compound or another heat sink around the package, but may simply include a direct leadframe attach (DLA) leadframe to which die are coupled covered by a coating material. In other implementations, a heat spreader may be included, but as the die may be directly coupled to the heat spreader and to the DLA leadframe, the need for separately soldered heat sinks to be coupled to an existing package design may be eliminated. In this document, a number of variations on these package structures and method implementations for forming them are discussed in further detail.

Referring toFIG.2, an implementation of a DLA leadframe28is illustrated that includes leads30,32. This DLA leadframe28does not have die attached thereto, but has two die attach regions34,36defined on it to which semiconductor die will ultimately be coupled. This DLA leadframe28has mold compound seals36,38molded around portions of the leads30,32prior to the die bonding according to a pre-molding process which will be subsequently disclosed. This DLA leadframe28in particular method implementations may have the semiconductor die coupled to the die attach regions34,36without the use of any other thermal transfer components (heat spreaders, heat sinks, etc.) allowing the heat from the semiconductor die to transfer, following application of a protective coating to be discussed further, directly to the coolant in which the DLA leadframe28contacts. In this way the size of the immersion cooling enclosure that contains the coolant can be made smaller than dual side cooling modules that utilize liquid flow that include separately bonded heat sinks and need additional dimensional size to incorporate seals between the two clamshell halves of the two outer portions of the liquid flow enclosure. The ability to cool in an immersion cooling setting without involving the use of additional process steps to bond heat sinks and seal outer portions of a liquid flow enclosure may result in reduced process costs and higher yield.

FIG.3illustrates an implementation of a DLA leadframe40that has heat spreaders42and die (not shown in the figure as they are covered by the heat spreader42) coupled thereto. In this implementation, the heat spreader takes the form of a flat portion of metallic material to which the die are bonded and which is then bonded with the die to the DLA leadframe40using a process which will be discussed hereafter. In this implementation, the mold compound seals46,48have been formed after the bonding of the heat spreader and a gap44is present between an edge of the heat spreader and the closest edge of the mold compound seals46,48.FIG.4is a side view of the assembly implementation ofFIG.3and shows the gap44along with the semiconductor die50,52coupled between the heat spreader42and DLA leadframe40. This view also shows how some of the leads54,56included a portion that angles to contact the heat spreader42to provide electrical connections to a side of the semiconductor die50,52via the heat spreader42.FIG.4also illustrates the cross sectional thickness of the mold compound seals46,48used to seal the coolant when the DLA leadframe40is assembled into an immersion cooling enclosure. As with the implementation inFIG.2, a coating material (not shown inFIGS.3and4) is applied to the heat spreader42and material of the DLA leadframe40to provide protection for the leadframe and the semiconductor die from the coolant. In some implementations the coating material does not have to be sufficiently thick to provide dielectric protection for electricity flowing through the leadframe/heat spreader because of the use of the immersion cooling enclosure and the use of a dielectric coolant. Because of this, the heat transfer rate between the heat spreader and the coolant may not be significantly impaired when compared to the heat transfer observed through use of a mold compound at a typical thickness over the DLA leadframe and other components.

Referring toFIG.5, another implementation of a DLA leadframe54is illustrated with heat spreaders56that contain pin fins58thereon. As illustrated inFIG.6, the use of the pin fins58results in an increase in surface area of the heat spreaders56thereby increasing the heat transfer rate possible from the heat spreaders and the die61,63bonded between the DLA leadframe54and the heat spreaders56. In various implementations, the heat spreaders may include a wide variety of fins and other projection types and shapes, such as, by non-limiting example, pins, cones, frustoconical shapes, rectangular protections, curved projections, or any other three dimensional fin or pin shape. As with the implementation illustrated inFIGS.3-4, a gap60is present between the edges of the heat spreaders56and the mold compound seals62,64. While the use of heat spreaders on one side of the DLA leadframes has been illustrated in this document, in other implementations, additional heat spreader(s) may be coupled to the largest planar side of the leadframes opposite the side to which the semiconductor die are coupled. In implementations where semiconductor die are coupled to both sides of the DLA leadframes, heat spreader(s) may also be used on both largest planar sides of the leadframes. The materials used for the heat spreaders, pin fin heat sinks, and DLA leadframes may be any of a wide variety of thermally/electrically conductive materials including, by non-limiting example, metals, metal alloys, alumina, copper, copper alloys, aluminum, aluminum alloys, or any other thermally conductive material.

Various implementations of DLA leadframes disclosed in this document may use a wide variety of leads. While in the figures in this document the use of leads at both ends of the leadframes is illustrated, leads may be present on only one side of the leadframes. Also, while the use of multiple leads on a side is illustrated, in other implementations, only one lead or two leads may be utilized in various DLA leadframe implementations.FIG.7illustrates a DLA leadframe66that includes a lead70that has a spring/spring portion68formed therein.FIG.8is a detail side view of the region marked7inFIG.7showing how in this implementation, the shape of the spring portion68takes the form of an S curve. However, in other implementations, the spring portion68may include any of a wide variety of other shapes, including, by non-limiting example, W curves, C curves, L curves, Z curves, curves, elbows, elongated portions, thinned portions, coiled portions or any other shape capable of accommodating movement of the lead. In some implementations, as illustrated inFIG.8, a portion of the lead70may include an angled portion that allows the lead to directly contact the semiconductor die72rather than electrically connect to the die through a heat spreader as previously illustrated. The mold compound seals74,76inFIGS.7-8are illustrated in partial see-through so that the interior structure of the leads is made visible that ordinarily is covered by the mold compound.

Referring toFIG.9, an implementation of a DLA leadframe78is illustrated prior to application of mold compound seals in a perspective view so that the various curved portions of the leads80,82are visible. The DLA leadframe78is illustrated adjacent two heat spreaders84,86to which semiconductor die88,90have been coupled. While the semiconductor die88,90are different semiconductor die types inFIG.9, in other implementations, they may be the same die type. Any of a wide variety of semiconductor die may be employed as well, including, by non-limiting example, power semiconductor die, processors, memory, IGBTs, diodes, rectifiers, metal-oxide field effect transistors (MOSFETs), gallium arsenide devices, high-electron-mobility transistors (HEMTs), passive components (capacitors, resistors, inductors, etc.) or any other semiconductor device or component.FIG.9illustrates discrete semiconductor die88,90each coupled to the heat spreaders84,86and then to the DLA leadframe78.FIG.14illustrates DLA leadframe132that has mold compound seals133,135pre-applied alongside heat spreaders137,141that have semiconductor die142,139coupled thereto, similar to the view inFIG.9.

In contrast with the discrete semiconductor die illustrated inFIG.9,FIG.10illustrates an implementation of a die module92in three views, which, from left to right, are a top view, side view, and bottom view. The die module92contains one or more semiconductor die which have been mechanically and electrically coupled together to form a discrete module with its own leads. In the implementation illustrated inFIG.10, leads94on the top and leads96on the bottom are used for temperature sensing for the semiconductor die included therein. Leads98on the top and100on the bottom are used to electrically connect with the gate(s) of the semiconductor die included in the die module92. Leads102on the top and104on the bottom are used to electrically connect with the source sense of the semiconductor die included in the die module92. Mold compound106is formed around the semiconductor die leaving the exposed leads/pads visible and providing mechanical support for the die. While a particular implementation of a die module92is illustrated inFIG.10, this is merely for the exemplary purposes of this disclosure as a wide variety of die modules containing any desired number of semiconductor die with any desired number of leads and mechanically supported using mold compound, leadframes, or combinations of mold compound and leadframes may be constructed using the principles disclosed herein. The die module92can be coupled on a heat spreader or coupled to a DLA leadframe in a similar way as the individual die in this document, so in every implementation disclosed herein, a die module could be used in place of the discrete semiconductor die illustrated.

Referring toFIGS.11-13, implementations of DLA leadframes108,110that have heat spreaders112,114that extend over the material of the respective mold compound seals116,118and120,122is illustrated. These heat spreaders112,114may be used with DLA leadframes where the mold compound seals are either pre-formed or formed during the assembly process as will be disclosed herein. The ability to use these larger heat spreaders112,114that do not have a gap between the mold compound seals116,118,120,122means that a larger surface area for both the planar heat spreader112and pin fin heat spreader114is available for heat transfer. The side view of the DLA leadframe110fromFIG.12illustrated inFIG.13shows that extending the heat spreaders involves increasing the length of the bent portions of the leads124,126to provide additional clearance and adjusting the thickness of the semiconductor die128,130. Adjusting the thickness of the semiconductor die128,130may involve thinning the semiconductor die to a desired thickness thicker than that used in previously illustrated implementations or may involve, by non-limiting example, increasing a thickness of a die attach film, die bonding material, use of a spacer, or any combination thereof.

Returning toFIG.1, three physically separate semiconductor packages2,4,6are illustrated installed in an immersion cooling enclosure8. In contrast,FIG.15illustrates three semiconductor modules132,134,136physically and collectively coupled together through the same mold compound seals138,140in immersion cooling enclosure143. This implementation illustrates how the ability to couple semiconductor modules together through the mold compound seals can allow for the formation of semiconductor packages that contain various desired numbers of modules. For example, the three semiconductor packages ofFIG.1could collectively form a half bridge assembly when installed into the immersion cooling enclosure8. However, the semiconductor package implementation illustrated inFIG.15could form a three phase inverter module. The ability to market semiconductor packages formed from multiple DLA leadframes as a single package allows for more compact packages and use of space in immersion cooling enclosures. It also allows customers to order and receive packages that have a desired number of and arrangement in the package of DLA leadframe modules, permitting customizing in some implementations. The mold compound seals138,140may be formed using any method disclosed in this document.

Referring toFIG.16, a flow diagram with various components of a semiconductor package at various points in an implementation of a method of forming a semiconductor package is illustrated. In the figure, heat spreaders144(which may be any type disclosed in this document) are illustrated with solder printed areas146. Semiconductor die148,150are then mounted to the solder printed areas146as illustrated in the next diagram to the right. Additional solder printed areas152are then printed on the surface of the semiconductor die as illustrated in the next diagram to the right. The heat spreaders144and a DLA leadframe156are then bonded together at the semiconductor die148,150using a jig158designed to align the heat spreaders and DLA leadframe. Following the use of the jig158, the assembly is placed in a reflow device like reflow oven160to undergo reflow soldering and/or flux cleaning. Following reflow and flux cleaning, the assembly then undergoes a molding/partial molding process where the mold compound seals162,164are formed around the leads of the DLA leadframe156. Following (or prior to, in various method implementations) a coating step is carried out that applies a coating material like any disclosed to this document over desired portions of the DLA leadframe and/or heat spreaders. The resulting semiconductor package is now ready for installation into an immersion cooling enclosure. This particular method implementation is referred to as a mold-after process. This method implementation also involves the use of a single flat heat spreader with one pass through a jig. This method implementation could be used with heat spreaders that are sized to create a gap with the mold compound seals and with heat spreaders that are sized to not create a gap with the mold compound.

Referring toFIG.17, another implementation of a method of forming a semiconductor package is illustrated with various components drawn after various portions of the method are completed. Heat spreaders166are first illustrated following solder printing of solder printed areas168. Semiconductor die170,172are then coupled to the solder printed areas as illustrated in the next figure to the right. Additional solder printed areas174,176are then formed on the semiconductor die as illustrated in the next figure to the right. A jig178configured to align the heat spreaders166to DLA leadframe180is then used to couple the heat spreaders166to the leadframe prior to reflow soldering and/or flux cleaning illustrated by reflow oven182. Mold compound seals184,186are then formed around leads of the DLA leadframe180using a partial molding process followed by a solder printing process to the side of the heat spreaders that is opposite the side of the semiconductor die170,172to form solder printed areas188,190. An additional jig192is then used to align an additional flat heat spreader and/or a pin fin heat sink194to the heat spreaders prior to reflow and cleaning by reflow oven182. In this way, an additional pin fin heat sink or flat heat spreader can be coupled to the heat spreader166. In various method implementations, similar processes could be used to couple an additional pin fin heat sink or flat heat spreader to the side of the DLA leadframe180opposite the side the semiconductor die were coupled to or to couple two heat spreaders with die at the same time or in separate steps. A coating step prior to or after molding is also included in various method implementations similar to the method implementation illustrated inFIG.16. A wide variety of method versions and variations may be constructed using the principles disclosed in this document.

A variation of the method implementations illustrated inFIGS.16and17may be carried out by using a DLA leadframe at the jig step that already has mold compound seals formed thereon. This method implementation is referred to as a pre-molding method which may allow for some process efficiency/reduction of thermal budget as the mold compound forming step does not require the presence of the semiconductor die to be in place. Such a method variation may be utilized in either method implementation disclosed inFIGS.16and17and the others disclosed herein.

Referring toFIG.18, an implementation of a method of forming a semiconductor package is illustrated that does not include a heat spreader. In this implementation, the semiconductor die196,198are prepared through printing of solder printed regions200,202for aligning using jig204with a DLA leadframe206. The resulting assembly is then reflowed and cleaned (as necessary) as indicated by reflow oven208. Following the reflowing of the die, a coating step is carried out to apply a coating to just the semiconductor die, the semiconductor die and portions of the adjacent DLA leadframe, or to the semiconductor die and all of the DLA leadframe that will be exposed to the coolant. A molding step is then carried out to form mold compound seals210,212like those illustrated in the other method implementations disclosed herein. The resulting semiconductor package illustrated inFIG.19thus does not include a head spreader or pin fin heat sink, but relies on heat transfer directly from the semiconductor die196,198(through the coating) to the immersion coolant. Because the thickness of the coating can be set to be thinner than would ordinarily be possible were immersion cooling in a dielectric coolant not being used (i.e., thicknesses less than the breakdown voltage of the coating at the operational voltage of the packages) better heat transfer than is possible using fully molded packages may be possible in this operating environment.

Referring toFIG.20, three semiconductor packages214,216,218are illustrated following assembly with the structure of a coating material220,222,224illustrated in partial see through view. For the semiconductor packages214,216, the coating material220,222covers the exposed material of the DLA leadframe, semiconductor die, and any heat spreader(s) used in the package. In the semiconductor package218, the coating material224covers just a region of the DLA leadframe surrounding the semiconductor die226,228, leaving portions of the DLA leadframe exposed to the coolant232when placed in the immersion cooling enclosure230. A wide variety of coating configurations covering all or various exposed portions of the package assemblies can be constructed using the principles disclosed herein.

The coating220,222,224can provide a large number of effects, such as, by non-limiting example, corrosion protection; ion gettering to extend longevity; physical protection during assembly; mechanical protection from coolant flow across the surface; particle/flake protection from particles from the enclosure, package, and/or assembly debris; and other positive effects resulting from protection of the material of the DLA leadframe and/or semiconductor die from the coolant232.

While the coating220,222is illustrated inFIG.20as being applied to all coolant-exposed surfaces of the packages214,216, in other implementations, one, all, or any number of the surfaces/sides of the packages214,216may be coated. The use of the coating may also allow for protection of wirebonds formed on/to the DLA leadframes as the coating may fully cover the wirebonds and mechanically and physically protect them from coolant flow and particles. The use of the coating may also assist with repairing of any components of the package during assembly as components can be tested individually first prior to full assembly. While a single layer of coating material is illustrated inFIG.20, multiple layers of coating material(s) may be applied in various implementations each made of the same or different materials. The coating may be formed of any of a wide variety of materials, including, by non-limiting example, epoxies, novolac resins, polymer films, aluminum oxide, titanium nitride, mold compound, any combination thereof, or any other material capable of being applied to the printed circuit board and resistant to the coolant material. In particular implementations, the ability to use aluminum oxide and/or titanium nitride as the coating may result in a thin film that provides adequate protection in the environment of the immersion cooling enclosure while facilitating additional heat transfer from the printed circuit board itself. In various implementations where aluminum oxide and/or titanium nitride are used in the coating, the thickness of the coating may be, by non-limiting example, between about 1 micron to about 5 microns, between about 1.5 microns to about 3 microns, or about 1.5 microns.

In certain method implementations, where the coating has two or more layers, multiple application steps may be sequentially carried out (along with a corresponding number of curing/drying steps). For example, a gel-type material may be applied in a first coating process followed by application of a mold compound through a molding process to form a two layer coating material. In some implementations, the application process for the coating may be a printing process. In other implementations, the application process may be a molding process. In other implementations, the coating may be applied using, by non-limiting example, spraying, dipping, dispensing, chemical vapor deposition, sputtering, physical vapor deposition, film application, or any other method for forming a layer of material over a leadframe, heat spreader, and/or semiconductor die.

In various semiconductor package implementations, while the pin fin heat spreaders/flat heat spreaders illustrated herein are illustrated as being suspended in the coolant, one or more of the pin fin heat spreaders/flat heat spreaders may contact the wall(s) of the immersion cooling enclosure. In some, the heat spreaders may be directly coupled with/through the walls through screwing/bonding; in others the heat spreader(s) may merely be in physical contact with the wall(s) of the immersion cooling enclosure. In such implementations, where sufficient mechanical support is available by/against the walls of the immersion cooling enclosure using the heat spreaders, no additional fixture or attaching mechanism may be used to further secure the semiconductor package therein. Also, in such implementations, additional conductive heat transfer is possible to the wall(s) of the immersion cooling enclosure. A wide variety of configurations of heat spreaders for various semiconductor package implementations that involve contact with/bonding with the wall(s) of the immersion cooling enclosure may be devised using the principles disclosed in this document.

Because of the use of DLA substrates for the various package implementations disclosed herein, no direct bonded copper (DBC) or integrated metal substrates (IMS) may be needed to form the packages. The use of immersion cooling may enable dual sided cooling of each package which does not need the additional thermal performance of use of DBC or IMS substrates. This can have the effect of reducing package cost and manufacturing steps. Improved junction to foot thermal resistance (RthJF) can also be observed where two phase immersion cooling is utilized due to increased cooling efficiency.

In places where the description above refers to particular implementations of immersion direct cooling modules and implementing components, sub-components, methods and sub-methods, it should be readily apparent that a number of modifications may be made without departing from the spirit thereof and that these implementations, implementing components, sub-components, methods and sub-methods may be applied to other immersion direct cooling modules.