Patent Application
20070295482
The invention will be better understood from a reading of the following detailed description, taken in conjunction with the accompanying figures in the drawings in which:
For simplicity and clarity of illustration, the drawing figures illustrate the general manner of construction, and descriptions and details of well-known features and techniques may be omitted to avoid unnecessarily obscuring the invention. Additionally, elements in the drawing figures are not necessarily drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help improve understanding of embodiments of the present invention. The same reference numerals in different figures denote the same elements.
The terms “first,” “second,” “third,” “fourth,” and the like in the description and in the claims, if any, are used for distinguishing between similar elements and not necessarily for describing a particular sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances such that the embodiments of the invention described herein are, for example, capable of operation in sequences other than those illustrated or otherwise described herein. Furthermore, the terms “comprise,” “include,” “have,” and any variations thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements is not necessarily limited to those elements, but may include other elements not expressly listed or inherent to such process, method, article, or apparatus.
The terms “left,” “right,” “front,” “back,” “top,” “bottom,” “over,” “under,” and the like in the description and in the claims, if any, are used for descriptive purposes and not necessarily for describing permanent relative positions. It is to be understood that the terms so used are interchangeable under appropriate circumstances such that the embodiments of the invention described herein are, for example, capable of operation in other orientations than those illustrated or otherwise described herein. The term “coupled,” as used herein, is defined as directly or indirectly connected in an electrical or non-electrical manner.
In one embodiment of the invention, a heat spreader for use in conjunction with a semiconducting device comprises a body having a first surface and a second surface that are spaced apart from each other, a first metal layer coating substantially all of the body, a second metal layer over a portion of the first metal layer at the first surface, and a lip protruding from the second surface. In a particular manifestation, the heat spreader comprises a microchannel comprising a base plate and a cover disposed over the base plate, where the base plate comprises spaced-apart first and second surfaces and a plurality of fins at the second surface, and the cover comprises a third surface having a cavity therein capable of receiving the plurality of fins, a fourth surface spaced apart from the third surface, and a lip or other grip adjacent to the fourth surface.
Referring now to the drawings,
As illustrated in
In one embodiment, body 110 comprises copper. In the same or another embodiment, metal layer 120 comprises nickel and metal layer 130 comprises gold. As an example, metal layer 120 can have a thickness of approximately 3 micrometers and metal layer 130 can have a thickness of approximately 0.2 micrometers. A reason for including metal layer 120 of nickel, as known in the art, is that a nickel layer can act as a diffusion barrier to prevent the gold from diffusing into the copper. As is also known in the art, a gold layer, because gold does not oxidize, provides a good wetting surface for solder that may be applied during the assembly of a package containing heat spreader 100.
The presence of lip 140 provides several advantages. Among them are that lip 140 serves as a handle that may be gripped while heat spreader 100 is being worked with, that lip 140 acts as a stiffener that prevents heat spreader 100 from flexing or bending, and that lip 140 acts as a bonding surface, a stand-off, or a fiducial for a manifold or the like. Additional advantages provided by lip 140 will be discussed in detail below.
It was mentioned above that in one embodiment of the invention heat spreader 100 is a microchannel. Accordingly,
In the embodiment illustrated in
Microchannel 200 may still further comprise a metal layer 230 that coats substantially all of surfaces 211, 221, and 223 and a metal layer 240 over a portion of metal layer 230 at or near surface 211, as illustrated. As an example, metal layers 230 and 240 can be similar to, respectively, metal layers 120 and 130 that were shown in
Cover 220, plurality of fins 213, and base plate 210 may be made of copper, silicon, aluminum, diamond, tungsten, silver, or the like, or of composites or alloys of the foregoing materials. As known in the art, copper is less brittle, and therefore easier to work with, than silicon so that a microchannel having the foregoing or other components made of copper instead of silicon may possibly lead to better thermal and other performance of the microchannel. As an example, the greater ease of workability of copper means that adjacent ones of plurality of fins 213 may be spaced apart from each other by a distance of between approximately 50 and 100 micrometers. Such close spacing of adjacent fins enables an increased number of fins in a given volume, leading to an increased surface area and a corresponding increase in heat transfer efficiency when compared to a silicon microchannel.
In the embodiment illustrated in
One such other embodiment is illustrated in
As illustrated in
The foregoing discussion has alluded to several potential advantages made possible by the presence of grips or similar features on an upper surface of a microchannel cover or the like. Some of the potential advantages represent solutions to problems that may arise in the manufacture of microchannels or other heat spreaders. These potential advantages, together with a description of some of the problems they may solve, will now be discussed in greater detail.
In a typical arrangement, a heat spreader is bonded to a die or another package component using a solder thermal interface material (TIM) process or the like. The bonding surface of the heat spreader, corresponding for example to surface 111 of heat spreader 100 and to surface 221 of microchannel 200, must be substantially flat in order to prevent the creation of voids in the solder TIM bondline after package assembly. In this context, “substantially flat” can mean that no height variation greater than approximately 35 micrometers is allowed. Any warping that takes the flatness of the bonding surface beyond that upper spec limit of approximately 35 micrometers can lead to increased and unacceptable solder voiding and can significantly decrease thermal performance.
In applications or embodiments where the heat spreader is a microchannel certain additional complications can arise, especially where the microchannel is made of copper. For example, cutting the grooves in the base of a copper microchannel (in a process that creates the fins) causes the underlying copper plate to warp during manufacturing. In addition, the cutting process imparts a large amount of plastic deformation, or cold work, into the copper. The other components of the copper microchannel, including the cover, are typically copper stampings that have also experienced a large amount of plastic deformation. Unfortunately, especially for large parts such as premium server products for which copper microchannels as large as approximately 50 by 50 millimeters or even 100 by 100 millimeters may be used, very little plastic deformation is required in order to cause warpage beyond the upper spec limit for flatness.
Copper has a re-crystallization temperature that depends on the amount of plastic deformation to which the copper has been exposed. Copper that has undergone a significant degree of plastic deformation, which for the reasons set forth above includes copper in a typical copper microchannel, can easily have a re-crystallization temperature in the range of approximately 250 to 300 degrees Celsius. A re-crystallization temperature in this range is problematic for copper microchannels because: (1) warpage and deformation of copper components are known to occur when copper re-crystallizes; and (2) copper microchannels typically undergo a brazing process during their manufacture at temperatures that, if not yet precisely defined, almost certainly exceed 250 degrees Celsius. Differences in the coefficients of thermal expansion (CTE) between the copper and the braze material, which may be silver paste or the like, may cause further dimensional changes as the microchannel cools from the brazing temperature to room temperature. The result is that the sub-components of a copper microchannel, including the base plate and the cover, will very likely fall outside of the upper spec limit for flatness after manufacturing, meaning that, after normal manufacturing, the microchannel will very likely not be sufficiently flat for solder TIM voiding to be avoided.
When the base plate and the cover of the microchannel are brazed together, drips of braze may occasionally end up on the bonding surface of the microchannel, which can interfere with the bondline on the assembled package. In addition, the exposed copper will likely be oxidized during brazing, making subsequent plating operations, as with nickel and gold, more difficult.
The foregoing and other problems may be overcome, and the bonding surface of the microchannel may be brought within the upper spec limit for flatness, through the performance of a grinding or polishing operation that follows the manufacture of the microchannel. It is the presence of grips such as those described above that makes such grinding or polishing possible. As mentioned earlier, the grips act as a handle such that the microchannel may be securely grasped during grinding or polishing and also act as a stiffener that prevents the microchannel from flexing. Note that if the grips were placed on the bonding surface they would interfere with the grinding or polishing operations; hence the placement of the grips on the surface opposite the bonding surface. Also note that either or both of the grinding and polishing operations may be used.
Another advantage attributable to the grips is that grinding or polishing the bonding surface will remove any drips of braze and any oxidized copper from that surface that resulted from the brazing operation. Furthermore, grinding or polishing the bonding surface of the microchannel or other heat spreader removes some of the bonding surface, resulting in a thinned bonding surface which will improve thermal performance. As an example, with reference to
Yet another advantage of the grips is possible in an embodiment like those shown in
A step 620 of method 600 is to form a grip adjacent to the second surface, which surface, at least in one embodiment, is opposite the bonding surface of the heat spreader. As an example, the grip can be similar to one or more of lip 140, grip 224, grip 412, and grip 512. As another example, the grip can be created using techniques of stamping, casting, grinding, machining, or the like.
A step 630 of method 600 is to flatten the first surface. As an example, step 630 can comprise at least one of polishing the first surface and grinding the first surface. As another example, step 630 is performed while the heat spreader is being held onto by the grip formed in step 620. In one embodiment, step 630 also removes braze material and/or oxide from the first surface.
A step 720 of method 700 is to provide a cover comprising a third surface having a cavity therein capable of receiving the plurality of fins, a fourth surface spaced apart from the third surface, and a grip adjacent to the fourth surface. As an example, the cover, the third surface, the cavity, the fourth surface, and the grip can be similar to, respectively, cover 220, surface 221, cavity 222, surface 223, and grip 224, all of which were first shown in
A step 730 of method 700 is to dispose the cover over the base plate. As an example, step 730 can comprise positioning the cover over the base plate such that the plurality of fins are in the cavity, placing a brazing material on at least a portion of at least one of the base plate and the cover, and brazing the base plate and the cover to each other.
A step 740 of method 700 is to flatten the first surface. As an example, step 740 can comprise at least one of polishing the first surface and grinding the first surface. As another example, step 740 is performed while the heat spreader is being held onto by the grip provided in step 720. In one embodiment, step 740 also removes from the first surface braze material and/or oxide that may be there as a result of the performance of step 730 or another step. In the same or another embodiment step 740 comprises flattening the first surface to a flatness of at least approximately 35 micrometers.
A step 750 of method 700 is to coat at least a portion of the base plate with a first metal layer. As an example, the first metal layer can be similar to metal layer 120 in
A step 760 of method 700 is to place a second metal layer over a portion of the first metal layer. As an example, the second metal layer can be similar to metal layer 130 in
A step 920 of method 900 is to provide a cointube capable of receiving at least the first microchannel and the second microchannel. As an example, the cointube can be similar to cointube 800 shown in
A step 930 of method 900 is to place the first microchannel in the cointube and a step 940 of method 900 is to place the second microchannel in the cointube above the first microchannel such that the third surface of the second microchannel contacts the lip of the first microchannel. In this way the first surface of the second microchannel, which is where the second metal layer of the second microchannel is located, will not contact the first microchannel. Thus, the second metal layer is protected so that it is not scratched or damaged during transport. Such protection is important because scratches or the like on the second metal layer are likely to lead to problems with solderability.
System 1000 further comprises a cooling loop 1030 that has a portion 1031 adjacent to microchannel 1020 and a portion 1032 spaced apart from portion 1031, and a cooling device 1040, such as a cooling fan or the like, positioned adjacent to portion 1032 in which a coolant (not shown) circulates. In one embodiment system 1000 still further comprises a pump 1050 coupled to the cooling loop.
Although the invention has been described with reference to specific embodiments, it will be understood by those skilled in the art that various changes may be made without departing from the spirit or scope of the invention. Accordingly, the disclosure of embodiments of the invention is intended to be illustrative of the scope of the invention and is not intended to be limiting. It is intended that the scope of the invention shall be limited only to the extent required by the appended claims. For example, to one of ordinary skill in the art, it will be readily apparent that the heat spreader and related structures and methods discussed herein may be implemented in a variety of embodiments, and that the foregoing discussion of certain of these embodiments does not necessarily represent a complete description of all possible embodiments.
Additionally, benefits, other advantages, and solutions to problems have been described with regard to specific embodiments. The benefits, advantages, solutions to problems, and any element or elements that may cause any benefit, advantage, or solution to occur or become more pronounced, however, are not to be construed as critical, required, or essential features or elements of any or all of the claims.
Moreover, embodiments and limitations disclosed herein are not dedicated to the public under the doctrine of dedication if the embodiments and/or limitations: (1) are not expressly claimed in the claims; and (2) are or are potentially equivalents of express elements and/or limitations in the claims under the doctrine of equivalents.
