The present invention relates to thermal control of electronics generally and more specifically to heat spreaders.
Thermal control of electronic systems is important to make sure that the systems can perform properly for their specified life cycles. If solid state devices are permitted to exceed their maximum allowable operating temperatures, equipment life may be drastically reduced.
The two main mechanisms for thermal control in terrestrial electronics are convection and conduction. Convection uses air flow around a component to remove heat from the component. A heat sink including a plurality of fins may be used to increase the heat removal rate. Conduction spreads the heat energy across the device (chip, package, circuit board, or the like.). Heat spreaders are frequently used to enhance conduction, to provide a more uniform temperature distribution across a device. A heat spreader typically includes a high conductivity thermal pad, made of a material such as copper or aluminum.
Heat spreaders have been used within packages, such as flip chip ball grid array (FC-BGA) packages.
For high power applications, conventional heat spreaders are limited in meeting both thermal performance and reliability specifications. Non-uniform power distribution and density can strongly affect thermal control of junctions and cause failure of chip functionality. When power is non-uniform and power density is high, existing methods do not have sufficient thermal conductivity and surface contact area to achieve thermal performance and are not able to address the hot spot issue. An improved heat spreader is desired.
In some embodiments, a heat spreader has first and second regions. The second region lies substantially in a plane. At least a portion of the first region of the heat spreader has an out-of-plane dimension greater than an out-of-plane dimension of the second region. The heat spreader is sized and shaped to be placed with the first region of the heat spreader proximate to a first region of a semiconductor die that dissipates more power than a second region of the die distal from the first region die during operation.
In some embodiments, a package comprises a semiconductor die and a heat spreader. The semiconductor die has first and second regions. The first region dissipates more power than the second region during operation. The die has a surface in a plane. The heat spreader has first and second regions. The first region of the heat spreader is proximate to the first region of the die. The second region of the heat spreader is distal from the first region of the die. At least a portion of the first region of the heat spreader has an out-of-plane dimension greater than an out-of-plane dimension of the second region of the heat spreader.
In some embodiments, a packaging method comprises providing a semiconductor die having first and second regions, and coupling a heat spreader to the die. The first region dissipates more power than the second region during operation. The heat spreader has first and second regions. The first region of the heat spreader is proximate to the first region of the die. The die has a surface in a plane. At least a portion of the first region of the heat spreader has an out-of-plane dimension greater than an out-of-plane dimension of the second region.
This description of the exemplary embodiments is intended to be read in connection with the accompanying drawings, which are to be considered part of the entire written description. In the description, relative terms such as “lower,” “upper,” “horizontal,” “vertical,”, “above,” “below,” “up,” “down,” “top” and “bottom” as well as derivative thereof (e.g., “horizontally,” “downwardly,” “upwardly,” etc.) should be construed to refer to the orientation as then described or as shown in the drawing under discussion. These relative terms are for convenience of description and do not require that the apparatus be constructed or operated in a particular orientation. Terms concerning attachments, coupling and the like, such as “connected” and “interconnected,” refer to a relationship wherein structures are secured or attached to one another either directly or indirectly through intervening structures, as well as both movable or rigid attachments or relationships, unless expressly described otherwise.
The package 300 includes a package substrate 304, to which the heat spreader stiffener ring 310 is bonded (e.g., by solder or conductive adhesive) before baking, preventing substrate warpage. One preferred thermally conductive material is a conductive adhesive material, such as a silver filled epoxy. In the example, the substrate 302 is an organic substrate, such as a glass/epoxy substrate. The substrate may have a plurality of levels, with electrical paths between layers provided by interconnect vias (not shown). The die 302 is positioned with its active face facing the package substrate 304, and a plurality of solder balls 306 on the die are reflowed to form electrical and mechanical connections, so that the integrated circuit die 302 is flip-chip bonded to the substrate 304. The space between the die 302 and substrate 304 is flushed with a solvent, such as water, and an underfill 308 is applied to prevent loss of contact during thermal cycling. The underfill material 308 may be an epoxy or other known underfill material. The top section 332 of the heat spreader is interfaced to the rear surface of the die 302 using a thermal interface material 314, such as an adhesive, a conductive adhesive such as a silver filled epoxy, thermal grease, solder or a phase change material. The preferred material 314 for connecting the top of the heatspreader to the rear surface of the die depends on the chip power levels and therefore, epoxy, thermal grease and phase change material are all preferred for their respective power levels. The top section 332 of the heat spreader is also bonded to the ring 310 of the heat spreader using solder or a conductive adhesive such as a silver filled epoxy. The substrate 104 has a plurality of solder balls 116, for forming the mechanical and electrical connection between the package 100 and a printed circuit board (PCB), not shown.
The die 302 has a first region 303 and a second region 309 (best seen in
The heat spreader 332 has a first region 335 and a second region 331. The first region 335 of the heat spreader 332 is proximate to the first region 303 of the die 302. The second region 331 of the heat spreader 332 is distal from the first region 303 of the die 302. At least a portion 333 of the first region 335 of the heat spreader 332 has an out-of-plane dimension T1 (in the Z direction) greater than an out-of-plane dimension T2 of the second region 331.
The portion 333 of the first region may be a single contiguous area (as discussed below with reference to
A layer 314 of a thermal interface material is provided between the die 302 and the heat spreader 332. The at least one protrusion 333 protrudes at least partially through the thermal interface material. The protrusions 333 provide a low-thermal-resistance path between the heat sink 332 and the hot spot in the first region 303 of the die 302. This increases the rate at which energy can be conducted between the hot spot and the heat spreader 332, and decreases the temperature difference between the hot spot and the heat spreader, for any given ambient temperature and amount of power dissipated by the die 302.
One of ordinary skill in the art understands that the improvement in the thermal resistance between the heat spreader and die is greatest if the protrusion(s) 333 extend(s) as close as possible to the rear face of the die 302. Preferably, the length of the protrusion(s) 333 is selected to be sufficiently short to leave a small gap between the protrusions 333 and the die 302, to accommodate any expected thermal expansion or stress deflection in the die 302. In some embodiments, the protrusion(s) 333 may extend all the way to the rear of the die.
In some embodiments, the heat spreader 310, 332 is made of copper. Heat spreaders as described above can provide the desired metal columns or bumps with high thermal conductivity (such as copper with conductivity K=395 w/m-k and Cu C with K=350 W/m-k). In other embodiments, other high conductivity materials may be used for the heat spreader, where the material has a coefficient of thermal expansion compatible with that of the die 302. Although a material with a substantially different coefficient of thermal expansion (such as aluminum) could be used for the heatspreader 411, an elastic thermal interface material would then be used to accommodate the expansion of the heatspreader, and still conduct heat well.
Although exemplary shapes are shown for the protrusions 323, 333 in the first region of the heat spreader, a variety of other shapes may be used, including, but not limited to, a prism having any desired number of sides, a pyramid having any desired number of sides, a cone, a frustum, an elliptic paraboloid, an elliptic cylinder, or other arbitrary three-dimensional shape.
Although two packages 300 and 400 having two heat spreader configurations are described above, the invention is not limited to these heat spreader configurations. A region having a greater out-of-plane dimension, such as a thicker region (e.g., 305 as shown in
Heat spreaders as described above may be fabricated using conventional technologies, such as molding, stamping, or extrusion.
Although the invention has been described in terms of exemplary embodiments, it is not limited thereto. Rather, the appended claims should be construed broadly, to include other variants and embodiments of the invention, which may be made by those skilled in the art without departing from the scope and range of equivalents of the invention.