Method and system for thermomechanically decoupling heatsink

Abstract

A structure and method of mounting a heat sink having a body and mounting points configured so as to connect to a mounting medium, at least one of the mounting points being configured to allow movement in a thermally-induced expansion direction.

Claims

1. A heat sink comprising: a body; and mounting points configured so as to connect to a mounting medium, wherein at least one of the mounting points is configured to allow movement in a thermally-induced expansion direction, wherein the mounting points include a mounting portion of the body and a mounting member configured to connect the mounting portion to the mounting medium, wherein the mounting member comprises a shaft including a non-cylindrical flexing portion between two cylindrical portions, and wherein the non-cylindrical flexing portion includes a plurality of flexing parts and a dampening material disposed on at least one of the plurality of flexing parts.

2. A heat sink according to claim 1, wherein the flexing portion is oriented such that a stiffness in a direction oriented along the thermally-induced expansion direction at the mounting member is less than a stiffness along an other direction at the mounting member.

3. A heat sink according to claim 1, wherein the non-cylindrical flexing portion includes a rectangular portion of the mounting member.

4. A heat sink according to claim 1, wherein the non-cylindrical flexing portion includes two flexing parts, the flexing parts comprising a material different from a material of the two cylinder portions.

5. A heat sink comprising: a body; and mounting points configured so as to connect to a mounting medium, wherein at least one of the mounting points is configured to allow movement in a thermally-induced expansion direction, wherein the mounting points include a mounting portion of the body and a mounting member configured to connect the mounting portion to the mounting medium, wherein the mounting member comprises a shaft including a non-cylindrical flexing portion between two cylindrical portions, and wherein the non-cylindrical flexing portion includes two flexing parts, the flexing parts comprising a material different from a material of the two cylinder portions.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The foregoing and other purposes, aspects and advantages will be better understood from the following detailed description of exemplary non-limiting embodiments of the invention with reference to the drawings, in which:

(2) FIG. 1 illustrates a related art heat sink mounting arrangement on an organic module carrying a microprocessor unit;

(3) FIG. 2 illustrates a related art heat sink mount for multiple surface mounted modules;

(4) FIGS. 3a and 3b show power cycle and thermal cycle profiles and the corresponding impact of deformation on TIM and solder joints;

(5) FIG. 4 illustrates an example of in-plane heat sink assembly set on top of a microprocessor and memory module using rigid mounting posts;

(6) FIG. 5 illustrates a plan view of the in-plane heat sink (shown transparent) supported by four corner mounting posts having a circular cross section;

(7) FIG. 6 illustrates an estimate of the temperature distribution after 10 seconds following a power on condition;

(8) FIG. 7 shows the thermomechanically driven deformation (exaggerated scale) of the system;

(9) FIGS. 8a, 8b and 8c schematically show the coupling mechanism as the temperature is increased or decreased from the stress free room temperature state;

(10) FIGS. 9a, 9b and 9c illustrate a modified mounting structure where a flexure element is added to a rigid mount. The soft bending axis is aligned with Y-axis, and the design is sensitive to X-directional shock applied to PCB;

(11) FIG. 10 illustrates an orientation of the soft bending axis of an exemplary embodiment for maximum flexibility along the thermal expansion vector while providing stiffness against X-Y shock;

(12) FIGS. 11a and 11b illustrate a cascaded heat sink with three mounting locations. To decouple, one point is made of no-slip mount and the remaining mounts are made with slip mounts;

(13) FIG. 12 illustrates various exemplary design options for mounting posts that could replace a standard mounting post to provide thermomechanical decoupling;

(14) FIG. 13 shows a simplified graph of cumulative strain energy vs. thermal cycles with and without heat sink decoupling;

(15) FIGS. 14a and 14b show an in-line heat sink providing thermal management for two surface mounted modules. The mismatch in coefficient of thermal expansion could drive cyclic strain of solder joints;

(16) FIG. 15 illustrates a slotted in-line heat sink that provides compliance in X-direction;

(17) FIGS. 16a and 16b illustrate the details of a slip-enabling heat sink mount. The second end of the heat sink (not shown) is secured by a no-slip design;

(18) FIGS. 17a and 17b illustrate another slip-enabled mount using split-post. The split-post provides compliance; and

(19) FIGS. 18a, 18b and 18c illustrate a close up view of the split-post. The split-post is attached to PCB similar to a surface mounted component.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE INVENTION

(20) The present invention addresses the limitations encountered in the conventional method of mounting a heat sink to a PCB. The conventional mounting posts essentially impose a no-slip boundary condition which strongly couples a heat sink to a printed circuit board detrimentally. Thermally driven expansion and contraction of key components are coupled and the ensuing deformation strains critical components unfavorably. The present invention provides several design methods that can constructively decouple a heat sink from PCB without compromising the shock robustness of the computer system.

(21) For example, in case of an in-plane heat sink, four rigid mounting posts can be replaced by four flexural posts. The flexural posts are oriented to present least resistance to thermal expansion vector at mounting point while providing high stiffness against linear shock along X or Y axis and rotational shock about Z-axis. Optionally, to damp out any dynamic oscillatory movement between heat sink and PCB, a dampening material can be sandwiched between the flexural elements.

(22) In the case of an in-line heat sink, two mounting post or rivets may be replaced by one mounting post and a flexural joint or by one rivet and a slip joint.

(23) In case of cascaded in-line heat sink, one joint can be made of a mounting post and all others are replaced by flexured joints. Alternatively, one joint is made of a rivet and all others by slip-enabled joints. In general, one fixed boundary condition and several slip boundary conditions are utilized.

(24) In case of an in-line heat sink, a rigid heat sink may be asymmetrically slotted to increase compliance along the X-axis while conventional mounting posts or rivets are used on both ends.

(25) It is, therefore, an exemplary feature of the present invention to provide a structure and method for a thermomechanically decoupled heat sink.

(26) Referring now to the drawings, and more particularly to FIGS. 8a-18, there are shown exemplary, non-limiting, embodiments of the method and structures according to the present invention.

(27) FIGS. 8a, 8b and 8c show the deformation process due to heating and cooling of a heat sink of an in-plane system. FIG. 8a shows a schematic representation of an assembled system represented by FIG. 4 in which there is no substantial residual stress imparted. FIG. 8b shows the power on heating cycle in which the heat sink 404 temperature rises rapidly ahead of the PCB 403, and forces the mounting posts 405 to move outwards. The bending moment thus created causes the PCB 403 and associated laminate and die into a convex shape. The power on process would cause a tensile strain on the TIM. Once the dwell period is reached, after an elapsed period of time, the expansion process would reach a steady state and no further deformation of TIM is anticipated.

(28) However, during the cool down or power down phase the heat sink 404 would contract much more quickly compared to PCB 403 and a reverse deformation occurs as shown by FIG. 8c. During the cooling phase, for example, below the stress free assembly temperature, the PCB 403, laminate and die could become concave and cause tearing of TIM at the center rather than at the edge of a die.

(29) An exemplary embodiment of the invention provides a design that can decouple the heat sink expansion effect from the PCB structure by utilizing flexured mounting posts.

(30) FIGS. 9a, 9b and 9c illustrate a basic concept applied to an in-plane heat sink 904. The flexured posts 905, for simplicity, are oriented in such a way that the easy or soft bending axis is along the Y-axis facilitating reduced compliance for X-directional motion. The flexure section 920 is illustrated in FIGS. 9a, 9b and 9c. Thus, any relative expansion of the heat sink 904 along X-direction will not force the PCB 903 to either bend or stretch.

(31) In addition, the Y-directional expansion of the heat sink 904 would still couple the PCB 903 because the bending stiffness of the post 905 for Y-movement is still high. However, any shock applied to PCB 903 in the X-direction will translate to substantial relative motion between the heat sink 904 and PCB 903, causing TIM 908 to tear. Thus, additional design innovation is required to mitigate these challenges.

(32) In order to simultaneously reduce the bending stiffness to thermal expansion while presenting substantial stiffness against X-Y shock, the invention takes advantage of the nature of expansion process. As the temperature rises, the heat sink 1004 mounting points (e.g., A, B, C and D) displace along the diagonal vectors as shown in FIG. 10. Therefore, the bending resistance needs to be reduced only along the diagonal directions at the mounting points (e.g., at A, B, C and D).

(33) By orienting the easy bending axis S orthogonal to the respective diagonals the thermally driven expansion process is decoupled from the PCB. In an exemplary embodiment, the soft (easy) bending axis S is orthogonal to the direction of expansion (e.g., Ea) at the mounting point (e.g., A). This technique can be done with any number of mounting posts 1005 on any shape heat sink 1004. In an exemplary embodiment the mounting posts 1005 support the heat sink 1004 in both the positive Z (up) direction and the negative Z (down) direction.

(34) Since the specification of shock direction is either along the X or Y axis, the four flexural mounting posts can be designed to offer the required stiffness against shock induced motion. A HS mounting post with a circular cross-section has a bending stiffness that is proportional to its moment of area:
I.sub.circle=()*pi*R.sup.4
where R is the radius of a circular post. Observe that the stiffness of this system is identical in all directions, and is independent of the mounting orientation. Thus, with a mounting post having a circular cross section it is impossible to develop a decoupling design while ensuring robustness against shock.

(35) However, if a flexured section 1020 as shown in FIG. 10 is introduced into the mounting post 1005 two distinct stiffness components are obtained. For a flexural section 1020 with a rectangular geometry (bt with b>t), as shown in FIG. 10, the soft axis S and hard axis H (call S-H as the principal axes) will have following moment of area:
I.sub.soft=(bt.sup.3)/12
I.sub.hard=(b.sup.3t)/12
Thus by maintaining b>>t (for example say by a factor of 10) a stiffness ratio of 100 between soft (easy) axis S and hard (stiff) axis H can be obtained.

(36) The orientation of flexures 1020 shown in FIG. 10 also enhances shock robustness. When the principal axis H of the hard-soft flexure 1020 is rotated with respect to the global X-Y axes by an angle a, then the effective stiffness along global X and Y axes are now given by the following equations:
I.sub.x=(bt/12)*(b.sup.2 Cos.sup.2a+t.sup.2 Sin.sup.2a)
I.sub.y=(bt/12)*(b.sup.2 Sin.sup.2a+t.sup.2 Cos.sup.2a)
For simplicity, for a system with a square heat sink, angle a will be 45 degrees, and the corresponding stiffness will be:
I.sub.x=I.sub.y=(b.sup.3t/24)
Thus, the stiffness against shock vs. thermomechanical coupling can be differentiated by a factor given by:
I.sub.soft/I.sub.x=2*(t/b).sup.2
For a case where (t/b)=( 1/10), the stiffness ratio can be as much as 1:50.

(37) Thus, it is possible to present two drastically different stiffnesses to the thermomechanical system and shock-response system using soft flexural posts 1020 whose orientations are made coincidental with the respective thermal expansion vector (e.g., Ea) of the mounting point (e.g., A).

(38) In an exemplary embodiment of the invention an in-plane heat sink can be mounted on a flexured mounting post. In an exemplary embodiment one or more components (e.g., microprocessors, memory, etc.) can be used.

(39) FIG. 12 shows an array of mounting post designs. Compared to a standard post 1251 with circular cross section, the same post can be thinned at the center zone to provide flexure action, which is referred to as a split post 1252. In another design, a flexure 1261 made of appropriate material with higher fatigue life can be embedded between two sections of a conventional circular post 1253. Finally, two parallel flexures 1262 with dampening material 1271 sandwiched in between circular posts 1254, as shown in FIG. 12, can be considered.

(40) FIG. 13 graphically illustrates the relationship between cumulative strain energy and number of strain cycles applied to a material. In one design, where the heat sink is coupled to the PCB, the expected life time is not met. By decoupling the heat sink, the strain energy dissipated per cycle is reduced and the desired life time is met.

(41) In FIG. 2, a representative 4-module in-line assembly on an organic printed circuit board 203 with a heat sink (HS) 204 was described. During a thermal cycle, the strain within the solder 209 of a module is generated as a result of external and internal thermal expansion/contract process. The strain generation mechanism for in-line as well as in-plane system is very similar, but TIM 208 failure in the in-line case is less severe because conductive tape instead of a cured thermal interface material can be inserted at the interfaces, and tearing is therefore not encountered. However, solder joint 209 fatigue failure can be a problem in the in-line heat sink systems.

(42) FIGS. 14a and 14b show two stages of an in-line CSP 1401 structure. Prior to the HS 1404 attachment at room temperature, the solder is assumed to be stress free. Once the HS 1404 is loaded and riveted at its ends (in order to retain the preload on the thermally conductive tape) substantial stress in the solder joint is generated. The stress due to preload is not strictly cyclic, and does not contribute to fatigue life of the material. However, once the assembly is completed, thermal cycling produces cyclic stress similar to that of in-plane heat sink system.

(43) Use of rivets 1405 reduces the cost of assembly. The rivets 1405, however, produce a nearly slip free joint which undesirably couples the heat sink 1404 to the PCB 1403. Observe that the mounting posts discussed under in-plane design can be interchangeably used for the in-line design. Only two mounting posts 1405 are required for the in-line assembly.

(44) A finite element model of an in-line heat sink with four surface mount modules was built to analyze and compare the effect heat sink mounting with and without slip boundary condition. It was estimated that a slip-enabled mounting could reduce the strain energy density in solder joints due to thermal cycling by as much as 25%.

(45) FIG. 16a shows a schematic of a HS 1604 assembly with a slip-joint used on a module 1601. The second end (not shown) is either riveted or mounted on a stiff post. The invention provides solution to the following two problems simultaneously. Clearance between the mounting post 1605 and HS 1604 allows the heat sink to move along X-direction without substantial resistance. Thus, the differential expansion between the organic board 1603 and the HS 1604 is decoupled.

(46) This can be accomplished by providing an elongated hole 1618 which can optionally be filled with a compliant material 1619. Note that the second mounting point with a no-slip design provides the required shock robustness along the X-direction. The clearance along the Y-direction is minimized so that Y-directional linear shock induced force is transferred to the heat sink as a balanced force through both mounting points. In an exemplary embodiment, the elongated hole 1618 allows relative movement of the heat sink in the X-direction but not in the Y-direction.

(47) The second feature is that the preloaded spring 1617 in the Z-direction presents a constant force on the thermal interface material 1608. Preloading through spring action is commonly used in the industry, but the shock robustness is largely overlooked by a loosely tolerant spring loaded mounting system.

(48) In an exemplary aspect, for shock robustness of an in-line system, the shock vector should pass through the center of gravity of the heat sink and the mounting point with one fixed boundary condition. Otherwise there can be a torque that will force relative movement between the heat sink and the PCB. In the event that there is a slip boundary condition along the X-axis and a fixed boundary condition along the Y axis (e.g., see FIG. 16b) using a design, for example as denoted by elongated hole 1618 the shock vector along the Y-axis is balanced by forces generated by mounting posts on both sides. In this case, the shock vector is not passing through the center of gravity of the fixed boundary condition. When using flexures the same general principle may apply. The soft direction of displacement should be along the X-axis while the hard or stiff direction should be along the Y-axis.

(49) FIG. 17a shows a snap-on method. This solution facilitates easy removal of HS 1704 for re-workability. An exemplary aspect of this configuration is that the split-post 1705 is supported by a platform 1717 that is surface mounted to the organic board 1703 like any other electronic component. It distributes local stress on the PCB 1703 that may result from other forms of mechanical mounting operation. FIG. 17b is a schematic representation of a snap-on fixture 1705 subsequently used in other illustrations.

(50) Also illustrated is a spring or wave washer 1716 to impart a preload. Even though the two mounting posts (1705) appear identical, in order to survive shock, one post should be stiffer than the other to bear shock induced force while the flexible second post accommodates the thermally-induced expansion. The flexible post can be designed to have higher flexibility in X-direction (for expansion) and higher stiffness in Y-direction to bear part of the Y-direction shock and rotational shock load about Z-axis.

(51) FIG. 18 shows more details of the snap-on method. By adjusting the relative stiffness of the snap post, for example by adjusting the thickness or width of each leg 1720, it is possible to have more or less stiffness in any desired direction. In addition, the four posts 1720 can be rotated or manufactured in different directions (e.g., coincidental with the X and Y direction, or 45 degrees from the X and Y direction) when

(52) FIGS. 11a and 11b show a cascade of in-line heat sinks to support a larger number of surface mounted modules. The examples shown support eight surface mounted modules. As illustrated in FIG. 11b, the center mounting point is retained with a no-slip joint, and the remaining mounting points are slip-enabled. In the illustrated embodiment of FIG. 11b, there would be clearance in the X-direction on both end through holes 1118.

(53) FIG. 15 shows a modified in-line heat sink 1504 with multiple slots 1523 cut near one mounting location. Slots 1523 are cut in the Y-direction and they provide compliance in the X-direction without impeding the thermal performance of the heat sink 1504. By providing compliance in the X-direction, the thermo-mechanical coupling is minimized while allowing conventional mounting methods, such as low cost rivets 1521, to be employed.

(54) While the invention has been described in terms of exemplary embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the appended claims.

(55) Further, it is noted that Applicant's intent is to encompass equivalents of all claim elements, even if amended later during prosecution.