Abstract:
A heat sink for dissipating heat of an electronic device comprises a base, a first fin group comprising a plurality of fins stacked together, a second fin group comprising a plurality of fins and overlapping the first fin group, and a sinuous heat pipe attached to the base. The first and second fin groups are respectively engaged with the heat pipe twice at different locations of the first and second fin groups.

Description:
BACKGROUND 
   1. Field of the Invention 
   The present invention relates to a heat dissipation device for an electronic device, and particularly to a heat sink having at least a serpentine heat pipe. 
   2. Description of Related Art 
   Along with the continuous boom of computer industry, an electronic device, such as a central processing unit (CPU), is generating more and more heat. To ensure normal operation of the electronic device, accumulated heat must be removed away in time. Various heat sinks therefore have been devised to dissipate heat from electronic devices. 
   Conventionally, a typical heat sink conducts and dissipates heat by metal thermal conduction, and generally comprises a heat spreader for contacting the CPU for absorb heat therefrom and a plurality of fins provided on the heat spreader for dissipating heat. Limited to inherent characteristic of metal and cost consideration, heat conduction is not sufficiently fast and heat is mainly accumulated in the heat spreader. Heat dissipation capability of the heat sink does not satisfy the requirement of a high-powered heat-generating electronic device. 
   Nowadays, heat pipes, due to their excellent heat transfer performance, have been widely applied to heat dissipation of electronic devices. A heat pipe is a sealed metal pipe, with a low-pressure therein, in which a wick structure is formed and working fluid is filled. The heat pipe transfers heat from one end to the other by repeated phase change of working fluid between vapor and liquid states. A heat pipe has a high heat transfer performance due to fast circulation of working fluid and can transfer heat over a long distance. 
   Taiwan patent No. 532758 discloses a typical heat sink using heat pipes. The heat sink comprises a heat spreader for contacting a heat source, a plurality of parallel fins erecting on the heat spreader, and two U-shaped heat pipes attached to the heat spreader and extending through the fins to transfer heat from the heat spreader to the fins. The heat sink draws heat from the heat source via the heat spreader, one part of the heat is directly conducted upward to the fins; the remainder is indirectly transferred to the fins via the heat pipes. Generally, to maximize heat dissipation, the fins are made as large as possible; on the other hand, each heat pipe has a small cross-section size due to the limitations of manufacture cost and method; each of the heat pipes is brought to extend through a small circular hole defined in each fin and contact the fin. The contact between the heat pipes and the fins in the conventional heat sink is not adequate to offer a required heat transferring from the heat pipes to the fins. Vapors in heat pipes cannot sufficiently dissipate their heat to the fins through the contact between the fins and the heat pipes, whereby vapors in the heat pipes cannot totally condense into liquid and flow back to ends of the heat pipes thermally contacting with the heat spreader. Accordingly, utilization of heat transfer of the heat pipes is not high and heat dissipation of the heat sink is limited. 
   Therefore, it is desired to evolve an improved heat sink to overcome above-mentioned problems. 
   SUMMARY 
   Accordingly, what is needed is to provide an improved heat sink with heat pipes which has an excellent heat transfer performance by increasing contact area between the heat pipes and fins of the improved heat sink. 
   A heat sink for dissipating heat of an electronic device comprises a base, a first fin group comprising a plurality of fins stacked together, a second fin group comprising a plurality of fins and overlapping the first fin group, and a sinuous heat pipe attached to the base. The first and second fin groups are respectively engaged with the heat pipe twice at different locations of the first and second fin groups. The heat pipe has an evaporating portion engaging with the base and a bottom of the first fin group, a first condensing portion engaging with the a top of the first fin group and a bottom of the second fin group, and a second condensing portion engaging with an upper portion of the second fin group. The evaporating portion and the first and second condensing portions are parallel to each other. 
   Other advantages and novel features of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings, in which: 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an exploded, isometric view of a heat sink in accordance with a preferred embodiment of the present invention; 
       FIG. 2  is a partly assembled view of  FIG. 1 ; and 
       FIG. 3  is a fully assembled view of  FIG. 1 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Reference will now be made to the drawing figures to describe a heat sink in accordance with a preferred embodiment of the present invention. 
   Referring to  FIG. 1  to  FIG. 3 , the heat sink in accordance with the present invention is used for mounting to a CPU (not shown) to dissipate heat therefrom. The heat sink comprises a heat spreader  10 , a fin group  20  adhered to the heat spreader  10 , and two heat pipes  30  attached to the heat spreader  10  and the fin group  20 . 
   The heat spreader  10  is a flat metal plate, preferably made of copper, having a bottom face (not visible) for contacting the CPU and a top face (not labeled) opposite to the bottom face and defining two parallel grooves  12  each having a semi-circular section. 
   The fin group  20  is adhered to the top face of the heat spreader  10  and comprises a first fin group  24  and a second fin group  26 . The first fin group  24  contacts the heat spreader  10  and comprises a plurality of parallel first fins  240  stacked together. The first fin group  24  defines a pair of adjacent grooves  244  at a bottom thereof and a pair of separate grooves  246  at a top thereof. The first fin group  24  defines two sets of flanges  242  bent perpendicularly from top and bottom edges of the first fins  240  for spacing adjacent first fins  240 . The grooves  244  are so defined that they cooperate with the grooves  12  to form two straight pipe-shaped passages when the bottom flanges  242  are attached to the top face of the heat spreader  10 . When the first fin group  24  is mounted on the heat spreader  10 , the fins  240  extend vertically upwardly from the heat spreader  10 . The second fin group  26  is mounted on and overlaps the first fin group  24 . The second fin group  26  comprises a plurality of parallel second fins  260  stacked together. The second fin group  26  and the first fin group  24  are combined together in such a manner that the first fins  240  are parallel to the second fins  260 . The second fin group  26  defines a pair of separate grooves  266  at a bottom thereof and a pair of separate slots  264  adjacent a top thereof. Two sets of flanges  262  are bent perpendicularly from top and bottom edges of the second fins  260  for spacing adjacent second fins  260 . An annular collar  268  is perpendicularly extended from each second fin  260  at a circumference of each slot  264 . The grooves  266  are so defined that they cooperate with the grooves  246  to form two straight pipe-shaped passages when the bottom flanges  262  are engaged with the top flanges  242  of first fin group  24 . 
   The heat pipes  30  each are bent to have an S-shaped configuration and comprises three parallel horizontal portions, namely, an evaporating portion  32 , a first condensing portion  34  and a second condensing portion  36 , accordingly to their functions, respectively. 
   The evaporating portion  32  is accommodated in said passage between the heat spreader  10  and the first fin group  24  and is thermally engaged with the heat spreader  10  and the first fin group  24 . The evaporating portion  32  contacts with the flanges  242  at the bottom of the first fin group  24  forming a corresponding groove  244 . The first condensing portion  34  extends through the passage between the first fin group  24  and the second fin group  26 , and thermally engages with the first fin group  24  and the second fin group  26 . The first condensing portion  34  contacts with the flanges  242  at the top of the first fin group  24  forming a corresponding groove  246  and the flanges  262  at the bottom of the second fin group  26  forming a corresponding groove  266 . The second condensing portion  36  extends through the slot  264  of the second fin group  26  and thermally engages with the second fin group. The second condensing portion  36  contacts with the collars  268  of the second fin group  26  forming a corresponding slot  264 . 
   To ensure that heat can be effectively transferred from the heat spreader  10  to the heat pipes  30  and evenly distributed in the fin group  20 , the evaporating portions  32  of the heat pipes  30  are intimately attached to the middle portion of the heat spreader  10  and the condensing portions  34 ,  36  extend through the fin group  20 . 
   When applied to the CPU, the heat sink draws heat from the CPU via the heat spreader  10 . Part of the heat is straightway conducted to the bottom of the first fin group  24  for dissipation. The other of the heat is transferred along the heat pipes  30  to the top of the first fin group  24  and the bottom of the second fin group  26  via the first condensing portion  34 , and to the upper portion of the second fin group  26  via the second condensing portion  36 , and then dissipated to the surrounding air by the fins  240 ,  260 . Each fin group  24 ,  26  absorbs heat from the heat pipes  30  at multi-points. Heat transfer from the heat pipes  30  to the fin group  20  is remarkably increased due to increased contact area between the heat pipes and the fins; accordingly, heat dissipation efficiency of the heat sink is improved/enhanced. 
   It is easy to modify the heat sink in accordance with the present invention by removing the heat spreader  10  from the heat sink, whereby a direct heat transfer path is established between the evaporating portions  32  of the heat pipes  30  and the CPU. In this situation, it is preferred that the evaporating portions  32  of the heat pipes  30  is processed to have a flat face coplanar with the bottom flanges  242  of the first fin group  24  so as to have a better contact with the CPU. 
   It is also clearly seen that the heat pipes  30  can be shaped to other configuration and contacts the first and second fin groups  24 ,  26  more than twice for establishing a more effective heat transfer from the heat pipe  30  to the fin group  20 . 
   It is believed that the present invention and its advantages will be understood from the foregoing description, and it will be apparent that various changes may be made thereto without departing from the spirit and scope of the invention or sacrificing all of its material advantages, the examples hereinbefore described merely being preferred or exemplary embodiments of the invention.