fig1 is a cross - sectional view illustrating a preferred embodiment of a variable thermal exchanger or flow control device or variable cooling heat sink 23 , of this invention , with at least one variable thermal component 13 , in a minimum separation position from heat sink or spreader 10 . the variable thermal component 13 , is preferably secured to the heat sink 10 , by at least one securing device 16 . it is preferred that the variable thermal component 13 , is preferably secured to the heat sink 10 , by the securing device 16 , in the center , so as to allow movement at each end of the variable thermal component 13 . it is also preferred that the variable thermal component 13 , further comprises of at least one flap , such as , a first flap 12 , a second flap 14 , and so on . for some applications the thermal coefficient of expansion ( tce ) of the first flap 12 , could be the same as the tce for the second flap 14 . however , for some other applications the tce for the first flap 12 , could be different than the tce for the second flap 14 . similarly , for some applications the physical dimension of the first flap 12 , could be the same as that for the second flap 14 . however , for some other applications the physical dimension of the first flap 12 , could be different than that for the second flap 14 . material for the heat sink or spreader 10 , is typically selected from a group comprising aluminum , aluminum nitride , copper , copper - tungsten composites , silicon carbide , thermally conductive polymers , to name a few . material for the variable thermal component 13 , is typically selected from a group comprising aluminum , steel , copper , brass , nickel , iron , polymers , metal composites , to name a few . the securing device 16 , is typically selected from a group comprising nail , screw , rivet , adhesive , to name a few , or it can be made as an integral part of the flap 12 and / or 14 . in a preferred embodiment of the present invention , the flow control device 23 , comprises of at least two plates or flaps 12 and 14 , with different thermal coefficient of expansion or tce , and joined over one surface to form a bimetallic strip 13 , in the manner shown in fig1 . it should be noted that the term “ bimetallic strip ” is used even when one or more materials forming the strip are not metal , or when more than two materials are used . fig2 is a cross - sectional view illustrating a preferred embodiment of the invention , with the inventive flaps 12 and 14 , of the variable cooling component 13 , in an intermediate separation position from the heat sink 10 . fig3 is a cross - sectional view illustrating a preferred embodiment of the invention , with the inventive flaps 12 and 14 ., of the variable cooling heat sink 23 , in a maximum separation position from the heat sink 10 . in this maximum separation position the flaps 12 and / or 14 , would be able to stop or divert any fluid 33 , and / or channel any fluid flow 35 . this diverting and / or channeling of the fluid flow 33 and / or 35 , will of course depend upon the other environmental constraints in the vicinity . an alternative use of the flow control device 23 , on heat sinks can also be advantageous when cooling heat dissipating devices with natural convention . since natural convection cooling does not uses forced flow of a fluid to provide the required cooling capability , the action of a thermo - mechanically activated device is directed instead to modifying the heat sink area and / or producing interference on the natural convective flow over the heat sink . one such arrangement is shown in fig4 where the heat spreader 10 , is assembled with a plurality of flow control devices 23 . in this particular example , the thermo - mechanically activated means 13 , is made from at least two materials with different tce , where the material with high tce flap 12 , is placed near the heat spreader 10 , and the material with lower tce flap 14 , is at the other side of the bi - material device 13 . in this particular case , the flow control device 13 , is expected to be flat at room temperature and bend upwards when the temperature of the heat spreader 10 , increases , consequently increasing the heat exchange area and heat dissipating rate of the assembly . similarly , many other mechanical arrangements can be devised to provide the particular thermal control function described here to modulate natural convection cooling without deviating from the principles of this invention . the flaps 12 and 14 , have been shown to be of the same size , however for some applications the flap 12 could be longer than flap 14 , and for others the flap 12 , could be shorter than the flap 14 . similarly , the width and / or thickness of the flaps 12 and 14 , could be the same or it could be different . fluid cooled module 50 , such as , an air cooled module 50 , as shown in fig5 is used here to further illustrate the salient features of the present invention . this assembly 50 , includes the following main components : a first level electronic package or substrate 54 , an integrated circuit chip 56 , a thermal exchange device or heat sink 51 , having at least one flow control device 23 . the integrated circuit chip 56 , is electrically attached to the first level package 54 , with the use of solder balls 55 , and thermally attached to the heat sink 51 , with at least one thermal interface or media 57 , such as , a thermal grease 57 . the heat sink 51 , shown in fig5 also includes a heat spreading base 10 , with cooling fins 58 . the cooling fins 58 , define cooling fluid channels or passages 59 . in the particular embodiment of this invention shown in fig5 each cooling channel 59 , could have at least one flow control device 23 , 52 and 53 , which is used to modulate the air velocity distribution in each cooling channel 59 . the flow control device 23 , 52 and 53 , could have customized flow control flaps working in harmony or independently from each other . the flow control device 23 , shown in fig5 is fastened to the heat spreading base 10 , by at least one securing device 16 , such as , a screw 16 . for this application the first plate or flap 12 , has a lower tce than the second plate or flap 14 , and is also longer . the initial shape of the flow control device 23 , is directly dependent on the operating temperature of the cooling fluid used to cool the assembly and the expected operating temperature of the chip when dissipating maximum power . in the particular case of an air cooled device with an expected operating temperature of about 75 ° c ., the flow control device 23 , shown in fig5 has the expected shape when the assembly is at room temperature , mainly to block air flow through the heat sink 51 , and cooling channels 59 . by blocking the flow at low temperatures , the flow control device 23 , effectively decreases the heat dissipation capability of the heat sink 50 , and increases the operating temperature of the integrated circuit chip 56 , when running at low power dissipation rates . then , as the temperature of the assembly increases , the bimetallic strip 23 , 52 and 53 , is expected to become flat and allow air flow through the cooling channels 59 , of the heat sink 51 , when the temperature reaches approximately 70 ° c ., for the bimetallic strip 23 , 52 and 53 . the use of a thermally - activated flow control device 23 , 52 and 53 , as shown in fig5 allows then the control of the assembly operating point when the chip 56 , is on stand - by and dissipating a low power level such as the operating power . the increase in operating temperature , i . e ., when the chip 56 , uses power management will reduce the assembly fatigue caused by power - on thermal cycles . in this case , the reduction in power - on thermal cycles will be approximately proportional , depending on the flow control device 23 , design used by the heat sink 51 . the design and use of a flow control device 23 , in a heat sink 51 , becomes then an important design factor when estimating the useful life of the assembly . both flow control device 23 , structure and operating function described have been selected for the sole purpose of illustrate the functionality of the present invention , but it is understood that the same behavior can be achieved with many alternative structures and operating functions without departing from the teachings of this invention . for example , the flow control device 23 , can be built from only one material 12 , instead of using a bimetallic strip structure 13 . similarly , the material selected for the component 13 , could have different tce than the material used to build the heat spreading base 10 . the flow blockage can be accomplished by using the flow control device thermal induced movement to mechanically move a flow controlling flap . also , it is possible to control the flow in all cooling channels 59 , with only one flow control device 23 , if all the channels have mechanically interconnected flow controlling flaps . alternatively , the use of multiple flow control devices 23 , in each channel 59 , can easily increase the flow blockage efficiency . similarly , the use of a plate ( not shown ) to cover the heat sink open area , thus increasing the cooling channel definition , can also increase the effectiveness of the flow control device 23 , to block or enhance air flow . the flow control device 23 , as shown in fig5 consist of two basic units , namely , the first or upstream / downstream flap 52 , and the second or downstream / upstream flap 53 . clearly , only one of the two basic units is the minimum structure required to provide the flow control function described in this invention . however , a single flap or a half - flap may provide a lower flow blockage efficiency . a further advantage of using heat sinks with flow control devices 23 , as shown in fig5 is its capability to modulate the power dissipation rate of a heat sink locally , and have high dissipation rate on one side of the heat sink where more energy is being dissipated , while providing lower power dissipation capability on the other side of the heat sink where the heat dissipating device requires lower power consumption to operate , i . e ., the thermal exchanger 23 , 52 and 53 , can work in harmony with each other or independently . this capability , shown in the heat sink and flow control device of fig5 is available to the heat sink 51 , when multiple and independent flow control devices 23 , are placed on the heat sink to control the local cooling flow distribution . for example , the cooling flow going into a first cooling channel 59 , is modulated by the first flow control device flaps 52 and 53 , while the cooling flow going into the second cooling channel 59 , is modulated by the second flow control device flaps 52 and 53 . if the flow control device in any of the cooling channels 59 , is mechanically uncoupled with the flow control device 23 , in other cooling channel 59 , then each flow control device 23 , will respond independently to the local flow control device temperature , which in this case is determined by the heat sink / spreader temperature distribution . in this case , the heat sink of fig5 will react by changing its local cooling capability in each cooling channel and try to equalize the heat sink spreader temperature distribution . this thermal characteristic can become a critical need when the heat dissipating device has a severe non - uniform power dissipation distribution . a particular application of the advantageous capability of a heat sink with flow control devices to modulate its local heat dissipation rate is shown in fig6 which shows the use of this technology for the cooling of a multi - chip module ( mcm ). here , a fluid cooled module 60 , having multi - chip module 64 , contains at least one heat dissipating chip 66 , is cooled with a heat spreader 68 , such as , a cold plate 68 . typically , the cold plate 68 , is cooled with a fluid , such as , water , antifreeze , to name a few , which flows into the cold plate 68 , through flow inlet / outlet 74 , and flow outlet / inlet 76 , into at least one fluid chamber 69 , such as , a cooling fluid chamber 69 . the internal geometry of the cold plate 68 , typically is much more complex than the details given in fig6 and have been simplified here for the sole purpose of describing the teachings of this invention . however , it is understood that geometric departures from fig6 shown here are obvious and expected to provide the cooling function required . the cold plate 68 , as shown in fig6 has been provided with at least one local flow control devices 23 . it is preferred that there is at least one flow control device 23 , directly above each heat dissipating chip 66 . the flow control device 23 , are made to have maximum gain at the expected cooling flow average operating temperature . in this particular application , the mechanical movement required to produce high heat flow control is minimal because of the high heat flux dissipating capability expected and designed into the cooling assembly and because any small local flow perturbation will have a large impact to the local thermal dissipation rate . it should also be understood that the uses and applications of heat sinks of variable heat dissipating characteristics as described above can also be advantageously applied to the heating or cooling of non - electronic devices . the variable thermal transfer capability of this invention can also be applied to the thermal exchange between two non - contacting fluid streams with different temperatures , or for heating / cooling a chemical reactor with a heating / cooling fluid to control the temperature when the reactor process includes an endothermic reaction . in many of these applications , the use of thermo - mechanically actuated devices may be required to reduce the control dependency on active control elements of critical processes which may fail when the electric power is suddenly not available . the use of flow control devices in a heat sink is also advantageous from thermal transient point of view . the flow control devices described above block the flow when the chip power dissipation is low or zero , hence during the assembly power - on transient the assembly power dissipation is minimal at time zero . thus , the assembly will warm - up at its maximum temperature increase rate when the heat sink includes a high efficiency flow control device , and reach operational chip temperature 4 to 6 times faster than possible with the use of conventional heat sinks . this advantage can become critical when the heat dissipating assembly must preserve power but become active and ready in a short time , such as , those found in some mobile electronic applications . another , advantage of using a flow control device in a heat sink , as described in this invention , is its inherent insensitivity to change in the assembly operating temperature with changes in power dissipation level . the temperature to power sensitivity is given by the slope of operating power curve . the minimum slope is located when the power is at level . the operating curve shape is directly determined by the characteristics of the flow control device used in the given heat sink design . therefore , by selecting an oversized heat sink for a given maximum power dissipation level it is possible to make the power level to be the maximum expected power dissipation of the heat generating device . thus one would achieve the lowest sensitivity to power dissipation when the actual operating power deviates by a relatively small amount from the power level . since the thermally - activated mechanical movement of a bimetallic strip is actuated relative to the absolute value of its temperature , this maximum gain point is fully defined and selected during the design and assembly of the flow control device . this flow control device temperature dependency can also be used to reduce the sensitivity between the assembly operating temperature and the temperature of the cooling fluid . this is a useful heat sink characteristic of this invention , when the fluid temperature is not well known or dependent of all other heat dissipating components located upstream of the assembly are of interest . while the present invention has been particularly described , in conjunction with a specific preferred embodiment , it is evident that many alternatives , modifications and variations will be apparent to those skilled in the art in light of the foregoing description . it is therefore contemplated that the appended claims will embrace any such alternatives , modifications and variations as falling within the true scope and spirit of the present invention .