Patent Publication Number: US-2022216129-A1

Title: Lid allowing for liquid metal thermal interfacing materials in a lidded flip chip package

Description:
TECHNICAL FIELD OF THE DISCLOSURE 
     The disclosure relates generally to a heat-dissipating object for dissipating heat from a heat-generating object in an electronic device, and particularly to a lid for a lidded flip chip package to use a liquid thermal interface material (TIM). 
     BACKGROUND OF THE DISCLOSURE 
     In a flip chip package with a semiconductor chip being a heat-generating object, a thermal interface material (TIM) is usually used to fill the gap between the flip chip and a heat-dissipating object, like a lid or a heat sink for transferring the heat from one to the other. The types of the TIM basically include thermal pad, thermal grease, phase change material and liquid metal. A good TIM needs to have 1) a high thermal conductivity, 2) a good surface wetting capability for reducing the thermal contact resistance, 3) a good gap filling capability, and 4) a good thermal reliability in test or application. A liquid metal as TIM usually includes gallium and gallium alloy. The melting point of the gallium is about 29° C., and that of gallium alloy is even lower. The thermal conductivity of the liquid metal is much higher than the extensively used thermal pad or thermal grease. Furthermore, a liquid metal has much better capability for surface wetting and gap filling. So, of all the types of TIM, a liquid metal is an ideal TIM if only looking at the first three items. If a liquid metal can be used in a lidded flip chip package, the temperature of the lidded flip chip package can be reduced significantly as compared to other types of TIM. However, the conventional lidded flip chip packages based on a conventional lid of prior arts are limited to use a liquid metal as its TIM due to the pumping-out issue, that is, when the package is under a thermal cycling test or in its long term of application, the volume of the gap between the flip chip and the lid varies with temperature due to the warpage of the flip chip, causing the liquid metal TIM to be pumped out. The TIM pumping-out issue will cause an incomplete gap filling between the flip chip and the lid, reducing the thermal performance of TIM. And the more important thing is that because a liquid metal is electrically conductive, a small amount of TIM pumping-out may damage the whole electronic device. As a result, a liquid metal type of TIM has not been commercially used in a lidded flip chip package. In general, because of the similar reason, a liquid metal type of TIM is rarely interposed between a heat-dissipating object (a heat sink, for example) and a heat-generating object (a semiconductor chip, for example) in an electronic device. 
     For overcoming the TIM pumping-out issue for an electronic device such as a lidded flip chip package to use a liquid TIM, a lid coupled with a reservoir structure has been disclosed in the prior art, U.S. Ser. No. 10/643,924 B1. However, the lid of the prior art has two disadvantages in its application. To eliminate the two disadvantages, a lid and its lidded flip chip package are described in the present disclosure of the present invention. 
     SUMMARY OF THE DISCLOSURE 
     A lid allowing for a liquid thermal interface material (TIM) in a lidded flip chip package, comprising: the lid, consisting of a top piece, which has a top surface and a bottom surface and a plurality of pins; and a reservoir structure comprising: a reservoir, a seal ring, and a connecting hole, wherein the top piece of the lid has a ring-form of slot on its bottom surface and the seal ring is mounted in the ring-form of slot, the seal ring directly sealing a peripheral edge region at a top surface of the flip chip with a portion on the bottom surface of the top piece, providing a gap between a portion of the bottom surface of the top piece and a portion of the top surface of the flip chip, and the gap being entirely filled with the liquid, wherein the reservoir is a tunnel in the top piece of the lid, which has an opening to the ambient, and the tunnel being partially filled with the liquid, wherein the connecting hole has an end at a portion of the bottom surface of the top piece surrounded by the seal ring, and another end connecting to the reservoir, wherein the top piece of the lid has an injection hole and a plug, the injection hole has an end at a portion of the bottom surface of the top piece surrounded by the seal ring, and another end at a portion of the bottom surface of the top piece is closed by the plug, and wherein the plurality of pins extrude downwards from the portion of the bottom surface of the top piece outside the seal ring and form an array of pins, and an adhesive material fills among the pins and between the pins and the substrate of the lidded flip chip package so as to bond the lid with the substrate. 
     A method for making the lid allowing for a liquid thermal interface material (TIM) in a lidded flip chip package, comprising the following major steps:
         1) Prepare a piece of material,   2) Form a spiral pattern of slot and a ring-form of slot at the bottom surface of the piece of material,   3) Form two through holes for making a connecting hole and an injection hole,   4) Form a rectangular slot for connecting the two through holes,   5) Form a ring-form of covering piece, a bridging bar, a plug and a seal ring,   6) Bond the ring-form of covering piece onto the bottom surface of the piece of material, covering the spiral pattern of tunnel to form a spiral pattern of tunnel,   7) Mount the bridging bar into the rectangular slot for forming a connecting hole from the spiral pattern of tunnel to a portion of the bottom surface of the piece of material surrounded by the ring-form of slot,   8) Place the plug into the injection hole,   9) Mount the seal ring into the ring-form of slot.       

     The features and advantages of the embodiments of the present disclosure will become more apparent from the detailed descriptions in conjunction with the drawings below. The drawings and associated descriptions are to illustrate the embodiments of the present disclosure, not to limit the scope of what is claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram for illustrating a conventional lidded flip chip package of prior arts. 
         FIG. 2  is a schematic diagram for illustrating a lidded flip chip package of prior art, U.S. Ser. No. 10/643,924 B1. 
         FIG. 3  to  FIG. 6  are schematic diagrams for illustrating the features of a lid for a lidded flip chip package to use a liquid TIM of one preferred embodiment of the present invention. 
         FIG. 7  is a schematic diagram for illustrating a lidded flip chip package with liquid TIM based on the present lid of one preferred embodiment of the present invention. 
         FIG. 8  a schematic diagram for illustrating some more features of the lidded flip chip package with liquid TIM based on the present lid of one preferred embodiment of the present invention. 
         FIG. 9  is a schematic diagram for illustrating some more features of a lid for a lidded flip chip package to use a liquid TIM of another preferred embodiment of the present invention. 
         FIG. 10  is a schematic diagram for illustrating a lidded flip chip package with liquid TIM based on the present lid of another preferred embodiment of the present invention. 
         FIG. 11  to  FIG. 14  are schematic diagrams for illustrating the steps of a method for making a lid, wherein some tunnels, slots and holes are formed of one preferred embodiment of the present invention. 
         FIG. 15  to  FIG. 17  are schematic diagrams for illustrating the steps of the method for making a lid, wherein a ring-form of covering piece, a bridging bar, a plug and a seal ring are formed of one preferred embodiment of the present invention. 
         FIG. 18  is a schematic diagram for illustrating the steps of the method for making a lid, wherein the ring-form of covering piece, the bridging bar, the plug and the seal ring are assembled with the piece of material for forming the present lid of one preferred embodiment of the present invention. 
         FIG. 19  is a schematic diagram for illustrating the step to bond the present lid with a flip chip package for forming a lidded flip chip package of one preferred embodiment of the present invention. 
         FIG. 20  is a schematic diagram for illustrating the step to fill a liquid material into the gap and the tunnel in the lidded flip chip package of one preferred embodiment of the present invention. 
         FIG. 21  is a schematic diagram for illustrating the liquid system in the lidded flip chip package of one preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 1  and  FIG. 2  are used to explain some terms associated with a lid, a lidded flip chip package and a thermal interface material (TIM), and to describe the challenges about using a liquid TIM in a lidded flip chip package of prior arts. 
       FIG. 1  is a schematic diagram for illustrating a conventional lidded flip chip package. The numerical symbol  1000  in  FIG. 1  designates a conventional lidded flip chip package, in which the numerical symbol  100  designates a conventional lid, including a top piece  101  and a side wall  102 , the numerical symbol  160  designates a flip chip package, including a flip chip  161  and a substrate  164  with solder balls  165 , the bumps  162  and the under fill material  163 , the numerical symbol  166  designates an adhesive material for bonding the lid  100  onto the substrate  160 , and the numerical symbol  122  designates a thermal interface material (TIM) which is placed in the gap between the top piece  101  and the flip chip  161 . It is noted that the flip chip package  160  is not always flat, but warps downwards or upwards at low or high temperature due to the CTE (coefficient of thermal expansion) mismatch between the flip chip  161  and the substrate  164 , causing the volume change of the gap between the flip chip  161  and the top piece  101 . As a result, a liquid TIM is easy to be pumped out, causing a challenge for a conventional lidded flip chip package to use a liquid TIM. 
       FIG. 2  is a schematic diagram for illustrating a lid for a lidded flip chip package to use a liquid TIM of the prior art. The numerical symbol  2000  in  FIG. 2  designates the lid  200  and the lidded flip chip package  220  based on the lid  200 , wherein the lid  200  includes a reservoir structure, and the lidded flip chip package  220  includes a reservoir system based on the reservoir structure, deviating them from the conventional ones. In additional to the top piece  201  and side wall  202 , the lid  200  includes a reservoir structure, consisting of a seal ring  211  mounted on its bottom surface of the top piece  201 , a reservoir  212  in the top piece  201 , and a connecting hole  213 , starting from a portion of the bottom surface of the top piece  201  and connecting to the reservoir. The end of the connecting hole at the bottom surface of the top piece is called an inner end and the other end of the connecting hole is called an outer end. The seal ring region of the bottom surface of the lid means the portion of the bottom surface of the lid surrounded by the seal ring. The reservoir  212  is a tunnel for taking in a liquid material and releasing it again when needed. The  214  designates an opening of the reservoir  212 , which opens to the ambient. After attaching the lid  200  on the flip chip package  160  through the adhesive  166  and filling a liquid in the gap and reservoir, the lidded flip chip package  220  to use a liquid TIM of the prior art is formed, wherein the lidded flip chip package  220  includes a reservoir system, which consists of the reservoir  212  of the lid, the connecting hole  213  of the lid, a sealed gap  222 , and a liquid material  222 A/ 212 A, the gap between the lid  220  and the flip chip  161  is sealed at the peripheral edge region of the flip chip  161  by the seal ring  211 A, forming the sealed gap  222 , the reservoir  212  is a tunnel in the top piece of the lid, the connecting hole  213  is between the sealed gap  222  and the reservoir  212  for connecting one with the other, and a liquid material is entirely filled in the sealed gap  222 , and partially filled in the reservoir  212 . The  222 A and  212 A respectively designate the liquid material in the gap  222  and reservoir  212 , and the  212 B designates the empty space in the reservoir  212 . The liquid material  222 A forms a liquid TIM between the flip chip  161  and lid  200 . It is seen that when the volume of the sealed gap  222  gets smaller, the reservoir  212  can take in the excessive amount of liquid material from the sealed gap  222  to its empty space  212 B, keeping the pressure inside the sealed gap  222  not to be high, and when the volume of the sealed gap  222  gets larger, the reservoir  212  can release the needed amount of liquid material into the sealed gap  222  from its stored liquid material  212 A, keeping the sealed gap  222  to be entirely filled all the time. It is noted that the reservoir  212  needs to have an opening  214  so that the liquid material can flow between the gap and the reservoir when the gap volume changes. 
     The lidded flip chip package  220  based on the lid  200  designated by the numerical symbol  2000  in  FIG. 2  of the prior art has two disadvantages in its application. One is that it is not easy to fill and remove a liquid material into and from the reservoir structure. As a result, when the package  220  is analyzed by a cross-sectional cutting, the liquid may flow out, causing messy anywhere. And the other is that the delaminating risk between the adhesive  166  and the side wall  202  of the lid is high due to an additional stress caused by the compressed seal ring  211 A. The present invention is to eliminate the two disadvantages of the prior art by introducing an injection hole with a plug in the top piece of the lid and a plurality of pins to replace the side wall, which is described in conjunction with the drawings in the following. 
       FIG. 3  to  FIG. 6  are schematic diagrams for illustrating a lid of one preferred embodiment of the present invention. The numerical symbol  3000  in  FIG. 3  designates the cross-sectional view of the lid, in which the  300  designates a lid which includes an injection hole  301  and a plurality of pins  302  as a replacement of the side wall of the lid  200  of the prior art as showed in  FIG. 2 , and the  301 A in the lid  310  designates a plug for closing the injection hole  301 . The pins extrude downwards from the bottom surface of the top piece and may have various arrays. The numerical symbol  3100  in  FIG. 4  designates the bottom view of a lid of one preferred embodiment of the present invention, in which the numerical symbols  311  and  312  designate the top piece of the lid and the pins with a full array on the portion of the bottom surface of the top piece outside the seal ring  314 , and the  313  and  315  designate the bottom view of the connecting hole  213  and the injection hole  301 . The numerical symbol  3200  in  FIG. 5  designates the bottom view of a lid of one preferred embodiment of the present invention, in which the pins  322  form a peripheral array, providing a ring-form of cavity  321  surrounding the seal ring. The numerical symbol  3300  in  FIG. 6  designates the bottom view of a lid of another preferred embodiment of the present invention, in which the pins  330  form an array with some cavities  331 ,  332 ,  333 , and  334 . It is noted that the arrays of the pins can be flexibly designed according to the requirement of a specific application, and those showed in  FIG. 4  to  FIG. 6  are only some examples. 
       FIG. 7  is a schematic diagram for illustrating a lidded flip chip package of one preferred embodiment of the present invention. The numerical symbol  4000  in  FIG. 7  designates a lid allowing for a liquid TIM in a lidded flip chip package, comprising: the lid consisting of a top piece  201 , which has a top surface and a bottom surface, a plurality of pins  202 , and a reservoir structure comprising: a reservoir  212 , a seal ring  212 A, and a connecting hole  213 , wherein the top piece  201  of the lid has a ring-form of slot  411  on its bottom surface and the seal ring  211 A is mounted in the ring-form of slot  411 , the seal ring directly sealing a peripheral edge region at a top surface of the flip chip  161  with a portion on the bottom surface of the top piece, providing a gap  222  between a portion of the bottom surface of the top piece  201  and a portion of the top surface of the flip chip  161 , and the gap being entirely filled with the liquid  222 A, wherein the reservoir  212  is a tunnel in the top piece  201  of the lid, which has an opening  214 , which opens to the ambient, and the tunnel  212  being partially filled with the liquid  212 A, wherein the connecting hole  213  has an end at a portion of the bottom surface of the top piece  201  surrounded by the seal ring, and another end connecting to the reservoir  212 , wherein the top piece of the lid has an injection hole  301  and a plug  301 A, the injection hole has an end at a portion of the bottom surface of the top piece surrounded by the seal ring, and another end at a portion of the top surface of the top piece is closed by the plug  301 A, and wherein the plurality of pins  202  extrude downwards from the portion of the bottom surface of the top piece outside the seal ring and form an array of pins, and an adhesive material  402  fills among the pins and between the pins and the substrate  164  of the lidded flip chip package so as to bond the lid with the substrate. 
       FIG. 8  is a schematic diagram for illustrating some more features of the lidded flip chip package  4000  showed in  FIG. 7  of one preferred embodiment of the present invention. The numerical symbol  4100  in  FIG. 8  designates a lidded flip chip package, which further composites a foam block  412  in the tunnel and near the opening  214 . The foam block allows the air to pass freely but does not allow the liquid in the reservoir to leak out easily. When the liquid in the reservoir and the gap for a liquid TIM is a liquid metal such as a gallium or its alloys, the lidded flip chip package further composites another non-metallic liquid or grease material  411  at the outer end of the liquid metal in the reservoir for preventing the liquid metal from exposing to the air because liquid metal can be oxidized when being exposed to the air under high temperature and long time. The  411  can also be a movable block as an option. 
       FIG. 9  is a schematic diagram for illustrating the variations of the array of pins and the structure of the seal ring of the lid of one preferred embodiment of the present invention. The  302 A and  302 B designate some shorter pins and a cavity in the array of pins  302  of the lid  5000 , and the  511  designates a preferred seal ring, which has a structure to further seal at the sides of the flip chip and the under fill filet at the sides of the flip chip as showed in  FIG. 10  below. 
       FIG. 10  is a schematic diagram for illustrating a lid allowing for a liquid TIM in a lidded flip chip package of one preferred embodiment of the present invention. The numerical symbol  6000  in  FIG. 10  designates a lid allowing for a liquid TIM in a lidded flip chip package, wherein the seal ring  611 A seals at the peripheral edge region of the flip chip and further seals at the sides of the flip chip and the under fill filet at the sides of the flip chip, and the adhesive  602  entirely fills the full array of pins and cavity between the pins and the seal ring. 
       FIG. 11  to  FIG. 18  are schematic diagrams for illustrating a method for making a lid allowing for a liquid TIM in a lidded flip chip package of one preferred embodiment of the present invention. The numerical symbol  7000  in  FIG. 11  designates the steps for forming some slots and holes in a piece of material, in which the  700  designates the step to prepare a piece of material,  710  designates the step to form a ring-form of slot  712  and a spiral pattern of slot  711  in the bottom surface of the piece of material, wherein the spiral pattern of slot  711  is outside and surrounding the ring-form of slot  712 , and  720  designates the step to form an injection hole  723 , two through holes  721  and  722  for making a connecting hole through a bridging bar later, and a step-form of rectangular slot  724  and  725 .  FIG. 12  to  FIG. 14  are schematic diagrams for illustrating the bottom and top views of the piece of material after the previous steps. The numerical symbol  7100  in  FIG. 12  is for illustrating the ring-form of slot  712  and the spiral pattern of slot  711  from their bottom view, in which the  712 A and  711 A designate the ring-form of slot  712  and the spiral pattern of slot  711  from their bottom view. The numerical symbol  7200  in  FIG. 13  is for illustrating the through holes  721  and  722  and the injection hole  723  from their bottom view, in which the  721 A and  722 A designate the two through holes  721  and  722  and the  723 A designates the injection hole from their bottom view. The numerical symbol  7210  in  FIG. 14  is for illustrating the step-form of rectangular slot  724  and  725  from their bottom view, in which the  724 A and  725 A designate the rectangular slot  724  and  725  from their bottom view. The numerical symbol  7300  in  FIG. 15  is for illustrating the step for forming a ring-form of covering piece with an array of pins, in which the  730  and  730 A designate the bottom and cross-sectional views of the ring-form of covering piece with the array of pins, the  731  and  731 A designate the ring-form of covering piece, the  732  and  732 A designate the array of pins, and the  733  designates the location for the cross-section. The numerical symbol  7400  in  FIG. 16  is for illustrating the step for forming a seal ring, in which the  740  and  740 A designate the bottom and cross-sectional views of the seal ring, the  741  and  741 A designate the seal ring, and the  742  designates the location for the cross-section. The numerical symbol  7500  in  FIG. 17  is for illustrating the step for forming a bridging bar and a plug, in which the  750  and  760  designate the bridging bar and the plug, the  751  and  751 A designate the bridging bar from its top and side view, and the  761  and  761 A designate the plug  760  from its top and side view, respectively. The numerical symbol  8000  in  FIG. 18  is for the steps to assemble the parts made from the previous steps together to form the lid, in which the  810  designates the step to bond the ring-form of covering piece with an array of pins  751 A to the bottom surface of the piece of material  720  through an adhesive layer  811 , the  820  designates the step to mount the bridging bar  821  in the step-form of rectangular slot  724  through an adhesive material  822  filled in the step-form of rectangular slot  725 , and the  830  designates the step to mount the seal ring  730 A and the plug  761 A in the ring-form of slot and the injection hole. 
       FIG. 19  to  FIG. 21  are schematic diagrams for illustrating the further steps of the method for making the lid allowing for a liquid TIM in a lidded flip chip package of one preferred embodiment of the present invention. The numerical symbol  9000  in  FIG. 19  is for illustrating the step to attach the lid to a flip chip package to form a lidded flip chip package, in which the  910  designates the step to apply an adhesive material  912  to the array of pins, the arrow  913  designates to attach the lid onto the flip chip package  920  through the adhesive  912 , forming the lidded flip chip package  930 . The numerical symbol  9400  in  FIG. 20  is for illustrating the step to fill the liquid into the gap and reservoir, in which the  941  designates the step to fill the liquid into the gap and the tunnel type of reservoir after removing the plug, and then the  951  designates the step to close the injection hole by putting back the plug, forming the lidded flip chip package  950  with a liquid TIM  952 . The  952  and  953  designate the liquid in the gap and the tunnel, and the  954  designate the portion of the tunnel without the liquid. The numerical symbol  9600  in  FIG. 21  is for illustrating the liquid system in the lidded flip chip package  950  in  FIG. 20  from the top view of the liquid system, in which the  962  and  963  designate the liquid in the gap and the tunnel, the  964  designates the portion of the tunnel without the liquid, the  965  designates the opening of the tunnel to the ambient, and the arrow  966  means that the liquid can flow in and out when the gap volume changes. 
     Although the present invention is described in some details for illustrative purpose with reference to the specific embodiments and drawings, it is apparent that many other modifications and variations may be made without departing from the spirit and scope of the present invention.