Method for bonding heatsink to multiple-height chip

Abstract

A method and structure for thermally connecting a thermal conductor to at least one chip, the thermal conductor including a lower surface and at least one piston extending from the lower surface corresponding to each of the chips, each of the chips having an upper surface opposing each of the pistons, the chips being mounted on a substrate, the method comprising steps of metalizing the lower surface of the thermal conductor and the pistons, applying a solder to the lower surface of the thermal conductor, applying a thermal paste between the upper surface of the chips and the pistons, positioning the substrate and the thermal conductor such that the substrate is aligned with the thermal conductor, biasing the thermal conductor toward the substrate, biasing the pistons toward the chips such that the thermal paste has a consistent thickness between each of the chips and the pistons, reflowing the solder, such that the solder bonds the substrate to the thermal conductor and the pistons form a metallurgical bond with the thermal conductor, wherein after the reflowing step, the pistons and the thermal conductor form a unitary structure for maintaining the consistent thickness of the thermal paste between each of the chips and the pistons which achieves a considerably thinner thermal paste layer and greater thermal conduction.

What is claimed is:  
     
         1 . A method for thermally connecting a thermal conductor to at least one chip, said thermal conductor including a lower surface and at least one piston extending from said lower surface, each of said at least one chip having an upper surface opposing each of said at least one piston, said at least one chip being mounted on a substrate, said method comprising steps of: 
 metalizing said lower surface of said thermal conductor and said at least one piston;    applying a solder to said lower surface of said thermal conductor;    applying a thermal paste to said upper surface of said at least one chip;    positioning said substrate and said thermal conductor such that said substrate is aligned with said thermal conductor;    biasing said thermal conductor toward said substrate;    biasing said at least one piston toward said at least one chip, such that said thermal paste has a consistent thickness between each of said at least one chip and said at least one piston; and    reflowing said solder, such that said solder bonds said substrate to said thermal conductor and said at least one piston forms a metallurgical bond with said thermal conductor,    wherein after said reflowing step, said at least one piston and said thermal conductor form a unitary structure for maintaining said consistent thickness of said thermal paste between each of said at least one chip and said at least one piston.    
     
     
         2 . The method as in  claim 1 , wherein said metalizing step comprises a step of metalizing said lower surface of said thermal conductor and said at least one piston with a solder wettable metalization.  
     
     
         3 . The method as in  claim 1 , wherein said step of applying said solder comprises a step of applying solder preforms to areas of said lower surface of said thermal conductor adjacent said at least one piston.  
     
     
         4 . The method as in  claim 1 , wherein said step of biasing said thermal conductor toward said substrate comprises a step of temporarily clamping said thermal conductor to said substrate.  
     
     
         5 . The method as in  claim 1 , wherein said step of biasing said at least one piston toward said at least one chip comprises a step of inserting at least one spring between said thermal conductor and said at least one piston.  
     
     
         6 . The method as in  claim 1 , wherein said step of biasing said at least one piston toward said at least one chip comprises a step of supplying sufficient force between said at least one piston and said at least one chip to narrow said consistent thickness of said thermal paste to less than 5 mils.  
     
     
         7 . The method as in  claim 1 , wherein said step of biasing said at least one piston toward said at least one chip comprises a step of supplying sufficient force between said at least one piston and said at least one chip to narrow said consistent thickness of said thermal paste to about 3 mils.  
     
     
         8 . The method as in  claim 1 , wherein said step of biasing said at least one piston toward said at least one chip comprises a step of supplying sufficient force between said at least one piston and said at least one chip to narrow said consistent thickness of said thermal paste to less than 3 mils.  
     
     
         9 . The method as in  claim 1 , wherein during said reflowing step, said solder fills all gaps between said at least one piston and said thermal conductor.  
     
     
         10 . The method as in  claim 1 , wherein each of said at least one chip has a different height above said substrate and said step of biasing said at least one piston toward said at least one chip accommodates for said different height.  
     
     
         11 . A method for thermally connecting a thermal conductor to a multi-chip structure, said thermal conductor including a plurality of movable pistons opposing each of a plurality of chips of said multi-chip structure, said method comprising steps of: 
 applying a thermal paste between each of said chips and said pistons;    adjusting a position of said pistons such that said thermal paste has a consistent thickness between each of said chips and said pistons;    forming a metallurgical bond between said pistons and said thermal conductor such that said pistons are permanently fixed in a position to maintain said consistent thickness; and    bonding said thermal conductor to said multi-chip structure.    
     
     
         12 . The method as in  claim 11 , wherein said step of forming a metallurgical bond comprises steps of: 
 metalizing a lower surface of said thermal conductor and said pistons;    applying a solder to said lower surface of said thermal conductor;    assembling said thermal conductor and said multi-chip structure; and    reflowing said solder such that said solder forms said metallurgical bond.    
     
     
         13 . The method as in  claim 12 , wherein said metalizing step comprises a step of metalizing said lower surface of said thermal conductor and said pistons with solder wettable metallurgy.  
     
     
         14 . The method as in  claim 12 , wherein said step of applying said solder comprises a step of applying solder preforms.  
     
     
         15 . The method as in  claim 11 , wherein said step of adjusting a position of said pistons comprises a step of inserting springs between said thermal conductor and said pistons.  
     
     
         16 . The method as in  claim 11 , wherein said step of adjusting a position of said pistons comprises a step of applying a force between said pistons and said chips to narrow said consistent thickness of said thermal paste to less than 5 mils.  
     
     
         17 . The method as in  claim 11 , wherein said step of adjusting a position of said pistons comprises a step of applying a force between said pistons and said chips to narrow said consistent thickness of said thermal paste to about 3 mils.  
     
     
         18 . The method as in  claim 11 , wherein said step of adjusting a position of said pistons comprises a step of applying a force between said pistons and said chips to narrow said consistent thickness of said thermal paste to less than 3 mils.  
     
     
         19 . The method as in  claim 11 , wherein each of said chips has a different height and said step of adjusting said position of said pistons accommodates for said different height.  
     
     
         20 . A method for thermally connecting a thermal conductor to a multi-chip structure having a substrate and a plurality of chips mounted on said substrate, said thermal conductor including a lower surface and a plurality of pistons extending from said lower surface, each of said chips having an upper surface opposing a corresponding one of said pistons, said method comprising steps of: 
 metalizing said lower surface of said thermal conductor and said pistons with solder wettable metallurgy;    applying a plurality of solder preforms to said lower surface areas of said thermal conductor adjacent each of said pistons;    applying a thermal paste to said upper surface of said chips;;    positioning said substrate and said thermal conductor such that said substrate is aligned with said thermal conductor;    clamping said thermal conductor against said substrate;    biasing said pistons toward said chips with springs, such that said thermal paste has a consistent thickness between each of said chips and pistons; and    reflowing said solder preforms, such that said solder bonds said substrate to said thermal conductor, reacts with said solder wettable metalization to form a metallurgical bond between said pistons and said thermal conductor, and fills all gaps between said pistons and said thermal conductor,    wherein after said reflowing step, said pistons and said thermal conductor form a unitary structure for maintaining said consistent thickness of said thermal paste between each of said chips and said pistons.    
     
     
         21 . A multi-chip structure comprising: 
 a substrate;    a plurality of chips mounted on said substrate;    a thermal conductor mounted on said substrate and covering said chips, said thermal conductor including a plurality of fixed pistons, each of said pistons extending from said thermal conductor toward a corresponding chip of said chips; and    a thermal paste positioned between said pistons and said chips,    wherein each of said pistons extends from said thermal conductor a unique distance such that a distance between each of said pistons and said corresponding chip comprises a consistent distance and said thermal paste has a consistent thickness between all of said pistons and said chips.    
     
     
         22 . The multi-chip structure in  claim 21 , wherein said thermal conductor is bonded to said substrate with a seal comprising one of solder and a polymer.  
     
     
         23 . The multi-chip structure in  claim 21 , wherein said pistons are metallurgically bonded to said thermal conductor with the metalization layers reacted with solder.  
     
     
         24 . The multi-chip structure in  claim 17 , wherein each of said chips has a different height above said substrate, said pistons extending from said thermal conductor said unique distance accommodates for said different height.  
     
     
         25 . A multi-chip structure including a substrate having a plurality of chips and a thermal conductor, said thermal conductor including a lower surface and a plurality of pistons extending from said lower surface, each of said chips having an upper surface opposing a corresponding one of said pistons, said multi-chip structure being formed by a process comprising steps of: 
 metalizing said lower surface of said thermal conductor and said pistons;    applying a solder to said lower surface of said thermal conductor;    applying a thermal paste to said upper surface of said chips;;    positioning said substrate and said thermal conductor such that said substrate is aligned with said thermal conductor;    biasing said thermal conductor toward said substrate;    biasing said pistons toward said chips, such that said thermal paste has a consistent thickness between each of said chips and said pistons; and    reflowing said solder, such that said solder bonds said substrate to said thermal conductor and said pistons form a metallurgical bond with said thermal conductor,    wherein after said reflowing step, said pistons and said thermal conductor form a unitary structure for maintaining said consistent thickness of said thermal paste between each of said chips and each of said corresponding pistons.    
     
     
         26 . The multi-chip structure as in  claim 25 , wherein said metalizing step comprises metalizing said lower surface of said thermal conductor and said pistons with solder wettable metallurgy.  
     
     
         27 . The multi-chip structure as in  claim 25 , wherein said step of applying said solder to said lower surface of said thermal conductor comprises a step of applying solder preforms to areas of said lower surface of said thermal conductor adjacent said pistons.  
     
     
         28 . The multi-chip structure as in  claim 25 , wherein said step of biasing said thermal conductor toward said substrate comprises a step of temporarily clamping said thermal conductor to said substrate.  
     
     
         29 . The multi-chip structure as in  claim 25 , wherein said step of biasing said pistons toward said chips comprises a step of inserting springs between said thermal conductor and said pistons.  
     
     
         30 . The multi-chip structure as in  claim 25 , wherein said step of biasing said pistons toward said chips comprises a step of applying a force between said pistons and said chips to narrow said consistent thickness of said thermal paste to about 3 mils.  
     
     
         31 . The multi-chip structure as in  claim 25 , wherein during said reflowing step, said solder fills all gaps between said pistons and said thermal conductor.  
     
     
         32 . The multi-chip structure as in  claim 25 , wherein each of said chips has a different height above said substrate and said step of biasing said pistons toward said chips accommodates for said different height.  
     
     
         33 . The multi-chip structure as in  claim 25 , further comprising at least one standoff positioned between at least one of said the chips and said corresponding one of said pistons.