Stacked die package

Abstract

A stacked die package is formed by stiffening a flexible substrate, arranging a chip mounting pattern on one side of the substrate, and mounting chips according to the arranged pattern. A solder ball array on the opposite side of the substrate is routed to the chips, and the flexible substrate and stiffener are folded to align the chips in the stacked die package. The stiffener is removed by a single saw cut.

What is claimed is:  
     
         1 . A method for making a stacked die package, comprising: 
 mounting at least two chips on a first side of a flexible substrate;    forming solder balls on a second side of the substrate;    interconnecting the chips and the solder balls through the substrate;    stacking the chips to form the stacked die package; and    surface mounting the stacked die package.    
     
     
         2 . The method of  claim 1 , wherein the chips are surface mounted.  
     
     
         3 . The method of  claim 1 , wherein the chips are flip chips.  
     
     
         4 . The method of  claim 1 , wherein the substrate includes a sub panel format with multiple images of individual designs.  
     
     
         5 . The method of  claim 1 , wherein the flexible substrate is stiffened by a printed wiring board.  
     
     
         6 . The method of  claim 5 , and further comprising: 
 stiffening the substrate with a stiffener; and    cutting the stiffener away after folding the substrate to stack the chips.    
     
     
         7 . The method of  claim 6 , wherein cutting is performed with a single saw cut.  
     
     
         8 . The method of  claim 1 , wherein stacking the chips comprises: 
 folding the substrate to abut chips together top to top; and    adhering abutting chips to one another.    
     
     
         9 . The method of  claim 1 , and further comprising: 
 underfilling the flip chips after mounting and reflowing.    
     
     
         10 . The method of  claim 1 , wherein mounting comprises: 
 placing the substrate on a reflow boat;    laying out chips in a predetermined pattern; and    reflowing the chips to the substrate.    
     
     
         11 . A method for forming a stacked die package, comprising: 
 stiffening a flexible substrate with a stiffener frame;    forming an array of mounting elements on a first side of the flexible substrate;    positioning the substrate mounting elements down on a reflow boat;    mounting and reflowing a plurality of chips on a second side of the flexible substrate in a predetermined pattern;    stacking the chips; and    removing the substrate stiffener.    
     
     
         12 . The method of  claim 11 , wherein removing the stiffener comprises: sawing the stiffener away.  
     
     
         13 . The method of  claim 12 , wherein sawing is accomplished with a single saw cut.  
     
     
         14 . The method of  claim 11 , wherein stacking the chips comprises: 
 folding the substrate and stiffener in a predetermined pattern;    aligning chips to be stacked; and    adhering the stacked chips together.    
     
     
         15 . The method of  claim 14 , wherein adhering comprises: 
 applying an adhesive to the chips; and    folding the substrate to stack the chips.    
     
     
         16 . The method of  claim 11 , and further comprising: 
 underfilling the chips.    
     
     
         17 . The method of  claim 11 , wherein the stiffener is a printed wiring board frame disposed around a perimeter of the substrate.  
     
     
         18 . The method of  claim 11 , wherein the stiffener is a spacer.  
     
     
         19 . The method of  claim 11 , wherein the stiffener is a metal frame.  
     
     
         20 . A method of forming a stacked die package, comprising: 
 stiffening a substrate with a stiffener;    forming solder balls on a back side of a substrate;    placing the substrate on a reflow boat with the back side facing down;    mounting at least two chips in a predetermined pattern on a front side of the substrate;    dispensing adhesive on a portion of the chips;    removing a portion of the stiffener;    folding the substrate and remaining stiffener to align the remaining portion of the chips with the portion of the chips having adhesive; and    curing the adhesive.    
     
     
         21 . The method of  claim 20 , and further comprising: 
 removing the remaining portion of the stiffener after curing.    
     
     
         22 . The method of  claim 21 , wherein removing the remaining portion of stiffener is accomplished with a single saw cut.  
     
     
         23 . A method of making a stacked die package, comprising: 
 designing a substrate in sub panel format for folding to abut mounted chips top to top;    stiffening the substrate with a printed wiring board stiffener;    forming an array of solder balls on a side of the substrate;    mounting a plurality of chips on another side of the substrate in the sub panel arrangement;    folding the substrate to abut the mounted chips; and    removing the stiffener after adhering the chips top to top.    
     
     
         24 . A stacked die array, comprising: 
 a stacked array of chips adhered together on a flexible substrate folded to accommodate mounting;    a surface mount element on a side of the substrate opposite the chips; and a    through-substrate routing for electrically connecting the chips and the surface mount element.    
     
     
         25 . The stacked die of  claim 24 , wherein the flexible substrate is a sub panel format with multiple images of individual designs.  
     
     
         26 . The stacked die of  claim 24 , wherein the surface mount element is a solder ball array.  
     
     
         28 . An implantable medical device, comprising: 
 a device body;    a stacked chip array comprising: 
 a stacked array of chips adhered together on a flexible substrate folded to accommodate mounting;  
 a surface mount element on a side of the substrate opposite the chips; and  
 a through-substrate routing for electrically connecting the chips and the surface mount element.  
   
     
     
         29 . The implantable medical device of  claim 28 , wherein the stacked chip array is mounted by conventional surface mount technology to a circuit board.  
     
     
         30 . The implantable medical device of  claim 28 , wherein the surface mount element is an array of solder balls.