Low profile stacking system and method

Abstract

The present invention provides a system and method that mounts integrated circuit devices onto substrates and a system and method for employing the method in stacked modules. The contact pads of a packaged integrated circuit device are substantially exposed. A solder paste that includes higher temperature solder paste alloy is applied to a substrate or to the integrated circuit device to be mounted. The integrated circuit device is positioned to contact the contacts of the substrate. Heat is applied to create high temperature joints between the contacts of the substrate and the integrated circuit device resulting in a device-substrate assembly with high temperature joints. The formed joints are less subject to re-melting in subsequent processing steps. The method may be employed in devising stacked module constructions such as those disclosed herein as preferred embodiments in accordance with the invention. Typically, the created joints are low in profile.

1 . A stacked circuit module comprising: 
 a first integrated circuit element;    a second integrated circuit element;    a flexible circuit connector connecting the first and second integrated circuit elements, the first and second integrated circuits being connected to the flexible circuit connector by a plurality of high temperature joints, each of the plurality of high temperature joints having melting points between 232° C. and 312° C. inclusive.    
     
     
         2 . A high temperature joint in a stacked circuit module in which a first integrated circuit element is disposed above a second integrated circuit element, the high temperature joint comprising: 
 an alloy comprised of lead and antimony having the proportion of no more than 95% tin and at least 5% antimony, the alloy having a melting point between 235° C. to 260° C.    
     
     
         3 . A high temperature joint in a stacked circuit module in which a first integrated circuit element is disposed above a second integrated circuit element, the high temperature joint comprising: 
 an alloy comprised of lead and tin, the alloy having a melting point between 275° C. and 312° C.    
     
     
         4 . A high temperature joint in a stacked circuit module in which a first integrated circuit element is disposed above a second integrated circuit element, the high temperature joint comprising: 
 an alloy comprised of lead, the alloy having a melting point of between 235° C. and 312° C.    
     
     
         5 . A stacked circuit module comprising: 
 a first integrated circuit;    a second integrated circuit disposed above the first integrated circuit;    a flex circuit connecting the first and second integrated circuits, the first integrated circuit and the second integrated circuit being connected to the flex circuit through high temperature joints devised in accordance with  claim 4  and the flex circuit comprising; 
 first and second outer layers;  
 first and second conductive layers, between which there is an intermediate layer, the first and second conductive layers and the intermediate layer being interior to the first and second outer layers, the second conductive layer having demarked first and second flex contacts, the first flex contacts being accessible through first windows through the second outer layer and the second flex contacts being accessible through second windows through the first outer layer, the first conductive layer, and the intermediate layer.  
   
     
     
         6 . The stacked circuit module of  claim 5  in which the second flex contacts are accessible through module windows through the second outer layer.  
     
     
         7 . The stacked circuit module of  claim 5  in which the first and second conductive layers are metal.  
     
     
         8 . The stacked circuit module of  claim 5  in which selected ones of the first flex contacts are connected to selected ones of the second flex contacts.  
     
     
         9 . The stacked circuit module of  claim 5  in which selected ones of the first flex contacts are connected to the first conductive layer.  
     
     
         10 . The stacked circuit module flex circuit of  claim 7  in which the metal of the first and second conductive layers is alloy 110.  
     
     
         11 . The flex circuit of  claim 8  in which the connected selected ones of the first and second flex contacts are connected with traces.  
     
     
         12 . The flex circuit of  claim 5  in which selected ones of the first flex contacts and selected ones of the second flex contacts are connected to the first conductive layer with vias.  
     
     
         13 . The flex circuit of  claim 12  in which selected ones of the first flex contacts are connected to the first conductive layer with on-pad vias.  
     
     
         14 . The flex circuit of  claim 12  in which selected ones of the second flex contacts are connected to the first conductive layer with off-pad vias.  
     
     
         15 . A stacked circuit module comprising: 
 a first flex circuit having first and second conductive layers;    a second flex circuit having first and second conductive layers;    a first integrated circuit element and a second integrated circuit element, each of the first and second integrated circuit elements being connected to the first flex circuit and the second flex circuit with solder joints having a melting point equal to or greater than 235° C.    
     
     
         16 . A method of constructing a stacked circuit module comprising the steps of: 
 providing a first CSP having a plurality of ball contacts;    removing the plurality of ball contacts from the first CSP leaving a plurality of CSP pads on the first CSP;    providing a substrate having a plurality of substrate contacts;    applying a solder paste to the plurality of substrate contacts of the substrate, the solder paste being comprised of an alloy of lead having a melting point, the melting point being equal to or greater than 235° C.;    positioning the first CSP and the substrate relative to each other to place the CSP pads of the first CSP in contact with the substrate contacts of the substrate to which the solder paste has been applied;    heating the first CSP and the substrate to cause the temperature at the CSP contacts and substrate contacts to reach the melting point.    
     
     
         17 . The method of  claim 16  in which solder remains upon the plurality of CSP contacts after the removal of the ball contacts from the first CSP.  
     
     
         18 . The method of  claim 16  in which the substrate is a flexible circuit.  
     
     
         19 . The method of  claim 16  further comprising the step of connecting a second CSP to the substrate.  
     
     
         20 . The method of  claim 16  in which the substrate is a flexible circuit and further comprising the step of connecting a second CSP to the flexible circuit with an alloy of lead having a second CSP melting point equal to or greater than 235° C. and disposing the second CSP above the first CSP.  
     
     
         21 . The method of  claim 20  in which portions of the flexible circuit are disposed between the first and second CSPs.  
     
     
         22 . A method of constructing a stacked circuit module comprising the steps of: 
 providing a first CSP having a plurality of ball contacts;    removing the plurality of ball contacts from the first CSP leaving a plurality of CSP pads on the first CSP;    providing a substrate having a plurality of substrate contacts;    applying a solder paste to the plurality of CSP pads, the solder paste being comprised of an alloy of lead having a melting point, the melting point being equal to or greater than 235° C.;    positioning the first CSP and the substrate relative to each other to place the CSP pads to which the solder paste has been applied in contact with the substrate contacts of the substrate;    heating the first CSP and the substrate to cause the temperature at the CSP contacts and substrate contacts to reach the melting point.    
     
     
         23 . A circuit board to which is connected a stacked circuit module devised in accordance with claims  16  or  22 .  
     
     
         24 . The circuit board of  claim 23  in which the stacked circuit module devised in accordance with claims  16  or  22  is connected to the board with a solder having a melting point below 235° C.  
     
     
         25 . A stacked circuit module comprising: 
 a first integrated circuit;    a second integrated circuit in stacked disposition relative to the first integrated circuit;    a substrate through which the first and second integrated circuits are connected;    a plurality of high temperature joints that connect the first integrated circuit to the substrate and the second integrated circuit to the substrate, the high temperature joints each having a melting point greater than 230° C.    
     
     
         26 . A high-density circuit module comprising: 
 a first CSP having first and second lateral sides and upper and lower major surfaces and a set of CSP pads along the lower major surface;    a second CSP having first and second lateral sides and upper and lower major surfaces and a set of CSP pads along the lower major surface, the first CSP being disposed above the second CSP;    a pair of flex circuits, each of which pair having a first conductive layer and a second conductive layer, both said conductive layers being interior to first and second outer layers, and demarcated at the second conductive layer of each flex circuit there being upper and lower flex contacts, the upper flex contacts being connected to the CSP pads of the first CSP with first high temperature joints and the lower flex contacts being connected to the CSP pads of the second CSP with second high temperature joints, the first and second high temperature joints each having melting points above 230° C.    
     
     
         27 . The high-density circuit module of  claim 26  in which: 
 a chip-enable module contact is connected to an enable lower flex contact that is connected to a chip select CSP pad of the first CSP.  
 
     
     
         28 . The high-density circuit module of  claim 27  in which the connection between the enable lower flex contact and the chip select CSP contact of the first CSP is through an enable connection at the first conductive layer.  
     
     
         29 . The high-density circuit module of  claim 26  in which a first one of the flex circuit pair is partially wrapped about the first lateral side of the second CSP and a second one of the flex circuit pair is partially wrapped about the second lateral side of the second CSP to dispose the upper flex contacts above the upper major surface of the second CSP and beneath the lower major surface of the first CSP.  
     
     
         30 . The high-density circuit module of  claim 26  in which the first CSP expresses an n-bit datapath and the second CSP expresses an n-bit datapath, each of the flex circuits of the flex circuit pair having supplemental lower flex contacts which, in combination with the lower flex contacts, provide connection for the set of module contacts and a set of supplemental module contacts to express a 2n-bit module datapath that combines the n-bit datapath expressed by the first CSP and the n-bit datapath expressed by the second CSP.  
     
     
         31 . A method of populating a circuit board comprising the steps of: 
 obtaining a high-density circuit module comprised of: two or more packaged integrated circuits;    a flexible circuit for connection between the two or more packaged integrated circuits, the connection between the two or more packaged integrated circuits being implemented through HT joints, the HT joints having a melting point range of between X and Y degrees Centigrade, where X is less than Y;    attaching the high-density circuit module to the circuit board with solder joints, the solder joints having a melting point range of between A and B degrees Centigrade, where A is less than B and A and B are both less than X.    
     
     
         32 . A method of populating a circuit board comprising the steps of: 
 constructing a high-density circuit module having two or more integrated circuits with one integrated circuit disposed above another, the construction being implemented with a first solder having a melting point of X degrees; and    attaching the stacked circuit module to a circuit board with a second solder having a melting point of Y degrees where Y is less than X.