Low Profile Stacking System and Method

Abstract

The present invention provides a system and method that amounts 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 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 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 at 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.    
   
   
       2 . The method of  claim 1  in which solder remains upon the plurality of CSP contacts after the removal of the ball contacts from the first CSP.  
   
   
       3 . The method of  claim 1  in which the substrate is a flexible circuit.  
   
   
       4 . The method of  claim 1  further comprising the step of connecting a second CSP to the substrate.  
   
   
       5 . The method of  claim 1  in which the substrate is a flexible circuit and further comprising the step of connecting a second CSP to the flexible circuit with one 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.  
   
   
       6 . The method of  claim 5  in which portions of the flexible circuit are disposed between the first and second CSPs.  
   
   
       7 . A method of constructinig 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.    
   
   
       8 . 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.    
   
   
       9 . 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.