Low profile chip scale stacking system and method

Abstract

The present invention stacks chip scale-packaged integrated circuits (CSPs) into low profile modules that conserve PWB or other board surface area. Low profile structures provide connection between CSPs of the stacked module and between and to the flex circuitry. Low profile contacts are created by any of a variety of methods and materials including, for example, screen paste techniques and use of high temperature solders, although other application techniques and traditional solders may be employed for creating low profile contacts in the present invention. A consolidated low profile contact structure and technique is provided for use in alternative embodiments of the present invention. The CSPs employed in stacked modules devised in accordance with the present invention are connected with flex circuitry. That flex circuitry may exhibit one or two or more conductive layers. In some preferred embodiments, a form standard provides a physical form that allows many of the varying package sizes found in the broad family of CSP packages to be used to advantage while employing a standard connective flex circuitry design. In other embodiments, a heat spreader is disposed between the CSP and the flex circuitry thus providing an improved heat transference function without the standardization of the form standard, while still other embodiments lack either a form standard or a heat spreader and may employ, for example, the flex circuitry as a heat transference material.

1 . A method of devising a high-density circuit module comprising the steps of:
 providing a first-level CSP having contact sites along a major surface, an upper surface and a lower surface, and opposing lateral sides defining a lateral extent of the first-level CSP;   providing a second-level CSP having contact sites along a major surface;   providing flex circuitry comprising at least one flex circuit having first and second conductive layers, an intermediate layer between the first and second conductive layers, an outer set of plural flex contacts, and an inner set of plural flex contacts;   disposing high-temperature solder paste on the contact sites of the first-level CSP;   applying heat sufficient to connect selected contact sites of the first-level CSP to select ones of the inner set of plural flex contacts to form a reflowed contact having a height between 2 and 6 mils;   disposing high-temperature solder paste on the CSP contact sites of the second-level CSP;   applying heat sufficient to connect select ones of the second-level CSP contact sites to select ones of the outer set of plural flex contacts to form a reflowed contact having a height between 2 and 6 mils; and   wrapping the flex circuitry partially about at least one of the opposing lateral sides of the first-level CSP such that the outer set of plural flex contacts are disposed above the upper surface of the first-level CSP and the inner set of plural flex contacts are disposed below the lower surface of the first-level CSP, the first conductive layer being closer to the first-level CSP than is the second conductive layer subsequent to the step of wrapping the flex circuitry.   
   
   
       2 . The method of  claim 1  in which selected ones of the inner and outer sets of plural flex contacts are at the second conductive layer. 
   
   
       3 . The method of  claim 1  in which selected ones of the inner and outer sets of plural flex contacts are at the first conductive layer. 
   
   
       4 . The method of  claim 1  in which in which the inner and outer sets of plural flex contacts for connection of the second-level and first-level CSPs, respectively, are exhibited at a conductive layer of the flex circuitry. 
   
   
       5 . A method of devising a high-density circuit module comprising the steps of:
 providing a first-level CSP having a top major surface and a bottom major surface and contact sites along the bottom major surface;   providing a second-level CSP having contact sites along a major surface;   providing flex circuitry comprising at least one flex circuit having first and second conductive layers, an intermediate layer between the first and second conductive layers, an outer set of plural flex contacts, an inner set of plural flex contacts, and a thickness of about 4 mils or less;   disposing high-temperature solder paste on the first-level CSP contact sites;   applying heat sufficient to connect selected contact sites of the first-level CSP to select ones of the inner set of plural flex contacts to form a reflowed contact having a height between 2 and 6 mils;   disposing high-temperature solder paste on the contact sites of the second-level CSP;   applying heat sufficient to connect selected contact sites of the second-level CSP to select ones of the outer set of plural flex contacts; and   wrapping the flex circuitry partially about the first-level CSP such that the first conductive layer is closer to the first-level CSP than is the second conductive layer subsequent to the step of wrapping the flex circuitry.   
   
   
       6 . A method of devising a high-density circuit module comprising the steps of:
 providing a first CSP having a lateral extent defined by opposing lateral sides, and having a plurality of ball contacts disposed along a major surface;   providing flex circuitry comprising at least one flex circuit having first and second conductive layers, an intermediate layer between the first and second conductive layers, and a plurality of selected flex contacts each penetrated by an orifice;   disposing the first CSP proximal to the flex circuitry to place the plurality of ball contacts adjacent to the plurality of flex contacts;   applying heat sufficient to melt the plurality of ball contacts such that the ball contacts melt and reflow partially through respective ones of the orifices to present outer contact surfaces;   attaching a second CSP to the flex circuitry; and   wrapping the flex circuitry partially about the first-level CSP such that after the attaching and wrapping steps the second CSP is in a stacked disposition above the first CSP with the first conductive layer being closer to the first CSP than is the second conductive layer.   
   
   
       7 . The method of  claim 6  in which the flex circuitry is comprised of two flex circuits. 
   
   
       8 . A method of devising a high-density circuit module comprising the steps of:
 providing a first CSP having contact sites along a major surface;   providing a second CSP having contact sites along a major surface;   providing flex circuitry having plural flex contacts and at least two conductive layers; and   connecting selected contact sites of the first CSP to a first set of the plural flex contacts so that the shortest distance from the major surface of the first CSP to a surface of the flex circuitry is between 1 and 6 mils inclusive.   
   
   
       9 . The method of  claim 8  further comprising the step of connecting selected contact sites of the second CSP to a second set of the plural flex contacts so that the shortest distance from the major surface of the second CSP to a surface of the flex circuitry is between 1 and 6 mils inclusive. 
   
   
       10 . A method of devising a high-density circuit module comprising the steps of:
 providing a first CSP having a plurality of ball contacts disposed along a major surface;   providing flex circuitry having a plurality of selected flex contacts each penetrated by an orifice;   disposing the first CSP proximal to the flex circuitry to place the plurality of ball contacts adjacent to the plurality of flex contacts; and   applying heat sufficient to melt the plurality of ball contacts to pass through the respective orifices to form consolidated contacts each with an inner flex portion and an outer flex portion.   
   
   
       11 . The method of  claim 10  further comprising the step of disposing a second CSP above the first CSP and connecting the first and second CSPs with the flex circuitry. 
   
   
       12 . The method of  claim 10  in which the flex circuitry has two conductive layers. 
   
   
       13 . The method of  claim 10  in which the outer flex portion has a volume substantially greater than the volume of the inner flex portion. 
   
   
       14 . A high-density circuit module comprising:
 a first CSP having an upper surface and a lower surface, and along the lower surface there are plural first CSP low profile contacts, each of which plural first CSP low profile contacts extends no more than 7 mils from the surface of the first CSP;   a second CSP in stacked disposition with the first CSP, the second CSP having an upper surface and a lower surface, and along the lower surface there are plural second CSP low profile contacts, each of which plural second CSP low profile contacts extends no more than 7 mils from the surface of the second CSP;   a flex circuit that connects the first CSP and the second CSP, the flex circuit having at least two conductive layers and a portion that is disposed between the first and second CSPs.   
   
   
       15 . The high-density circuit module of  claim 14  in which the shortest distance from the lower surface of the second CSP to the upper surface of the first CSP that passes through one of the plural second CSP low profile contacts is less than 11 mils. 
   
   
       16 . A high density circuit module comprising:
 a first CSP;   a second CSP, the second CSP being disposed above the first CSP;   flex circuitry connecting the first CSP and the second CSP, the flex circuitry comprising
 a conductive layer overlaid by an outer layer, 
 plural flex contacts disposed along the conductive layer and accessible through the outer layer, and 
 an orifice in at least one of the plural flex contacts, which orifice has a median opening extent of DO; and 
   a consolidated contact comprising an inner flex portion and an outer flex portion delineated by the orifice, the consolidated contact disposed to pass through the orifice and provide a connection between the first CSP and the conductive layer of the flex circuitry, with the outer flex portion being disposed adjacent to the outer layer of the flex circuitry and having a median lateral extent of DCC and DCC is larger than DO.   
   
   
       17 . A high density circuit module comprising:
 a first CSP;   flex circuitry having a first side and a second side; and   a second CSP in a stacked relationship with the first CSP, the second CSP having plural consolidated contacts, each of which consolidated contacts is one piece of metal comprising
 an inner portion disposed between the first side of flex circuitry and the second CSP, 
 an outer portion that is larger than the inner portion, which outer portion is disposed along the second side of the flex circuitry, and 
 a portion disposed through the flex circuitry and in connection with a conductive layer of the flex circuitry. 
   
   
   
       18 . The high density circuit module of  claim 17  in which the flex circuitry comprises an outer layer that is disposed at least in part between the conductive layer of the flex circuitry and the outer portion of at least one of the consolidated contacts. 
   
   
       19 . The high density circuit module of  claim 17  in which the flex circuitry comprises at least two conductive layers, and one or more of the plural consolidated contacts is in electrical connection with only one of the conductuctive layers of the flex circuitry.