Abstract:
The present invention mechanically integrates a flexible printed circuit pre-disposed with solder and flux and two or more leaded integrated circuit packages into an assembly that does not require a solder reflow process prior to the reflow cycle to attach the assembly to a printed circuit module. Each IC device includes: (1) a package having a top, a bottom and sides; and (2) external leads that extend out from one or more sides for electrical connectivity to a printed circuit module. Each flexible circuit includes: (1) a multi-segment pattern for each IC connection where there is a segment for: (a) attaching a package lead to the flexible printed circuit; (b) a segment for attaching a preformed piece of solder and flux; (c) a bridge for the solder to flow when heated to the package lead attach segment; (2) solder and flux and (3) adhesive to bond the flexible printed circuit to the packages and bond the packages together.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to integrated circuit devices. More particularly, this invention relates to a flexible circuit interposer for a stacked integrated circuit module. 
       BACKGROUND OF THE INVENTION 
       [0002]    Designers of electronic component ranging from portable consumer electronics to massive computer platforms have constantly strived to reduce the size of the systems. The reasons vary from the convenience of carrying one&#39;s music library around in one&#39;s pocket to limiting the interconnect network in order to reduce the loading so that electrical signals may operate at higher speeds. 
         [0003]    Various methods have been employed over the years to reduce the size of systems starting with integrating circuits onto a piece of silicon, then integrating multiple circuits into a single device. However, where multiple instances of a particular device was employed and the size of the die was at the point where no more could be integrated on the die, as is often the case with memory devices, the designers started stacking devices one atop another with an interconnect scheme that electrically connected common signals while isolating and re-routing unique signals. 
         [0004]    The state of the art advanced and technologies were then developed that allowed the integration of multiple instances of the silicon die to be integrated into a single package. This provided the designers with components that had multiple instances of the silicon die in a single package without the need for an electrical and mechanical stacking. However, the trend to reduce the size of systems has outpaced the technology of integrating multiple die into a single package. The industry once again finds itself stacking like devices in a system. 
         [0005]    A new requirement has been placed on electronic systems in recent years. Environmental concerns over the use of potentially hazardous substances in electronic systems have led to initiatives to eliminate the use of these substances. A key component in the solder that was used to electrically and mechanically connect semiconductor packages to modules and, of particular concern, stacks of devices together is lead (Pb). 
         [0006]    While lead (Pb) in solder enabled low melting temperature solder. Lead free solders have much higher melting points. Typical lead-free solders require temperatures of up to 265° C. The silicon semiconductor devices do not fare well at high temperatures. Multiple cycles through reflow ovens at lead-free reflow temperatures are having an adverse effect on the semiconductor devices. Some of the failures due to lead-free solder reflow cycles are data retention (memory devices), bond wire corrosion and hard failures of the devices. This has caused the manufactures of the semiconductors to specify a maximum number of reflow cycles that the devices experience. 
         [0007]    Many stacking technologies used today require multiple reflow cycles to assemble the stacked module. In some cases the number of reflow cycles may exceed the specified maximum for the devices that are being stacked. Then the stacked assemblies have to be attached to modules where they could experience two or more reflow cycles. 
         [0008]    Accordingly, what is needed is an improved apparatus for electrically and mechanically coupling stacked integrated circuit devices that reduces or eliminates high-temperature reflow cycles from the stack assembly process. 
       SUMMARY OF THE INVENTION 
       [0009]    The present invention aggregates multiple leaded package devices into stacked module subassemblies without the need for reflow cycles to electrically and mechanically bond the devices together prior to being assembled onto PCB. When the stacked module is placed on the module and run through the reflow oven pre-dispensed solder and flux in the stack bond the leads together at the same time that the stacked module subassembly is being soldered to the module. 
         [0010]    The present invention increases the capacity of the device footprint, minimizes the interconnection network length, and provides ample power to all devices without the need of high temperature reflow cycles in the assembly process. The present invention can be used advantageously to increase the total memory capacity of portable consumer electronics or a computing system. 
         [0011]    In a preferred embodiment implemented in accordance with the present invention a flexible printed circuit approximately equal to the length of the side of the semiconductor package to be stacked where electrical leads are disposed is patterned on a first side with an electrically conductive material to align with the foot of the leads of an upper device. The flexible printed circuit is patterned on the second side an electrically conductive material to align with the shoulders of the leads of the lower device in the stack. These patterns have a one to one correspondence with the leads of the packages to be stacked. The electrically conductive pattern on the flexible printed circuit is divided into three segments. The first segment is the area that comes into contact with the lead of the package being stacked and its size is in accordance with good surface mount practices. The second segment is located adjacent to the first segment and is connected to the first segment by a third segment. The second segment is sized to have a preformed piece of solder and flux attached of sufficient size that when heated will flow across the third section coating the first section and a sufficient portion of The features and advantages of the invention will become apparent upon reading the following specification, when taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  depicts the present invention. 
           [0013]      FIG. 2  shows lead bearing edge view of present invention. 
           [0014]      FIG. 3  shows non-lead bearing edge view of present invention. 
           [0015]      FIG. 4  depicts flexible circuit interposer. 
           [0016]      FIG. 5  depicts the types of contact pads used on interposer module 
           [0017]      FIG. 6  depicts the interposer sub assembly 
           [0018]      FIG. 7  Depicts a cross section of the flex circuit through the footprint and solder reservoir. 
           [0019]      FIG. 8  shows the process flow for creating the interposer sub-assembly. 
           [0020]      FIG. 9  shows an exploded view in cross section of a contact bearing edge of the present invention. 
           [0021]      FIG. 10  shows the interposer subassembly mounted on the lower device in cross section of a contact bearing edge of the present invention. 
           [0022]      FIG. 11  shows in cross section of a contact bearing edge the present invention the assembled stack. 
           [0023]      FIG. 12  shows in cross section of a contact bearing edge the present invention placed on a module prior to the solder reflow cycle. 
           [0024]      FIG. 13  shows in cross section of a contact bearing edge the present invention placed on a module after the solder reflow cycle. 
           [0025]      FIG. 14  depicts an alternative embodiment of the present invention mounted on a module 
           [0026]      FIG. 15  shows an exploded view in cross section of a contact bearing edge of an alternative embodiment of present invention. 
           [0027]      FIG. 16  shows in cross section of a contact bearing edge an alternative embodiment of present invention. 
           [0028]      FIG. 17  shows in cross section of a contact bearing edge an alternative embodiment of the present invention placed on a module after the solder reflow cycle 
           [0029]      FIG. 18  depicts the process flow for creating the present invention. 
           [0030]      FIG. 19  depicts the process flow for creating an alternative embodiment of the present invention. 
           [0031]      FIG. 20  shows an exploded view in cross section of a contact bearing edge of yet another alternative embodiment of present invention. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENT 
       [0032]    The following embodiments introduce new construction concepts directed at providing higher-density memory solutions with fewer solder re-flow cycles leading to a higher reliability module. The embodiments disclosed herein may be broadly classified as “Multi-Chip Modules” in that they are comprised of multiple packages in a vertical stack. 
         [0033]      FIG. 1  depicts a top view of the stacked module  1  of the present invention. Visible is a device  10  that is known in the industry as a Thin Small Outline Package (TSOP) type-1. Semiconductors that are available in this package are typically flash devices. One skilled in the art will appreciate that the present invention may be employed to stack any leaded package. 
         [0034]    The upper device  10  is the only device  10  visible in this view because the lower device  10  is placed directly below the upper device  10 . The flexible interposer assembly  11  is visible where it protrudes beyond the extent of the lead bearing edge  103  of the device  10 . 
         [0035]      FIG. 2 , view B-B of  FIG. 1 , is an elevation of the lead  101  bearing side  103  of the stacked module  1 . Here it starts to be come clear how the stacked module  1  interconnects the upper and lower devices  10 . The leads  101  of the upper device  10  are aligned over contacts  110  on the interposer sub-assembly  11 . The interposer subassembly  11  had previously been positioned over the lower device  10  with the contacts  110  aligned with the leads  101 . 
         [0036]    While the preferred embodiment is to stack two of the same package types one skilled in the art will be appreciated how the present invention may be used to stack different package types. 
         [0037]      FIG. 3 , view C-C of  FIG. 1 , is an elevation of the non-lead bearing side  104  of the stacked module  1  mounted to a Module  20 . In this view the connection of the lead  101  of the lower device  10  is connected to the flexible circuit interposer  11  with solder  14  and the lead  101  of the upper device  10  is connected to the interposer through solder  14 . Additionally the lower device  10  is bonded to the interposer subassembly  11  with an adhesive  13  and the upper device  10  is bonded to the interposer subassembly  11  with a second layer of adhesive  13 . 
         [0038]      FIG. 4  shows the flexible circuit module  111  that is used to create the interposer  11  sub-assembly. Shown is a flexible circuit  111  with a top surface  118  that is bonded to an upper device  10  and a bottom surface  119  that is bonded to the lower device  10 . Disposed along the edges of the flexible circuit  111  are rows of contacts  110  that align with the leads  101  of the devices  10  to be stacked. In addition to the contacts there may be etch that connects the contact pad  110  with a via to route to a circuit on the other side of the module, or to another pad on the current side of the flex circuit. 
         [0039]      FIG. 5   a  details the contact pad  110  that is a key part of the stacked module  1 . The contact pad  110  is divided into three separate sections. First is the contact region  114 . The contact region  114  is the area where the bond to the device  10  lead  110  is made. The second region  116  is a reservoir region where a solder and flux composite is attached prior to assembling the stack. The third region is a bridge  115  between the contact region  114  and the reservoir region  116 . In an alternative embodiment the solder flux composite may be replaced with a solder ball and a flux applied separately. 
         [0040]    The reservoir region is positioned such that when the stacked module  1  is heated in a solder reflow oven capillary action and gravity will draw the molten solder and flux across the bridge  115  to form an electrical and mechanical bond between the contact region  114  and the device lead  101 . 
         [0041]    The contact may have additional features as shown in  FIG. 5   b  and  FIG. 5   c . In  FIG. 5   b  a piece of etch  112  extends from the reservoir region  116  to connect to another feature on the flexible circuit module  111 .  FIG. 5   b  shows a typical implementation of the contact  110  where a short run of etch  112  connects the reservoir to a pad w/via  117 . This arrangement is typically used with an identical contact  110  on the opposite side of the flexible circuit  111 . In this configuration the contacts  110  on the top side  118  and the bottom side  119  of the flexible circuit  111  are electrically connected through the via  117 . Another application of this configuration of  FIG. 5   c  is where this configuration is used on the bottom side  119  and the via connects to etch  112  on the top side  118  that re-routs the signal brought into the stacked module  1  on a lead  101  on the lower device and used by the upper device  10  on a different lead  101 . 
         [0042]    The completed interposer sub-assembly  11  is shown in  FIG. 6 . Disposed on the reservoir region  116  of the contact pad  110  is a preformed piece of solder-flux composite  14 . On the region of the flexible circuit that is sandwiched between the two packages to be stacked one or more pieces of adhesive  113  are placed. The adhesive  13  on the bottom side  119  bonds the lower device  10  to the interposer sub-assembly  11  and the adhesive  13  on the upper side  118  bonds the upper device  10  to the interposer subassembly  11  thus bonding the upper device  10  to the lower device  10  until the stacked module  1  is placed on a module and the solder-flux composite  14  bonds the leads  101  together during a solder re-flow procedure. 
         [0043]      FIG. 7  shows the cross section view D-D from  FIG. 6 . Here the placement of contacts  110  opposite each other on the top side  118  and the bottom side  119  of flex circuit  111 . The via  117  electrically connects the contacts  110 . On the reservoir region  116  the solder-flux composite  14  is attached. 
         [0044]      FIG. 8  presents a sequence of process steps used to assemble the interposer  11 . In process step  81  The bare flexible printed circuit  111  has an adhesive disposed on side  118  and in Process step  82  the preformed solder and flux composite  14  are attached to the reservoir region  116  of the contacts  110 . Then in step  83  the flexible circuit is flipped over and an adhesive is disposed on site  119  and the preformed solder and flux composite  14  are attached to the reservoir region  116  of the contacts  110 . The interposer subassembly  11  is now ready to assemble the stacked module  1 . 
         [0045]      FIG. 9  is an exploded view of the stacked module  1  through the cross section A-A from  FIG. 1 . There are upper and lower devices  10  with leads  101  protruding from a lead bearing edge  103 . The leads  101  from the upper and lower devices  10  align with contact pads  110  in the flexible circuit  111 . Attached to the contact  110  is a preformed piece of a solder-flux composite  14 . 
         [0046]      FIG. 10  shows the step  82  in creating the stacked module  1 . The contact  110  has been aligned with the lead  101  and the adhesive  13  has bonded the interposer  11  to the lower device  10 . The preformed solder-flux piece  14  may or may not contact the lead  101  at this point depending on where on the lead bearing edge  103  the lead  101  protrudes. 
         [0047]      FIG. 11  shows the completed stacked assembly. The upper device  10  has been positioned over the lower device  10  and Interposer  11  sub-assembly from  FIG. 10 . At this point the stacked assembly has not been subjected to a solder reflow cycle. The preformed solder-flux composite  14  is still in the preformed state. 
         [0048]      FIG. 12  shows the stacked assembly  1  of  FIG. 11  placed on a PCB  20 . The leads  101  of the lower device  10  are used to electrically and mechanically bond the stacked module  1  to the module  20 . This is done by aligning the leads  101  of the lower device  10  with the pads  21  on the module  20 . Disposed on the pad  21  prior to placing the stacked module  1  is a solder paste  15 . 
         [0049]      FIG. 13  shows a view of the stacked module  1  through the cross section A-A from  FIG. 1  after the solder reflow process. The solder paste  15  has melted and bonded to the lead  101  of the lower device  10  to the pad  21  of the module  20 . The pre-formed solder flux composite  14  attached to the bottom side  119  of the interposer  11  has melted and flowed from the reservoir  116  across the bridge  115  and bonded the lead  10  to the contact pad  114 . The pre-formed solder flux composite  14  attached to the top side  118  of the interposer  11  has melted and flowed from the reservoir  116  across the bridge  115  and bonded the lead  10  to the contact pad  114 . 
         [0050]      FIG. 14  shows an alternative embodiment of the present invention In this embodiment the leads  102  of the upper device  10  have been re-shaped prior to being placed in the stacked assembly  2 . 
         [0051]      FIG. 15  is an exploded view of the alternative embodiment of the stacked module  2  through the cross section A-A from  FIG. 1 . The leads  101  from the lower device  10  protruding from a lead bearing edge  103  are un-modified. The leads  102  from the upper device  10  protruding from a lead bearing edge  103  are straightened. 
         [0052]      FIG. 16  shows the completed alternative embodiment of the present stacked module  2 . The upper device  10  has been positioned over the lower device  10  and Interposer  11  sub-assembly from  FIG. 10 . The re-shaped lead  102  of the upper device  10  has deflected the interposer  11  so that the contact  110  on the bottom side  119  of interposer  11  has come into contact with the lead  101  from the lower device  10  and the contact  110  on the upper surface  118  and come into contact with the lead  102  of the upper device  10 . 
         [0053]      FIG. 17  shows a view of alternative embodiment of the stacked module  2  through the cross section A-A from  FIG. 1  after the solder reflow process to mount the assembly  2  to the module  20 . The solder paste  15  has melted and bonded to the lead  101  of the lower device  10  to the pad  21  of the module  20 . The pre-formed solder flux composite  14  attached to the bottom side  119  of the interposer  11  has melted and flowed from the reservoir  116  across the bridge  115  and bonded the lead  101  to the contact pad  114 . The pre-formed solder flux composite  14  attached to the top side  118  of the interposer  11  has melted and flowed from the reservoir  116  across the bridge  115  and bonded the lead  102  to the contact pad  114 . 
         [0054]      FIG. 18  presents a sequence of process steps used to assemble the stacked module  1 . The lower device  10  is placed in a fixture or on a surface in step  181  to support it during the assembly process. In the next process step  182  the interposer subassembly  11  is placed on the lower device from the previous step  181 .  FIG. 10  depicts the result of this step  182 . In the next step  183  the adhesive  13  bonding the interposer subassembly  11  to the lower device  10  is activated. The adhesive may be a pressure sensitive adhesive (PSA) where the placement of the interposer subassembly  11  on the lower device  10  will activate the adhesive or it may be a thermal set adhesive such as 3M&#39;s APAS where a relatively small rise in temperature will partially set the adhesive. 
         [0055]    The next step  184  is to place the upper device  10  on the lower device  10  and interposer subassembly  11 . As in the previous adhesive activation step  183  the adhesive  13  bonding the upper device  10  to the interposer subassembly  11  is activated. 
         [0056]    With the devices  10  stacked and mechanically bonded together into the stacked module  1  the assembly is placed on a module  20  that has had solder paste  15  disposed on the surface mount pads  21  in the next step  186 . The module  20  and stacked module  1  are then subjected to temperatures sufficient to melt the solder paste ( 15 ) and the solder and flux composite ( 14 ) in the present invention. 
         [0057]    The process shown in  FIG. 19  is the same as the process of  FIG. 18  with the addition of an intermediate step  194  to reshape the leads  101  of the upper device  10 . This process flow shows the reshape step  194  prior to the device  10  being placed on the device  10  and interposer subassembly  11 . One skilled in the art can appreciate that this step  194  could also be done after the upper device  10  is placed on the lower device  10  and interposer subassembly  11 . 
         [0058]      FIG. 20  depicts yet another embodiment of the stacked module  3 . Here multiple devices  10  are stacked with an interposer  11  between each device  10 . 
         [0059]    It will be seen by those skilled in the art that various changes may be made without departing from the spirit and scope of the invention. For example, while the stacked module  1  has primarily been described in terms of a first and a second IC device, skilled persons will recognize that a stacked module  1  of the present invention may include multiple stacked IC devices coupled together by flexible circuit conductors mounted between adjacent devices. 
         [0060]    Furthermore, in the depicted embodiment, Thin Small Outline Packaged (TSOP) devices with leads extending from one pair of oppositely-facing peripheral sides are shown. However, the invention can be used with any commercially available packaged devices and other devices including but not limited to TSOP, custom thin, and high lead count packaged integrated circuit devices. 
         [0061]    Accordingly, the present invention is not limited to that which is expressly shown in the drawings and described in the specification.