Abstract:
Parallel surfaces are interfacially mechanically bonded and optionally electrically, and/or thermally connected using an interposer fabricated from a flexible laminates, such as flex PWB. The bond/connection points for the device made on one side of the interposer are displaced in the X and Y axis from the bond/connection points on the other or obverse side for the interposer and the board. The optional electrical/thermal connection through the interposer is made through one or more traces and vias in the flex board.

Description:
BACKGROUND 
     This invention relates to techniques for forming a compliant interfacial mechanical, thermal and/or electrical bond interface between planar surfaces. 
     A single chip module (SCM) or multichip module (MCM) for military or space applications typically has a surface mount ceramic single chip package (SCP) or multichip package (MCP) for hermeticity, reliability, thermal and double-side printed wiring board assembly (PWA) requirements. 
     A peripheral leaded ceramic quad flat pack (CQFP) package with a high pin count requires a large ceramic body and an even larger PWB footprint. Even with a small lead pitch of 20 mils, a square CQFRP package with 144 to 576 pins requires a ceramic body size of about 0.80 to 2.96 inches on a side, a PWB footprint size of about 1.16 to 3.32 inches on a side (about 0.36 inches larger than the ceramic body on each side due to formed peripheral leads) and has a surface interconnect density (number of PWB contacts divided by the PWB footprint area) of 107 to 52 contacts per square inch. In contrast, a square ceramic area array (CAA) package with 144 (12×12) to 576 (24×24) pins, such as a land grid array (LGA), ball grid array (BGA), column grid array (CGA) or pin grid array (PGA), with a 50 mil grid pitch requires a ceramic body and PWB footprint size of 0.60 to 1.20 inches on a side with a constant interconnect density of 400 p ins per square inch. 
     In comparis on, these CAA packages require only 0.27 to 0.13 times the area and offer 3.74 to 7.65 times the interconnect densities of their equivalent CQFP packages. The PWB footprint area and interconnect density advantages of CAA over CQFP packages increases with pin count. 
     Surface mount attachment of large heavy ceramic packages to a PWB for military and space applications presents several challenges in terms of electrical and thermal erformance, mechanical integrity, reliability, assembly and rework. For one, the temperature coefficient of expansion (TCE) differences between the package body and PWB materials, along with PWB deflections, under worst case shock, vibration, acceleration and temperature cycling environments can cause distortions and high stress levels in the attachment and interconnect interfaces. 
     The attachment and interconnect interfaces of a surface-mounted CQFP package are typically separate. The attachment interface is comprised of interfacial solder and/or epoxy between the ceramic body and the PWB. The interconnect interface is comprised of relatively long gull-lead-formed peripheral leads which provide X-axis, Y-axis and Z-axis compliance. 
     The attachment and interconnect interfaces of CAA packages (LGA, BGA, CGA or PGA) are one and the same. With conventional vapor phase solder surface-mount assembly, the attachment/interconnect interface is comprised of an interfacial array of short direct vertical solder ball, solder column or pin connections. This array of rigid vertical interfacial connections has some X-axis and Y-axis compliance (amount dependent upon connection aspect ratio or length divided by cross-sectional area), but offers little or no Z-axis compliance. For an area array package to be used in military and space applications, its interfacial attachment/interconnect interface should be modified to provide enough X-axis, Y-axis and Z-axis compliance to absorb distortions of the interfacial interface between the package and the PWB and to reduce stress on solder (or other adhesive) joints that leads to attachment and/or connection failures. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an improved method of interfacially mechanically bonding two surfaces with one or more compliant paths that may also be electrically and/or thermally conductive, such as mounting and interconnecting an electronic device to a PWB. 
     According to the invention, a compliant (flexible) interposer is used to form part of one or more interfacial electrical, mechanical and/or thermal attachment/conduction paths between two surfaces, such as a PWB and a SCM or MCM (LGA, BGA, CGA or PGA), a connector, a heat sink or another PWB. Assuming that the surfaces to be connected and the interposer lie parallel to a horizontal X-Y plane, each electrical, mechanical and/or thermal path between the two interposer surfaces is made through a path comprised of two vertical columns (attachment interfaces) of adhesive material (one between each surface and the interposer) and a horizontal and possibly a vertical path through the interposer itself. Each path through the interposer is comprised of a pair of attach pads, one on each side and displaced from each other in the X and Y directions, at least one surface or buried trace to provide a horizontal connection and one or more vias to provide a vertical connections if the attach pads are not on opposite sides of the same conductor layer. 
     The example depicts the case of the horizontal path comprised of two opposing surface traces with vias connecting them. Each surface trace has an integral (mask-defined) attachment pad at one end and an integral via pad at the opposite end. Each via connects the attach pad portion of one surface trace to the via pad portion of the opposing surface trace. Therefore, each mask defined attach pad has redundant trace-via and via-trace conduction paths to its mating mask-defined (X,Y displaced) attach pad on the opposite side of the interposer. 
     According to one aspect of the invention, conductive or non-conductive bumps (e.g. solder, epoxy or thermoplastic) are first applied to one or more pairs of the attach pads on opposite sides of the interposer and are subsequently bonded to mating attach pads on a PWB and a device to form one or more electrical, mechanical and/or thermal connection paths between the PWB and device. 
     According to one aspect of the invention, the surface trace width is tapered from the full diameter of the integral attach pad at one end to the full diameter of the integral via pad at the other end to minimize electrical and thermal resistance between attach pad pairs. 
     A feature of the present invention, it is especially useful in improving the reliability of interfacial interfaces under high G-force applications, such as space craft and high performance aircraft. 
     Other objects, benefits and features of the invention will apparent to one of ordinary skill in the art from the drawing and following description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is top plan view of an interposer embodying the present invention. 
     FIG. 2 is a cross-section along line  2 — 2  in FIG.  1 . 
     FIG. 3 is a cross-section along line  3 — 3  in FIG.  1 . 
     FIG. 4 is a cross-section along line  4 — 4  in FIG. 1 
     FIG. 5 is a top plan view along line  5 — 5  in FIG.  4   
    
    
     DESCRIPTION 
     Referring to FIG.  1  and FIG. 4, an interposer  10  is fabricated using a flex PWB  12  with a plurality of integral connection paths  13 , each comprised of a pair of directly opposing surface traces  14 ,  15  (e.g. foil conductors) and a pair of vias  16 ,  18  (conductive lined and/or filled passages) that extend in the Z direction, through the flex PWB  12 . Focusing in particular on just one connection path  13 , each surface trace  14 ,  15  is comprised of an integral mask-defined attach pad  20 ,  22  at one end and a smaller integral via pad  24 ,  26  at the other (opposite) end. The attach pads are displaced from each other in the X and Y directions, e.g. by half a grid. Each via  16 ,  18  connects the center of each attach pad  20 ,  22  for each trace  14 ,  15  to the center of the via pad  24 ,  26  on the opposite side of the flex circuit board  12 . 
     FIGS. 4 and 5 illustrate that the surface traces  14 ,  15  on the top and bottom of the flex PWB  12  and the vias through the flex PWB  12  create redundant connection paths within the interposer between each pair of attach pads  20 ,  22 , which provides an electrical, mechanical and/or thermal connection path between one surface  32  (e.g. a main PWB) and another surface  28  (e.g. a device). A solder or polymer bump  30  is formed on one attach pad  20  over via  16 . Likewise, a solder or polymer bump  30  is formed on the other attach  22  over via  18 . Solder bumps are one way to interfacially attach one surface  28  (e.g. a device) to the interposer  10  and the interposer  10  to another surface  32  (e.g. a PWB) forming one or more electrical, mechanical and/or thermal connection paths between the surfaces  28  and  32 . The solder bumps can be formed by screen printing or stenciling solder paste on the attach pads of both sides, followed by a vapor phase reflow or by a controlled solder dipping process. Polymer bumps can be formed by screen printing or stenciling polymer onto the attach pads of both sides followed by a partial cure. Surface solder or polymer mask layers  36  should cover all but the attach pad areas  20  and  22  of the surface traces  14 ,  15  and vias  16 ,  18 . This limits solder or polymer whetting and bonding to attach pads  20 ,  22 , promotes self-alignment of the device to interposer and interposer to PWB interfaces and vents the vias. Vias are vented so that whetting and capillary action fills the holes with solder or polymer as they are applied. Conventional techniques can be used to fabricate the connection paths  13  in the interposer  10  that make electrical and/or mechanical connections between one surface  32  (e.g. a PWB) and another surface  28  (e.g. a device) at various connection points, as described. For instance, assuming the flex PWB is double-clad cast polyamide, standard PWB processes are used to drill and plate holes with copper  34  to form vias and pattern surface traces  20 ,  22 . All but the attach pad and via holes are masked  36  and solder or polymer  30  is applied to the top and bottom of the interposer  10 . In its liquid form (during reflow or partial cure) the solder or polymer fills the vias  14 ,  16  and forms a hemispherical bump  30  due to capillary action and a combination of surface tension and adhesion to the attach pads  20 ,  22 . 
     Because the attach pads to one surface  28  (e.g. a device) are displaced in the X and Y directions relative to the attach pads to the other surface  32  (e.g. a PWB), mechanical stresses due to flexing of the interface in the Z direction are better tolerated as compared to a direct vertical connections in the Z direction between the surfaces  28 ,  32 . The conductive traces  20 ,  22  in the X-Y plane form a jog or cantilever in each connection path that absorbs Z-axis deflections and reduces stress on solder or polymer joints. The amount of X-axis and Y-axis compliance is defined by the combined height of the vias and the solder or polymer bumps when the device and wiring board on attached to the interposer. Another assembly processes would include pre-forming solder or polymer bumps on the attach pads of the surfaces  28 ,  32  to be attached instead of on the attach pads of the interposer  10 . Another assembly process would include applying solder or polymer paste to the attach pads of either the interposer  10  or attach surfaces  28 ,  32 , placing the interposer  10  between the surfaces  28 ,  32  and reflowing or curing the solder or polymer bonds in a single step (i.e. no pre-bumping). 
     It should be appreciated that an interposer  10  construction with a single conductor layer is possible, where the attach pad pairs  20 ,  22  are integral parts of a common trace  14 / 15 , vias  16 ,  18  are not required. In an application, where only mechanical solder bonding is required (no electrical or thermal connection required), only the solderable attach pads  20 ,  22  and the mask layers  36  are required in the interposer  10  construction. 
     Likewise, in the case where only mechanical polymer bonding is required (no electrical or thermal connection required), only the mask layers  36  are required in the interposer  10  construction. 
     In some applications, where only mechanical bonding and a single electrical and/or thermal conduction path is required, only the mask layers  36 , over a continuous flexible electrically and/or thermally conductive sheet (e.g. copper), would be required in the interposer  10  construction. 
     One skilled in the art may make modifications, in whole or in part, to described embodiment of the invention and its various functions and components without departing from the true scope and spirit of the invention.