Abstract:
Integrated circuit modules made in accordance with the methods of the present invention have multiple Ball-Grid Array (BGA) packages mounted to a substantially planar support substrate. Each package is inclined at an angle to the support substrate and partially overlaps another package. The first package or row of packages overlaps a wedge that is provided for support. A flexible substrate is mounted to each BGA package and has a portion that extends away from the package to adhere to the support substrate, for communication between each package and signal lines on the support substrate. Optionally, a portion of the substrate that extends away from the integrated circuit package can be bent back at a 180° angle to allow the pads on the top surface of the flexible substrate to attach to mating pads on the support substrate. Precise control of the impedance of select signal traces is accomplished through the use of ground traces that run parallel with the select signal, a ground plane layer, and alternating signal-substrate mounting pads with GND-substrate mounting pads. One aspect of the present invention has the package leads located in close proximity to one side of the package for optimally short trace lengths. Another aspect provides a method for routing signals for optimal signal length and elimination of stubs. A preferred embodiment of a flexible substrate has an accordion portion formed in the flexible substrate to provide elasticity for preventing stretching and distortions of the flexible substrate during automated assembly. A method is provided for connecting signals lines of each flexible substrate in series. Various embodiments of package lead mounting pads formed in the flexible substrate that provide electrical and thermal coupling to package leads including ring-shaped pads, flexible tabs, and apertures for removing flux, excess solder, and debris.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to multi-package integrated circuit modules. In particular, this invention relates to Dual In-line Memory Modules (DIMM), comprised of Ball-Grid-Array (BGA) or other near chips scale or chip scale packages, that have increased density, enhanced reliability, and precision impedance control for high speed memory applications. The DIMM module physical size and the system interface conform to one of two specifications for 168 pin, 8-byte wide DIMMs; the first generation buffered DRAM DIMM, and the second generation unbuffered DRAM DIMM 
     DIMMs based on RAMBUS™ technology are capable of data transfer rates up to 800 Mhz using low voltage swings of 800 mv. Data is transferred across a 16-bit bus on both edges of a 400 Mhz-clock signal. The high-speed operation of a RAMBUS CHANNEL™ requires dense packaging and high quality transmission lines. The RAMBUS CHANNEL™ high-speed signals require matched transmission lines that have the identical propagation characteristics and travel across a uniform, parallel layout. These high-speed signals require a uniform matched impedance of 25 to 65 ohms over the entire length of each signal-transmission line after all components are installed. Multiple DRAM components that are electrically coupled to a single data transmission line present periodically spaced capacitive loads, which significantly effect the impedance of a portion of the transmission line. The periodic spacing between these capacitive loads on the transmission line is referred to as pitch. Decreasing the pitch has the effect of decreasing the impedance of the transmission line as a whole. Other parameters which effect the impedance of a signal transmission line are the properties and thickness of the material located between the line and either an adjacent ground plane, ground line, or signal line. Increasing the dielectric thickness or decreasing the signal line width has the effect of increasing the impedance of the effected portion of the transmission line. RAMBUS™ layout guidelines require the high-speed channel signals to be routed on a single signal layer with no vias, and with data signals routed in parallel with clock signals. Vias typically have a physical shape that creates a capacitance effect in the order of 1 pF; for this reason, the RAMBUS CHANNEL™ guidelines state that vias are not allowed on high-speed transmission lines. These guidelines allow for 20% variation in dielectric thickness, signal line trace widths, and copper thickness. Signal line trace length among the 13 high-speed signals can vary by 100 mils (2.5 mm). Deviations from the guidelines that maintain the desired transmission line characteristics are acceptable. 
     Ball-Grid-Array (BGA) Integrated circuit packages typically have leads that extend from the bottom surface of a rectangular solid casing in a two-dimensional grid pattern. Package leads provide electrical and thermal coupling to one or more integrated circuit dies that are embedded within the protective casing. The external portion of each lead has a relatively large ball of solder. Typically, the protective casing completely surrounds the embedded die, but in some BGA packages, the protective casing does not cover the inactive top surface of the die. Near-chip scale packages provide 1.0 mm center-to-center lead spacing. Chip-scale packaging, having 0.5 mm center-to-center lead spacing, offers excellent electrical characteristics including low capacitance and thermal design. 
     Prior methods of manufacturing memory modules unnecessarily limit the number of integrated packages that can be placed on a standard DIMM module while maintaining electrical characteristics required for the high-speed transmission-lines of RAMBUS CHANNEL™. 
     Known methods for manufacturing multi-package modules, typically have signal transmission lines with inconsistent impedance in portions as well as structures and multiple-dissimilar materials comprising a single transmission line that can cause reflections of propagating high-speed signal transitions, such as the data transfers that occur each 1.25 nanosecond period on a RAMBUS CHANNEL™. These reflections compromise the integrity of the system by increasing the settling time of the signal. This settling time is commonly referred to as ringing. Ringing is high frequency signal oscillation, typically in the 100 MHZ to 1 GHz range, that can occur when signals quickly transition from one voltage level to another as a result of reflections. This ringing can create cross talk. Crosstalk is undesired noise added to a signal resulting from a magnetic field radiated from adjacent signals. Ringing also increases the overall electromagnetic radiation from the system. When ringing couples over as noise to power input signals or increases ground bounce it will slow the response from the high-speed devices such as memory. More precise control of the impedance of the transmission lines is necessary to reduce ringing. 
     SUMMARY OF THE INVENTION 
     The methods and apparatus of the present invention enable the economical manufacture of multi-package modules comprised of chip-scale packages, and provide enhanced thermal characteristics, while maintaining high-quality transmission lines required for RAMBUS CHANNEL™. Integrated circuit modules, made in accordance with the methods of the present invention, include multiple chip scale packages mounted to a support substrate. Each package is inclined at an angle to the support substrate and partially overlaps another package. The first package or row of packages may be supported by a wedge structure on the support substrate to establish the angle of inclination of the packages. A flexible substrate or flex circuit is mounted to the bottom surface of each package and has a portion that extends away from the mounted integrated circuit packages to adhere to the support substrate. It will be apparent to those skilled in the art of integrated package manufacturing that the methods and apparatus described herein are applicable to a broad range of modules comprised of multiple integrated circuit packages. 
     One embodiment of the present invention has chip scale packages mounted to the upper surface of a flexible substrate. Package lead pads formed in the flexible substrate electrically couple to select package leads. A portion of each flexible substrate extends away from the integrated packages and includes circuit interconnect pads located on the lower surface for electrical and thermal coupling to corresponding interconnect pads on the support substrate. Another embodiment of a flexible substrate of the present invention, has the extended portion folded back, at a 180° angle to itself, allowing circuit interconnect pads on the upper surface of the flexible substrate to couple with corresponding interconnect pads on the support substrate. 
     An alternative embodiment provides two chip scale packages mounted to opposite sides of two adjacent flexible substrates, each of which is coupled to a different one of the package&#39;s leads. The extended portion of each flexible substrate attaches to the support substrate. 
     Different specific aspects of the present invention can be used singularly or in combination with other methods of the present invention for precision control of the impedance in each portion of select transmission signal lines formed in the flexible substrate. One method utilizes one or more electrical ground (GND) traces that are located in close proximity and parallel to a substantial portion of the signal trace. Another method has a ground (GND) plane layer separated from the signal layer by a dielectric layer. One method arranges interconnect pads, which couple signal lines on the flexible substrate to signal lines on the support substrate, such that interconnect pads that are to carry high speed signals are located adjacent to interconnect pads that are designated to be coupled to GND. 
     Different preferred embodiments are described herein for the package lead mounting pads, or receptacles, formed in the flexible substrate, for coupling select package leads to select signal lines formed in the flexible substrate. One embodiment of the present invention provides apertures or receptacles in the flexible substrate that are adapted to receive package leads. A ring-shaped pad, formed in the flexible substrate, surrounds a substantial portion of each aperture. The application of heat causes the solder on the distal end of the package leads to flow, electrically and thermally coupling each lead to a ring-shaped pad. Alternately, a flexible tab formed so as to extend into each aperture may be provided for coupling to the package leads. Each flexible tab is formed with a package lead mounting pad for connection to a package lead. 
     In the preferred embodiment, a layer of thermally conductive adhesives having a thickness approximately equal to the nominal length of a package lead is provided on the lower surface of the package. Select void areas in the adhesive allow package leads to extend through the layer and firmly mate with the package lead mounting pads. In all embodiments, it is preferred to use what is known in the industry as a no-clean solder paste, such as Alpha Metals Ultraprint 78, which eliminates the need for post-solder cleaning in the area of the lead to pad coupling. If conventional solder is used, it is preferred to provide an aperture around the flexible tab or ring pad that is wide enough to allow removal of flux, debris, and excessive solder by rinsing or similar procedure. Alternately, the aperture can have a package mounting pad extending from one side of the aperture to the other leaving two void areas on either side for the vacuum suction of flux, debris, and excessive solder. 
     One aspect of the present invention for minimizing the length of signal lines utilizes ball-grid-array packages where the package leads are located in close proximity to one side of the package. The package is mounted to a flexible substrate or flex circuit with the package leads in close proximity to the extended portion of the flexible substrate. 
     Another aspect of the present invention provides for a section of the support substrate to be formed substantially thinner than the remainder to decrease the overall height of the module. Yet another aspect of the present invention provides an accordion-like portion formed in the flexible substrate for increased elasticity to facilitate registration of the flexible substrate with the support substrate without imposing stress due to stretching or distortion during automated assembly. 
     The present invention provides a method for the optimal routing of signals through the support substrate to each flexible substrate. Each flexible substrate includes two groups of interconnect pads for connecting to a corresponding set of interconnect pads on the support substrate. Signal lines enter each flexible substrate through a first group of pads, route through conductive traces to select package leads, and exit through the a second group of pads. Signals lines in each flexible substrate are connected to the corresponding signal lines in other substrates through conductive traces formed in the support substrate. Typically, each signal line is connected, in series, with a signal line in one of the other flexible substrates, to form a single signal transmission line, having one starting point, one end point and no stubs. Preferably, the two groups of pads are located near opposing ends of the extended portion of each substrate and traces on the support substrate electrically connect groups of pads on adjacent flexible substrates that are on the same end. 
     The present invention provides inclined packages which allows for increased variability in the height of a package. The methods and apparatus of the present invention provide a variety of techniques for precise impedance control of every portion of select signal transmission lines that may be tailored for a range of specific applications. Prior methods did not provide for optimal short signal transmission lines and typically required structures and multiple interconnections of dissimilar materials that create reflections in high-speed signal transitions. The improvement in impedance control provided by the present invention significantly improves the operational reliability of the system. 
     Other methods for providing high density stacked modules are disclosed in U.S. Pat. Nos. 5,279,029, 5,367,766, and 5,455,740, 5,480,959, and 5,592,364, all of which are assigned to the common assignee of the present invention and all incorporated herein by reference. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 a  is a three-dimensional view of one embodiment of the multi-package module of the present invention; 
     FIG. 1 b  is a three-dimensional view of an embodiment of a single flexible substrate carrying a row of packages for use in the module of the present invention; 
     FIG. 1 c  is an exploded view of a detail from FIG. 1 a , which illustrates an enlarged view of an accordion portion of a flexible substrate of the present invention; 
     FIG. 2 a  is a cross-section of one embodiment of the support substrate of FIG. 1 a  where a portion of the substrate is thinner in the middle than at the edges; 
     FIG. 2 b  is a planar view of a support substrate, with reference arrows to illustrate the preferred method for routing signals; 
     FIGS. 3 and 4 are side views of alternative embodiments of the module of FIG. 1 a;    
     FIGS. 5 a  and  5   b  are partial side views of alternate configurations of a module of the present invention; 
     FIG. 6 is a three-dimensional view of a flexible substrate with one mounted package according to one aspect of the present invention; 
     FIG. 7 is a cross-section along line A—A of FIG. 6 for illustrating an aspect of the present invention; 
     FIG. 8 is a cross-section along line B—B of FIG. 6 for illustrating an aspect of the present invention; 
     FIGS. 9-11 are different alternative cross-sections along line A—A of FIG. 6 for illustrating various alternative embodiments of the present invention; 
     FIG. 12 a  is an alternative cross-section along line A—A of the flexible substrate of FIG. 6 illustrating an embodiment of a flexible substrate and other aspects of the present invention; 
     FIGS. 12 b - 12   d  are planar views of portions of a flexible substrate of the present invention illustrating alternative package lead mounting pad configurations; and 
     FIG. 13 is a another alternative cross-section along lines A—A of FIG. 6 for illustrating various aspects of the present invention. This illustration shows a flexible substrate flanked by the package shown in FIG. 6 and a second package. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the description that follows, numeric reference characters followed by a character from the alphabet reference elements that are a specific embodiment or subset of the element referenced by the numeric character alone. For example, element  65   a  is a subset of element  65  and has all the properties and attributes of element  65 . 
     Referring now to the drawings, FIG. 1 a  illustrates a preferred method for mounting multiple chip scale or BGA packages  50  to a support substrate  80  to form a module M. The embodiment illustrated includes five flexible substrate  60  mounted to a single support substrate  80 . It should be noted, however, that the methods and apparatus of the present invention are not limited to any particular number of packages, or flexible substrates which could be mounted to a specific support substrate. 
     Again with reference to FIG. 1 a , a first portion  63  of each flexible substrate  60  provides mounting for four chip scale or ball-grid-array packages  50  aligned side-by-side to form a single row. Alternately, in accordance with the methods of the present invention, each flexible substrate  60  may have more than one row of packages  50  or merely a single package  50 . FIG. 1 b  shows an alternate view of one of the flexible substrates  60 , shown in FIG  1   a , with four mounted packages  50 . As can be seen in FIG. 1 b , a second portion  64  of each flexible substrate  60  extends away from the packages  50 , and includes interconnect pads  69  for electrical and thermal coupling with corresponding interconnect pads  82  located on the support substrate  80 , as shown in FIG. 2 b.    
     Referring now to FIG. 3, flexible substrates  60  with mounted packages  50  are mounted on the support substrate  80  so that packages  50  are inclined at an angle A with respect to the mounting surface of the support substrate  80 . To define the angle A, the first row of packages  50  are supported by a wedge  90 , which may be adhered to or otherwise formed with the support substrate  80 . The angle of inclination of each subsequently mounted row of packages  50  is established by mounting subsequent rows to overlap a portion of the previously mounted row of packages  50 . 
     Throughout this specification where adhesives are referred, such as adhesive  70 ,  71 ,  72  and  73  shown in FIGS. 3-13, for mounting packages  50  and substrates  60 ,  80 , such adhesives are preferably electrically insulating with enhanced thermal conductivity equal to or greater than the thermal conductivity of the outer portions of the package  50  for facilitating direct heat exchange. Adhesive layers  70 ,  71 ,  72  and  73  may be, for example, an epoxy, such as Rogers Corp. R/flex 8970 which is B-staged phenolic butyryl epoxy that may be laminated at a temperature of 130 degrees centigrade and cured at a temperature of 175 degrees centigrade. Alternately, the adhesive layers  70 ,  71 ,  72  and  73  may be a 2 mil thick sandwich of polyamide film, such as Kapton™ which includes a 0.5 mil thick layer of adhesive on both sides (a three-layer system). 
     In the preferred embodiment, flexible substrate  60  is formed from a commercially available material commonly known as flexible circuit boards or flexcircuits. Such flex-circuits are typically built to custom specifications to relatively close material and physical layout tolerances to provide consistent and accurate electrical characteristics such as impedance. As shown in FIGS. 7,  8  and  1   b , commercially available flexible substrates  60  are typically comprised of one or more thin layers of conductive material that are die cut to form ground planes  67 , signal traces  65 , and pads  69  (FIG. 1 b ) to couple to support substrate  80  and pads  68  to couple to package leads  52  (FIG.  7 ). The conductive elements such as traces  65  and ground plane  67  typically flank a thin (typically 1 to 11 mils thick) layer of dielectric material  62  such as polyimide which results in thin planar circuit boards  60  that are flexible. 
     Referring now to FIGS. 2 a  and  2   b , suitable support substrates  80  are made of alternating thin layers of rigid dielectric material such as FR4 and conductive material such as copper (not illustrated). The support substrate  80  may provide for mounting additional electrical components (not illustrated) on it and may include an edge connector for industry compatible connectivity. The side with the edge connector has a thickness compatible with the standards for DIMM connectors, which is typically 0.050 inches. To reduce module height, the section  81  of the support substrate  80  which underlays the packages  50  may be thinned as much as possible without compromising the rigidity of the resultant module. Such sections  81  of support substrates  80  made from layers of FR4 material and copper may be as thin as 0.015 inches without compromising rigidity. 
     The combination of a flexible substrate  60  connected to a support substrate  80  provides enhanced reliability. The flexible substrate  60  is tolerant of the expansions and contractions associated with temperature variations and the support substrate  80  provides the mechanical strength tolerance preferably to endure stresses associated with handling and shock. 
     FIG. 2 b  shows a planar view of a support substrate  80  and illustrates the preferred method for routing signals through the support substrate  80  to each flexible substrate  60 . The second portion  64  of each flexible substrate  60  has two groups of interconnect pads  69   a ,  69   b  which are located in close proximity to opposing longitudinal ends  45   a ,  45   b , as shown in FIG. 1 b . Signal lines enter each flexible substrate  60  through a first groups of interconnect pads  69   a  and exit through a second group of pads  69   b . Select interconnect pads  69   a  are each coupled to a select trace within flexible substrate  60  that is routed to select package leads  52  and to a second select interconnect pad  69   b . Referring now to FIG. 2 b , the support substrate  80  includes eight groups of pads  82  for mounting four flexible substrates  60 . Double-line arrows R 1 -R 5  show the direction that signal transmission lines are routed through the support substrate  80  and solid-line arrows T 1 -T 4  show the direction that signal transmission lines are routed through each flexible substrate  60 . Signal lines originate from one area of the support substrate  80 , as shown by arrow R 1 , and are routed through the support substrate  80  to a first group P 1  of pads  82 . Signals are routed though through the first flexible substrate  60  (not illustrated) to a second group P 2  of pads  82  as shown by arrow T 1 . Signals are routed though the support substrate  80  as shown by arrow R 2  to the nearest group of pads P 3  for entering the next adjacent flexible substrate  60  (not shown). In a similar manner, signals are routed, as shown in FIG. 2 b , through each flexible substrate  60  and then exit the support substrate as indicated by arrow R 5 . In other words, each trace  65 , in each flexible substrate  60 , that is electrically coupled to one interconnect pad  69   a  and to one interconnect pad  69   b , is electrically coupled in series to a corresponding trace  65  in each of the other flexible substrates  60  to form a single signal transmission line having one origination point on the support substrate  80  and one end point on the support substrate  80 . The groups of pads of any two adjacent flexible substrates  60  that are electrically coupled are located near the same end  45   a  or  45   b . This method eliminates signal line stubs that can cause reflections and provides for optimally short signal lines. 
     Referring now to FIGS. 1 b  and  2   b , the preferred method for mounting a flexible substrate  60  to a support substrate  80  utilizes an automated placement machine. The automated placement machine positions a first group of pads  69   a  of each flexible substrate  60  to a mating group of pads  82  on the support substrate  80 , then positions the second group of pads  69   b  of each flexible substrate  60  to a second group of pads  82  on the support substrate  80 . As shown in FIG. 1 c , the flexible substrate preferably includes an expanding portion  42  formed in the flexible substrate  60  to provide longitudinal flexibility to facilitate registration during automated assembly. The expanding portion  42  has one or more folds, as shown in FIG. 1 c , that can flatten when the automated placement machine pulls the flexible substrate  60  to position it on the second group of pads  82  on the support substrate  80 , and exerts a spring like force in resistance to the pulling action. 
     FIGS. 3 and 4 show alternative embodiments of the integrated circuit module shown in FIG.  1 . Integrated circuit packages  50  are mounted to the top surface of a flexible substrate  60  with select package leads  52  electrically coupled to select package lead pads  68 , as illustrated in detail in FIGS. 7-10. A portion  64  of each flexible substrate  60  extends away from the package  50  to provide electrical and thermal connectivity between the package leads  52  and the support substrate  80 . FIG. 3 illustrates one embodiment of the present invention where a portion  64  of the flexible substrate  60  has interconnect pads  69  located on the bottom surface for electrical and thermal coupling to mating interconnect pads  82  on the support substrate  80 . Interconnect pads  69  on the flexible substrate  60  preferably are provided with a coating of solder which flows when heat is applied and any suitable means may be used to hold substrate  60  in position over support substrate  80  until registration is achieved. To improve density, the portion  64  of the flexible substrate  60  that mates with the support substrate  80  can overlap a similar portions  64  of other flexible substrates  60  as shown in FIG.  3 . 
     FIG.  4 . illustrates an alternative embodiment of the present invention where a portion  64  of the flexible substrate  60  is doubled-back at a 180-degree angle. In this embodiment, interconnect pads  69  are located on the top surface of substrate  60 , so that the doubled-back portion provides coupling between pads  69  and interconnect pads  82  on the support substrate. This method has the disadvantage of increasing signal length. However, the signal length is increased equally for all signals as required for the RAMBUS CHANNEL™. 
     Also as shown in FIGS. 3 and 4, a wedge  90  is formed from plastic or other suitable such as FR4 and is mounted or formed integrally with support substrate  80 . The wedge  90  provides mechanical support for the flexible substrate  60  and packages  50  at the desired angle A. The angle A between the major plane of the integrated circuit packages  50  and the top major surface of the support substrate  80  may be as great as 90 degrees. The angle A may also be as small as possible while allowing an edge of one package  50  to overlap the edge of another package  50 . 
     Alternate embodiments of the present invention are shown in FIGS. 5 a  and  5   b . The embodiment illustrated by FIG. 5 a  offers the advantage of having packages  50  attach to the same surface of the flexible substrate  60  that is also attached to the support substrate  80 . FIG. 5 b  shows two packages, each mounted to a separate flexible substrate  60  with package leads  52  electrically coupled to the flexible substrates  60 . The two flexible substrates  60  are mounted together using a thin layer of adhesive  73  and each flexible substrate  60  has a portion  64  that connects to the support substrate  80 . 
     FIG. 7 shows a cross-sectional view taken on line A—A of FIG. 6 of one aspect of the present invention. The internals of the package  50  are not illustrated in FIGS. 6-13 for simplicity. This embodiment utilizes a flexible substrate  60  comprised of one thin conductive signal layer  61  and a thin dielectric layer  62 . Each signal layer  61  is typically comprised of a 1.4 mil layer of copper that is die cut to form package lead pads  68 , interconnect pads  69  (FIGS. 1 b ,  3 - 5 ), and elongated traces  65 . Interconnect pads  69  are electrically and thermal coupled to package lead mounting pads  68  through elongated thin strips of copper material  65  commonly referred to as either traces  65  or signal lines  65 . A layer of solder resist (not illustrated) can be used to cover select portions of exposed conductive layers  61 , 67 . 
     FIG. 8 shows a cross-sectional view taken on line B—B of FIG. 6 to illustrate a method for controlling the impedance of select signal traces  65   a . This method utilizes traces  65   b , formed in the signal layer  61 , that are designated to be connected to electrical ground (GND). The characteristic impedance of a portion of a signal trace  65   a  is determined by methods well known in the arts by having one or more GND traces  65   b  in close proximity and parallel to the portion of the signal trace  65   a . The distance and material between the signal trace  65   a  and the GND trace  65   b  are selected to obtain the desired characteristic impedance of the signal trace  65   a  which is preferably between 25 and 50 ohms. 
     FIGS. 9 and 10 illustrate an alternative embodiment for controlling impedance of select signal traces  65   a  that can be used alone or in combination with other methods described herein. The flexible substrate  60  is formed with a ground (GND) plane layer  67  separated from the signal layer  61  by a thin high-dielectric layer  62  that is typically 1 mil to 11 mils thick. The thickness and material of the dielectric material  62  are selected to obtain the desired characteristic impedance of a signal trace  65  formed in the signal layer, which is preferably 25 to 50 ohms. Yet another alternative embodiment that provides for increased area for routing signals  65   a  is illustrated by FIG.  10 . In this embodiment, a flexible substrate  60  is formed with two signal layers  61 . Each signal layer  61  is separated from the ground layer  67  by a dielectric layer  62 . 
     An embodiment of the present invention which minimizes the length of signal traces  65   a  and for maintaining consistent signal trace  65   a  length is illustrated by FIG.  11 . This embodiment utilizes packages  50  that have package leads  52  located in close proximity to one of the sides of the package. The package is oriented on the first portion  63  of the flexible substrate  60  so that the leads  52  are close to the substrate mounting portion  64  of the flexible substrate  60 . 
     FIG. 12 a  shows substrate mounting pads  69 , which preferably are balls of solder similar to package leads  52 . Preferably rigid spacers  40  formed of an electrically insulating and optionally thermally conductive material are positioned between the substrate pads to allow pressure to be applied to the substrate when heat is applied to prevent undesired electrical connections forming between adjacent pads  69 . In this embodiment, pads  69  on the flexible substrate  60  are arranged such that select pads  69   a , which are electrically coupled to signal traces  65   a , are adjacent to one or more pads  69   b , which are electrically coupled to the GND plane layer  67 . This method provides impedance control as well shielding that decreases crosstalk. 
     FIGS. 12 a  and  13  illustrate a preferred embodiment of the present invention, which provides the advantages of being tolerant to variations in package lead length, eliminates vias on signal traces, and results in short traces. This embodiment may be used in conjunction with the methods described herein for precision impedance control. A flexible substrate  60  is formed with at least one copper layer  67  and at least one signal layer  61  separated by a thin dielectric layer  62 . Apertures  66  extending through the flexible substrate  60  are each adapted to receive one package lead  52 . Optionally, after the flexible substrate  60  is mounted to the bottom surface of a package  50 , the distal end of all the package leads may be trimmed off. Pads  68   b , shown in FIG. 12 b , are formed in the signal layer  61  and surround a substantial portion of each aperture  66 . Typically each pad  68   b  is ring-shaped, or partially-ring-shaped as shown in FIG. 12 b , where the inner diameter of each ring-shaped pad  68   b  is defined by the perimeter edge of the aperture  66 , and the ring-shaped pad  68   b  forms a partial flange around the aperture  66 . The application of heat causes the solder on the package leads  52  to flow, electrically and thermally coupling the package leads  52  to the ring-shaped pads  68   b , as shown in FIG.  13 . Traces  65   a  formed in the signal layer electrically connect select ring-shaped pads  68   b  to select interconnect pads. The aperture  66  may be wider in the portion that extends through the GND plane layer  67  to electrically isolate package leads  52  from the GND plane layer  67 . Solder resist coating (not illustrated) may be used to shield the exposed GND plane  67 . Alternatively, electrically insulating adhesive  70  may be used to prevent solder from the leads  52  from electrically coupling to the GND plane  67 . 
     One method for assembling a module includes the steps of: (1) depositing a thin layer of adhesive on the support substrate  80 ; (2) depositing a thin layer of adhesive  70  on the top surface of a first flexible substrate  60  leaving select areas void; (3) mounting a first set of one or more packages  50  to the flexible substrate  60  where apertures  66  formed in the flexible substrate  60  receive the package leads  52 ; (4) trimming off a portion of the distal end of all package leads  52 ; (5) applying heat to cause the solder on the package leads  52  to flow to electrically couple select package leads  52  to ring-shaped pads  68   b  surrounding each aperture  66 ; (6) depositing a thin layer of adhesive  72  on the top exposed surface of the wedge  90 ; (7) mounting the flexible substrate  60 , with one or more mounted packages  50 , to the support substrate  80  where a portion  64  of the flexible substrate  60  is adhered to pads on the support substrate  80  and a portion  63  of the flexible substrate, underlying the circuit packages, is adhered to the wedge  90 ; (8) repeating steps 3 through 6 for a second flexible substrate  60  and second set of one or more packages  50 ; (9) depositing a thin layer of adhesive  72  to the top surface  53  of the previously mounted packages  50 ; and (10) mounting the second flexible substrate  60 , with one or more mounted packages  50 , to the support substrate  80  where a portion  64  of the second flexible substrate  60  is adhered to the support substrate  80 , and the portion  63  of the second flexible substrate  60  underlying the circuit packages  50  is adhered to a portion of the top surface  53  of each of the first set of one or more packages  50 . 
     FIGS. 12 c  and  12   d  show alternate configurations for pads  68 . FIG. 12 shows a preferred embodiment where the pad is formed on a flexible tab  68   c  that extends into an aperture  41  from within through the flexible substrate  60 . As shown in FIG. 13, a double-sided adhesive film  70  is attached to the bottom surface  54  of the package  50 . The adhesive film  70  is stamped to form apertures for package leads  52 . Adhesive film  70  is electrically insulating and thermally conductive and has a thickness that is about the nominal length of a package lead  52 . Pad  68   c  flexes with pressure, applied by the received package leads  52 , insuring a firm connection while being tolerant to variations in package lead  52  length. In this embodiment, the flexible substrate  60  is oriented to the package with the conductive layer including pads  68   c  facing the bottom  54  of the package  50 . An aperture  41  partially surrounds pads  68   c  for allowing flux, excess solder, and debris to be removed by vacuum suction. Each aperture  41  has a width that is constrained by the package lead  52  pitch and number of traces  65  routed between package leads. FIG. 12 d  shows an alternative configuration for pad  68  where the pad  68   d  extends across an aperture  41  that is provided for vacuum suction removal of flux, excess solder, and debris. 
     The foregoing disclosure and description of the invention are illustrative and explanatory of the preferred embodiments. Changes in the size, shape, materials and individual components used, elements, connections and construction may be made without departing from the spirit of the invention. The specification and drawings are, accordingly, presented by way of example only, and not limitation. Thus, the breath and scope of the invention should not be limited by the exemplary embodiments described herein, but should be defined only in accordance with the following claims and their equivalents.