Abstract:
A circuit board assembly comprising a laminate substrate and a surface mount device having a CTE less than that of the laminate substrate and attached with at least one solder joint to a first surface of the laminate substrate. The assembly further includes a localized stiffener attached to a second surface of the laminate substrate so as to be directly opposite the circuit device. The localized stiffener is formed of a material and is shaped so that, when attached to the laminate substrate, the stiffener is capable of increasing the thermal cycle fatigue life of the one or more solder joints that attach the device to the substrate.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     (1) Field of the Invention  
         [0002]     The present invention generally relates to surface-mount circuit devices. More particularly, this invention relates to a laminate circuit board substrate modified to have a localized stiffener on a surface opposite a surface-mount circuit device so as to improve the fatigue life of one or more solder joints securing the device to the substrate.  
         [0003]     (2) Description of the Related Art  
         [0004]     Electronic circuit assemblies are often required to be capable of surviving in hostile operating environments, including those commonly found in automotive and aerospace applications. Such assemblies often employ surface-mount (SM) integrated circuit (IC) devices, which are generally characterized as being electrically and mechanically attached to the substrate of a circuit assembly with one or more terminals or leads that are soldered to conductors on the substrate surface. A prominent example of a SM IC device is a flip chip which has bead-like terminals typically in the form of solder bumps along the perimeter of the chip. After registering a flip chip to its corresponding conductor pattern on a substrate, heating above the liquidus temperature of the solder causes the solder bumps to reflow, forming solder joints that secure the chip to the substrate and electrically interconnect the chip circuitry to the conductor pattern.  
         [0005]     Due to the numerous functions typically performed by the microcircuitry of a flip chip, a relatively large number of solder joints are typically required, resulting in the joints being crowded along the edges of the chip. To reduce device profile and the overall size of circuit board assemblies, a current IC packaging trend is for smaller solder joints to reduce bump pitch, which also reduces device standoff height resulting in solder joints that are less compliant. Such size constraints result in solder joints of minimal size and therefore reduced strength. Complicating this is the fact that solder joints are subject to thermal stresses as a result of temperature fluctuations in the working environment of the assembly and differences in coefficients of thermal expansion (CTE) of the various materials used in the construction of the assembly. A CTE mismatch particularly exists for flip-chip-on-board (FCOB) processes in which a flip chip is mounted to an organic laminate circuit board, such as a printed wiring board (PWB) or printed circuit board (PCB). As a result of their multilayer laminate construction and compositions, such substrates typically have CTE&#39;s in the circuit plane of about 17 ppm/EC, which is significantly higher than CTE&#39;s of the materials (e.g., silicon, alumina, quartz, etc.) of which SM devices are formed, e.g., typically not higher than about 10 ppm/EC. Thermal stresses arising from this CTE mismatch can potentially fatigue and fracture the solder joints, particularly if the device is subject to many temperature excursions, high temperatures, and/or intense vibration.  
         [0006]     To reduce and distribute stresses on their solder joints, SM devices mounted to laminate organic substrates are typically underfilled to encapsulate their solder joints. For example, epoxy resins containing a glass filler have been used as underfill materials for SM IC devices, including flip chips. The glass filler reduces the CTE of the underfill material in order to mitigate the thermal mismatch between the flip chip and circuit board. Other approaches to improving solder joint fatigue life include modifying the solder composition and increasing the solder bump (joint) height. While such approaches have the ability to improve solder joint fatigue life, further improvements would be desirable.  
       BRIEF SUMMARY OF THE INVENTION  
       [0007]     The present invention is directed to a circuit board assembly comprising a laminate substrate and a surface mount device having a CTE less than that of the laminate substrate and attached with at least one solder joint to a first surface of the laminate substrate. According to the invention, the circuit board assembly includes a localized stiffener attached to a second surface of the laminate substrate so as to be directly opposite the circuit device. The localized stiffener is formed of a material and is shaped so that, when appropriately attached to the laminate substrate, is capable of increasing the thermal cycle fatigue life of the one or more solder joints that attach the device to a laminate substrate. For this purpose, the localized stiffener has at least one lateral dimension in a plane parallel to the first and second surfaces of the laminate substrate that is greater than a corresponding lateral dimension of the device. Furthermore, the CTE of the localized stiffener is less than the CTE of the laminate substrate, and the modulus of elasticity of the localized stiffener is equal to or greater than that of the laminate substrate.  
         [0008]     According to the invention, the localized stiffener is able to increase the longevity of a surface-mount device mounted to a laminate substrate by locally inhibiting the thermal expansion and shrinkage of a region of the substrate directly beneath the device, thereby reducing the stresses on the one or more solder joints that attach the device to the substrate. Such improvements can be achieved in combination with more conventional approaches, such as device underfilling, modified solder compositions, and thinner laminate substrates. The localized stiffener can also be utilized to reduce yield losses attributable to excessive warpage and stresses that can occur during the circuit board assembly process.  
         [0009]     Other objects and advantages of this invention will be better appreciated from the following detailed description. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is a cross-sectional view of a portion of a laminate circuit board substrate on which a flip chip is mounted to a first surface thereof, and on which a localized stiffener is attached directly opposite the flip chip in accordance with a preferred aspect of this invention.  
         [0011]      FIG. 2  is a cross-sectional view similar to  FIG. 1 , but with the stiffener of  FIG. 1  replaced with a second flip chip attached opposite the first chip in accordance with a second embodiment of this invention.  
         [0012]      FIG. 3  is a perspective view of an alternative configuration for a stiffener suitable for use as the localized stiffener of  FIG. 1 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0013]      FIG. 1  represents a portion of a circuit board assembly  10  comprising a substrate  12  on whose surface  14  a surface mount component  18 , such as a flip chip, is attached. The substrate  12  is a laminate organic substrate, such as a printed circuit board (PCB) or a printed wiring board (PWB). An example of a suitable substrate material is known as FR-4, available in various thicknesses (e.g., about 0.015 to about 0.065 inch (about 0.38 to about 1.65 mm)) and typically having a CTE of about 17 ppm/EC in the plane of the substrate. FR-4 boards are typically a glass-reinforced or woven fiberglass-reinforced epoxy resin laminate available from various sources.  
         [0014]     As known in the art, the component  18  may be formed of a semiconductor material, such as silicon, in whose surface integrated circuits are formed. Other potential materials for the component  18  include alumina, beryllia, quartz, etc. As is conventional, the component  18  is electrically and physically connected by a number of solder joints  20  to solderable pads (not shown) defined on the surface  14  of the laminate substrate  12 . The solder joints  20 , which are typically formed by reflowing solder bumps, support the component  18  above the surface of the substrate  12  as shown. The solder is typically a eutectic or near-eutectic tin-lead solder, though the use of other solder compositions is also within the scope of this invention.  
         [0015]     Because of the different materials used to form the substrate  12 , component  18  and solder joints  20 , a mismatch in coefficients of thermal expansion (CTE) exists. In particular, the CTE of the component  18  is much less than that of the substrate  12 . This mismatch generates stresses during temperature excursions that are concentrated in the solder joints  20 , which if sufficiently severe leads to fatigue fracturing of the joints  20 . To mitigate the adverse effect of such stresses, an underfill material  22  is shown as covering the solder joints  20  and completely filling the void between the substrate  12  and component  18 . In accordance with conventional practice, the underfill material  22  is also shown as completely surrounding the peripheral boundary  34  of the component  18 .  
         [0016]     The above-described assembly  10  is merely intended to generally represent one of various circuit assemblies to which this invention applies. Therefore, the teachings of this invention are not limited to the specific configuration shown in  FIG. 1 , and can be applicable to electronic assemblies that utilize essentially any type and combination of surface mount technology (SMT) packages and various terminal designs, as well as overmold circuits and components.  
         [0017]     To further mitigate the adverse effect of the CTE mismatch between the component  18  and the laminate substrate  12 ,  FIG. 1  shows the circuit board assembly  10  as further comprising a localized stiffener  24  on a surface  16  of the laminate substrate  12  directly opposite the component  18 . The stiffener  24  is represented as being attached to the substrate  12  with a bonding material  26  that may be, for example, a polymer adhesive, solder, etc., as further discussed below, though lamination of the stiffener  24  to the substrate  12  is also within the scope of this invention. The role of the stiffener  24  is to locally stiffen that portion of the laminate substrate  12  directly beneath the component  18  so as to reduce the extent to which the local portion of the substrate  12  expands and contracts during temperature excursions, with the effect of reducing stresses on the chip solder joints  20 . In particular, the stiffener  24  preferably serves to locally increase the biaxial bending stiffness of that portion of the laminate substrate  12  between the component  18  and the stiffener  24 . For this purpose, the stiffener  24  is formed of a material having a CTE that is less than the CTE of the laminate substrate  12 , and preferably something between the CTE&#39;s of the substrate  12  and component  18 . For example, for a silicon component  18  (CTE of about 2.3 ppm/EC) attached to an FR-4 substrate (in-plane CTE of about 17 ppm/EC), a suitable CTE range for the stiffener  24  is about 0 to about 16 ppm/EC. Furthermore, in order to adequately counteract the thermal expansion and contraction of the substrate  12 , the stiffener  24  preferably has a modulus of elasticity that is greater than that of the laminate substrate  12 , which is typically on the order of about 18 to 31 GPa. Consequently, the stiffener  24  preferably has a modulus of elasticity of at least 18 GPa, and more preferably at least 300 GPa. To further promote the stiffening effect of the stiffener  24 , the peripheral boundary  34  of the component  18  preferably lies entirely within the footprint of the peripheral boundary  36  of the stiffener  24 , such that each of the transverse lateral dimensions of the component  18  (i.e., the dimensions of the component  18  in a plane parallel to the substrate surfaces  14  and  16 ) are less than the corresponding transverse dimensions of the stiffener  24 .  
         [0018]     Various materials are capable of providing the above-noted physical and mechanical properties desired for the stiffener  24 , including silicon, alumina, silicon nitride, silicon carbide, stainless steel, molybdenum, Fe—Ni alloys (e.g., Alloy 42), tungsten, etc. If formed of a material with suitable thermal properties, e.g., a thermal conductivity of at least 10 W/mK, the stiffener  24  can also serve to dissipate heat from the component  18 . As shown in  FIG. 1 , this potential role of the stiffener  24  can be promoted by forming conductive vias (e.g., plated through-holes)  38  within the substrate  12  to improve thermal coupling of the component  18  and stiffener  24 . The bonding material  26  shown in  FIG. 1  as attaching the stiffener  24  to the surface  16  of the substrate  12  can also contribute to the localized stiffness of the substrate  12  and/or promote heat transfer to the stiffener  24 . For example, stiffness can be enhanced if the bonding material  26  is formed of a nonductile solder, epoxy, filled epoxy, or another structural adhesive, and thermal conduction can be enhanced if the bonding material  26  is formed of solder or a thermally-filled epoxy. Stiffness can also be promoted by encapsulating the stiffener  24  by what is known as glob top encapsulation, depicted in  FIG. 1  as an adhesive compound  28  that entirely encapsulates the stiffener  24  and the bonding material  26  attaching the stiffener  24  to the substrate  12 . Suitable materials for the adhesive compound  28  include epoxies, filled epoxies, or another structural adhesive.  
         [0019]     Notably, if formed of silicon, the stiffener  24  can be a rejected chip  124 , as represented in  FIG. 2 . With this approach, a chip  124  that would otherwise be scrapped can be put to advantageous use. The stiffener chip  124  is shown as being attached to the surface  16  of the substrate  12  with multiple solder joints  126 , similar to the solder joints  20  that attach the flip component  18  to the substrate  12  though without the requirement for electrically connecting the chip  124  to the substrate  12 , i.e., the chip  124  is not electrically functional. As with the component  18 , the solder joints  126  attaching the stiffener chip  124  to the substrate  12  space the chip  124  from the substrate surface  16  to define a gap, and this gap is preferably filled with an underfill material  128  that completely fills the gap to reduce stresses on the solder joints  126 , as well as completely surrounds the peripheral boundary  136  of the chip  124 .  
         [0020]     The stiffeners  24  and  124  represented in  FIGS. 1 and 2  generally have rectangular shapes in a plane parallel to the surfaces  14  and  16  of the laminate substrate  12 . In addition, the stiffeners  24  and  124  are sized so that the peripheral boundary  34  of the component  18  is superimposed entirely within the footprint of the peripheral boundary  36  of the stiffeners  24  and  124 , as discussed previously.  FIG. 3  represents one of multiple other configurations that are possible for stiffeners within the scope of this invention. In each case, it is preferred that each transverse lateral dimension of the stiffener is greater than the corresponding transverse dimension of the component  18 . In  FIG. 3 , a stiffener  224  is depicted as having a cross-shape configuration defined by two pairs of opposing legs  30  and  32  that establish the lateral dimensions of the stiffener  224 . The stiffener  224  may be attached to the substrate  12  so that each pair of opposing legs  30  and  32  is parallel to one of the lateral dimensions of the component  18 , in which case the lateral dimension established by each pair of legs  30  and  32  is preferably greater than the lateral dimension of the component  18  with which the pair of legs  30  and  32  is parallel. Alternatively, the cross-shaped stiffener  224  can be oriented on the substrate surface  16  so that each pair of legs  30  and  32  is transverse to one of the lateral dimensions of the component  18 , i.e., the legs  30  and  32  extend from corner to corner of the component  18 . In this orientation, the legs  30  and  32  again preferably project beyond the lateral dimensions of the component  18 .  
         [0021]     In view of the increased stiffness of the substrate  12  beneath the component  18  resulting from the presence of stiffeners within the scope of this invention, various design possibilities are available for the laminate substrate  12  and the circuit board assembly  10  generally. For example, in applications where thicker substrates (e.g., at least 0.031 inch (about 0.79 mm) have been required to provide adequate stiffness for surface-mount devices, stiffeners of this invention can permit the use of relatively thin substrates, e.g., less than 0.031 inch (0.79 mm) whose flexibility is compatible with other design considerations including heat removal with flip chip pedestals. Processing issues such as yield losses due to excessive warpage and stresses during board assembly can also be mitigated by appropriately locating stiffeners of this invention on laminate substrates. As such, in addition to improving the reliability of surface-mount devices the stiffeners of this invention can serve to provide various processing and reliability-related requirements of circuit board assemblies utilizing laminate substrates.  
         [0022]     While the invention has been described in terms of a preferred embodiment, it is apparent that other forms could be adopted by one skilled in the art. Accordingly, the scope of the invention is to be limited only by the following claims.