Abstract:
A circuit assembly comprising a substrate formed to have one or more apertures that define one or more compliant members in the substrate, and to which a circuit device can be attached so as to reduce thermally-induced stresses in the device and in solder joints securing the device to the substrate. The compliant members are sufficiently compliant to permit relatively large surface-mount devices to be attached to an organic substrate without sacrificing reliability.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     Not applicable.  
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH  
       [0002]     Not applicable.  
       BACKGROUND OF THE INVENTION  
       [0003]     (1) Field of the Invention  
         [0004]     The present invention generally relates to substrates on which circuit devices are mounted. More particularly, this invention relates to a circuit assembly having a substrate equipped with one or more compliant structures to which a circuit device can be attached so as to reduce thermally-induced stresses in the device and in the solder joints that secure the device to the substrate.  
         [0005]     (2) Description of the Related Art  
         [0006]     Surface-mount technology (SMT) offers the advantages of increased reliability and lower manufacturing costs. Surface-mount devices (SMD) used in SMT processes are available in a range of chip or die (package or case form) sizes, with larger SMD&#39;s including the 2512 chips (about 6.5×3.25 mm), diodes, inductors, capacitors, resistors, varisters, etc. In many applications, large SMD&#39;s are desirable as they can replace numerous smaller SMD&#39;s (such as the 1206, 805 and 603 chips, etc.), thus reducing the need for capital equipment to place SMT components. Large SMD&#39;s also offer the possibility to replace traditional stick-lead components, which have a higher initial cost and higher cost associated with placement on a circuit board assembly.  
         [0007]     Though having the above advantages, it is not conventional practice to place large SMD&#39;s on organic substrates (circuit boards) because of the significant mismatch in the coefficients of thermal expansion (CTE) between the organic materials used to form such substrates and the silicon or ceramic materials of SMT devices. Thermal cycling of a circuit assembly with continuous power cycles causes accumulative fatigue in the solder joints that attach a circuit device to its substrate, as well as within the body of the device and within the body of the substrate. This accumulative fatigue mechanism includes intergranular precipitation and alloy separation in the solder joints, which accelerates solder joint fatigue. The significant difference in thermal expansion between an SMD and substrate promotes solder joint fatigue, and can be sufficient to cause cracking of more brittle-bodied ceramic SMT components, such as surface-mount capacitors. In effect, the substrate materials are much stronger than the devices mounted to them. This disproportionate strength is also due to differences in mass moment of inertia of the devices and substrate, because SMD&#39;s present a much smaller moment of inertia in the stack-up with the substrate than does the substrate. The larger the component, the higher the stress and thus the likelihood for a shorter life from solder joint fatigue.  
         [0008]     In view of the above, though their use is advantageous for a variety of reasons, large SMD&#39;s are not conventionally mounted to organic (e.g., FR4, Chem 1, Chem 3, etc.) circuit boards utilized in high temperature applications (e.g., temperatures of about 100° C. and higher).  
       BRIEF SUMMARY OF THE INVENTION  
       [0009]     The present invention is directed to a circuit assembly comprising a substrate formed to have one or more apertures that define one or more compliant members in the substrate, and to which a circuit device can be attached so as to reduce thermally-induced stresses in the device and in solder joints securing the device to the substrate. The compliant members are sufficiently compliant to permit relatively large surface-mount devices to be attached to an organic substrate without sacrificing reliability.  
         [0010]     Generally, the circuit assembly of this invention includes the substrate and a surface-mount device mounted thereto, multiple electrically-conductive pads present on at least one device attachment region of the substrate, and solder joints bonding the surface-mount device to the pads. The entire substrate, including the device attachment region and a second region outside the device attachment region, can be formed of a first material, e.g., an organic. The surface-mount device comprises a package (chip) formed of a material (e.g., silicon or a ceramic) having a lower coefficient of thermal expansion than the substrate material. At least one aperture is formed in the substrate, and is located and configured so as to cause the device attachment region (or at least a portion thereof) to be more compliant than the second region of the substrate, though both are formed of the same material. In this manner, the surface-mount device, mounted to the substrate through the pads located within a compliant region of the device attachment region, is subjected to lower thermally-induced stresses as compared to mounting the surface-mount device to the second (and less compliant) region of the substrate.  
         [0011]     In view of the above, the present invention enables large surface-mount devices (SMD&#39;s) to be placed on organic substrates (circuit boards), though a significant CTE mismatch exists between the organic substrate and SMD. More particularly, the compliant region(s) to which the SMD is attached reduces thermally-induced stresses that lead to fatigue fracturing of the solder joints that attach the SMD to the substrate and can also lead to fatigue cracking of the SMD. Accordingly, the processing and packaging advantages associated with large SMD&#39;s can be achieved on organic (e.g., FR4, Chem 1, Chem 3, etc.) circuit boards, even when service temperatures are about 100° C. or higher.  
         [0012]     Other objects and advantages of this invention will be better appreciated from the following detailed description. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]      FIG. 1  is a plan view of a region of a circuit board substrate showing a device attachment region defined between a pair of apertures in accordance with an embodiment of the present invention.  
         [0014]      FIG. 2  is a plan view of the substrate region depicted in  FIG. 1 , after filling the apertures with a fill material and attaching an SMD to the device attachment region.  
         [0015]      FIGS. 3 through 8 ,  10  and  11  are plan views of circuit board substrate regions showing device attachment regions defined with apertures in accordance with alternative embodiments of the present invention.  
         [0016]      FIG. 9  is a cross-sectional view of the substrate region depicted in  FIG. 8 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]      FIGS. 1 through 11  depict portions of a circuit assembly  10  with device attachment regions configured in accordance with various embodiments of the present invention, for the purpose of attaching surface-mount devices (SMD&#39;s) to a substrate, referred to herein as a circuit board  12 . The SMD&#39;s can be of any type known or subsequently developed, though of particular interest to the invention are relatively large SMD&#39;s such as the 2512 chip (package or case form), with dimensions of about 6.5×3.25 mm. Furthermore, the circuit board  12  can be formed of a variety of materials, though of particular interest are circuit boards formed of organic materials, such as FR4, Chem 1, and Chem 2, which are generally glass-reinforced or woven fiberglass-reinforced epoxy resin laminate materials available from various commercial sources.  
         [0018]      FIGS. 1 and 2  represent a first embodiment of this invention, in which a pair of substantially U-shaped slots (apertures)  14  with parallel sets of legs  15  are formed in the circuit board  12  by any suitable process, such as punching. As most readily seen in  FIG. 1 , a device attachment region  16  is defined by and between the slots  14 , in which an SMD  18  is attached to the circuit board  12  as shown in  FIG. 2 . The device attachment region  16  comprises a pair of oppositely-disposed members  20  separated and cantilevered from a central region  22  of the attachment region  16 . Three sides of each member  20  form a peripheral border  24  delineated by one of the slots  14 , while the remaining side is generally a boundary  26  that separates the member  20  from the central region  22 . As a result, each member  20  is more readily able to flex in the direction normal to the plane of the circuit board  12  than a region  32  of the circuit board  12  outside (e.g., immediately surrounding) the device attachment region  16 . Depending on the width of the members  20  (transverse to the alignment direction of the slots  14 ) and the thickness of the circuit board  12 , the members  20  can be significantly more compliant than the region  32  of the circuit board  12  immediately outside the attachment region  16 .  
         [0019]      FIG. 1  also shows a rectangular-shaped central aperture  28  as being present in the central region  22  and longitudinally extending in opposite directions into the members  20 . The central aperture  28  is preferably centered symmetrically within the device attachment region  16 , so as to be equidistant in the longitudinal direction from each slot  14  and equidistant in the transverse direction from the legs  15  of each slot  14 . The central aperture  28  further increases the compliance of the members  20 , particularly by reducing the cross-sectional area of the board  12  at the boundary  26  of each member  20 . The aperture  28  is between a pair of electrically-conductive bond pads  34  located on the members  20 , to which the SMD  18  is physically and electrically attached with solder joints  19  in a conventional manner. The bond pads  34  are electrically coupled to the region  32  of the circuit board  12  with electrically-conductive runners  30 , such as copper traces. As shown in  FIG. 1 , the runners  30  approach the attachment region  16  from opposite directions, bifurcate and follow the outward edges  36  of the slots  14  to the inward edges of the slots  14  (defined by the borders  24  of the members  20 ), and then merge and continue inward toward the central region  22  until the pads  34  are encountered. The portions of the runners  30  along the edges of the slots  14  are preferably formed by plating the walls of the slots  14 .  
         [0020]     The dimensions of the features shown in  FIG. 1  can be varied, depending on the particular application and the size of the SMD  18 . If the SMD  18  is a 2512 chip, suitable dimensions include: width of the slots—about 0.8 mm; transverse and longitudinal dimensions of the device attachment area  16  (between the slots  14 )—about 4.2×7.9 mm; transverse and longitudinal dimensions of the central aperture  28 —about 2.0×4.0 mm; transverse and longitudinal dimensions of the central region  22  (between the boundaries  26 )—about 4.2×2.1 mm. Those skilled in the art will appreciate that the size and number of bond pads  34  will depend on the particular size and type of SMD  18 .  
         [0021]     As noted above,  FIG. 2  shows the SMD  18  placed on the device attachment region  16  and attached to the bond pads  34 , such that the central aperture  28  is directly beneath the SMD  18 . According to an optional aspect of the invention,  FIG. 2  also shows the U-shaped slots  14  filled with a suitable fill material  38 , such as a solder mask material or hole-filling material. The purpose of the fill material  38  is to stabilize the cantilevered members  20  and potentially tailor the compliance of the members  20 .  
         [0022]      FIGS. 3 through 11  depict additional configurations for device attachment regions in accordance with embodiments of this invention. In these Figures, consistent reference numbers are used to identify similar structures, but with a numerical prefix (1, 2, or 3, etc.) added to distinguish the particular embodiment from other embodiments of the invention.  
         [0023]     In  FIG. 3 , the transverse dimensions of the slots  114 , device attachment region  116  and central aperture  128  have each been increased in proportion to their longitudinal dimensions, and runners  130  are routed between the slots  114  instead of along the edges of the slots  114 . Otherwise, the embodiment of  FIG. 3  is generally identical to the embodiment of  FIGS. 1 and 2 .  
         [0024]     The embodiment of  FIG. 4  differs from that of  FIG. 3  by replacing the rectangular-shaped central aperture  128  with a circular-shaped central aperture  228 , and by sizing each U-shaped slot  214  to have legs  215  that differ in length. In combination, the circular-shaped central aperture  228  and the legs  215  of different lengths render the bond pads  234  more accessible with the runners  230 , which are again routed between the legs  215  instead of along the edges of the slots  214 .  
         [0025]      FIG. 5  is similar to the embodiment of  FIG. 4 , but differs by having slots  314  with a more arcuate C-shape, instead of the more rectilinear U-shape depicted in  FIGS. 1 through 4 . A benefit to the C-shaped slots  314  is that they consume less circuit board area than the U-shaped slots of previous embodiments.  
         [0026]      FIG. 6  depicts a configuration in which the size of the central aperture  428  is significantly larger than adjacent U-shaped slots  414 , such that nearly the entire central region  422  is occupied by the aperture  528 . In addition, to ensure adequate compliance of the members  420 , the shape of the central aperture  428  has been altered to compensate for the relatively smaller size of the slots  414 . More particularly, the slots  414  are substantially equal in shape and size but with relatively shorter legs  415  than in previous embodiments, and the central aperture  428  is significantly wider than the slots  414  in the direction in which the slots  414  and aperture  428  are aligned. Finally, the central aperture  428  defines two oppositely-disposed U-shaped edges  429  that face the slots  414 , such that the central region  422  is effectively as compliant as the member  420 . Consequently, this embodiment permits the bond pads  434  to straddle both the compliant members  420  and the central region  422 .  
         [0027]     In  FIG. 7 , two slots  514  and a central aperture  528  of substantially the same size and shape are aligned in a row. The slots  514  and aperture  528  are depicted as having an oval shape, oblong transverse to their direction of alignment. The device placement region  516  (effectively the area between the slots  514 ) comprises two compliant members  520  delineated by and between the central aperture  528  and each of the slots  514 . As such, each compliant member  520  has opposing peripheral borders  524  delineated by the slots  514  and aperture  528 , and opposing boundaries  526  that are not delineated by the slots  514  and aperture  528  so as to be contiguous with the remainder  532  of the circuit board  12  outside the device placement region  516 . As such, the compliant members  520  are not cantilevered but instead are effectively bridges, and the central region of previous embodiments is essentially eliminated in the embodiment of  FIG. 7 .  
         [0028]      FIGS. 8 and 9  depict a similar embodiment to that of  FIG. 7 , but with the slots  614  and aperture  628  being rectilinear instead of oval.  FIG. 9  also shows a pair of SMD&#39;s  618  attached with solder joints  619  to opposite surfaces of the circuit board  12 .  
         [0029]      FIG. 10  depicts an embodiment in which the three apertures  514  and  528  of  FIG. 7  are effectively interconnected to form a single continuous S-shaped aperture  713 . The device placement region  716  is effectively the area between a pair of oppositely-disposed transverse portions  714  of the aperture  713 . A central transverse portion  728  of the aperture  713  delineates a pair of compliant members  720  with each of two the transverse portions  714 . Each of the compliant members  720  has peripheral borders  724  delineated on three sides by the aperture  713 , and a boundary  726  that is not delineated by the aperture  713  and therefore contiguous with the remainder  732  of the circuit board  12 . As such, the compliant members  720  are again cantilevered, in contrast to the bridge-type compliant members of  FIGS. 7 through 9 .  
         [0030]     Finally,  FIG. 11  depicts an embodiment in which the three apertures of previous embodiments are replaced by multiple circular-shaped apertures  813 . The device placement region  816  is effectively the area between a pair of oppositely-disposed sets  814  of aperture  813 . A central set  828  of apertures  813  delineates a pair of compliant members  820  with each of two other sets  814  of apertures  813 . While each apertures  813  is discrete, they are sufficiently close together to cause the compliant members  820  to be significantly more compliant than the remainder  832  of the circuit board  12  surrounding the device placement region  816  between the sets  814  of apertures  813 . Each compliant member  820  is defined by borders delineated by the apertures  813 , and further defined by boundaries formed by bridges between adjacent apertures  813 . Though they consume more circuit board area as compared to previous embodiments, an advantage to the apertures  813  of this embodiment is that they can be readily formed by simple drilling or punching operations.  
         [0031]     In an analytical investigation of the present invention, a three-dimensional finite element analysis (FEA) simulation was performed to assess the potential of this invention to improve the life expectancy of solder joints. The FEA simulation modeled an organic circuit board having a thickness of 1.6 mm, a standard 2512 SMD chip having length and thickness dimensions of 6.6 mm and 0.6 mm, bond pads with length and thickness dimensions of 0.6 mm and 0.0175 mm, and solder connections with a thickness dimension of 0.02 mm. The material properties of these components are summarized in the following Table 1.  
                               TABLE 1                                       Chip   Modulus   372,413 MPa               Poison&#39;s ratio   0.23               CTE   5.6 ppm/° C.           Board   Modulus    24,400 MPa               Poison&#39;s ratio   0.26               CTE    18 ppm/° C.           Bond Pad   Modulus   120,000 MPa               Poison&#39;s ratio   0.3636               CTE    17 ppm/° C.                      
 
         [0032]     The analysis compared a conventional (slotless) circuit board to circuit boards with device attachment regions defined by slots and a central aperture in accordance with  FIG. 1 , with dimensions summarized in Table 2.  
                                                 TABLE 2                                   DESIGN A   DESIGN B                                    Width of the U-shaped slots:   0.8   mm   0.8   mm       Outer transverse dimension   6.5   mm   5.0   mm       of each U-shaped slot:       Outer longitudinal dimension   3.7   mm   3.7   mm       (leg length) of each U-shaped slot:       Combined outer longitudinal   9.5   mm   8.8   mm       dimension of the U-shaped slots:       Dimensions of the   3.0 × 4.0   mm   2.0 × 4.0   mm       central aperture -       Transverse × longitudinal:                  
 
         [0033]     The analysis employed a temperature profile based on a sixty-minute temperature cycle with fifteen-minute ramps and dwells using temperature extremes of −40° C. and +125° C. The results of this analysis are summarized in Table 3, with reported values being cycles completed before the occurrence of solder joint cracking (“failure”), with “nominal reports indicating the average Weibull number of cycles to “failure.” 
                                             TABLE 3                           WORST CASE   BEST CASE   NOMINAL                                Slotless circuit board   1259   2481   1909       Design A   3967   7814   6011       Design B   &gt;10,000   &gt;10,000   &gt;10,000                  
 
         [0034]     Consequently, the FEA simulation showed that a 2512 chip can exhibit a significantly extended life expectancy if mounted to an organic circuit board with slots configured in accordance with this invention, particular if the Design B dimensions are used.  
         [0035]     In view of the above, it can be seen that the present invention provides a technique for modifying substrates to which SMD&#39;s are to be attached, by which the substrate includes stress-relieving compliant structures that reduce the deleterious stresses that arise from differences in CTE&#39;s within the circuit assembly, particularly the substrate and the SMD chip. The slots and/or apertures used to form the compliant structures also serves to better match the moment of inertia of the SMD with that of the modified substrate. By reducing the relative strength ratios of the SMD and substrate with compliant structures, solder joints that attach the SMD to the substrate will have less accumulative strain energy with each thermal cycle, and the resulting damage due to thermal fatigue will be proportionally decreased. Slots and apertures of a wide variety of sizes and shapes can be readily formed by existing circuit board processes, including drilling and punching. As such, the invention can be readily implemented at relatively low cost, while maintaining the same circuit functionality and reducing concerns for solder joint fatigue.  
         [0036]     A significant advantage of the present invention is enabling the use of large SMD (e.g., 2512 chip, etc.), which under many circumstances will reduce the component count (i.e., one 2512 chip in place fifteen or more 0803 chips), simplify assembly processing, and reduce the number of component placement machines required for a particular circuit board assembly.  
         [0037]     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.