Abstract:
A method and structure for reducing thermally induced strains on the solder joints that couple a ball grid array (BGA) module to a circuit card, so as to improve the fatigue life of the BGA module. The thermally induced strains arise from a mismatch in thermal expansion coefficient between the dielectric substrate of the BGA module and the dielectric board of the circuit card. The method generates void annular regions around portions of the BGA dielectric substrate to which the BGA solder balls are to be attached and/or around portions of the circuit card dielectric material to which the BGA module is to be attached. This results in the formation of dielectric islands or peninsulas that bound the solder balls of the BGA module after installation on the circuit card. The dielectric islands or peninsulas thus formed serve to increase the effective height over which the differential expansion is accommodated, thereby reducing the strains throughout the solder joints. Additionally, the void annular regions provide space for the deformation of the dielectric islands or peninsulas, thereby increasing their compliance and transferring strain from the solder joints to the dielectric islands or peninsulas.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates to a structure and associated method of fabrication that reduces thermally induced strains in solder joints associated with solder balls that couple a ball grid array (BGA) module to a circuit card. 
     2. Related Art 
     A contemporary circuit board configuration may have a BGA module mounted on a circuit card. A BGA module comprises a substrate of dielectric material, such as ceramic or plastic, having both a top surface and a bottom surface. An array of solder balls are attached to a corresponding array of conductive pads on the bottom side of the BGA module, while one or more chips are attached to the top side of the BGA module. The pertinent circuit card is any pre-wired board comprising dielectric material and an array of conductive pads on the board. An example of a circuit card is a motherboard of a computer. The circuit card pads serve as attachment points for accommodating one or more BGA modules. Accordingly, a BGA module that is mounted on a circuit board has each solder ball attached to a conductive pad on the BGA module itself and also to a conductive pad on the circuit card. These conductive pads are respectively affixed to the dielectric substrate of the BGA module and the dielectric board of the circuit card. Thus, each solder ball is a structural element that is mechanically attached to dielectric sheets of material on each side of the solder ball. 
     When the circuit card is heated or cooled, the solder ball is subject to strains that arise from the differential rate of thermal expansion of the supporting dielectric structures. For example, a typical thermal expansion coefficient of the circuit card is 14 to 22 ppm/° C. (ppm denotes parts per million), while a ceramic substrate of a BGA module may have a smaller thermal expansion coefficient of approximately 6 to 11 ppm/° C. If the BGA module uses a plastic substrate material, the effective thermal expansion coefficient of the plastic substrate, at locations where a silicon chip constrains expansion of the substrate, is typically about 7 ppm/° C. While the preceding materials, and corresponding thermal expansion coefficients, typify BGA substrates and circuit cards, materials could be characterized by a reversed relationship in which the BGA substrate has a higher thermal expansion coefficient than that of the circuit card to which the BGA module is attached. 
     Unfortunately, strains on the solder balls resulting from the aforementioned differential thermal expansion may cause fatigue failure in the BGA solder joints. U.S. Pat. No. 5,726,079 (Johnson, Mar. 10, 1998), which is hereby incorporated by reference, discloses an approach that could be used to reduce the effect of differential thermal expansion. With this alternative approach, a chip mounted on a substrate is encased peripherally with a dielectric material that is mechanically bonded to the substrate. Additionally, the chip is mechanically coupled to a conductive plate located above the top side of the chip, wherein the conductive plate comprises a material such as a stainless steel. Because of the mechanical bonding between the conductive plate and the substrate through the encased dielectric material, bending of the structure due to the differential thermal expansion is eliminated and the thermal expansion of the conductive plate mitigates the thermal expansion of the substrate of the BGA module. Thus the material of the conductive plate would be selected to have a thermal expansion coefficient that would eliminate the bending that arises from the mismatched thermal expansions of the substrate of the BGA module and the dielectric board of the circuit card. Although this method would greatly extend the BGA fatigue life, it would also increases the product&#39;s cost because of the cost of the conductive plate and because of the process steps and associated equipment required to fabricate the encasing structure. 
     SUMMARY OF THE INVENTION 
     The present invention provides an inexpensive method, and an associated electrical structure, that reduces the thermally induced strain on solder balls of BGA modules, wherein the BGA module is mounted on a circuit card. The essence of the method is to increase the effective length of the connecting structure, between the BGA module and the circuit card, that deforms due to differential thermal expansion. This results in a reduction in strain in the connecting structure and, in particular, in the solder ball. Thus, increasing the length over which a given displacement acts effectively reduces the strain throughout the solder ball. In particular, the method of the present invention forms annular voids by removing material from the dielectric substrate of the BGA module so as to leave the solder balls affixed to regions of dielectric material, wherein the regions of dielectric material are each substantially surrounded by an annular void thus formed. Annular voids may be similarly formed around the regions of the dielectric board of the circuit card, wherein the regions are underneath the conductive pads of the circuit card to which the BGA module will be attached. Thus, the method forms peninsulas of dielectric regions if the formed annular voids substantially, but not completely, surround the dielectric regions. Alternatively, the method forms islands of dielectric regions if the formed annular voids totally surround the dielectric regions. A peninsula thus formed will be unseparated from the remaining dielectric substrate (or board), thereby leaving a thin connecting dielectric strip between the peninsula and the remaining dielectric substrate (or board). 
     Generally, the present invention provides a method for forming at least one electrical structure, comprising the steps of: 
     providing a substrate including a dielectric layer having a first surface, and at least one electrically conductive pad attached to the first surface; and 
     removing a first portion of the dielectric layer to form a void portion of the dielectric layer, wherein the void portion substantially surrounds a second portion of the dielectric layer, and wherein the pad is positioned on the second portion. 
     The present invention generally provides at least one electrical structure, comprising: 
     a substrate including a dielectric layer having a first surface; 
     at least one electrically conductive pad attached to the first surface; and 
     a void portion of the dielectric layer, wherein the void portion substantially surrounds a second portion of the dielectric layer, and wherein the pad is positioned on the second portion. 
     The present invention has several advantages. The present invention protects the integrity of solder joints to extend the fatigue life of BGA modules. The method of the present invention is inexpensive compared with other methods, such as that of U.S. Pat. No. 5,726,079 (discussed previously in Related Art section). Moreover, the present invention does not preclude using other methods and could be used in conjunction with other methods with little added cost. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 depicts a front cross-sectional view of a BGA module attached to a circuit card. 
     FIG. 2 depicts a front cross-sectional view of a BGA module attached to a circuit card, with void regions surrounding dielectric material under both the BGA pad and the circuit card pad, in accordance with the present invention. 
     FIG. 3 depicts a front cross-sectional view of a BGA module attached to a circuit card, with a void region surrounding dielectric board material under the circuit card pad, in accordance with the present invention. 
     FIG. 4 depicts a front cross-sectional view of a BGA module attached to a circuit card, with a void region surrounding dielectric substrate material under the BGA pad, in accordance with the present invention. 
     FIG. 5 depicts a top view of a substrate having a pad with a connecting strip, in accordance with the present invention. 
     FIG. 6 depicts FIG. 5 with a void region substantially, but not totally, surrounding dielectric material underneath the pad. 
     FIG. 7 depicts a top perspective view of a substrate with a void region substantially, but not totally, surrounding dielectric material underneath a pad on the substrate. 
     FIG. 8 depicts a top view of a substrate having a pad, in accordance with the present invention. 
     FIG. 9 depicts FIG. 8 with a void region surrounding dielectric material underneath the pad. 
     FIG. 10 depicts a cross-sectional side view of the configuration of FIG. 9, showing a wiring pattern that is routed to the pad. 
     FIG. 11 depicts a top perspective view of a substrate with a void region totally surrounding dielectric material underneath a pad on the substrate. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates a front cross-sectional view of a BGA module  20  attached to a circuit card  30 . The BGA module  20  comprises a dielectric substrate  22 , a chip  26  attached to a top side  27  of the substrate  22 , a BGA pad  24  attached to a bottom side  29  of the substrate  22 , and a solder ball  28  of height L attached to the BGA pad  24 . The circuit card  30  comprises a dielectric board  32  and a circuit card pad  34  attached to a top side  37  of the board  32 . The circuit card may be any pre-wired board comprising dielectric material, such as a motherboard of a computer. The solder ball  28  is attached to pad  34  on circuit card  30 , thereby connecting the BGA module  20  to the circuit card  30 . The thermally induced strain on the solder ball  28  generated during thermal cycles are distributed over the height L. The intent of the present invention is to modify the substrate  22  and/or the board  32  so as to redistribute the strain over a height greater than L in order to increase the fatigue life of the configuration of FIG.  1 . 
     FIG. 2 illustrates an embodiment of the present invention that increases the effective height over which the strain is distributed when compared with the configuration of FIG.  1 . FIG. 2 shows a front cross-sectional view of a BGA module  40  attached to a circuit card  50 . The BGA module  40  comprises a dielectric substrate  42 , a chip  46  attached to a top side  47  of the substrate  42 , a BGA pad  44  attached to a bottom side  49  of the substrate  42 , and a solder ball  48  of height H is attached to the BGA pad  44 . The solder ball  48  may be attached to the BGA pad  44 , inter alia, prior to the chip  46  being attached to the top side  47  of the substrate  42 . The circuit card  50  comprises a dielectric board  52  and a circuit card pad  54  attached to a top side  57  of the board  52 . The solder ball  48  is attached to the circuit card pad  54 , thereby connecting the BGA module  40  to the circuit card  50 . An annular void  43  of height ΔH 1  within the substrate  42  surrounds substrate material underneath the BGA pad  44 . An annular void  56  of height ΔH 2  within the board  52  surrounds board material underneath the circuit card pad  54 . Without the annular voids  43  and  56 , the thermally induced strain would be distributed over height H in accordance with the prior art. With the annular voids  43  and  56  of the present invention, however, the deformation is distributed over the greater height H+ΔH 1 +ΔH 2 , thereby reducing the strain throughout the solder ball  48 . The annular voids  43  and  56  respectively provide space so that the substrate material underneath the BGA pad  44  and the board material underneath the circuit card pad  54  are less constrained, thereby increasing their compliance and alleviating the thermally induced strain in the solder ball  48 . 
     FIG. 3 illustrates an embodiment of the present invention that increases the effective height over which thermal strain is distributed relative to the configuration of FIG.  1 . FIG. 3 shows a front cross-sectional view of a BGA module  60  attached to a circuit card  70 . The BGA module  60  comprises a dielectric substrate  62 , a chip  66  attached to a top side  67  of the substrate  62 , a BGA pad  64  attached to a bottom side  69  of the substrate  62 , and a solder ball  68  of height Y that is attached to the BGA pad  64 . The circuit card  70  comprises a dielectric board  72  and a circuit card pad  74  attached to a top side  77  of the board  72 . The solder ball  68  is attached to the circuit card pad  74 , thereby connecting the BGA module  60  to the circuit card  70 . An annular void  76  of height ΔY within the board  72  surrounds board material underneath the circuit card pad  74 . Without the annular void  76 , the thermally induced strain would be distributed over height Y in accordance with the prior art. With the annular void  76  of the present invention, however, the deformation is distributed over the greater height Y+ΔY, thereby reducing the strain throughout the solder ball  68 . The annular void  76  provides space so that the substrate material underneath the circuit card pad  74  is less constrained, thereby increasing its compliance and alleviating the thermally induced strain in the solder ball  68 . 
     FIG. 4 illustrates an embodiment of the present invention that increases the effective height over which strain is distributed when compared with the configuration of FIG.  1 . FIG. 4 shows a front cross-sectional view of a BGA module  80  attached to a circuit card  90 . The BGA module  80  comprises a dielectric substrate  82 , a chip  86  attached to a top side  87  of the substrate  82 , a BGA pad  84  attached to a bottom side  89  of the substrate  82 , and a solder ball  88  of height Z that is attached to the BGA pad  84 . The circuit card  90  comprises a dielectric board  92  and a circuit card pad  94  attached to a top side  97  of the board  92 . The solder ball  88  is attached to the circuit card pad  94 , thereby connecting the BGA module  80  to the circuit card  90 . An annular void  83  of height ΔZ within the substrate  82  surrounds substrate material underneath the BGA pad  84 . Without the annular void  83 , the thermally induced strain is distributed over height Z in accordance with the prior art. With the annular void  83  of the present invention, however, the thermal shear stresses are distributed over the greater height Z+ΔZ, thereby reducing the strain throughout the solder ball  88 . The annular void  83  provides space so that the substrate material underneath the circuit card pad  84  is less constrained, thereby increasing its compliance and alleviating the thermally induced strain in the solder ball  88 . 
     If an annular void in FIGS. 2-4 substantially but not totally surrounds dielectric material underneath a pad, a peninsula of dielectric material under the pad will have been defined by the annular void. Alternatively, if an annular void in FIGS. 2-4 totally surrounds dielectric material underneath a pad, an island of dielectric material under the pad will have been defined by the annular void. 
     FIGS. 5-7 illustrates a process of the present invention which forms an annular void that substantially but not totally surrounds dielectric material under a pad to create a peninsula of dielectric material. The process begins with the configuration of FIG. 5, which shows a top view of a pad  112  on a substrate  110  with a wiring pattern  114  that is attached to the pad  112 . The substrate  110  represents either a dielectric substrate of a BGA module, or a dielectric board of a circuit card, as described for FIGS. 1-3. Next, FIG. 6 shows the result of forming an annular void  116  that substantially but not totally surrounds dielectric material underneath the pad  112 . As a result, a peninsula  119  of dielectric material is created under the pad  112 , wherein a strip  115  of dielectric substrate material connects the peninsula  119  to the remainder  118  of the substrate  110  (see FIG. 5 for substrate  110 ). The strip  115  serves to mechanically support the wiring pattern  114 , which electrically couples the pad  112  to the substrate  110  or to internal circuitry of the BGA module or circuit card that comprises the substrate  110 . 
     The peninsula  119  is shown in FIG. 6 to comprise a larger area than the area of pad  112  which rests on the peninsula  119 . FIG. 7 shows a perspective view of an alternative configuration of a substrate  128  with an annular void  126  that substantially but not totally surrounds a peninsula  129  of substrate material, wherein the area of the peninsula  129  is approximately the same as the area of the pad  122  that rests on the peninsula  129 . A strip  125  of dielectric substrate material connects the peninsula  129  to the substrate  128  and serves to mechanically support a wiring pattern  124  that is attached to the pad  122 . The wiring pattern  124  electrically couples the pad  122  to the substrate  128  or to internal circuitry of the BGA module or circuit card that comprises the substrate  128 . 
     FIGS. 8-11 illustrates a process of the present invention which forms an annular void that totally surrounds dielectric material under a pad to create an island of dielectric material. The process begins with the configuration of FIG. 8, which shows a top view of a pad  212  on a substrate  210 . The substrate  210  represents either a dielectric substrate of a BGA module, or a dielectric board of a circuit card, as described for FIGS. 1-3. Next, FIG. 9 shows the result of forming an annular void  216  that totally surrounds dielectric material underneath the pad  212 . As a result, an island  219  of dielectric material is created under the pad  212 , leaving a remainder  218  of the substrate  210  (see FIG. 8 for substrate  210 ). FIG. 10 illustrates a cross-sectional view of the configuration of FIG. 9, showing a wiring pattern  222  within a via  220 , wherein the via  220  is contained within the island  219 . The wiring pattern  222  is routed upward to connect the pad  212  with internal circuitry of the BGA module or circuit card that comprises the substrate  210 . 
     The island  219  is shown in FIG. 9 to comprise a larger area than the area of pad  212  which rests on the island  219 . FIG. 11 shows a perspective view of an alternative configuration of a substrate  238  with an annular void  236  that totally surrounds an island  239  of substrate material, wherein the area of the island  239  is approximately the same as the area of the pad  232  that rests on the island  239 . There is a wiring pattern (not shown) similar to that in FIG. 10, wherein the wiring pattern is within a via that is contained within the island  239 , and wherein the wiring pattern is routed upward to connect the pad  232  with internal circuitry of the BGA module or circuit card that comprises the substrate  238 . 
     The annular voids of the present invention in FIGS. 2-11 may be formed by any method known to those of ordinary skill in the art. In particular, the annular voids my be formed by laser ablation using any one of various types of lasers as are known to those skilled in the art. A practical laser for this purpose is a frequency tripled Neodumuim YAG laser using ultraviolet emission, high peak power, high repetition rate, and a focused beam of 6 to 50 μm in diameter. Generally, dielectric polymers absorb best at ultraviolet energies and the thermal damage to the nonablated portions is minimal. A useful wave length in the ultraviolet range is 355 mn. Because the areas to be scanned are usually larger than the beam size, a raster scan may be created to cover the area to be scanned. Scan spacing is typically 80% of the spot size in order to provide some overlap of the guassian beam. Depending on the material, repetition rates of 1,000 Hz to 20,000 Hz can be utilized, with the 1,000 Hz rate providing the highest power per pulse of 12 KW which drops to 0.5 KW at 20,000 Hz. An example of a specific process is ablating a circular pad on a carrier made of a glass-cloth reinforced epoxy material such as FR-4, with a 14 μm beam scanned over the target area at 2,000 Hz, 8.3 KW per pulse, and a pulse spacing of 11 μm. Each scan pass removes approximately 20 μm of material. To remove 200 μm (0.008 inch), 10 scan passes would be needed. As an alternative to the preceding Neodumuim YAG laser, other laser technologies (e.g., CO 2  and Excimer) can be used to achieve similar results. 
     Although the preferred embodiments described herein pertain to annular void regions surrounding dielectric matter underneath a pad on a BGA module or on a circuit card, the present invention applies to any configuration having annular void regions surrounding dielectric matter underneath a pad on a dielectric substrate. 
     While preferred and particular embodiments of the present invention have been described herein for purposes of illustration, many modifications and changes will become apparent to those skilled in the art. Accordingly, the appended claims are intended to encompass all such modifications and changes as fall within the true spirit and scope of this invention.