Abstract:
A method for designing structures with complimentary dynamic warp characteristics for attachment of a component to a PC board is disclosed. The method may include determining characteristics of thermally induced dynamic warp of the PC board and of the first component, analyzing and comparing differences between the dynamic warp characteristics of the PC board and the first component and selecting design modifications to match PC board and the first component dynamic warp characteristics. Selecting design modifications may include determining if the first component dynamic warp characteristics can be changed, determining if matching the dynamic warp characteristics of the PC board and the first component can be achieved by modifying the design of at least one of the PC board and the first component. The result of the method may be modified dynamic warp characteristics of at least one of the PC board and the first component.

Description:
TECHNICAL FIELD 
       [0001]    The present disclosure relates to structures for attaching components to printed circuit (PC) boards. In particular, this disclosure relates to improved component to PC board connection reliability. 
       BACKGROUND 
       [0002]    As integrated circuits (ICs) increase in complexity over time and include more active devices, the number of signals needed to connect an IC to other components in an electronic system increases. More complex ICs often consume increasing amounts of power, which in turn also requires a larger number of electrical connection points on the IC to adequately supply current. 
         [0003]    Ball grid array (BGA) electronic packages are often used to connect IC die to a printed circuit (PC) board. BGA packages are used to interconnect both signals and power between the PC board and the IC die, through an arrangement of solder ball connections at the interface between the BGA package and the PC board. The need for increasing numbers of connections to complex ICs has caused an increase in the interface areas of both the BGA package and the PC board, in order to provide more BGA ball connection sites. 
         [0004]    Electrical signal connections to ICs through electronic packages are typically not redundant, so each signal connection from the IC to a PC board (or other connecting structure) is essential for the IC&#39;s intended operation. A faulty signal connection, in the form of an open, a short, or an intermittent can cause an IC to catastrophically malfunction. The reliability of IC interconnections to PC boards and other structures is therefore critically important to ensure the proper function of the IC in an electronic system. 
       SUMMARY 
       [0005]    One embodiment is directed to a method for designing component attachment structures with complimentary dynamic warp characteristics for attachment of a first component in a first locality on a PC board. The method includes determining the warp characteristics (including magnitude and direction of warp) of thermally induced dynamic warp of the PC board and of the first component. The method also includes analyzing and comparing differences between the dynamic warp characteristics of the PC board and the first component and selecting design modifications to match the dynamic warp characteristics of the PC board and the first component. Selecting design modifications may include determining if the first component dynamic warp characteristics can be changed, determining if matching the dynamic warp characteristics of the PC board and the first component can be achieved by modifying the design of at least one of the PC board and the first component. The method also includes selecting design characteristics of at least one of the PC board and the first component to modify, and modifying the design of at least one of the PC board and the first component. The result of the method may be modified dynamic warp characteristics of at least one of the PC board and the first component. 
         [0006]    Another embodiment is directed to an electrical assembly of a PC board and an attached component. During a thermal excursion, the PC board and component may dynamically warp in the same direction, with approximately the same magnitude of warp. 
         [0007]    Aspects of the various embodiments may provide increased reliability of electrical connections in assemblies of PC boards and components. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    The drawings included in the present application are incorporated into, and form part of, the specification. They illustrate embodiments of the present invention and, along with the description, serve to explain the principles of the invention. The drawings are only illustrative of embodiments of the invention and do not limit the invention. 
           [0009]      FIGS. 1A ,  1 B and  1 C depict cross-sectional views of a component and PC board through three steps of a reflow process. 
           [0010]      FIGS. 2A ,  2 B and  2 C depict cross-sectional views of a component and PC board, illustrating three combinations of BGA and PC board warp. 
           [0011]      FIG. 3  is a graph illustrating an exemplary BGA reflow temperature profile. 
           [0012]      FIG. 4  is a diagram depicting the warp of a BGA package at various points along the BGA reflow temperature profile of  FIG. 3 . 
           [0013]      FIG. 5  is a flow diagram illustrating determining, analyzing, selecting and modifying warp characteristics to create design structures with complimentary warp characteristics, according to embodiments of the invention. 
           [0014]      FIG. 6A through 6I  depict cross-sectional views of PC board, illustrating design changes to alter warp characteristics, according to embodiments of the invention. 
       
    
    
       [0015]    In the drawings and the Detailed Description, like numbers generally refer to like components, parts, steps, and processes. 
       DETAILED DESCRIPTION 
       [0016]    In general, the embodiments describe a method for determining, analyzing, and modifying warp characteristics of a component and a PC board that may provide increased soldered connection reliability. An electronic system according to the invention may have improved reliability over a range of operating conditions including temperature. 
         [0017]    For the purposes of discussion, a “component” may be an electronic package that includes a planar array of connection points used to make connections to a corresponding area on a PC board. Connecting structures are used to create connections that are both electrical and mechanical between the component and PC board. Connecting structures may include BGA solder balls, land-grid array (LGA) structures, hybrid LGA (HLGA) structures, solder columns, and other mechanical interconnects such as springs, pogo-pins, or elastomeric materials. A component may also comprise a connector or socket element that makes use of the above connecting structures. It is understood that the principles of the invention may apply to a wide range of types of components. 
         [0018]    The particular types of electronic components relevant to the embodiments are known as “area array devices”, and include a dense array of conductive sites on a substantially planar surface designed to mate with a corresponding set of sites on a PC board or other structure. This type of component often includes a laminate substrate which provides one substantially planar surface for the purpose of active or passive electronic device attachment, and the opposite substantially planar surface for the array of mating conductive sites. A substantially planar surface is planar within the normal process tolerances of semiconductor package and PC board processing facilities. An exemplary flatness specification may be 0.004 inches (4 mils) on a device 2 inches or larger per side, however this may vary depending on manufacturing methods and connection type(s). 
         [0019]    A ball grid array (BGA) substrate is a type of area array device (package) used in conjunction with complex ICs, which may provide a large number and high density of electrical connections. Multiple electrical and mechanical connections are formed between the BGA substrate and PC board by partially melting (reflowing) solder balls previously attached to metal pads on the substrate. BGA packages with over 2,000 solder balls connections are commercially available and larger packages are contemplated. 
         [0020]    Increasingly complex integrated circuits (ICs or chips) require greater numbers of electrical connections on the chip packages they are mounted on in order to meet the IC&#39;s signal IO and power needs. A chip package acts as an intermediate electrically conductive layer between the chip and the PC board and provides multiple electrical attachment points for connection to a PC board. A chip package is one example of a component. 
         [0021]    A PC board is generally understood to be a rigid planar laminate structure comprised of one or more insulating or dielectric layers, and one or more conductive layers, which provides one or more surfaces on which to mount electronic components, and a means to interconnect the components. For the purposes of discussion and illustration, the above definition of a PC board will be used, however, embodiments of the invention may employ another type of rigid structure in place of a PC board. Such structures may include but are not limited to connectors, component sockets, and interposers. 
         [0022]    The characteristics of a PC board or other structure that apply to the embodiments are rigidity and at least one planar surface which contains an array of contact sites which form a mating pattern to those found on the opposing surface of a component. The contact sites are used to make electrical and mechanical connections to the component. 
         [0023]    The footprint dimensions of large ball-count BGA packages may be greater than 50 mm×50 mm. Solder balls attached to BGA substrates have a generally spherical shape prior to a reflow operation. An industry trend of decreasing solder ball diameter to accommodate smaller solder ball pitches has resulted over time in decreasing spacing between the BGA module and the PC board. 
         [0024]    The BGA interface area on both the BGA package and the PC board are planar, and the trend of decreasing spacing between them has made the BGA to PC board interface very susceptible to defects caused by planar deviations such as warp. Even a slight amount of warp mismatch between the component and the PC board may result in defects in the BGA ball interfaces, which are essential in providing stable electrical and mechanical connections between the module and PC board. 
         [0025]    Electronic packages and PC boards are typically constructed from a laminate of materials which may include various conductor and dielectric layers, each material type having its own unique coefficient of thermal expansion (CTE). When heated, each material type may expand at a different rate, causing warp of the component or PC board. 
         [0026]    A preferred design methodology for electronic packages and PC boards involves using a vertical arrangement of conductor and dielectric layers (stackup) that is symmetrical about an axis drawn through the center of a cross section of the laminate layers, and parallel to the layers.  FIG. 6A  illustrates an essentially symmetrical laminate cross section, including axis of symmetry  118 , conductor layers  112 , dielectric layers  110 , solder mask coating  114 , and BGA connection pads  108 . These features will be described in detail with reference to  FIG. 6A . Aspects of the symmetry include the number, arrangement, thicknesses and types of layers on each side of the axis of symmetry. 
         [0027]    A symmetrical stackup may minimize or eliminate warping of the PC board or package during assembly operations involving temperature excursions, such as a reflow process. Design or manufacturing constraints however may prohibit the stackup from being entirely symmetrical, and some warping of either the PC board, the component, or both may occur as a result. 
         [0028]    Warp has two important attributes; phase (or direction) and magnitude. Warp phase refers to the direction (upwards or downwards from a reference plane) of the PC board or component deflection. Warp magnitude  212  ( FIG. 2B ) is defined as the largest vertical component or PC board deflection that can be measured relative to a reference plane, often a flat surface. Warp magnitude is measured within the field of BGA connection pads  108  ( FIG. 2A ) on the PC board or package surface. 
         [0029]    Dynamic (thermally induced) warp is the deformation of a component (chip package) or a PC board as it experiences a thermal excursion, for example during a reflow process. A PC board or component may have a certain cross-sectional profile at room temperature, then deflect (movement of edges and/or surfaces) a certain amount either upwards (concave) or downwards (convex) during heating, then return to the original profile once it returns to room temperature. Concave deflection has a positive 2 nd  derivative as seen from a side view, such as a “cup” shape. Convex deflection has a negative 2 nd  derivative as seen from a side view, such as a “cap” shape. Referring now to  FIG. 2A ,  2 B,  2 C, BGA module  210  depicts concave warp, and PC board  220  illustrates convex warp. Dynamic warp may be exacerbated by larger component footprints and PC board mounting areas. 
         [0030]    If the dynamic warp of a PC board and a component being attached to it during a reflow operation are different (mismatched) in phase or magnitude, a number of types of BGA reflow defects may occur which may later result in mechanical or electrical failures. Defects may occur when one or more solder balls on the BGA substrate separate from the solder paste during a reflow operation, due to mismatch between the PC board and substrate dynamic warp characteristics. Exemplary warp magnitudes of a component may be between 0.254 mm and 0.508 mm, though higher magnitudes are possible. An exemplary maximum permissible package warpage may range from 0.10 mm to 0.25 mm, depending on solder ball size and manufacturing process used. 
         [0031]    One type of defect, known as a “head-in-pillow” defect occurs when BGA substrate solder balls are separated from the solder paste on the PC board during the reflow process, due to dynamic warp mismatch. Solder paste typically contains flux, a chemical agent to clean and prevent oxidation of mating metallic surfaces during the reflow process. When the solder ball is separated from contact with the solder paste during the heating of a reflow operation, the lack of flux may allow an oxide layer may be grown on the outer surface of the solder ball. This oxide layer may subsequently prevent proper bonding of the solder ball to the solder paste, resulting in a defective (weak) mechanical connection. 
         [0032]    Defects such as the head-in-pillow type may manifest themselves immediately, as an intermittent or open, or may be latent, only appearing after some period of thermal cycling of the component and PC board. Because a head-in-pillow may initially appear as a valid connection, it may not be easily detected using classical test methods. The latent failure of a single BGA connection may result in catastrophic system failure, as every signal connection is critically important to many electronic systems. 
         [0033]    A material&#39;s coefficient of thermal expansion (CTE) is defined as the change of unit length per change in unit temperature for that type of material. A commonly used unit of measure for CTE is parts-per-million per degree Celsius (ppm/° C.). A material may expand or contract linearly in proportion to the CTE times the change in temperature experienced. A larger CTE indicates a greater material expansion than a smaller CTE, for an identical temperature excursion. The dynamic warp of a component or PC board is largely dependant on the CTEs, dimensions, and arrangement of the various materials comprising its laminate structure. Dynamic warp and dynamic warp mismatch may also be influenced by temperature gradients across a component or PC board, due to uneven heating during a reflow operation. 
         [0034]    The table below lists some representative materials used in the fabrication of electronic packages and PC boards. The coefficient of thermal expansion (CTE) ranges and values shown are intended to be exemplary, and are understood to not be limiting. One skilled in the art of PC board and electronic package design will understand that a large number of material types are available for use, each having a specific CTE range. 
         [0000]    
       
         
               
               
               
             
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                 CTE range 
               
               
                 Material Name 
                 Usage 
                 (ppm/° C.) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Silicon die 
                 Active electronic circuits 
                 2.3 
               
               
                 Unfilled Polymer 
                 Chip encapsulant 
                 10-25 
               
               
                 Lead-free solder 
                 BGA solder ball 
                 19-23 
               
               
                 Tin/lead solder 
                 BGA solder ball 
                 21 
               
               
                 Copper 
                 PC board signal and power 
                 17-18 
               
               
                   
                 interconnection 
               
               
                 FR-4 
                 PC board base material (epoxy resin + 
                 13-14 
               
               
                   
                 glass) 
               
               
                   
               
             
          
         
       
     
         [0035]    For simplicity of illustration, the following figures depict a BGA component and its relationship to an exemplary PC board; however, the principles of the invention may be applied to other types of components and PC boards or other rigid structures. 
         [0036]      FIGS. 1A ,  1 B and  1 C are cross-sectional views of a BGA component  120  and a PC board  130  depicting three consecutive steps of a reflow process. It should be noted that a reflow process devoid of any component or PC board warp is depicted, which though desirable, may not necessarily be achievable in practice. 
         [0037]      FIG. 1A  includes the BGA component  120  which is comprised of a chip  102 , a substrate  104 , and solder balls  105 . The solder balls  105  are attached to the substrate  104  and are arranged in a regular array on the lower planar surface of the substrate  104 . The substrate  104  provides multiple electrical connections (not shown) between the chip  102  and the solder balls  105 . 
         [0038]      FIG. 1A  also includes the PC board  130 , which is comprised of a laminate of alternating dielectric layers  110  and conductor layers  112 . BGA connection pads  108  are formed on the top surface of the PC board  130 , and a layer of solder paste  106  is deposited on each of the BGA connection pads  108 . The lower planar surface of the PC board  130  is covered by a solder mask coating  114  to protect surface metallization during its assembly and from oxidation and damage. 
         [0039]      FIG. 1A  illustrates the BGA component  120  aligned with the PC board  130  so that the array of solder balls  105  is vertically aligned with and opposes the array of BGA connection pads  108  below it. This alignment is often performed by automated machinery, which then lowers the BGA component  120  onto the PC board  130 . 
         [0040]      FIG. 1B  illustrates the BGA component  120  placed on the PC board  130  so that the array of solder balls  105  is both aligned with the array of BGA connection pads  108  below it, and impressed into the solder paste  106  on top of the connection pads  108  (see inset drawing for magnified view).  FIG. 1B  depicts the component  120  and PC board  130  combined into a pre-reflow assembly  140 , ready for a reflow operation. 
         [0041]    The pre-reflow assembly  140  is subsequently exposed to a heat source such as a reflow oven, where the temperature of the assembly  140  is carefully controlled and monitored over time. (See  FIG. 3 ). The purpose of heating the assembly  140  is to melt and fuse the solder of the solder balls  105  with the solder paste  106 , which simultaneously attaches to the connection pads  108 . 
         [0042]      FIG. 1C  shows the reflowed assembly  150 , after the reflow operation (elevating then subsequently lowering assembly temperature) is completed. The solder balls  105  and solder paste  106  have melted together to form a barrel-shaped reflowed BGA ball  116 , which is securely attached to both substrate  104  and BGA connection pads  108 . It should be noted that in this illustration, featuring both a planar BGA component  120  and PC board  130 , that all the reflowed BGA balls  116  are similar in shape and dimensions, and all form stable connections between their respective sites on the substrate  104  and BGA connection pads  108 . The reflowed assembly  150  contains no BGA connection electrical defects, such as shorts, opens, or intermittent connections caused by warpage. 
         [0043]    PC board  130  has the same number of conductor layers  112  (including BGA connection pads  108 ) and dielectric layers  110  on either side of the axis of symmetry  118 . The conductor layers  112  are all the same thickness, and the dielectric layers  110  are all the same thickness. The layer arrangement shown represents a desirable, symmetric configuration that may minimize warp when the PC board is heated. 
         [0044]      FIGS. 2A ,  2 B and  2 C are cross-sectional views illustrating three types of warp relationships between a BGA component  210  and PC boards  220 ,  240 , and  260 .  FIGS. 2A ,  2 B and  2 C are exemplary illustrations only; many other combinations of warp relationships between components and PC boards are possible, including a large variety of warp phase and magnitude combinations. 
         [0045]    It should be noted that  FIGS. 2A ,  2 B and  2 C are static illustrations of the warp of a BGA substrate and a mating PC board. These illustrations may depict the warp characteristics at a certain temperature, but those characteristics, including phase and magnitude may vary as the temperature of the substrate and the PC board changes. 
         [0046]      FIG. 2A  depicts a BGA component  210  and a PC board  220  with warp that is out of phase; the BGA component  210  edges deflect upwards, while the PC board  220  edges deflect downwards. The difference in warp phase may cause separation of the solder balls  105  and the solder paste  106 , which may create a defect in certain solder ball connections during a reflow operation. 
         [0047]    Similarly,  FIG. 2B  depicts a BGA component  210  and a PC board  240  with warp that is out of phase; the BGA component  210  edges deflect upwards, while the PC board  240  edges do not deflect. The difference in warp phase may cause separation of the solder balls  105  and the solder paste  106 , which may create a defect in certain solder ball connections during a reflow operation. 
         [0048]      FIG. 2C  depicts a BGA component  210  and a PC board  260  with warp that is in phase. BGA component  210  edges deflect upwards in the same direction and with the same magnitude as the edges of PC board  260 . The type of warp relationship between a PC board and a component with the same warp phase and magnitude (within normal manufacturing tolerances) over a temperature excursion is known as complimentary dynamic warp. The identical warp phase and similar magnitude does not cause separation of the solder balls  105  and the solder paste  106 , which may help to ensure stable, defect-free solder ball connections during a reflow operation. The warp characteristics (magnitude and phase) of both the BGA component  210  and PC board  260  must remain within manufacturing tolerances during the entire reflow operation (temperature excursion—see  FIG. 3 ) in order to ensure defect-free connections. 
         [0049]      FIG. 3  is a graph illustrating a BGA reflow temperature profile during an exemplary reflow operation. The graph depicts the temperature of the solder ball and solder paste interface between the BGA substrate and the PC board. During the reflow operation, the temperature of the profile is typically precisely controlled and monitored in a reflow oven. Heat may be applied to the substrate and PC board through convection, forced hot air or other gasses, infrared heating elements, vapor phase heating, or other means. The temperature excursion of a reflow process (difference between peak temperature  330  and room temperature  310 ) is typically greater than 200 degrees C., but may vary depending on the specific solder alloy compositions of the solder balls  105  and solder paste  106 . 
         [0050]    The temperature profile is divided up into several zones, each having a unique purpose. During the ramp-up zone  315 , the component and PC board temperature is rapidly raised from room temperature  310  to a temperature that approximates a soaking temperature  320 . The temperature rise may be uneven across the PC board and component due to the method(s) of heating, the specific heat of each material, and other factors, and during the ramp-up zone  315  may cause differences in warp magnitude between them. 
         [0051]    A soaking zone  325  allows time for the PC board and component temperatures to stabilize and equalize. The pre-heating zone  345  comprises the ramp-up zone  315  and the soaking zone  325 . During the reflow zone  335 , the solder ball and solder paste is raised to the peak temperature  330 , at which the paste and solder ball merge, and wet the surfaces of the BGA connection pads  108  on the PC board. The temperature then returns to the soaking temperature  320 . During the cool-down zone  350  the solder temperature begins to return from the soaking temperature  320  to room temperature  310 . 
         [0052]      FIG. 4  is a diagram depicting the warp of an exemplary BGA substrate  410  at various points along the BGA reflow temperature profile of  FIG. 3 .  FIG. 4  also illustrates how a type of defect may be created by mismatched dynamic warp characteristics of a BGA substrate and a PC board. For ease of illustration,  FIG. 4  depicts dynamic warp of just a BGA substrate  410 ; the PC board is shown with no dynamic warp. Warp combinations may include any combination of magnitude and phase differences, some of which are depicted in  FIG. 3 . 
         [0053]    Starting at room temperature  310 , assembly  400  depicts the BGA substrate  410  and PC board  415  interface. The inset diagram shows a solder ball  105  impressed into and aligned with areas of solder paste  106 , similar to  FIG. 1B . All solder balls on the BGA are similarly impressed into and aligned with areas of solder paste  106 . 
         [0054]    Assembly  420  depicts the and PC board  415  interface as the solder ball  105  and solder paste  106  temperature are raised to the final soaking temperature  320 . The inset diagram shows solder ball  105  separated from an area of solder paste  106 , due to differences in dynamic warp of the BGA substrate  410  and the PC board  415 . The increase in temperature from room temperature  310  to final soaking temperature  320  has caused the BGA substrate  410  to deflect more than the PC board  415 , causing the separation on the solder balls  105  at the edge of the BGA substrate  410 . 
         [0055]    As the interface temperature is further raised to the peak reflow temperature  330 , assembly  440  depicts the BGA substrate  410  and PC board  415  interface. The inset diagram shows a solder ball  105  further separated from an area of solder paste  106 . A BGA ball oxide layer  402  has begun to form on surface of the solder ball  105 . If the solder ball  105  remained impressed into the solder paste  106 , the flux present in the solder paste would chemically prevent oxide from forming, allowing the solder ball  105  and solder paste  106  to melt together in response to the applied heat. Both the separation of the solder ball  105  and solder paste  106  and the BGA ball oxide layer  402  may now prevent a proper reflow process. 
         [0056]    As the temperature decreases from the peak reflow temperature  330  to the post-reflow temperature  320 , assembly  460  depicts the BGA ball oxide layer  402  growing thicker, while the BGA substrate  410  warp decreases, moving solder ball  105  closer to solder paste  106 . As the temperature continues to decrease, the BGA substrate  410  continues to deflect downwards, eventually bringing the surface of BGA ball oxide layer  402  in contact with an area of solder paste  106 . By the time the BGA ball oxide layer  402  is in contact with an area of solder paste  106 , the flux that was contained in the solder paste has been consumed in cleaning the micro-spheres of solder in the solder paste  106 , and not enough remains to remove the BGA ball oxide layer  402 , and allow the solder paste  106  and the solder ball  105  to properly melt together. 
         [0057]    Again referring to assembly  460 , the solder ball  105  (with oxide layer) may be pressed into the solder paste  106 , but may create an open connection, or a very weak electrical connection due to the electrically insulating nature of the oxide. This type of defect is known as a “head-in-pillow” defect, and may be very difficult to detect using traditional test methods. A head-in-pillow defect is one type of a cold solder joint defect. Other defect types such as shorts, opens, and other forms of intermittent connections are also possible as a result of component and PC board warp mismatch. The above listed defect types may manifest themselves immediately, or at some point in time after the reflow operation. 
         [0058]      FIG. 5  is a flow diagram that illustrates determining, analyzing, selecting and modifying PC board and component warp characteristics to create design structures with complimentary dynamic warp characteristics, according to embodiments of the invention. While PC board manufacturing tolerances (values) may vary by technology, they are specified to ensure consistent contact between mating parts of a reflow connection throughout a reflow operation. 
         [0059]    A desirable outcome of modifying (tuning) components and PC boards to produce complimentary dynamic warp is contact between solder balls on a BGA package and corresponding solder paste on a PC board throughout a reflow process ( FIG. 3 ). The continuity of contact that complimentary dynamic warp may produce may create more reliable BGA reflow connections, and thus more reliable IC operation in electronic systems. Defects such as head-in-pillow, opens and shorts may be reduced or eliminated.  FIG. 1C  and  FIG. 2C . depict exemplary complimentary dynamic warp characteristics at two different temperatures of a reflow profile, such as  FIG. 3 . 
         [0060]    The process  500  moves from start  502  to determine warp characteristics  504 . At determine warp characteristics  504 , the initial (before modifications) dynamic warp characteristics of the component and the PC board are ascertained. Warp data regarding phase, magnitude, and other characteristics may be gathered from any combination of measurements, component or PC board supplier specifications, software modeling results, or other sources. Warp measurement methods may include the shadow moire method, the laser reflection method, or X-ray imagery. Component or PC board supplier specifications may include a table of warp values over a range of reflow profile temperatures. The warp characteristics of each individual component and PC board mounting site are determined. 
         [0061]    After warp characteristics are determined (operation  504 ) magnitudes and phases of the component and PC board warp are analyzed and compared to each other (operation  506 ), and differences between them are determined. 
         [0062]    The results of operation  506  are then used as a basis of the selection operation  510 , which determines whether the component, the PC board, or both should be modified, and which types of modifications are to be made. Operation  510  may use historical recorded warp characteristics and thermo-mechanical warp simulation results from operations  504  and  506  to determine which design modifications will provide appropriate complimentary dynamic warp between the component and the PC board. 
         [0063]    Types of design modifications may include any combination of altering the vertical thickness of conductor or dielectric layers, the number or arrangement of conductor or dielectric layers, the types of materials used and wiring density. The above listed design modifications are further illustrated in  FIG. 6A  through  FIG. 6I . 
         [0064]    The selection of operation  510  may include determining the economic feasibility of modifying the component&#39;s warp characteristics. The component may manufactured by a company that is unwilling to change its warp characteristics, or it may be prohibitively difficult or expensive to do so. 
         [0065]    Again referring to selection operation  510 , if only PC board modifications are chosen, the board design data and material types may be modified in operation  512 . If only component modifications are chosen, the component design data and material types may be modified in operation  514 . If both board and component modifications are chosen, then operations  508  and  514  are both performed, modifying the design data and material types of both the PC board and component. 
         [0066]    Operations  508 ,  512 , and  514  may include modifying design data that is stored in electronic design automation (EDA) programs, as well as specifying various materials with CTEs suitable for the construction of one or more of the PC board and the component. Design data may include but is not limited to wiring shapes that define material placement, material types and dimensions, and stackup configurations. 
         [0067]    Once all board and component design modifications are completed (operations  508 ,  512 , and  514 ), the board and component are analyzed for signal integrity (SI) and power structure robustness in operation  516 . SI and power distribution analysis may employ commercially available circuit analysis software. The analysis of  516  may be necessary because modifying the design data, including wiring, material types and dimensions, and layer arrangement may adversely affect the electrical properties of the design, and create unforeseen functional problems. If power distribution or SI criteria are not met, as determined in deciding operation  518 , then the process  500  may need to be repeated starting at operation  506 , where the warp characteristics of the modified PC board are re-analyzed and compared against each other. Adjustments may be made in select operation  510  to balance the needs of warp adjustment against SI and power distribution criteria. If power distribution and SI criteria are met, then the process  500  may end at block  520 . 
         [0068]    It is noted that the process depicted by  FIG. 5  may be applied to multiple component attachment sites on a PC board with each interface area independently adjusted to create complimentary dynamic warp characterizes for that interface. PC board modifications are not strictly limited to the area underneath a component, but may be larger or smaller than the component attach area. 
         [0069]      FIG. 6A through 6I  depict cross-sectional views of a PC board, illustrating design changes that may alter dynamic warp characteristics, according to embodiments of the invention. Similar changes may be made to the structure of an electronic component or component substrate. The changes illustrated are exemplary embodiments that reference typical materials properties shown in Table 1, and do not limit the invention in any way. Actual practice of the invention may use any combination of the changes illustrated in  FIG. 6A through 6I  and other suitable types of design modifications or material types. 
         [0070]      FIG. 6A  represents a cross-sectional view of an unmodified PC board, having six conductor layers  112 , six dielectric layers  110 , BGA connection pads  108  and solder mask coating  114 . The above listed features of  FIG. 6A  are present in  FIG. 6B through 6I , except where noted.  FIG. 6A  depicts an exemplary PC board cross-section that is largely symmetrical across the axis of symmetry  118 , and exhibits minimal dynamic warp in response to temperature excursions. 
         [0071]    All changes depicted in  FIG. 6B through 6I  are in reference to the axis of symmetry  118 , and are relative to  FIG. 6A . Design changes generally involve changing the overall CTE of the portion of the laminate structure on one side of the axis of symmetry  118 , to promote warping either upwards (concave) or downwards (convex) during a reflow process. The relative thickness of a laminate material and the material&#39;s CTE (See Table 1) both influence the amount of dynamic expansion and warp produced by that material In general, any changes described or depicted in relation to  FIG. 6B through 6I  may be migrated across the axis of symmetry  118  to produce dynamic warp in an opposite direction (phase). 
         [0072]      FIG. 6B  shows the replacement of two conductor layers  112  above the axis of symmetry  118  with two thicker conductor layers  602 . This may cause downward dynamic warp of the edges of PC board  620 . 
         [0073]      FIG. 6C  shows conductor layers  112  above the axis of symmetry  118  with a higher wiring density (ratio of wire to dielectric area on a conductor layer) than conductor layers  112  below the axis of symmetry  118 . This may cause downward dynamic warp of the edges of PC board  630 . 
         [0074]      FIG. 6D  shows the replacement of two conductor layers  112  below the axis of symmetry  118  with two thicker conductor layers  608 . This may cause upward dynamic warp of the edges of PC board  640 . 
         [0075]      FIG. 6E  shows additional conductor layer  610  and additional dielectric layer  611  below the axis of symmetry  118 . As depicted, this may not change the dynamic warp of the edges of PC board  650 , but may influence the overall warp of the PC board  650  depending on the particular arrangement, thickness, and CTEs of other conductor and dielectric layers above it. 
         [0076]      FIG. 6F  shows the replacement of two dielectric layers  110  above the axis of symmetry  118  with two thicker dielectric layers  612 . This may cause upward dynamic warp of the edges of PC board  660 . 
         [0077]      FIG. 6G  shows two additional dielectric layers  614  and additional conductor layers  112  below the axis of symmetry  118 . The additional dielectric layers  614  have a different CTE than other dielectric layers  110 , and the additional conductor layers  112  have a CTE that is identical to other conductor layers  112 . If the CTE of the additional dielectric layers  614  is greater than the CTE of dielectric layers  110 , this may cause upward dynamic warp of the edges of PC board  670 . If the CTE of the additional dielectric layers  614  is less than the CTE of other dielectric layers  110 , this may cause downward dynamic warp of the edges of PC board  670 . 
         [0078]      FIG. 6H  shows the addition of a sash structure  616  (comprised of copper) surrounding BGA connection pads  108  on the top of PC board  680 , which may cause downward dynamic warp of the edges of PC board  680 . Sash may be added to either the top or bottom surface of a PC board, as needed. 
         [0079]      FIG. 6I  shows the addition of partial pre-preg patch  618  to the bottom of PC board  690 . Pre-preg is typically a partially cured, glass-free epoxide sheet used for bonding printed circuit board layers. Embodiments may include using a partial patch, which corresponds to a part or the entirety of the field of BGA connection pads  108 . Other embodiments may include using a full patch that covers the entire lower surface of the PC board  690 . Since the CTE of pre-preg materials are typically greater than the overall CTE of the other layers of PC board  690 , the pre-preg patch  618  may cause upward dynamic warp of the edges of PC board  690 . However, if the pre-preg patch  618  were to have a CTE less than the overall CTE of the other layers of PC board  690 , it may cause downward dynamic warp of the edges of PC board  690 . 
         [0080]    Although the present invention has been described in terms of specific embodiments, it is anticipated that alterations and modifications thereof may become apparent to those skilled in the art. Therefore, it is intended that the following claims be interpreted as covering all such alterations and modifications as fall within the true spirit and scope of the invention.