Abstract:
A method for controlling the height, shape and placement of solder connections of a surface mount circuit device, such as a flip chip, by way of controlling the extent to which solder is able to flow on a conductor during reflow, but without the conventional use of a solder mask in a manner that results in a portion of the mask remaining beneath the device. Each conductor is defined to include a bond pad and a reduced-width portion adjacent the bond pad. The width of each reduced-width portion is sufficiently narrower than the width of its adjacent bond pad to impede the flow of molten solder onto the reduced-width portion from a solder bump registered with the bond pad. As a result, when reflow soldering a circuit device to the circuit board, molten solder substantially remains on the bond pads and, for a given solder bump size, the bond pads determine the shape and height of the resulting solder connections.

Description:
TECHNICAL FIELD 
     The present invention generally relates to surface mount (SM) circuit devices that are attached to conductor patterns with solder connections formed by reflow soldering methods. More particularly, this invention relates to a method for promoting the stand-off height of surface mount circuit devices on laminate circuit boards by controlling the height of their solder connections without the conventional use of a solder mask as a solder stop. 
     BACKGROUND OF THE INVENTION 
     A flip chip is generally a monolithic surface mount (SM) semiconductor device, such as an integrated circuit, having bead-like terminals formed on one of its surfaces. The terminals, typically in the form of solder bumps, serve to both secure the chip to a circuit board and electrically interconnect the flip chip circuitry to a conductor pattern formed on the circuit board, which may be a ceramic substrate, printed wiring board, flexible circuit, or a silicon substrate. Due to the numerous functions typically performed by the microcircuitry of a flip chip, a relatively large number of solder bumps is required. The solder bumps are typically located at the perimeter of the flip chip on electrically conductive pads that are electrically interconnected with the circuitry on the flip chip. The size of a typical flip chip is generally on the order of a few millimeters per side, resulting in the solder bumps being crowded along the perimeter of the flip chip. 
     Because of the narrow spacing between adjacent solder bumps and conductors, soldering a flip chip to its conductor pattern requires a significant degree of precision. Widely employed for this purpose are reflow soldering techniques, which typically entail precisely depositing a controlled quantity of solder on a flip chip using methods such as electrodeposition, and then heating the solder above its liquidus temperature to form the characteristic solder bumps on the surface of the chip. After cooling to solidify the solder bumps, the chip is soldered to the conductor pattern by registering the solder bumps with their respective conductors and then reheating, or reflowing, the solder so as to metallurgically adhere, and thereby electrically interconnect, each solder bump with its corresponding conductor, forming what will be referred to herein as a solder connection. 
     Placement of the chip and reflow of the solder must be precisely controlled not only to coincide with the spacing of the terminals and the size of the conductors, but also to control the height of the solder connections after soldering. As known in the art, controlling the height of solder connections after reflow is often necessary to prevent the surface tension of the molten solder from drawing the flip chip excessively close to the substrate during the reflow operation. Sufficient spacing between the chip and its substrate, termed the stand-off height, is desirable for allowing penetration of cleaning solutions for removing undesirable processing residues, promoting the penetration of mechanical bonding and encapsulation (underfill) materials between the chip and its substrate, and enabling stress relief of the solder connections during thermal cycles. Solder bump position and height are generally controlled by the amount of solder deposited on the flip chip to form the solder bump and by the use of solder stops that limit the surface area over which the solder bump is allowed to reflow. Solder stops are typically formed by a solder mask on laminate substrates and printed dielectric on ceramic substrates. For laminate circuit boards, the solder mask is applied over the conductor pattern and an opening is formed in the mask to expose a limited portion of each conductor, which then serves as a bond pad for the solder bumps. 
     While solder stops are widely used in the art, trends in the industry have complicated their ability to yield solder connections that provide an adequate flip chip stand-off height. As flip chips become more complex, the number of bumps that must be accommodated along the chip perimeter has increased. In turn, the conductors to which the bumps are registered and soldered have become more closely spaced and narrower, e.g., a pitch of about 0.010 inch (about 250 micrometers) or less and line widths of about 0.004 inch (about 100 micrometers), yielding a line spacing of about 0.006 inch (about 150 micrometers) or less. Fine solder bump and conductor pitches complicate the design and fabrication of solder stops, particularly on laminate substrates with the result that pitches of less than 0.010 inch have not been widely used. Solder connections having adequate stand-off height have also become more difficult to consistently produce, which increases the difficulty of removing residues from between the chip and substrate, underfilling the chip with bonding and encapsulation materials, and promoting stress relief in the solder connections during thermal cycling. This difficulty is particularly evident on laminate circuit boards, because the requirement for a solder mask as a solder stop requires that a portion of the mask remains beneath the chip, which reduces the stand-off height of the chip by the thickness of the mask. For example, on a fine pitch pattern of 0.010 inch, the height of each solder connection may be about 0.0036 inch (about 90 micrometers), but the stand-off height is only about 0.003 inch (about 75 micrometers) for a typical solder mask thickness of about 0.0006 inch (about 15 micrometers). 
     Accordingly, it would be desirable if a method were available that was able to increase the stand-off heights of flip chips and other surface mount devices, and particularly those devices requiring a fine pitch solder bump pattern. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method for controlling the height, shape and placement of solder connections of a surface mount circuit device, such as a flip chip, by way of controlling the extent to which solder is able to flow on a conductor during reflow, but without the conventional use of a solder mask in a manner that results in a portion of the mask remaining beneath the device. As a result, solder connections formed by the method of this invention are able to achieve a greater stand-off height for the device, which promotes the penetration of cleaning solutions, mechanical bonding and encapsulation materials between the chip and its substrate, and enables stress relief during thermal cycles. Because bonding and encapsulation materials typically adhere better to circuit board materials than solder mask materials, another advantage of the invention is that underfill materials are able to bond directly to a circuit board rather than the mask. 
     According to this invention, the above is achieved by forming on a circuit board a conductor pattern defined by a number of conductors, with each of the conductors defining a bond pad and a reduced-width portion adjacent the bond pad. The width of each reduced-width portion is sufficiently narrower than the width of its adjacent bond pad to impede the flow of molten solder onto the reduced-width portion from a solder bump registered with the bond pad. As a result, when reflow soldering a circuit device to the circuit board, molten solder substantially remains on the bond pads and, for a given solder bump size, the bond pads determine the shape and height of the resulting solder connections. In this manner, the reduced-width portion of each conductor serves as a solder stop, because each reduced-width portion limits the flow of molten solder on the conductor. 
     In view of the above, a significant advantage of this invention is the elimination of the prior art requirement for conventional solder stops to limit the flow of solder on the conductors. On laminate circuit boards, the elimination of a solder stop eliminates the need for a solder mask that would otherwise lie between the circuit device and board and, as a result, would reduce the stand-off height of the device by the thickness of the mask. Accordingly, the stand-off height of a surface mount surface device on a laminate board is maximized for a given solder bump size, which promotes the penetration of cleaning solutions, mechanical bonding and encapsulation materials between the chip and its substrate, promotes the adhesion of bonding and encapsulation materials, and enables stress relief during thermal cycles. 
     Other objects and advantages of this invention will be better appreciated from the following detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1,  2  and  3  are plan views of portions of circuit boards with conductor patterns in accordance with three embodiments of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Shown in FIG. 1 is a portion of a circuit board  10  in accordance with a first embodiment of this invention. Conductors  12  define a conductor pattern on the circuit board  10 , with each conductor  12  configured in accordance with this invention to have a bond pad  14  delineated by a reduced-width portion  16  that separates the bond pad  14  from the remainder of the conductor  12 . The circuit board  10  is shown prior to placement of a flip chip or other surface mount device on the board  10 , by which solder bumps on the chip would be registered with and then reflowed on the bond pads  14 . Optional “dummy bumps”  18  in accordance with commonly assigned U.S. Pat. No. 5,400,950 are shown within an interior region  20  of the circuit board  10  surrounded by the conductor pattern. These bumps  18  are not electrically active, but are used if additional mechanical lift is desired to promote the stand-off height of the chip on the board  10 . 
     As typical in the art, after registration with the bond pads  14 , the solder bumps of the chip are reflowed in any suitable manner to form solder connections that physically and electrically connect the chip to the conductors  12 . Suitable solder alloys include, but are not limited to, tin-based, lead-based and indium-based alloys, with notable examples being tin-lead alloys containing about ten to about sixty percent tin, with possible alloying additions of antimony, silver, etc. These alloys can be reflowed at sufficiently low temperatures to avoid thermal damage to the circuitry of the chip and circuit board  10 . The solder alloy for the bumps is typically screen printed or electrodeposited on contact pads on the chip, and must be accurately deposited in limited amounts such that, after reflow, the solder bumps will be substantially of equal size and will accurately and uniquely register with the bond pads  14  when the chip is registered with the conductors  12 . 
     The conductors  12  are formed of a solderable material, which denotes a material to which solder will metallurgically bond and reliably adhere for purposes of making an electrical interconnection, as determined in the art using known test methods. A preferred conductor material for laminate circuit boards (e.g., FR4) is planar copper deposited by plating or by lamination of a copper foil, with a suitable thickness being about 0.015 to about 0.040 millimeters. While the invention has particular applicability for laminate circuit boards, conductors configured in accordance with this invention can be printed or otherwise formed on the surface of other circuit board materials, including ceramic and silicon substrates and flexible circuits, as is known in the art. 
     A solder mask  22  is shown as surrounding the bond pads  14 , the reduced-width portions  16 , and the interior region  20  of the circuit board  10 . The solder mask  20  is preferably a photoresist material so that an opening  24  in the mask  22  can be defined by known photoengraving techniques. The opening  24  in the mask  22  would typically be required on a laminate circuit board to expose a limited portion of the conductors  12  for the purpose of defining the bond pads  14 , resulting in the opening  22  being roughly a square-shaped trench corresponding to the square pattern formed by solder bumps located along the perimeter of a chip. However, in accordance with this invention, the solder mask  22  is excluded from the interior region  20  of the board  10 , and the conductors  12  are truncated to define the interior end of each bond pad  14 , while the opposite end of each pad  14  is delineated by the interface with its reduced-width portion  16 . 
     According to the invention, the reduced-width portions  16  serve as solder stops by physically impeding the flow of molten solder beyond their respective bond pads  14 , and preventing solder flow onto the remainder of the conductors  12  set apart from the bond pads  14  by the reduced-width portions  16 . As the molten solder coalesces during reflow, the surface tension of the solder constrains the final shape of the solder connections formed by the bumps in accordance with the size and shape of the bond pads  14 . Because the solder mask  22  is not required within the interior region  20 , the stand-off height of a chip attached to the bond pads  14  by reflow soldering is effectively increased by the width of the solder mask  22 . With this increase in stand-off height, the penetration of cleaning solutions, mechanically bonding and encapsulation materials is promoted during processing of the board  10 , and stress relief of the solder connections is promoted during thermal cycling of the circuit board  10 . Because bonding and encapsulation materials typically adhere better to circuit board materials than solder mask materials, another advantage of this invention is that underfill materials are able to bond directly to the circuit board  10  rather than the solder mask  22 . 
     In view of the above, the transverse widths of the bond pads  14  and reduced-width portions  16  are critical to achieving the objects of this invention, while the width of the remainder of each conductor  12  is not. Because the surface area of each bond pad  14  determines the manner in which the molten solder will flow, the length of each bond pad  14  is important, while the length of each reduced-width portion  16  should be sufficient to prevent molten solder from flowing over the portion  16  and onto the remainder of the conductor  12 . The square corners shown in FIG. 1 at the junction between each bond pad  14  and its reduced-width portion  16  improve the ability of the portion  16  to impede the flow of solder across the junction. In practice, the surface areas, widths and lengths of the bond pads  14  are preferably the same, as are the widths and lengths of the reduced-width portions  16 . To be sufficiently narrower than its bond pad  14  for the purpose of effectively impeding the flow of molten solder, the maximum width for a reduced-width portion  16  should be about 75% the width of its bond pad  14 , and preferably about 40% to about 70% of the width of the bond pad  14 . As an example, for conductors  12  on a 0.008 inch pitch (200 micrometers), a suitable length and width for a bond pad  14  is about 0.007 inch (about 175 micrometers) and about 0.004 inch (about 100 micrometers), respectively, a suitable length for each reduced-width portion  16  is about 0.007 inch (about 175 micrometers), and a maximum width for each reduced-width portion  16  is believed to be about 0.003 inch (about 76 micrometers), with a suitable width being about 0.002 inch (about 50 micrometers). 
     The bond pads  14  of FIG. 1 are shown as being rectangular and linearly aligned on the circuit board  10 . As a result, solder connections formed on the bond pads  14  will generally have an oblong shape whose major axis is in the longitudinal direction of each conductor  12 . 
     In a second embodiment of the invention shown in FIG. 2, conductors  112  are formed to have circular bond pads  114  set apart from the remainder of each conductor  112  by, a reduced-width portion  116  whose intersection with the conductor  112  defines a square transverse edge  118 . The bond pads  114  are arranged in a staggered pattern on a circuit board  110  as a result of adjacent reduced-width portions  116  having different lengths. To maximize the spacing between circular-shaped solder connections subsequently formed at each bond pad  114 , pads  114  with shorter reduced-width portions  116  are preferably aligned adjacent to the reduced-width portions  116  of adjacent conductors  112 , as depicted in FIG.  2 . In this embodiment, the maximum width of each reduced-width portion  116  is about 50% of the diameter of its bond pad  114 , and preferably about 33% to about 42% of the diameter of the pad  114 . As an example, for conductors  112  on a 0.008 inch pitch (200 micrometers), a suitable diameter for a bond pad  114  is about 0.006 inch (about 150 micrometers), and a maximum width for each reduced-width portion  116  to adequately impede the flow of molten solder is believed to be about 0.003 inch (about 76 micrometers), with a suitable width being about 0.0025 inch (about 63.5 micrometers). To produce the desired staggered arrangement of the pads  114 , the lengths of adjacent reduced-width portions  116  preferably differ by the diameter of the bond pads  114 . e.g., about 0.006 inch (about 175 micrometers) for the pattern just described. It is foreseeable that the pads  114  could be staggered greater distances apart, with the maximum length of each reduced-width portion  116  generally being limited by the allowable temperature rise and resistance increase of its conductor  112 . 
     As with the embodiment of FIG. 1, a solder mask  122  is shown as being limited to surrounding the bond pads  114  and reduced-width portions  116  of the conductors  112 . The solder mask  122  is excluded from the interior region of the circuit board  110  surrounded by the bond pads  114 , where the mask  122  would conventionally be required as a solder stop, and instead the reduced-width portions  116  serve as solder stops by physically impeding the flow of molten solder beyond the bond pad  114 , and preventing solder flow onto the remainder of the conductors  112  set apart from the bond pads  114  by the reduced-width portions  116 . The benefit of excluding the solder mask  122  from the interior region of the board  110  is again the increased stand-off height of a chip attached to the bond pads  114  by reflow soldering. An additional benefit of the embodiment of FIG. 2 is that the staggered arrangement of solder connections on the bond pads  114  provides for greater clearance between connections for a given conductor pitch, which reduces the risk of shorts when fine conductor pitches (e.g., 0.010 inch or less) are used. 
     A third embodiment of this invention is shown in FIG. 3, in which conductors  212  have bond pads  214  defined between pairs of reduced-width portions  216 , each of which intersects its conductor  212  to define a square transverse edge  218 . The length and width of each reduced-width portion  216  is again sized to prevent molten solder from flowing across the portion  216  and onto the remainder of the conductor  212 , as indicated by the solder connections  220  depicted in FIG.  3 . As with the previous embodiments of FIGS. 1 and 2, a solder mask is not used to delineate the bond pads  214 , and therefore can be excluded from beneath a flip chip attached to the bond pads  214  with the solder connections  220 . Similar to the embodiment of FIG. 1, a maximum width for a reduced-width portion  216  is about 75% of the width of its bond pad  214 , and preferably about 40% to about 70% of the width of the pad  214 . An example of suitable dimensions for conductors  212  configured in accordance with FIG. 3 are, for a conductor pitch of about 0.008 inch (about 200 micrometers), bond pad lengths and widths of about 0.007 inch (about 150 micrometers) and about 0.004 inch (about 100 micrometers), respectively, and lengths and widths for the reduced-width portions  216  of about 0.002 inch (about 50 micrometers). 
     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.