Patent Publication Number: US-2005121806-A1

Title: Method for attaching circuit elements

Description:
FIELD OF THE INVENTION  
      The present invention relates to fabrication of circuits and/or circuit boards. More particularly, but not exclusively, the present invention describes improved methods for attaching circuit elements in a circuit assembly.  
     BACKGROUND OF THE INVENTION  
      There are a variety of conventional methods for bonding circuit elements such as resistors, capacitors, inductors, diodes, and semiconductor elements to a substrate or printed circuit board (PCB).  
      One method for attaching a semiconductor device, such as an integrated circuit die, to a substrate is disclosed in U.S. Pat. No. 6,498,400 to Yamakawa et al. Yamakawa discloses using a hot melt adhesive sheet having spacer to bond a circuit element to a circuit assembly. The hot melt adhesive includes an adhesive such as a silicone-modified epoxy resin adhesive, a silicone adhesive, and/or a silicone-modified polyimide adhesive. The hot melt adhesive is displaced onto the substrate before the circuit element is placed on the circuit assembly.  
      U.S. Pat. No. 4,661,192 to McShane discloses electrically connecting an integrated circuit die to a lead frame using a conductive epoxy. The conductive epoxy provides an electrical bond between electrical leads of the circuit components and the metallic pads or “landing areas” of the PCB.  
      Other conventional techniques for attaching circuit components to a circuit board include soldering or wire bonding the electrical connections. In many instances a non-conductive adhesive is placed on the substrate before placement of the circuit element to provide a non-conductive bond between non-conductive portions of the circuit element and the board or substrate.  
      Turning to  FIGS. 1A and 1B , a method for attaching a circuit element  10  to a circuit board  20  according to the related art includes: (i) applying a conductive bonding material  30  to the electrical terminals  12  of circuit element  10  and/or the conductive pads  24  of the circuit board  20 ; (ii) positioning element  10  onto board  20  in a manner that the electrical connections (i.e., terminals  12  and pads  24 ) of element  10  and board  20  are properly aligned; and (iii) applying heat to the circuit assembly to promote formation of a conductive bond  25  ( FIG. 1B ) between terminals  12  and landing areas  24  from conductive bonding material  30 .  
      Circuit element  10  is an electrical element such as a capacitor, inductor, diode, transistor or resistor, which is electrically connected to circuit board  20  and includes a body  11  and one or more terminals  12  (additionally referred to herein as “terminations”) for establishing electrical connection with board  20 .  
      The circuit board  20  in the illustrated example includes a laminate substrate  22  having one or more metallized or pre-tinned conductor lands or pads  24  (also referred to as “landing areas”) for establishing electrical connection with terminations  12  of circuit element  10 . Conductive bonding material  30  is a material such as a conductive adhesive (e.g., silver or gold epoxy) or solder and is applied to one or both of pads  24  and/or terminations  12 . When heat is applied to bonding material  30  and it is allowed to cool, a rigid electrically conductive bond  35  results (for example, solder flows and hardens and/or conductive epoxy is cured).  
      As shown in  FIG. 1B  pads  24  and/or bonding material  30  may elevate circuit element  10  above the laminate substrate  22  resulting in a gap  40 . Consequently, circuit element  10  may only be supported by, and/or connected to, board  20  by its electrical connections. This arrangement potentially results in an unstable and/or relatively fragile mounting of element  10  on board  20 .  
      To provide additional bonding strength, referring to  FIGS. 2A and 2B , another method of the related art proposes applying a second, non-conductive adhesive  50  (e.g., a chip bonder) at room temperature between areas of the element body  11  and substrate  22  (i.e., where gap  40  exists) before element  10  is put in place. When dried and/or cured, adhesive  50  forms an additional bond between circuit element  10  and substrate  22  and thus the shear strength of the overall bond between element  10  and board  20  is increased.  
      This conventional method, for example, where two strips of conductive epoxy are applied to attach the terminations  12  to the landing areas  24  and a strip of non-conductive epoxy  50  is applied to attach the circuit element body  11  to the substrate, suffers from one or more of the following problems.  
      When the finished part or assembly is subjected to subsequent thermal cycling, the chip bonder  50  has been shown to expand and potentially cause a disconnect between the circuit element terminations  12  and the board landing areas  24 . Thermal expansion of the chip bonding adhesive during thermal cycling additionally may result in a loss bond shear strength.  
      Conventional non-conductive adhesives, such as chip bonders, have low glass transition temperatures (Tg) which inhibits the adhesive&#39;s ability to absorb stress resulting from thermal cycling. The non-conductive epoxy has a high coefficient of thermal expansion (C.T.E.) that emphasizes stress in the Z-axis (i.e., the direction perpendicular to the substrate) during thermal cycling. It is the stress in the Z-axis that tends to lift the circuit element from the circuit board and sever its electrical connections.  
      Consequently, it would be desirable to have a process for attaching circuit elements to a substrate with greater shear strength than the shear strength of bonds achieved from element attachment using soldering or conductive epoxy alone. Additionally, it would be desirable to have a process for attaching circuit elements to a substrate with a greater reliability and lower tendency to disconnect and/or lose shear strength when subject to thermal cycling as compared to element attachment using chip bonding adhesives.  
     SUMMARY OF THE INVENTION  
      The present invention alleviates one or more of the foregoing problems by a process for attaching a circuit element to a substrate including: applying a conductive bonding material to conductors of the circuit element and/or substrate; placing the circuit element in position over the substrate; seating the circuit element on the substrate via the conductive bonding material; heating the bonding material to promote formation of a conductive bond; and while heating, applying a low viscosity encapsulant around the area of the circuit element near the board.  
      According to certain embodiments of the present invention, the conductive bonding material is a conductive epoxy. In other embodiments of the present invention, the conductive bonding material is solder.  
      According to various other aspects of the invention, the low viscosity encapsulant is a flip chip underfill material.  
      Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims.  
      It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.  
      The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate several embodiments of the invention and together with the description, serve to explain the principles of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIGS. 1A and 1B  are side views of a circuit assembly using an element attach technique of the related art.  
       FIGS. 2A and 2B  are side views of a circuit assembly using another element attach technique of the related art.  
       FIG. 3  is a flow diagram for a method of attaching circuit elements to a substrate according to one embodiment of the present invention.  
       FIGS. 4A, 4B  and  4 C are side views of a circuit assembly during various stages of the method for attaching circuit elements shown in  FIG. 3 .  
       FIG. 5  is a flow diagram for a method of attaching circuit elements to a substrate according to another embodiment of the present invention. 
    
    
     DESCRIPTION OF THE EMBODIMENTS  
      Reference will now be made in detail to the present exemplary embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts.  
      A method for forming a circuit assembly by attaching a circuit element to a desired location (e.g., substrate, printed circuit board, lead frame, or other semi-permanent mounting surface) in accordance with one or more methods of the present invention provides greater bond shear strength than attaching circuit elements using conductive bonds alone. Moreover, the methods of the present invention produce a circuit assembly which is less susceptible to circuit element disconnect than conventional methods using non-conductive epoxy. Additionally, a circuit element attached to a substrate in accordance with the inventive methods has a shear strength that is not significantly altered by thermal cycling.  
      Turning to  FIGS. 3 and 4 , a method  300  ( FIG. 3 ) of forming a circuit assembly  400  ( FIGS. 4A, 4B  and  4 C) by attaching a circuit element  410  to a desired location  420  includes: applying  310  an amount of conductive bonding material  430  to electrical conductors  412  of the circuit element  410  and/or the landing areas  424  at the desired circuit element location  420 ; positioning  320  the circuit element  410  at the desired location  420  (e.g. on substrate  422 ); heating  330  at least a portion of the assembly  400  to promote formation of a conductive bond  435 ; and applying  340  an amount of non-conductive underfill encapsulant  460  around the circuit element  410 ; and heating  350  to promote formation of a non-conductive bond.  
      The amount and timing for applying the conductive bonding material  430  varies depending on the type of material used. For example, if conductive bonding material  430  is a conductive epoxy, such as a gold or silver epoxy, the epoxy may be applied before positioning element  410  in place on substrate  422  (i.e., where terminations  412  are aligned with and contact landing areas  424 ). Alternatively, if bonding material  430  is a conductive solder, then a solder flux may be placed on the electrical connections prior to positioning and placement of the circuit element  410  on substrate  422 , and the solder may be applied before or after circuit element  410  is in position. If solder paste is used for bonding material  430 , it may be applied prior to placement of circuit element  410  on substrate  422 .  
      In any event, solder or conductive epoxy  430  both utilize elevated temperatures to promote formation of a conductive bond  435  ( FIG. 4B ). That is, at least a portion of the assembly  400  and/or bonding material  430  is heated to flow solder or gel cure the conductive epoxy. Heating  330  may be performed in accordance with conventional techniques for forming the conductive bonds  435 . For example, solder reflow may be achieved using infrared, vapor phase or hot air convection oven reflow methods. In preferred embodiments, where conductive epoxy is used for conductive bonding material  430 , the circuit assembly is heated on a hot plate to promote a snap-cure or gel-cure of the conductive epoxy.  
      When element  410  is positioned  320  at its desired location  420  (e.g., on substrate  422  with terminations  412  aligned on pads  424 ), preferably, although not necessarily, a slight gap  440  exists between the body  411  of circuit element  410  and an opposing surface of substrate  422 . The heat applied during formation of the conductive bonds  435  and the existence of this slight gap  440  between body  411  and substrate  422  result in a situation where additional bonding (i.e., forming bonds in addition to the conductive bonds  435 ) may be achieved using a low viscosity underfill encapsulant  460  and without significant changes in assembly steps or device handling.  
      Low viscosity underfill material  460  is preferably a non-conductive encapsulant selected to have good wicking characteristics at the temperature ranges used during soldering or snap curing of conductive epoxy. In preferred embodiments of the invention, material  460  is an underfill encapsulant used for flip-chip applications.  
      A preferred underfill material in accordance with one aspect of the present invention is a high purity, low stress liquid epoxy encapsulant designed for enhanced adhesion to integrated circuit passivation materials. Preferred underfill materials are selected to rapidly underfill devices with as little as ½ mil gap at temperatures in the range of 80° C. to 140° C. Uncured properties of an example underfill material at 25° C., Cone and plate (Brookfield, cps) CP52; speed  20  include a viscosity in the range of 2000-2,500 centipoises. One preferred underfill material included the following cured characteristics: 
          a Coefficient of Linear Thermal Expansion (C.T.E) of 45_ppm/° C. (Alpha 1) and 143_ppm/° C. (Alpha 2),     an extractable ionic content of less than 10 ppm for Chloride or Sodium;     a Glass Transition Temperature (Tg) of approximately 140° C.; and/or     a flexural modulus at 25° C. of approximately 5.6 GPa.        

      Advantages of using an underfill encapsulant, as opposed to a chip bonding adhesive, in the methods of the present invention include: (i) the underfill material has a lower viscosity as compared to conventional bonding adhesives (and thus a greater ability to spread through small spaces or gaps); (ii) the ability of the underfill material to wick up and under the sides of the circuit element  410  during low to moderate heating temperatures (which may result in an increase in bonding surface area between element  410  and substrate  422  than as compared to traditional chip bonding adhesives); (iii) a greater wetting ability of the underfill material to element  410  and substrate  422 ; (iv) a greater glass transition temperature (Tg) of the underfill increases the ability of the material to absorb stress and thus allows for greater variation of temperatures and longer durations for thermal cycling; (v) a uniform filler shape and size allow a higher flow rate of underfill material as compared to conventional adhesives; (vi) the medium to high elastic modulus of the underfill material absorbs the strain caused by the mismatch of the coefficient of thermal expansion (C.T.E.) between circuit element  410  and substrate  422 ; and (vii) the C.T.E. of the underfill material may be closer matched to the C.T.E. of the conductive epoxy, substrate and/or element body, thus the strain in the Z-axis during thermal cycling may be reduced.  
      Referring to  FIG. 5 , an exemplary method  500  for attaching circuit elements to a substrate includes: cleaning  510  the substrate and/or circuit element terminals if desired; if needed, mixing  515  conductive epoxy resin (preferably in accordance with the manufacturer&#39;s specifications); applying  520  an amount of the conductive epoxy to the landing areas on the circuit board and/or circuit element terminals; placing  525  the circuit element in position over the landing areas; seating  530  the circuit element on the substrate by pressing it down with a finger or soft implement; repositioning  535  the circuit element if necessary; optionally, adding 540 an additional amount of conductive epoxy to cover exposed conductor surfaces; gel curing  545  conductive epoxy at an elevated temperature; dispensing  550  a low viscosity encapsulant around and under the circuit element near the substrate surface; and curing  555  the conductive epoxy and underfill material at an elevated temperature to achieve a rigid bond. Additional cleaning (not shown) may be subsequently performed if desired.  
      Cleaning  510  may be performed in any manner to remove debris and contaminates from the substrate landing areas and/or circuit element terminals. In the preferred embodiment cleaning  510  is an isopropyl alcohol clean. Certain epoxies require mixing two individual components to form the epoxy while other epoxies are available as a single material. Accordingly, if needed, the conductive epoxy is mixed  515  before application  550  of conductive epoxy to assembly  400 .  
      The conductive epoxy resin used for the present invention is preferably selected to be compatible with the type of conductors subject to bonding (e.g., gold conductors bonded with gold epoxy and silver conductors with silver epoxy) and may be applied  520  in any manner suitable to dispense an amount of epoxy resin to a desired area. This may include for example, manual or automated dispensing using a brush, blotter or syringe (e.g., a pneumatically assisted hypodermic syringe). Screen printing the conductive epoxy in the appropriate areas, similar to solder paste screen printing, may also be used to apply the conductive epoxy.  
      Circuit element  410  is placed  525  and seated  530  and repositioned  535 , if necessary, on substrate  422 . Placement of circuit element  410  may be performed manually or by an automated machine such as a pick and place machine. Additional conductive bonding material may be applied  540  if desired to provide additional conductive bonding of terminals  412  with landing areas  424 .  
      In preferred embodiments, although not required, the conductive epoxy is snap cured to a gel consistency (referred to as “gel cured”) at an elevated temperature (for example, between 65° C. and 85° C. for 10-12 minutes). Gel curing the conductive epoxy may be performed using a hot plate or other method for heating a circuit assembly. Heating the circuit assembly on a hot plate may be advantageous over alternate methods for heating, such as using a convection oven, because the user (or automated machine) may readily access the circuit assembly during heating for application  550  of the underfill material.  
      The underfill material is dispensed  550  around the circuit element preferably after the conductive epoxy has cured to a gel consistency but at a similar elevated temperature which facilitates the gel cure of the conductive epoxy. Application of the underfill material at this elevated temperature promotes the wicking of the material to fill gap  440  ( FIG. 4   b ) and gelling of the underfill material from its liquid application form.  
      Assembly  400  is then cured  555  at an increased elevated temperature to promote full curing of the conductive epoxy and underfill material. Full curing  555  may be performed using a convection oven or other method for raised temperature curing. In preferred embodiments the full cure may be performed in a convection oven at a temperature range of 155° C. to 175° C. for a period of approximately ninety minutes.  
      It should be recognized that the gel cure and full cure temperatures and durations for the conductive epoxy and/or underfill material will depend on the amount and type of the bonding materials used and corresponding manufacturer&#39;s recommended specifications.  
      The following example details the production, testing and comparison of (i) circuit assemblies built in accordance with the methods of the present invention; and (ii) and circuit assemblies fabricated according to the techniques of the related art.  
     EXAMPLE  
      Two materials (an underfill encapsulant and a chip bonder (i.e., non-conductive epoxy)) were comparison tested for staking a chip capacitor to a laminate substrate. Seven test boards were built with board #4 serving as a control specimen in which the capacitor was attached without staking (i.e., no bonding other than the conductive bonds) and without being subjected to temperature cycling. The capacitor was sheared from Board 4 to determine the shear strength of the conductive bonds not yet subject to temperature cycling.  
      The substrates built, some of which were subjected to multiple temperature cycles of −65° C. to 150° C., are identified in the following table:  
                           TABLE 1                           Conductive               Board #   Attachment   Staking   Temp Cycles                  1   Ag (Silver) filled   Underfill encapsulant   140X           epoxy       2   Ag filled epoxy   Underfill encapsulant   None       3   Ag filled epoxy   None   140X       4   Ag filled epoxy   None   None       5   Ag filled epoxy   Chip bonder   140X       6   Ag filled epoxy   Chip bonder   None       7   Ag filled epoxy   None   140X                  
 
      Substrates #1, #3, #5 and #7 were subjected to multiple temperature cycles. During the first few sets of ten temperature cycles, the capacitance from each capacitor was probed from the back of the substrates to ensure each capacitor was still electrically connected.  
      Substrates #2 and #6 were staked and sheared without any temperature cycling so that the degradation of shear strength due to temperature cycling could be evaluated. After temperature cycling of relevant boards, all capacitors were sheared from their boards and the following observations were determined.  
      The shear test demonstrated that the chip bonder (i.e., non-conductive epoxy) significantly degraded after temperature cycling. The shear strength for capacitors staked with the non-conductive epoxy was nearly the same as the shear strength of a capacitor that had not been staked at all. By way of contrast, the shear strength of the capacitor staked with the underfill material after temperature cycling, was very similar to its shear strength prior to temperature cycling.  
      Accordingly, it was observed that use of an underfill encapsulant was superior to the use of non-conductive epoxies for staking non-integrated circuit elements, such as capacitors, inductors, resistors, diodes, transistors and mechanical switches or relays, to a substrate or circuit board.  
      Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.