Abstract:
A method for preparing an integrated circuit for connection to a surface, the integrated circuit including lead contacts and leadless contacts, is provided. The method includes providing the integrated circuit, applying a first solder paste to the leadless contacts, forming solder balls on the applied solder paste, heating the solder balls, thereby removing at least a portion of the first solder paste and bringing the solder balls into electrical contact with the leadless contacts, the base of the solder balls being generally aligned in a plane, and bending the lead contacts into gull wings, with the base of the gull wings being substantially coplanar with the plane. The base of the gull wings and the base of the at least one of the solder balls collectively generally define a contact plane for potential future contact with the surface.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The instant application claims priority to U.S. Provisional Application 61/064,337 filed on Feb. 28, 2008, the disclosure of which is expressly incorporated herein in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a connection methodology. More specifically, the present invention relates to methodologies for connecting hybrid chips to printed wiring boards where the chips contain both leads and leadless contacts. 
         [0004]    2. Discussion of Background Information 
         [0005]    An integrated circuit (“IC”) typically comes in two varieties. One variety includes ICs with metal leads extending therefrom that carry power, ground, input and output signal. The metal leads are often rigid and bent into a shape known as a “gull wing.” The other variety uses “leadless” contacts, in which conductive pads are integrated into the surface of the IC. Varieties of methods are known for connecting the leads, leaded ICs, or conductive pads of leadless ICs to printed circuit boards. 
         [0006]    Recently a hybrid chip has been introduced that utilizes both gull wing leads and leadless contact pads on the bottom of the chip.  FIGS. 1A-1C  show an example of such a hybrid chip  102 . It is difficult to mount hybrid chip  102  to a printed circuit board, as known connection methodologies for the gull wing and the leadless pads can conflict with each other. 
       SUMMARY OF THE INVENTION 
       [0007]    According to an embodiment of the invention, a method for preparing an integrated circuit for connection to a surface, the integrated circuit including lead contacts and leadless contacts, is provided. The method includes providing the integrated circuit, applying a first solder paste to the leadless contacts, forming solder balls on the applied solder paste, heating the solder balls, thereby removing at least a portion of the first solder paste and bringing the solder balls into electrical contact with the leadless contacts, the base of the solder balls being generally aligned in a plane, and bending the lead contacts into gull wings, the base of the gull wings being substantially coplanar with the plane, wherein the base of the gull wings and the base of the at least one of the solder balls collectively generally define a contact plane. 
         [0008]    The above embodiment may include various features. The method may include: determining, before the bending and after the heating, a lateral distance between the lead contacts and the base of at least one of the solder balls; and/or electrically connecting the lead contacts and leadless contacts to the surface. The electrically connecting may include applying a second solder paste to the surface, soldering the base of the gull wings to the surface, and heating the solder balls, thereby removing at least a portion of the second solder paste and bringing the solder balls into electrical contact with the surface, wherein the integrated circuit will be in electrical contact with the surface through both the leadless contacts and the lead contacts. The providing the integrated circuit may include providing an integrated circuit with the lead contacts attached to the body of the integrated circuit and extending laterally away from the body. The forming solder balls may include forming solder balls of about 10 mils in diameter and/or forming solder balls of substantially equal size. 
         [0009]    According to another embodiment of the invention, a method for preparing an integrated circuit for connection to a surface, the integrated circuit including lead contacts and leadless contacts, is provided. The method includes providing the integrated circuit, applying a first solder paste to the leadless contacts, connecting bent leads to the applied solder paste, soldering the bent leads, thereby removing at least a portion of the first solder paste and bringing the bent leads into electrical contact with the leadless contacts, the base of the bent leads being generally aligned in a plane, and bending the lead contacts into gull wings, the base of the gull wings being substantially coplanar with the plane, wherein the base of the gull wings and the base of the at least one of the bent leads collectively generally define a contact plane. 
         [0010]    The above embodiment may have various features. The method may include: determining, before the bending and after the soldering, a lateral distance between the lead contacts and the base of at least one of the bent leads; and/or electrically connecting the lead contacts and leadless contacts to the surface. The electrically connecting may include applying a second solder paste to the surface, soldering the base of the gull wings and the base of the bent leads to the surface, thereby removing at least a portion of the second solder paste and bringing the bent leads into electrical contact with the surface, wherein the integrated circuit will be in electrical contact with the surface through both the leadless contacts and the lead contacts. The providing the integrated circuit may include providing an integrated circuit with the lead contacts attached to the body of the integrated circuit and extending laterally away from the body. The bent leads may be either S-leads or C-leads. The connecting may include providing the bent leads on a fixture, and orientating the fixture to bring the bent leads into alignment with the leadless contacts; the bent leads made be removed from the fixture after the bent leads are connected to the integrated circuit. 
         [0011]    According to yet another embodiment of the invention, a method for preparing an integrated circuit for connection to a surface, the integrated circuit including lead contacts and leadless contacts, is provided. The method includes providing the integrated circuit, applying a conductive epoxy to the leadless contacts, the base of the epoxy being generally aligned in a plane, and bending the lead contacts into gull wings, the base of the gull wings being substantially coplanar with the plane, wherein the base of the gull wings and the base of the at least one of the bent leads collectively generally define a contact plane. 
         [0012]    The above embodiment may include various optional features. The method may include: determining, before the bending and after the applying, a lateral distance between the lead contacts and the base of at least one of the bent leads; and/or electrically connecting the lead contacts and leadless contacts to the surface. The electrically connecting may include applying a second solder paste to the surface, soldering the base of the gull wings to the surface, curing the conductive epoxy, wherein the integrated circuit will be in electrical contact with the surface through both the leadless contacts and the lead contacts. The providing the integrated circuit may include providing an integrated circuit with the lead contacts attached to the body of the integrated circuit and extending laterally away from the body. 
         [0013]    According to still yet another embodiment of the invention, a method for preparing an integrated circuit for connection to a surface, the integrated circuit including lead contacts and leadless contacts, is provided. The method includes providing the integrated circuit, applying a first solder paste to the leadless contacts, placing preformed conductive metal pieces on the first solder paste, soldering the metal pieces, thereby removing at least a portion of the first solder paste and bringing the metal pieces into electrical contact with the leadless contacts, the base of the metal pieces being generally aligned in a plane, and bending the lead contacts into gull wings, the base of the gull wings being substantially coplanar with the plane, wherein the base of the gull wings and the base of the at least one of the metal pieces collectively generally define a contact plane. 
         [0014]    The above embodiment may have various optional features. The method may include: determining, before the bending and after the soldering, a lateral distance between the lead contacts and the base of at least one of the metal pieces; and/or electrically connecting the lead contacts and leadless contacts to the surface. The electrically connecting may include applying a second solder paste to the surface, soldering the base of the gull wings and the metal pieces to the surface, wherein the integrated circuit will be in electrical contact with the surface through both the leadless contacts and the lead contacts. The providing the integrated circuit may include providing an integrated circuit with the lead contacts attached to the body of the integrated circuit and extending laterally away from the body. 
         [0015]    Other exemplary embodiments and advantages of the present invention may be ascertained by reviewing the present disclosure and the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    The present invention is further described in the detailed description which follows, in reference to the noted plurality of drawings by way of non-limiting examples of certain embodiments of the present invention, in which like numerals represent like elements throughout the several views of the drawings, as follows. 
           [0017]      FIGS. 1A-1C  are top, side and bottom views of a hybrid integrated circuit with leads and leadless contacts in which the leads have been bent into gull wings. 
           [0018]      FIG. 2  is a side view of the steps of an embodiment of the invention for attaching a hybrid chip to a printed wiring board. 
           [0019]      FIG. 3  is a flow chart of the process steps of  FIG. 2 . 
           [0020]      FIG. 4  is a side view of the steps of an embodiment of the invention for attaching a hybrid chip to a printed wiring board. 
           [0021]      FIG. 5  is a flow chart of the process steps of  FIG. 4 . 
           [0022]      FIG. 6  is a side view of the steps of an embodiment of the invention for attaching a hybrid chip to a printed wiring board. 
           [0023]      FIG. 7  is a flow chart of the process steps of  FIG. 6 . 
           [0024]      FIG. 8  is a side view of the steps of an embodiment of the invention for attaching a hybrid chip to a printed wiring board. 
           [0025]      FIG. 9  is a flow chart of the process steps of  FIG. 8 . 
       
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0026]    The particulars shown herein are by way of example and for purposes of illustrative discussion of the embodiments of the present invention only and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the present invention. In this regard, no attempt is made to show structural details of the present invention in more detail than is necessary for the fundamental understanding of the present invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the present invention may be embodied in practice. 
         [0027]    Referring now to  FIGS. 2 and 3 , a methodology for attaching a hybrid chip  202  is shown. At step  302 , a hybrid chip  202  is provided that includes a plurality of leads  204  in a first orientation and a plurality of contact pads  206  ( FIG. 1C ). The initial orientation (for this embodiment and the later embodiments below) is preferably a connection to the body of chip  202  and extending laterally away from chip  202  without bends. However, other initial orientations may be used. 
         [0028]    Solder paste  208  (thickness exaggerated for illustration) is applied to conductive contact pads  206  at step  304 . At step  306 , solder balls  210  are then applied on top of solder paste  208 . The solder balls  210  are then heated to bond with conductive pads  206  at step  308 ; this tends to remove solder paste  208 , such that it is no longer shown in  FIG. 2 . 
         [0029]    Each solder ball  210  is preferably about 10 mils in diameter when deposited, although they are expected to expand as solder flows during soldering. Each solder ball  210  is preferably made from a material with a melting point of 290 degrees or above. Pure copper or a 10/90 alloy of tin and lead are suitable for this environment. 
         [0030]    The lower surface of the solder balls  210  will roughly define a base plane  212  at which the solder balls  210  will later connect to a printed wiring board. At step  310  the distance between that plane and leads  204  is then determined, and at step  312  the leads  204  are bent into a second orientation that includes gull wings  214 . The lateral wing portions  216  of gull wings  214  lie in the base plane  212 , thus forming a collective contact plane. 
         [0031]    At step  314 , solder paste  218  is applied using a stencil at the appropriate locations on a printed wiring board  220 . At step  316 , wings  214  are then soldered onto their respective portions of solder paste  218 , while the solder balls  210  are heated to form connections onto the printed circuit board  220 . The connections at step  316  can be simultaneously or in any order. 
         [0032]    Solder balls  210  tend to have minimal compliancy and tend to crack under stress. The connections of  FIG. 2  are thus preferable for environments with minimal thermal expansion and/or minimal thermal expansion cycles. 
         [0033]      FIGS. 4 and 5  show another embodiment of the invention. At step  502 , a hybrid chip  402  is provided that includes a plurality of leads  404  in a first orientation and a plurality of contact pads  406  (as in  FIG. 1C ). Solder paste  408  (thickness exaggerated for illustration) is applied to contact pads  406  at step  504 . At step  506 , a plurality of pre-bent conductive leads  410 , such as copper alloy C-leads or S-leads, are then secured in fixtures and pressed against solder paste  408 . Solder is then applied to connect the bent leads  410  to pads  406  at step  508 . Once bent leads  410  are attached, the connection to the securing fixture can be removed. 
         [0034]    Each bent lead is preferable 0.40 mils high, and made from a copper alloy. In the alternative, small form factor bent leads of the type shown in co-pending U.S. patent application Ser. No. 11/979,487 (filed on Nov. 7, 2007, the disclosure of which is herein incorporated by reference in its entirety) can be used. 
         [0035]    The lower portion of the connected bent leads will roughly define a base plane  412  at which the bent leads  410  will later contact the printed wiring board. At step  510  the distance between that plane and leads  404  is then determined, and at step  512  the leads  404  are bent into a second orientation that includes gull wings  414 . The wing portions  416  of gull wings  414  lie in the base plane  412 , thus forming a collective contact plane. 
         [0036]    At step  514 , solder paste  418  is applied using a stencil at the appropriate locations on a printed wiring board  420 . At step  516 , wings  414  and leads  410  are then soldered onto their respective portions of solder paste  218 . The connections at step  516  can be made simultaneously or in any order. 
         [0037]    Bent leads have a compliancy that allows them to shift during thermal stress. This makes the connection of  FIG. 4  particularly useful for harsh environments subject to considerable thermal expansion and/or repeating thermal expansion cycles. 
         [0038]      FIGS. 6 and 7  show another embodiment of the invention. At step  702 , a hybrid chip  602  is provided that includes a plurality of leads  604  in an initial orientation and a plurality of contact pads  606  (as in  FIG. 1C ). Conductive epoxy  608  is applied to contact pads  606  at step  704 . The lower portion of the epoxy will roughly define a base plane  612  at which the epoxy  608  will later contact the printed wiring board. At step  710  the distance between that plane and leads  604  is then determined, and at step  712  the leads  604  are bent into a second orientation that includes gull wings  614 . The wing portions  616  of gull wings  614  lie in the base plane  612 , thus forming a collective contact plane. 
         [0039]    At step  714 , solder paste  618  is applied using a stencil at the appropriate locations on a printed wiring board  420  that correspond to the contact points for wing portions  616 . At step  716 , wings  614  are soldered onto their respective portions of solder paste  618 . At step  718 , the epoxy is cured. The connections at steps  716  and  718  can be made simultaneously or in any order. 
         [0040]    Conductive epoxy is more compliant than solder but less compliant than bent leads. It is thus suitable for use in environments with moderate to high thermal expansion and/or cycles of thermal expansions, although not to the same extent as bent leads. Thus, for example, this connection methodology is not preferable for avionics applications. 
         [0041]    Referring now to  FIGS. 8 and 9 , another methodology for attaching a hybrid chip  202  is shown. At step  902 , a hybrid chip  802  is provided that includes a plurality of leads  804  in an initial orientation and a plurality of contact pads  806  ( FIG. 1C ). Solder paste  808  (thickness exaggerated for illustration) is applied to contact pads  806  at step  904 . At step  906 , pre-slugs of conductive metal  810  are then applied on top of solder paste  808 . Solder is applied at step  908 ; this tends to remove the solder paste, such that it is no longer shown in  FIG. 8 . 
         [0042]    Each slug  810  is preferably about 5 mils in height, although other heights could be used.  FIG. 8  shows slug  810  as rectangular, but other shapes, such as cylindrical, could be used. Each slug  810  is preferably made from a material with a higher melting point than the solder. A copper alloy with a sufficiently high melting point so as not to melt during the soldering process is sufficient for this. 
         [0043]    The lower portion of the slugs  810  will roughly define a base plane  812  at which the slugs  810  will contact the printed wiring board. At step  910  the distance between that plane and leads  804  is then determined, and at step  912  the leads  804  are bent into a second orientation that includes gull wings  814 . The wing portions  816  of gull wings  814  lie in the base plane  812 , thus forming a collective contact plane. 
         [0044]    At step  914 , solder paste  818  is applied using a stencil at the appropriate locations on a printed wiring board  820 . At step  916 , wings  814  are then soldered onto their respective portions of solder paste  818 , while the slugs  810  are heated to form connections onto the printed circuit board  820 . The connections at step  916  can be made simultaneously or in any order. 
         [0045]    It is noted that the foregoing examples have been provided merely for the purpose of explanation and are in no way to be construed as limiting of the present invention. While the present invention has been described with reference to certain embodiments, it is understood that the words that have been used herein are words of description and illustration, rather than words of limitation. Changes may be made, within the purview of the appended claims, as presently stated and as amended, without departing from the scope and spirit of the present invention in its aspects. Although the present invention has been described herein with reference to particular means, materials and embodiments, the present invention is not intended to be limited to the particulars disclosed herein; rather, the present invention extends to all functionally equivalent structures, methods and uses, such as are within the scope of any current or future claims. 
         [0046]    Various claims below recite terms for which the following additional discussion may be relevant. For example:
       “base” is a relative term generally referring to the lower ends of structures when in the orientation shown in  FIG. 1 .   “before” and “after” refer to the order of steps, but do not require (nor exclude) that the identified order of steps follow directly or indirectly via intervening steps.   “lead” is used in the context of a lead of a circuit a circuit, as opposed to the metal Pb. This does not require nor exclude that the lead may be made of Pb or include Pb.