Abstract:
A package includes a substrate mechanically supporting circuitry. A conductive cover (e.g., a metal sheet) is over the circuitry so that the circuitry is exposed below an opening in the conductive cover. A bent down corner of the conductive cover is inserted into a hole in the substrate. A solder ball is placed on the other end of the hole. During a subsequent heating, the solder ball is drawn up through the hole. When cooled, the conductive material grasps onto the tip of the bent down corner, thereby establishing a good connection between the conductive cover and the newly formed conductive via. As a finger approaches the circuitry (e.g., a fingerprint detection circuit), the finger first discharges electrostatic charge into the cover, not into the circuit, thereby protecting the circuit. In another package, the cover is composed of a highly resistive material, to slowly dissipate the electrostatic charge. Thus, the induced parasitic currents in the circuit are relatively low and damage to the circuit is avoided.

Description:
FIELD OF THE INVENTION 
     The present invention relates to packaging technology, and more specifically, to an electrostatic discharge protection package and method. 
     BACKGROUND OF THE INVENTION 
     Electrostatic discharge (“ESD”) is the flow of electrostatic charge from a charged object. For example, a capacitor experiences ESD from one capacitor terminal to the other if the accumulated charge, and thus the voltage, across capacitor terminals is sufficiently large. 
     One source of ESD is, for example, the finger of a human being. As human beings engage in every day activities, electrostatic charge often accumulates in their bodies. This electrostatic charge is discharged from, for example, the finger when the finger contacts another object capable of receiving the charge. ESD can be damaging if the receiving object is circuitry containing minute wires, such as in, for example, VLSI and ULSI circuits. Fortunately, most circuits are protected by packaging and/or computer cases to avoid direct human contact. However, other circuits are not isolated from direct human contact or contact with other objects capable of accumulating electrostatic charge. 
     Therefore, what is desired is a package and packaging method for ESD protection in circuits that are not isolated from objects capable of accumulating electrostatic charge. 
     SUMMARY OF THE INVENTION 
     In accordance with one embodiment of the present invention, a package includes a substrate. Circuitry sensitive to electrostatic discharge is formed on a device mounted to the substrate. The circuitry may be, for example, VLSI or ULSI circuitry having minute wires that are damaged by even very low currents. 
     A conductive cover is positioned over the circuitry so that the circuitry is exposed below an opening of the conductive cover. The conductive cover may be, for example, a rectangular metal sheet having a rectangular hole in its center, the rectangular hole positioned above the circuitry. A portion of the conductive cover, such as a bent down corner of the conductive cover, is inserted into a conductive via. In one embodiment, each of the four corners of the rectangular conductive cover is bent down and the tips of the four corners are inserted within a corresponding conductive via. 
     In a method of fabricating a package, the conductive via is formed by inserting the portion (e.g., the bent down corner prong) of the conductive cover into the upper portion of a hole formed through the substrate. A conductive ball (e.g., a metal solder ball) is formed on the lower portion of the hole. During a subsequent heating step, the conductive material of the conductive ball is drawn up into the hole due to capillary forces, thereby forming the conductive via. The conductive material molds around the bent down corner prong. When the conductive via cools, the conductive material grasps the corner prong, thereby establishing a reliable electrical and mechanical connection between the conductive cover and the conductive via. 
     As a finger, or other object capable of accumulating electrostatic charge, approaches the circuitry, the finger first contacts the conductive cover, thereby draining the electrostatic charge in the finger. When the finger contacts the circuitry, electrostatic charge does not discharge from the finger into the circuit. Thus, the circuit is protected from electrostatic discharge. 
     In accordance with another embodiment of the invention, a package includes a substrate. Circuitry is formed on a device which is mounted to the substrate. A cover is positioned with respect to the substrate such that an opening of the cover lies over the circuitry. The cover is composed of a highly resistive material which dissipates (i.e., slowly conducts) the electrical charge it receives through a conductive via and into a charge drain. The cover may be, for example, a dissipative ring deposited around the circuitry and may be composed of an epoxy resin with conductive fillers. 
     The resistance between the charge drain and a contact surface of the cover for contact with a finger is relatively high (e.g., approximately 1000 ohms). Since the electrostatic charge drains relatively slowly from the cover, induced parasitic currents within the circuitry are relatively low and thus the circuitry is less likely to be damaged by induced parasitic currents. 
    
    
     These and other objects, features and advantages of the present invention will be more readily apparent from the detailed description of the various embodiments set forth below taken in conjunction with the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is an isometric view of a finished package according to a first embodiment of the present invention. 
     FIG. 1B is a cross sectional view of the package of FIG. 1A along cross section line  1 B— 1 B of FIG.  1 A. 
     FIG. 1C is a cross sectional view of the package of FIG. 1A along cross section line  1 C— 1 C of FIG.  1 A. 
     FIG. 2A is an isometric view of the package of FIG. 1A after an initial stage of packaging. 
     FIG. 2B is a cross sectional view of the package of FIG. 2A along cross section line  2 B— 2 B of FIG.  2 A. 
     FIG. 2C is a cross sectional view of the package of FIG. 2A along cross section line  2 C— 2 C of FIG.  2 A. 
     FIG. 2D is a cross sectional view of an alternative embodiment of the package of FIG. 2A in a flip chip configuration. 
     FIG. 3A is an isometric view of the package of FIG. 2A after a subsequent stage of fabrication. 
     FIG. 3B is a cross sectional view of the package of FIG. 3A along cross section line  3 B— 3 B of FIG.  3 A. 
     FIG. 4 is an exploded isometric view of the package of FIG. 3A after a subsequent step of fabrication. 
     FIG. 5A is an isometric view of a package according to a second embodiment of the invention. 
     FIG. 5B is a cross sectional view of the package of FIG. 5A along cross section line  5 B— 5 B of FIG.  5 A. 
     FIG. 5C is a cross sectional view of the package of FIG. 5A along cross section line  5 C— 5 C of FIG.  5 A. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Several elements in the following figures are substantially similar. Therefore, similar reference symbols are used to represent substantially similar elements. 
     Package  100  is first generally described with reference to FIGS. 1A,  1 B and  1 C. Subsequently, the details of package  100  are described with reference to the fabrication steps of FIGS. 2A,  2 B,  2 C,  2 D,  3 A,  3 B and  4 . 
     Referring to FIGS. 1A,  1 B and  1 C, package  100  includes a device  110  mounted on a substrate  120 . Device  110  contains circuitry  112  which is sensitive to electrostatic discharge (“ESD”), and which will not be isolated from contact with an object capable of accumulating electrostatic charge. Circuitry  112  may be, for example, a fingerprint detection circuit onto which a user is to press a finger  101  (FIG.  1 A). 
     A conductive cover  130  overlies the periphery of device  110  and has a contact surface  131  defining an opening  132  such that circuitry  112  is exposed under opening  132 . Conductive cover  130  may be, for example, a copper plate. 
     Conductive cover  130  is structurally supported by prongs  134   a,    134   b,    134   c  and  134   d  disposed in corresponding conductive vias  122   a,    122   b,    122   c  and  122   d  in substrate  120 . These conductive vias  122   a,    122   b,    122   c  and  122   d  are contact terminals configured to be electrically coupled to a charge drain  150  (e.g., a ground or fixed voltage mechanism). Conductive cover  130  may also be structurally supported by electrically insulating insulation ring  140 . Conductive cover  130  is coupled to a charge drain  150  through prongs  134   a,    134   b,    134   c  and  134   d  such that when charge develops on any part of conductive cover  130  (e.g., contact surface  131 ), that charge is conducted to charge drain  150 . 
     During operation, as finger  101  moves toward circuit  112 , finger  101  contacts conductive cover  130  at contact surface  131  (FIG. 1A) which receives the electrostatic charge from finger  101 . After contacting conductive cover  130 , the electrostatically discharged finger contacts circuitry  112 . 
     Having described the general features of package  100 , the details of package  100  may best be understood by describing the method of fabricating package  100  as follows. 
     Referring to FIGS. 2A,  2 B and  2 C, device  110  is mounted to an upper surface  202  of a substrate  120  of package  100 . Device  110  may be attached to substrate  120  using, for example, a conventional adhesive  204 . 
     Substrate  120  may be any substrate capable of mechanically supporting device  110 . Substrate  120  may have hollow vias  206   a,    206   b,    206   c,  and  206   d  formed through respective corner regions  208   a,    208   b,    208   c  and  208   d  of substrate  120 . A corresponding one of conductive balls  214  is formed below at least one, but possibly all, of hollow vias  206   a,    206   b,    206   c  and  206   d.  FIG. 2C, for example, shows two conductive balls  214  formed below each of hollow vias  206   a  and  206   b.    
     Substrate  120  has terminals  210  which may be, for example, solder balls formed in or on a lower surface  212  of substrate  120 . Conductive balls  214  and terminals  210  may be deposited on lower surface  212  at the same time. Conductive balls  214  are ultimately drawn up, in a subsequent high temperature process, through hollow vias  206   a,    206   b,    206   c  and  206   d  due to capillary forces. Conductive balls  214  may be, for example, 0.35 inch diameter, composed of an alloy of 63% lead and 37% tin. 
     Substrate  120  has contact regions, such as bond pads (for clarity, only bond pads  226   a  and  226   b  are labeled), formed on, for example, upper surface  202  of substrate  120 . Device  110  has corresponding contact regions, such as bond pads (for clarity, only bond pads  216   a  and  216   b  are labeled), which may be formed on an upper surface  218  of device  110 . 
     Bond pads  226   a  and  226   b  of substrate  120  may be electrically coupled to one or more corresponding terminals  210  provided on substrate  120 . Bond pads  216   a  and  216   b  are coupled to circuitry  112  within device  110 . 
     Leads, such as bond wires  220   a  and  220   b,  electrically couple bond pads  216   a  and  216   b  to respective bond pads  226   a  and  226   b.  Alternatively, device  110  is electrically connected to substrate  120  in a leadless fashion such as shown in cross section in FIG.  2 D. Referring to FIG. 2D, contact regions  216   a  and  216   b  formed on a bottom surface of device  110  directly contact contact regions  226   a  and  226   b,  respectively, of substrate  120 . 
     Referring back to FIGS. 2A,  2 B and  2 C, after wire bonding, a small dam structure  222  of a fluid is deposited using, for example, a dispense system such as an MRSI 375-3S dispenser produced by MRS Technology, Inc., located at 10 Elizabeth Drive, Chemlsford, Mass. This fluid may be, for example, Hysol 4323. Dam structure  222  is deposited to circumscribe circuitry  112  of device  110  and may circumscribe as a square. 
     FIG. 2B shows that the cross sectional profile of dam structure  222  need not be large. However, the profile should be significant enough to contain later deposited material (e.g., insulation ring  140  described hereinafter) from overflowing dam structure  222  and contacting circuitry  112 . The dimensions of dam structure  222  may be, for example, 0.010 inches by 0.010 inches. 
     FIG. 3A is an isometric view of the package of FIG. 2A after a subsequent stage of fabrication. FIG. 3B is a cross sectional view of the package of FIG. 3A along cross section line  3 B— 3 B of FIG.  3 A. 
     Dam structure  222  is gelled by exposure to a temperature of approximately 150° C. for approximately 30 minutes under ultraviolet radiation. After dam structure  222  is gelled, insulation ring  140 , composed of a fluid that is electrically insulating (e.g., epoxy) and that has a moderate viscosity is deposited around the periphery of device  110  using, for example, the MRSI 375-3S dispenser. Insulation ring  140  is deposited to cover leads  220   a  and  220   b  and upper surface  218  of device  110  up to dam structure  222 . Although overflow of insulation ring  140  over dam structure  222  is not aesthetically pleasing, some overflow may occur so long as the functionality of circuitry  112  does not significantly deteriorate. 
     FIG. 4 is an exploded isometric view of the package  100  after a subsequent stage of fabrication. Insulation ring  140  is gelled by exposure to a temperature of approximately 150° C. for approximately 1 hour. Conductive cover  130  is then deposited over substrate  120 . Conductive cover  130  is shaped like a rectangular plate having a rectangular opening  132  formed in the approximate center of the plate. The corners of conductive cover  130  are bent approximately ninety degrees downward to form prongs  134   a,    134   b,    134   c  and  134   d.  The tips of each prong  134   a,    134   b,    134   c  and  134   d  are inserted into a respective hollow via  206   a,    206   b,    206   c  and  206   d.    
     The insulation ring  140  and dam structure  222  are further cured at a temperature of approximately 170° C. for approximately 1 hour. During this curing, referring now to both FIG.  2 C and FIG. 4, conductive balls  214  melt and are pulled into hollow vias  206   a,    206   b,    206   c  and  206   d  and around the tips of prongs  134   a,    134   b,    134   c  and  134   d  due to capillary forces. The material is cooled to form conductive vias  122   a,    122   b,    122   c  and  122   d  as shown in FIG.  1 C. This cooling causes the conductive vias  122   a,    122   b,    122   c  and  122   d  to grasp onto respective prongs  134   a,    134   b,    134   c  and  134   d,  thereby forming a strong electrical and mechanical connection between the conductive cover  130  and the conductive vias  122   a,    122   b,    122   c  and  122   d.  The resulting package is shown in FIGS. 1A,  1 B and  1 C. 
     In some applications, quickly conducting the charge from conductive cover  130  may result in damage to circuitry  112 . For example, a rapid voltage drop in conductive cover  130  might induce enough current in circuitry  112  to damage the minute wires of circuitry  112 . In a second embodiment described with reference to FIGS. 5A,  5 B and  5 C, charge is slowly dissipated from a cover rather than quickly conducted from the cover. 
     FIG. 5A is an isometric view of a dissipative package  500  according to a second embodiment of the invention. FIGS. 5B and 5C are a cross sectional views of dissipative package  500  of FIG. 5A along respective cross section lines  5 B— 5 B and  5 C— 5 C of FIG.  5 A. 
     Dissipative package  500  is structured similar to package  100 . However, dissipative package  500  contains a dissipative ring  530  instead of conductive cover  130  of package  100 . Furthermore, instead of conductive balls  214  being drawn through hollow vias  206   a ,  206   b ,  206   c  and  206   d,  some dissipative ring  530  material is pull into hollow vias  206   a,    206   b,    206   c  and  206   d  to form vias  122   a,    122   b,    122   c  and  122   d.    
     Dissipative ring  530  is deposited using, for example, the MRSI 375-3S dispenser. Dissipative ring  530  has a relatively low viscosity. Small dam structure  222  at least substantially prevents dissipative ring  530  from overflowing onto circuit  112 . Curing of dissipative ring  530  takes place at a temperature of approximately 175° C. for approximately 1 hour. 
     The total electrical resistance from the dissipative ring  530  to the charge drain  150  depends on the conductivity of dissipative ring  530 . In one embodiment, dissipative ring  530  is an epoxy resin having conductive fillers (25% aluminum and 45% carbon). 
     In package  500 , when a finger  101  contacts contact surface  131 , charge is not quickly conducted from dissipative ring  530 , but is slowly dissipated from dissipative ring  530  due to relatively high electrical resistance of the material between contact surface  131  and charge drain  150 . The higher the ohmic resistance between contact surface  131  and charge drain  150 , the slower the discharge to charge drain  150 . In one embodiment, the electrical resistance between contact surface  131  and charge drain  150  is approximately 1000 ohms and the dissipation occurs in a matter of microseconds. However, the electrical resistance may also be 10, 100, 1000, 10000, or even 100,000 ohms or more. 
     Since the charge dissipates slowly from dissipative ring  530 , the induced current in circuitry  112  is lowered, thereby protecting circuitry  112  from damage. 
     Although various specific embodiments are described above, these embodiments are illustrative only and not limiting. After having read this disclosure, one skilled in the art will recognize various modifications and variations that fall within the scope of the present invention. Thus, the invention is defined by the following claims.