Tool and method for welding to IC frames

Abstract

A device for forming an encapsulated end of a wire bonded to a metal surface. The device includes a clamp being an end surface of a bar is pressed against a surface of a p[ate. The end surface has a ridge formed around its edge. so that when a clamping force is applied to the bar against the plate, the metal sheet clamped between the plate surface and end surface of the bar, the clamping force is concentrated at an interface between the metal surface and ridge. When encapsulant is injected into the contact region, the ridge prevents contaminating material from migranting over interface between bar and plate where the welding step is to be performed. The bar is then withdrawn leaving a tunned through which the wire is positioned with the end of the wire in contact bondable to the clean surfave of metal. The wire is positioned in the tunnel and bonded to the metal surface.

Description

FIELD OF THE INVENTION

[0001] This invention relates to manufacturing integrated circuits and particularly to a tool and method for butt welding a wire to an integrated circuit frame.

BACKGROUND AND INFORMATION DISCLOSURE

[0002] Fabrication of semiconductor devices includes two areas of operations: forming electrical contacts between wires and terminals of components, and packaging, i.e., encasing the device in a plastic or ceramic enclosure to provide mechanical support and protection from the environment.

[0003] Contacts between a wire or pin and a pad (terminal) generally fall into two categories: permanent and temporary.

[0004] In permanent applications, the requirement is to secure an end of a wire to a pad. The pad is the surface of a lead frame or the terminal of a semiconductor die. Methods used to permanently bond the end of the wire to such terminations include soldering and ultrasonic welding.

[0005] Contacts of a temporary nature include contacts of conductors to terminals of a test circuit where contact is maintained only during a brief test procedure. In this application, the test wire is commonly supported in a manner wherein the wire maintains a resilient (springlike) force against the terminal of the device under test.

[0006] Numerous techniques for forming permanent and compressive bonds have been disclosed in the Patent literature. U.S. Pat. No. 3,460,481 discloses a technique whereby the wire severing operation comprises moving the needle with holding pressure sufficient to frictionally engage the wire and insufficient to deform the wire away from the bond area.

[0007] U.S. Pat. No. 3,73,481 to Lins et al discloses forming pin-like gold pedestal structures atop terminal portions of an n integrated circuit device by thermocompressing gold spheres and shaping the spheres with a heated vacuum holder.

[0008] U.S. Pat. No. 5,095,187 to Gliga discloses a wire-bonding technique involving a combination of heat, pressure and vibration. This patent discusses weakening or severing the wire by localized application of heat and how the severing operation may result in a broadened portion on the severed end of the wire

[0009] U.S. Pat. No. 6,062,459 to Sabyeying discloses a wirebond clamp for holding fingers of lead frames against a heater block. The clamp includes a base plate having an array of apertures for the lead wires.

[0010] U.S. Pat. No. 6,336,269 to Eldridge et al discloses a flexible elongate member (such as a gold wire) mounted on a contact area and is configured to have a springable shape, such as a wire stem having at least one bend.

[0011] Packaging is an important part of semiconductor technology wherein the object of the package is to confer mechanical stability and protection from the environment on a circuit comprising a plurality of electrical connections between electronic components.

[0012] Ceramic packages are required where long life of the encapsulated device is required. Plastic packaging of the circuit is practiced where cost is an important consideration with some degree of mechanical stability and protection from the environment.

[0013] Intensive research into packaging techniques has been directed toward the development of polymer composites that have improved thermal resistance, lower coefficients of thermal expansion, reduced moisture absorption characteristics. More recently efforts have been directed toward the development of three-dimensional packages. Numerous patents are directed toward these efforts.

[0014] For example, U.S. Pat. No. 5,985,693 to Leedy is directed toward fabrication of integrated circuits from flexible membranes formed of very thin low stress dielectric materials and semiconductor materials.

[0015] U.S. Pat. No. 6,147,335 to Von Arx discloses various components of thermal systems molded within a polymer sheath. The use of transfer and compression molding allows for the use of thermoset polymers containing very high levels of reinforcement fillers.

[0016] In a related configuration, it is required to form electrical connection of the ends of wires to respective fingers of a lead frame. Attempts to form such contacts involve forming a permanent bond of the end of a wire or pin to the broad surface of the finger. Encasing the contact in a plastic after the contact is formed is difficult to carry out because of the mechanical forces exerted on the contact during the subsequent molding process.

[0017] Ultrasonic bonding the tip of a wire or pin to the broad surface of a lead of a lead frame requires that the surface of the lead be scrupulously clean in order to obtain a strong bond.

SUMMARY OF THE INVENTION

[0018] It is an object of this invention to provide a tool and process for forming a permanent bond of the tip of one or more wires or pins to the surface of a metal sheet.

[0019] It is a further object of the invention that the contact(s) be encapsulated in a resin medium.

[0020] It is a further object that, in forming the contact to the surface of the metal to which the tips are bonded the surface of the metal be devoid of contaminating films including oxidized films that would diminish the strength of the bond. It is contemplated that a particular application of the method and tool be to the formation of wires and pins bonded to the leads of a led frame.

[0021] This invention is directed toward a process which begins with cleaning the surface of the lead frame and then providing a clamp whose one jaw is a flat plate and whose other jaw is a bar that is preferably round. The clamping surface of the bar, (i.e., the end of the bar) has a rim around its edge.

[0022] When the sheet is clamped between the bar and the plate, a casting medium is forced under high pressure into the cavity that includes the end of the bar forced into contact with the plate. The force of the rim of the bar is sufficient to deform the surface of the metal and prevent leakage of the encapsulating medium between the plate and the clamping (end) surface of the bar.

[0023] When the clamp (bar and plate} are withdrawn from the casting, there is left a package including the lead frame with attached lead encapsulated in the casting medium. An open column leads from the surface of the lead to the outer surface of the package.

[0024] In a second step of the process, a wire (or pin) is inserted into the open end of the column and into contact with the surface of the lead (of the lead frame) on the opposite end of the open column.

[0025] The ultrasonic energy is then applied to the wire so as to ultrasonically bond the wire to the frame.

[0026] The open column enclosing the bonded wire is then impregnated in a second encapsulating step so as to seal the wire in position welded to the lead frame thereby completing the packaging operation. A crucial feature in the foregoing process is the use of the hold down bar with the raised rim around the edge that effectively prevents the liquefied encapsulating medium from contaminating the surface of the lead which would otherwise interfere with formation of the bond between the surface of the lead and the wire tip.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] FIG. 1a shows a lead frame.

[0028] FIG. 1b shows the encapsulated circuit

[0029] FIG. 2 is a flow chart illustrating the method of the invention.

[0030] FIG. 3 shows details of the clamping surface of the bar.

[0031] FIG. 4 is a perspective view showing deformation of the lead surface and details of the mold structure.

[0032] FIG. 5 shows the clamp in the encapsulating mold.

[0033] FIG. 6 shows a sectional view of the lead surface without the benefit of the ridge illustrating contamination by encapsulant of the lead surface.

[0034] FIG. 7 is a sectional view showing the tunnel formed by the bar clamp.

[0035] FIG. 8 shows the wire positioned in the tunnel.

[0036] FIG. 9 shows the tunnel backfilled with encapsulant.

DESCRIPTION OF A PREFERRED MODE

[0037] Turning now to a discussion of the drawings, FIG. 1a shows a lead frame 10 (well known in the art) being a metal lamination etched or stamped to form an array of leads 12, each lead 12 having an outer end integrally joined to a surrounding frame 14. There is also shown an array of wires 16 (three are shown) each wire 16 bonded at a selected location for the respective wire. The circuit is entirely hermetically sealed in an encapsulating medium to form a molding 18 as shown in FIG. 1b.

[0038] The steps forming the encapsulated circuit of FIG. 1a are listed in the flow chart of FIG. 2 and illustrated in FIGS. 3-6.

[0039] In step 1, the lead frame with leads shown in FIG. 1a is formed by techniques well known in the art. The process includes leaving the surface of the lead frame 10 in a “clean” condition that is suitable for forming a bond of a wire 16 to the surface of the lead frame 14 using the ultrasonic bonding technique.

[0040] In step 2, as shown in the sectional view of FIG. 3, the lead 12 of lead frame 10 is clamped in a clamp including a plate 20 (one jaw) and an opposing jaw 22. The opposing jaw 22 is a round bar with one end 24 being the clamping surface.

[0041] The clamping surface 24 on the end of the bar 22 has a ridge 26 extending around the edge of the clamping surface 24.

[0042] FIG. 4 is a perspective view the surface 28 of the lead 12 deformed leaving a circular depression 29 in response to the force of the ridge 26 against the surface of the metal.

[0043] FIG. 5 is a sectional view showing that the plate 20 and bar 22 forming the clamp is inside the cavity 21 of a mold 23 to be filled with encapsulating medium. The bar is spring loaded (spring not shown) thereby providing a controlled predetermined force of the end of the bar against the surface of the metal lead when the mold is closed.

[0044] In step 3, an encapsulating medium is injected into the cavity. The encapsulating medium is any one of a number of polymer/ceramic encapsulating media that are well known in the art of injection molding. The ridge 26 of the bar 22 forced against the lead 12 forms an intimate contact between the ridge on the end face of bar so as to prevent encapsulating medium from being forced in between the bar and metal surface of the lead 12. FIG. 6 shows that, when there is no ridge on the end of the bar) the liquid encapsulating medium is forced between the end surface 24 of the bar and the lead 12.

[0045] In contrast to FIG. 6, the surface of the lead 12 shown in FIG. 5 remains clean and is ready for subsequent wire bonding thereto whereas the surface of the lead shown in FIG. 6 is contaminated by encapsulating medium and must be cleaned by additional steps before ultrasonic bonding can be performed to the surface.

[0046] In step 4, the clamping bar is withdrawn from the plate, leaving the encapsulated frame in the mold cavity. FIG. 7 is a sectional view showing the tunnel 30 left in the encapsulating medium 32 after the bar 20 is withdrawn from the plate. The tunnel 30 extends from the (exposed) surface 24 of the metal lead 12 to the outer surface of the molding 32.

[0047] In step 5, as shown in FIG. 8, a wire 16 is inserted into the tunnel 30 wherein one end of the wire is in contact with the surface of the lead 12 and the other end of the wire 16 extends out of the open end of the tunnel 30.

[0048] In step 6 either one or a combination of ultrasonic/bonding energy is applied to weld the tip of the wire to the lead.

[0049] In step 7, the wire is severed leaving an end of the wire extending out of the open end of the tunnel.

[0050] In step 8, as shown in the sectional view of FIG. 9, a second encapsulating step is performed to fill the tunnel with encapsulate 33 and securing the wire in place.

[0051] A major feature of the invention is the specially formed clamping bar with ridge formed on the perimeter of the clamping surface. This feature is very effective in eliminating several expensive steps that would otherwise be required to prepare the surface of the lead for bonding thereto.

[0052] Variations and modifications of this invention may be contemplated after reading the specifications and studying the drawings, which are within the scope of the invention. The scope of the invention includes not only the process of making the circuit but also the method of making the bar (clamp) having a ridge on the end. In one embodiment the bar is a tube. This configuration has the advantage of great convenience in being able to frequently sharpen the end of the bar (tube) to maintain the required ridge configuration.

[0053] It is contemplated that a single mold can be used for numerous versions of a circuit within the molded package. Such a mold is illustrated in FIG. 9 where there is shown a plate holding an array of bores. Selected bores each hold a spring and one end of a bar. The bores are selected according to the required circuit. Each Bore that is not selected for forming contact with a lead contains a plug.

[0054] It should be understood, in the context of the specification that the “plate 20 (one jaw) of FIG. 3 is any member of the mold that provides a support to the lead and the bar. In this context, the “plate” in one embodiment is an end of an opposing bar or the internal surfce of a mold or a member of any configuration that provides the required backup support.

[0055] In view of these versions, I therefore wish to define my invention by the appended claims.

Claims

1. A device for forming an encapsulated circuit, said circuit including a wire with an end and a metal sheet having a metal surface, said circuit including said wire having an end contacting said metal surface, said device comprising:

a plate having a plate surface;

a bar having an end surface perpendicular to and meeting an elongated side surface of said bar, whereby a comer edge of said bar is defined;

a ridge formed on said end surface around said comer edge of said bar;

means for applying a clamping force to force said ridge toward said plate surface whereby said ridge abuts an area of said metal surface when said metal sheet is clamped between said plate surface and said end surface providing that said clamping force is concentrated at an interface between said metal surface and said ridge;

an enclosure arranged to form a molding by injecting an encapsulating medium into a region within said enclosure that includes said interface between said interface and said ridge;

said molding, after being separated from said plate and said bar, includes said encapsulating medium containing a portion of said metal sheet and a tunnel through said encapsulating medium left after withdrawing said metal bar from said enclosure;

said tunnel extending from one end coincident with said metal surface and a tunnel opening on a surface of said molding providing that said wire is positionable in said tunnel with an end of said wire in contact with said metal surface.

2. The device of claim 1 wherein said clamping force is sufficiently great to deform said metal surface.

3. The device of claim 1 further comprising means for bonding said end of said wire positioned in said tunnel to said metal surface;

4. The device of claim 1 further comprising means for backfilling said tunnel with encapsulating medium.

5. The device of claim 3 further comprising means for backfilling said tunnel with encapsulating medium.

6. The device of claim 3 wherein said metal surface is plated with a solder alloy and said means for bonding includes means for passing a current through said wire having an end in contact with said metal surface, said current being sufficient to solder said end of said wire to said metal surface.

7. The device of claim 1 wherein said bar is a tube providing that said ridge is an edge of said tube.

8. The device of claim 2 wherein said means for bonding comprises means for injecting ultra sonic energy into said interface.

9. The device of claim 8 wherein said wire is a rigid metal pin and said means for injecting ultrasonic energy comprises:

a source of ultrasonic energy;

means for coupling said source of ultrasonic energy to an end of said pin extending from said tunnel.

10. The device of claim 1 wherein said metal sheet is a lead of a lead frame.

11. A method for forming an encapsulated circuit, said circuit Including a wire with an end making contact at a location on a metal surface of a metal sheet, said method including the steps in operable order:

providing a mold having a cavity containing a clamp, said clamp including a plate being one jaw, and a bar having an end clamping surface being an opposing jaw wherein said bar has a ridge extending around an edge of said clamping surface, said bar slidably supported to permit forcing said end clamping surface against said plate;

cleaning said metal surface sufficiently to permit forming a bond of said end of said wire to said metal surface;

clamping the metal sheet between said jaws with a clamping force at a location where said contact is required;

forming an encapsulation by injecting an encapsulating medium into said cavity wherein said encapsulating medium is prevented from covering said location by intimate contact between said ridge and said metal surface;

withdrawing said clamping bar from said mold, leaving said metal sheet encapsulated in a molding formed in said mold cavity, molding having a tunnel that extends from said location on said metal surface to a surface of the molding.

inserting a wire is inserted into said tunnel wherein one end of said wire is in contact with said metal surface and another end of said wire extends out of said open end of the tunnel;

applying at least one of ultrasonic and heat energy sufficient to weld a tip of said wire to said metal surface at said location;

severing said wire leaving an end of said wire extending out of said open end of said tunnel;

filling said tunnel in a second encapsulating step whereby said wire is secured in place.

12. The method of claim 11 wherein said clamping force is sufficient to form a depression at said location in said metal surface.