Patent Application
20140091465
Disclosed embodiments relate to leadframes for integrated circuit (IC) packages, and more particularly, to a leadframe having metal terminals including a metal coating on a base metal.
In the manufacture of semiconductor integrated circuits (ICs), semiconductor IC die (or chips) are mounted on a leadframe, followed by enclosing the IC die and part of the leadframe in a plastic casing to form an IC package. The IC package can be mounted on a printed circuit board (PCB) for interconnection of the electronic devices on the IC die with external circuitry. A leadframe should provide good bondability, molding compound characteristic, and solderability, so that it can facilitate the packaging process. To provide these characteristics, various coatings may be formed on the leadframe surface.
A conventional method for providing improved bondability for the interconnection between bond wires and bonding areas of a leadframe is to plate a metal such as silver (Ag) on the bonding areas including on the metal terminals within the package before wire bonding. Wire bonding is generally performed by a first bonding which forms a ball bond by placing a capillary over the bond pad of the IC die with a ball of the wire extending out of the capillary, and then a second bonding for bonding the ball to the bond pad. The capillary is then moved to a metal terminal (e.g., lead finger) of the lead frame to which a second bond is to be made with the wire travelling with respect to the capillary bore, and a stitch bond is made to the metal terminal (e.g., lead finger) using the capillary with the wire then being broken, leaving a small wire pigtail extending out of the capillary.
After the semiconductor IC is sealed in a plastic casing, in the case of a leaded plastic package, where the terminals comprise leads having internal leads portions which are encapsulated, the external lead portions may be plated with a layer of an alloy of tin/lead (Sn/Pb) to provide suitable solderability for the external lead portions of the IC package to allow ease of mounting on a PCB by soldering. Plating generally provides a smooth and constant thickness metal coating.
Disclosed embodiments recognize when the metal coating on bonding areas of metal terminals (e.g., leads or lead fingers) of a leadframe is provided by a metal paste dispensing apparatus such an ink-jet, the surface of the metal coating is significantly rougher as compared to an electroplated metal coating. Such rough/uneven surfaces can cause reduced contact area by the capillary and the bond wire during the second bonding process reducing the applied pressure, and as a result reducing the contact area of the stitch bond between the bond wire and metal terminal, leading to a reduced pull strength of the stitch bond.
Disclosed embodiments also recognize ink-jetting and dispensing have the flexibility to control both the volume dispensed and position. Sloped metal terminal coatings including sloped top faces are provided by controlling the dispensed metal coating volume as a function of position. By controlling the angle of the top metal terminal surface to reduce the angle between the terminal surface and the capillary/bond wire out from the capillary during wire bonding, the contact area of capillary and the bond wire to the top metal terminal surface is increased. As a result, wire bond ability, pull strength, shear strength and break mode, are all improved.
Reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, wherein:
Example embodiments are described with reference to the drawings, wherein like reference numerals are used to designate similar or equivalent elements. Illustrated ordering of acts or events should not be considered as limiting, as some acts or events may occur in different order and/or concurrently with other acts or events. Furthermore, some illustrated acts or events may not be required to implement a methodology in accordance with this disclosure.
Step 101 comprises dispensing a metal paste including metal particles in a solvent onto a bonding area of a plurality of metal terminals of a leadframe comprising a base metal and a center die pad. The dispensing provides a varying dispensed thickness (and thus varying volume) over the bonding area, with a range in thickness after solvent removal (step 102) of at least 1 μm, typically providing a thickness range between 2 μm and 8 μms. The base metal of the leadframe is generally copper or a copper alloy including Alloy 194, C7025, KCF125, EFTEC, or can be other than copper comprising such as a nickel/ferrite alloy (e.g., Ni-Fe 42 alloy). A typical thickness for the base metal is 0.15 mm to 0.30 mm. The metal particles in the metal paste can comprise metals such as silver, copper, aluminum or gold, or alloys thereof.
A computer controlled ink jet apparatus can be used for the dispensing. Other dispensing apparatus can include computer controlled needle dispensers (air, mechanical) and jet dispensers. These methods all dispense metal particles in a solvent (a metal paste), and can print a paste with high resolution.
In the case of ink-jet printing, the ink-jet printing action can be induced by various technologies known in the art, including piezoelectric or thermal ink jet printers. Ink-jet printing operates via a series of nozzles to shoot small droplets of liquid onto a surface with high precision. The nozzles are part of a print head that can be moved back and forth (e.g., by a stepper motor) with respect to the surface being printed. The surface being printed can also be moved relative to the print head.
Disclosed coatings having sloped (angled) top faces can be achieved by computer control of the dispensed metal coating volume as a function of position. For example, for a constant paste flow rate, slower translations or longer times result in higher thicknesses compared to faster translation/shorter times. Dispensed dot size may also be used to control dispensed thickness and thus dispensed volume.
Step 102 comprises evaporating the solvent to form a sloped metal coating including a first sloped top face and a second sloped top face angled relative to the first sloped top face. The first sloped top face is closer to the die pad as compared to the second sloped top face, the second sloped top face increases in coating thickness with decreasing distance to the die pad, and the first sloped top face decreases in coating thickness with decreasing distance to the die pad. Heat and/or ultraviolet light may be used for evaporating the solvent.
A typical average thickness for the sloped metal coating is from 3 μm to 10 μm, such as around 5 μm in one particular embodiment. As noted above, the thickness difference across the metal coating is typically 2 μm to 8 μm, such as 8 μm as a maximum thickness and 4 μm at a minimum thickness for a 4 μm thickness difference.
Step 103 comprises attaching a bottom side of a semiconductor die which includes a plurality of bond pads on a top side active surface to the die pad. A gluing agent/adhesive, such as a silver filled epoxy may be used for the attachment.
Step 104 comprises connecting the plurality of bond wires between the plurality of bond pads and ones of the second sloped top faces. The bonding connection is generally a direct connection (i.e. no solder needed). In the bonding process, a plurality bond wires, such as gold or aluminum wires, each having one end bonded to one bonding pad (not shown) on the semiconductor die and the other end bonded to the metal coating on the metal terminals (internal leads for a leaded package), are used for the interconnect. Known wire bonding techniques may be used.
Step 105 comprises encapsulating the semiconductor device in an encapsulating material, such as a polymer. An electrically non-conducting (dielectric) encapsulation polymer can be molded over the package in the encapsulation step. The packaged semiconductor device is then generally electrically tested.
Functionally, the leadframe 450 is divided into a package area, as the area enclosed by a dashed box pointed shown by reference numeral 462, which includes a bonding area (or called a coin area), as the area enclosed by a dashed box pointed out by reference numeral 460, therein and the internal leads 454. The bonding area 460 includes the die pad 322 and the free end (referred to as coin-lead tip) 464 of the internal leads 454. The coin-lead tip 464 of the internal leads 454 is where disclosed sloped metal terminal coatings are provided.
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Disclosed embodiments can be integrated into a variety of assembly flows to form a variety of different semiconductor IC devices and related products. The assembly can comprise single semiconductor die or multiple semiconductor die, such as PoP configurations comprising a plurality of stacked semiconductor die. The semiconductor die may include various elements therein and/or layers thereon, including barrier layers, dielectric layers, device structures, active elements and passive elements including source regions, drain regions, bit lines, bases, emitters, collectors, conductive lines, conductive vias, etc. Moreover, the semiconductor die can be formed from a variety of processes including bipolar, CMOS, BiCMOS and MEMS.
Those skilled in the art to which this disclosure relates will appreciate that many other embodiments and variations of embodiments are possible within the scope of the claimed invention, and further additions, deletions, substitutions and modifications may be made to the described embodiments without departing from the scope of this disclosure.
