Evaluation of wire bonding performance, process conditions, and metallurgical integrity of chip on board wire bonds

Chip on board wire bonding presents challenges to modern wire bonding technology which include smaller, closely spaced wire bond pads; bonding to soft substrates without special processing and pad construction; and diverse first bond and second bond metallurgies. These challenges are addressed by extensive bonding accuracy tests, a design of experiments approach for optimizing wire bond process parameters, reliability testing, and detailed materials characterization of the metallurgical integrity of the wire bonds. The thermo-mechanical integrity of the wire bond interconnects was evaluated by wire pull and hot storage tests. The methodology of materials analyses of the metallurgy of the wire bond interconnects is described. The paper illustrates a wire bond lift technique that is used to inspect for cratering damage and the “area-uniformity” of gold aluminum intermetallics. An improved understanding of the wire bonding process was achieved by showing the dependence of the visual appearance of the wire bonds on wire bond process parameters. INTRODUCTION Our chip on board wire bond applications require a fine pitch gold wedge bond process to interconnect a silicon die with aluminum bond pad metallization to an FR4 epoxy glass printed circuit board, (PCB) with electrolytic gold plating. Wire bonding on soft, easily deformed PCB substrates is a complex process which requires an understanding of multiple disciplines including: (1) the metallurgy and properties of the materials of construction; (2) the evaluation of mechanical integrity by wire pull and bond shear test methods; and (3) the methodology of reliability testing. Wire bond processes are traditionally optimized by conducting designed experiments (DOE’s), with wire bond machine set-up parameters, including ultrasonic power and time duration, bonding tool force, and stage temperature as control factors. The response factors in these DOE’s are typically wire pull and bond shear tests. Wire bond shear testing is not commonly used to test the strength of wedge bonds, but does provide complimentary data. Wedge bond shear testing will become more popular as the metrology and test standards are developed. However, wire bond integrity should not be characterized by only performing wire pull and bond shear tests. This paper describes several analytical techniques and the reliability testing used to evaluate the wire bonds. These techniques included: traditional wire pull tests; a visual characterization of the size and shape of the wire bonds; a grain structure evaluation of the bonding wire; and a “wedge-lift” method for evaluating intermetallic metallurgy and area uniformity of the wire bonds. Characterization of visual appearance of the wire bonds and bond width measurements are important, but often overlooked, indicators of wire bond quality. The reliability testing included two chamber thermal cycling and high temperature storage. Wire Bond Process Parameters Optimizing a wire bond process begins with a clear understanding of the machine set-up, the response variables involved, and their relationship to one another. Experimenting with these parameters is time well spent, and is an important step toward developing a robust wire bond process. An overview of thermosonic bonding and the machine set-up parameters is helpful for interpreting the results of wire bond DOE data. Thermosonic wedge bonding utilizes a combination of heat, pressure, and ultrasonic vibration to form a metallurgical bond between two materials. The four key machine settings are: bonding work stage temperature, ultrasonic transducer power, bonding tool force, and bonding time. Heat softens the gold bonding wire and gold board metallization. Power and time relate to the ultrasonic generator settings used to “ultrasonically soften,” the bonding wire. Tool force is the amount of weight applied to the bonding wire to mechanically couple the bonding wire to the bonding pad surface. During the bonding process, the bond tool oscillates back and forth to soften a short piece of bonding wire that is wedged between the tool and bonding pad