Smaller mesa devices, as well as today’s demand for faster processes, have resulted in greater challenges in terms of wire bonding. Overcoming the challenges associated with successfully applying wire bonds to small mesa chips requires the latest wire-bonding equipment, specially designed tools, and right expertise. In the tech brief, “Tips for Increasing Yields when Wire Bonding Small MESA Chips,” SemiGen discusses some of the issues that concern the wire bonding of small mesa chips, as well as how to solve these problems.
The tech brief gives a basic definition of wire bonding before explaining how it is performed in more than 40 million integrated circuits (ICs) each year. Wire bonding is essentially the predominant form of electrical die-to-package bonding. Wires with diameters ranging from one to three mils have traditionally been used, but to reduce parasitics, sub-1-mil diameter wires are now being applied more frequently. Nonetheless, wire-bond failures can be attributed to a number of semiconductor device issues.
Three types of wire bonding are mentioned in the note. High-temperature thermocompression is largely used to bond aluminum (Al) wires to Al or gold (Au) pads. Another method is ultrasonic welding at room temperature. Lastly, but certainly not least, there is thermosonic bonding, which accounts for more than 90% of wire bonding, according to the document.
To illustrate the challenges in terms of working with small mesa devices, the tech brief describes five common forms of wire-bond failures. They include cratering and peeling; over-stretching and over-twisting; untamed tails; poor adhesion; and missed targets.
An application may require specific evaluation methods and testing procedures. Various forms of testing include internal visual testing, ball-bond shear tests, constant acceleration tests, random vibration, mechanical shock tests, and stabilization bake. Moreover, the app note points out that customized wire-bond tools are often necessary to meet stringent application-specific evaluation parameters and standards. It concludes by presenting a table that contains a range of wire-bonding process variables.
SemiGen, 920 Candia Rd., Manchester, NH 03109; (603) 624-8311; www.semigen.net.