Challenges and optimization of 2nd bond process for reliable QFN packages

As device technology is moving towards nano-IC technologies and consumer market are aggressively demands for the miniaturization of electronic products with more functionality [1], the requirement for small IC components has significantly increased, particularly on QFN (Quad Flat Non-leaded) packaging. Two major advantages of QFN over other leaded packages are (1) cost to manufacturer. Smaller, thinner and lighter package size is required to achieve more units per lead frame and (2) improved performance ICs since smaller packages will have smaller routing area, hence better thermal performance [2]. Other benefits of the QFN packages are low inductance and capacitor, smaller package volume and no external leads compare to the conventional leaded packages. QFN wirebonding has its own special characteristic as compare to other leaded packages. QFN wire bond process set-up is much more complicated than other packages and highly influence by three factors. They are clamping, lead bouncing and lead design. The challenge of QFN package is obtaining its process window without compromising on the 2^nd bond quality. This paper specifically discusses the critical wirebonding parameters and capillary selection for QFN package using FA gold wire diameter of 1.0mils, breaking load range of 6.5 ~ 10.5g and elongation range of 2 ~6%. The main critical parameters discussed in this work are base ultrasonic power, base time and base force. Wire bonding process parameters are optimized to achieve wedge pull of > 4.0gf, no Non-Stick on Lead (NSOL), no heel crack and heel break observed. Destructive test such as wedge pull test is used to check the bonding quality. Failure modes are analyzed using high power optical scope microscope and Scanning Electronic Microscope (SEM).