Advances in Wire Bonding Technology to Overcome Resonance Conditions

Increasing number of semiconductor packages are using low modulus die attach adhesives. Low modulus adhesives can efficiently absorb and dissipate the mechanical and thermal stresses (e.g. due to CTE mismatch) than conventional adhesives resulting in better thermal performance and package reliability. However, wire bonding on a die attached with low modulus adhesive can be very challenging. Die attached with low modulus adhesive is more prone to die movement and vibration, which may lead to resonance condition. Resonance during bonding can cause low ball shear, low IMC, skidded and smashed bonds. This paper examines the challenges of wire bonding on devices attached using low modulus epoxy. A FEA model is developed to understand the effect of key factors such as die attach adhesive modulus, bond line thickness, die size and die thickness on different resonant modes that can occur during wire bonding. It is found that for a given die size, the die thickness, die attach modulus, and bond line thickness can significantly affect resonant modes. When a low modulus epoxy is used, a smaller and thinner die is prone to oscillation and resonance, and is more difficult to wire bond than a larger die. The results show that die and die attach system behaves similar to a mass-spring system, where the die is the mass and the die attach acts like the spring. The resonant frequency of such a system does not depend on an external applied force, but only on the mass and the spring constant (stiffness). Study results of wire bonding process on a device using low modulus adhesive are presented. The process parameters that are found to be most significant are ultrasonic (energy, frequency and control mode), bond force, and bond time. It is found that controlling the bond force and ultrasonic energy is key in optimizing the wire bonding process on these devices. If the force and ultrasonic are not controlled properly, a significant portion of the ultrasonic energy is lost in exciting the die and developing the resonance situation instead of bond formation. Since different devices can vibrate and resonate under different conditions, optimizing the bonding process can be tedious and time consuming. To simplify the process development, a response based process is developed with closed loop force and ultrasonic control. This new process feature helps control resonance issues such as smashed bonds while still achieving a robust process with good ball shear and IMC performance. Experimental results of the wire bonding process using the new process approach are presented.