Reliability improvement in low loop nailhead wirebonding for long Au wire in a high stress encapsulant material

There has been many innovations and technological advancement in the gold wire bond for semiconductor applications in recent years. The development of several categories and types of gold wire was triggered by the various needs of the end users in this industry. The right selection of wire for low loop bonding of long wire bond is critical to meet the final product quality and reliability requirement. Coupled with the product needs for a special but high CTE encapsulation material, the resultant stresses on the wire in a complete package and at elevated condition has dampened the wire bond reliability performance. In this detail study, the influence of the wire properties, the wire bond parameters, and the wire loop profile were analyzed and reviewed. To achieve the desired low loop wire height for such long wire length, two wire types with low HAZ, high tensile strength, suitable hardness and elongation properties were choosen for the initial evaluation. The result revealed that the wire with an assumed better control in dopant and drawing process has produced smaller process variation sigma in almost all of the performed quality tests. The next step was to study the influence of the HAZ length and the loop bending point on the product reliability. In this study, the HAZ length was maintained and the loop bending points were varied. In addition, a suitable wire bond capilary movement was established to achieve the desired loop height. The reliability screening test revealed that the units with low bending point within the HAZ length failed at early readouts. The separation was due to the grain boundary stress in the larger grain size at the beginning of the HAZ length. In conclusion, the reliability of long and low loop Au wire in high stress encapsulant material can be achieved by the proper selection of the wire type, wire bonding parameters, and loop profile with relation to the HAZ length.