The effect of fillet height and bondline thickness on the mechanical performance of a plastic package

The general direction in IC assembly has been towards thinner packages with larger number of leads. This has led to die back grinding to meet package thickness requirements. In addition, the line width of the circuitry on the die has also been shrinking, making the die footprint smaller. These moves have caused most packages to change in size but the package type has not changed. Some typical examples would be the QFP, SOIC and PBGAs. In all these instances, IC assembly houses have found themselves having to contend with issues such as the package form factor, wirebonding and reliability. Engineers are finding that the materials set that met all the requirements of the package in its earlier form do not necessarily fit the needs of the new dimensions of the shrunken package. This has led to the unenviable task of considering materials requalification. Die attach in particular has been a source of problems. Low modulus adhesives that met all requirements for larger and thicker dice now cause wirebonding issues. The very low modulus of the material that was essential for package reliability now causes a bouncing issue during die wirebonding. This study examines this problem from a mechanical viewpoint. A theory for the bouncing phenomenon is given while the actual change in die height is measured. The effect of the fillet height and the bondline on the bouncing phenomena is studied using a DTMA where the package is tested. Details of the test set-up are given. The influence of the temperature is also verified. Recommendations are made on how one can overcome the problem of bouncing during wirebonding.