Determining the tie bar break point in LFCSP

Strip Testing is popular in LFCSP as it addressed difficulty of handling small and unleaded packages. In LFCSPs, tie bars are very susceptible to mechanical stress after the lead trimming process because it is the only part that holds the package while being processed at laser marking and electrical test. A phenomenon called “stressed tie bar” is a condition when the tie bars are already deformed or damaged prior to singulation. A good indicator to this is the “crack” on the plastic covering underneath the package. Under this condition the affected units may result to potential reliability issues at Singulation. Possible defects include chipout, package cracking or missing leads depending on the angle of crack lines produced. This documentation is composed of a three-part study. The first uses theoretical computation of Shearing Stress and Strain to determine the maximum shearing that the tie bars can withstand prior to permanent deformation. The second part is a laboratory setup that uses actual load force to determine the required shearing stress to break the tie bars. This validates the findings made on the theoretical computation. The last phase of the study dwells into the characterization of the tie bars on different load force below the shearing strength. In this study, the 4×4 LFCSP was used. The theoretical computation showed that the tie bars have a maximum shearing strength of 8.075 Kg. This means that beyond this value, the tie bars will undergo phase change towards permanent deformation or the break point. This was validated through shearing simulation where the mean reading of the breakpoint is at 9.38 Kg. After determining the maximum shearing strength and correlation with the actual values, the next step is the characterization of stressed tie bars. This study revealed that if the mechanical stress were equally applied on the four tie bars, there is a minimal chance that the package will crack. On the other han- - d, if the stress is not distributed evenly, it will result to package chipping when the unit is singulated. The findings presented in this documentation were used on various applications. These include control for strip contacting, process improvements, and as a critical guide in the design considerations for the acquisition of various strip handlers.