Resolution of Tie Bar Protrusion Issue in Over-Molded Plastic Package

Abstract

The tie bars are critical component of the leadframe as it provides structural support to the entire package body during assembly. Its design must be strong enough to withstand the mechanical stresses but should be optimal to be effectively cut during singulation. Common issues such as premature singulation can be associated with poor designs. Tie bar protrusion, on the other hand is typically linked to tool wear-out or incorrect tooling tolerances. In the development of the Over-Molded Plastic (OMP) package, the tie bar protrusion was resolved by adding enhancement features on the cutting tool and via leadframe design change. The heatsink of the package is initially connected to the tie bars through riveting. After singulation, tie bars protruding outside the package outline poses risk of electrical shorting to the adjacent leads or with other components in the PCB. One key customer required a specific package size to be shorter than the standard (called short package OMP). This requirement led to the qualification of new mold tool and new leadframe design. Initial trial showed that the design requirement specification is not met because tie bar protrusions are appearing randomly. Mechanical stress modelling revealed a potential issue on the leadframe design. Further review with the leadframe supplier also revealed some manufacturing limitations that need to be comprehended for new designs to avoid similar issue. The leadframe for short package OMP was redesigned to correct the flaws identified from the stress modelling. In addition, a design improvement in the cutting tool was evaluated. Evaluation results showed that both solutions are successful in eliminating the protrusion issue. Because of these corrective actions, the business continuity with the key customer was guaranteed. The learnings also enabled better development lead times by eliminating the potential risk, thus meeting the first-time right target for new package development.