Magnetic performance degradation of giant magnetoresistive (GMR) sensors in magnetic recording heads often occurs after ESD exposure. This ESD damage phenomenon is not just limited to slider fabrication and head gimbal assembly (HGA) levels. It also can happen at head stack assembly (HSA) level. This study investigates ESD damage phenomena at HSA level under four types of experiments: tribo-charging from insulating parts of the HSA, EMI from external sources, direct charge, and the AC field around the HSA. The results indicate that the design and the connection of the flex printed circuit (FPC) has a significant impact on ESD protection for both HSA manufacturing and disk drive manufacturing.