A study of head stack assembly sensitivity to ESD

It is well known that both giant magnetoresistive (GMR) and MR head gimbal assemblies (HGAs) are susceptible to damage by electrostatic discharge (ESD) from human body model (HBM) and machine model (MM) transients. What is less well known is the susceptibility of a GMR or MR sensor once the HGA has been wired to the pre-amplifier and becomes part of an HSA (head stack assembly). We measured current transients across a current probe at the head position generated by applying an HBM voltage transient at the actuator body, flex or pins. The measured amplitude transfer function (mA through head vs. HBM voltage) through this particular preamp/head stack assembly was about 1 mA/33 V. The determination of damage thresholds for HSAs is essential to determine the level of ESD controls needed for post-HSA manufacturing processes in the production of hard disk drives.