A case study on hidden ESD events of GMR HGA dynamic test fixture

This paper reveals an ESD event that is usually overlooked during head gimbal assembly (HGA) dynamic test operation and how a solution to the problem has been established. Experiments were conducted to determine a hidden ESD source by using two different designs of dynamic test shoe. ESD events were captured during unloading and reloading of the HGA bridge flex circuit (BFC) pad to the old shoe fixture (original design). Tribocharging and field-induced charging are both suspects in the observed failures. These results are consistent with CDM type ESD failure. Results show how this event caused severe magnetic or melting damage to the GMR sensor. We were able to measure of about 155 V (negative charge) on BFC kapton after unloading from the old shoe dynamic electrical test (DET) fixture. Our fast digital oscilloscope captured 131.2 mA (max) at nanosecond current transients during metal contact of the MR exposed lead. This causes great yield loss at HGA level manufacturing for that particular prototype program. A solution was established by re-designing the BFC insertion nest and clamp of the shoe fixture and using dissipative base and clamp materials. An enhanced design is also incorporated whereby the fixture can stand alone during insertion of BFC to the nest and unloading from the shoe clamp. Therefore, this new fixture (modified design) minimizes the risk of tribocharge voltage against the BFC kapton and metal contact event.