Test Disk-Drive Heads for ESD Sensitivity

GMR heads—thin-film semiconductor devices—make just about the best ESD detector. So, GMR head manufacturers test their products for susceptibility to ESD, knowing a more robust design will give them an advantage in the marketplace. A head that can withstand a greater discharge without being destroyed or one whose transfer curve changes less will be more attractive to disk-drive manufacturers.

Two conditions caused the polarity reversal in the GMR sensor. First, the ESD event had to raise the sensor’s temperature to its blocking temperature. At the same time, the current from the discharge creates a magnetic field around the sensor.

Above the blocking temperature, the sensor’s internal materials reorient themselves in the direction of the magnetic field around the sensor. If you alternate the polarity of the discharge prior to each measurement, then the GMR head’s polarity will alternate with each measurement.

Because the GMR heads are so sensitive to damage from ESD, they can sustain damage from indirect discharges, too. The current from a nearby ESD event will generate magnetic fields that can couple into the wires between the GMR head and its preamp, creating a secondary current that travels through the sensor. That current can be strong enough to change a GMR sensor’s transfer curve (by heating the sensor to its blocking temperature) or even melt the sensor.

Similarly, if you’re performing an ESD test on GMR sensors, a remote ESD event can produce a current in your test cables and damage the sensor under test.³ Because of their sensitivity to direct and indirect ESD, GMR heads require proper handling and ESD control procedures.

According to Subrata Dey, vice president of advanced technologies at Read-Rite (Milpitas, CA), you should take every precaution when assembling GMR heads and when installing them into disk drives. Dey recommends the use of wrist straps and foot straps, ESD protective footwear and bench mats, clothing, chairs, and packaging. You should also, according to Dey, use ionizers in the lab when conducting ESD tests.  T&MW

FOOTNOTES
1. Wallash, Al, “Understanding ESD Damage to Magnetoresistive Recording Heads,” www.wallash.com/understa.htm.

2. Wallash, Al, and Young K. Kim, “Magnetic Changes in GMR Heads Caused by Electrostatic Discharge,” IEEE Transactions on Magnetics, Vol. 34, No. 4, July 1998, IEEE, Piscataway, NJ, www.ieee.org.  A version of the article is available at www.esdsv.org/papers.htm.

3. Wallash, Al, and Doug Smith, “Damage to Recording Heads Due to Electromagnetic Interference,” 1998 IEEE Symposium on EMC proceedings, p. 834, IEEE, Piscataway, NJ, www.ieee.org.