Vibration and Shock Capabilities of the FELIX PIND System

The shock is used in the PIND test to free particles that adhere to the cavity wall. The smaller particles are more prone to exhibit the property of adhesion and stop moving during the vibration cycle. The shock amplitude must be held to within 20%.

Unique to the SD PIND system is an “Active Shock”. The computer controls of the shaker motion which is active during the shock until the striker in the armature collides with the shock anvil deep inside the shaker. The shock is created by the stopping of the armature velocity creating a dynamic shock pulse that travels up the sensor to the DUT. The computer control allows a programming change to shock amplitude to accommodate the larger loads. Older mechanical shocks using a spring-loaded tapper actuated shock must be reset by manually adjusting the screws for any parts that weigh over 25 grams.

The SD PIND shock is calibrated and programmable from 500 to 1500 g’s and the display reads the calibrated value of the shock waveform during the test for the actual DUT. In addition there is a selectable boost for heavier parts or a negative boost to get lower values than the calibration range. In this way the dynamic  conditions are always monitored and accurate throughout the variety of test conditions and devices being tested.

Single and Multiple Crystal Sensors for PIND Testing with the FELIX System

The sensor is defined in terms of its longitudinal sensitivity in the physical parameter of pressure as -77.5dB+/-3dB ref 1V/microbar as described in the absolute calibration method of ANSI S1.2-1988, using a full-field three sensor underwater reciprocity calibration technique to accurately measure the crystal response.

Less accurate methods of sensitivity measurement used include capacitive pickup calibration or ultrasonic white noise calibration, which can be used to measure the sensor output but are only relative measurement methods and can made accurate only by referencing the absolute underwater calibration method.

All SD PIND sensors have a complete Faraday shield around each crystal to protect the sensor from unwanted stray electrical signals. This protection enhanced by the five-conductor, seven-layer flexible circuit that attaches the sensor to the shaker mounted connector eliminates the need for Transient Detectors with Spike indicators required on older PIND systems. The sensor peak sensitivity can be dampened by a variety of factors but the most common reason for sensors to lose sensitivity over time is the bond that holds the crystal to the front surface wear plate will begin to micro crack with use and age.

MULTIPLE CRYSTAL SENSORS
As the sensor crystal and the source of the acoustic wave get farther apart the measured energy is reduced. JEDEC Recommended Practice 114 graphically outlines the decay of detection is down to less than 50% at distances over 0.75 in from the impact site. For the PIND test it is then important that the lid of the part to be tested be placed a close to the crystal in the sensor as possible. The Model 100-5S155-4 sensor incorporates five separate impact detection crystals within the single sensor. The most sensitive area of the sensor are those areas where each detection crystal is located. For testing small parts, it is important to place the part directly on one of the four target areas.

Advanced PIND Detection with the FELIX System

Acoustic energy, generated by the particle impact with the cavity lid, propagates as an acoustic wave through the material until it reaches the sensor wear plate. The acoustic pressure causes it to deflect slightly pushing on the crystal, which then generates an electrical output. Please note that if the signal is forced to propagate further such as through the substrate, it will lose significant energy. For maximum sensitivity, Impact sensors use a piezoelectric element of Lead Zirconate Titanate (most often called PZT-5A) at peak resonance. These are simply the most sensitive detectors available capable of detecting surface displacements of less than 10-11 meters. Their exact sensitivity and resonant frequency can both vary at time of manufacture and over time with use. For military specifications, the frequency of resonance is allowed to vary from 150 to 160KHz.