Application reports

# 4 : Leak testing of crystal oscillators

A crystal oscillator is an electronic circuit that uses the mechanical resonance...

Application
A crystal oscillator is an electronic circuit that uses the mechanical resonance of a vibrating crystal of piezoelectric material to create an electrical signal with a very precise frequency. This frequency is commonly used to keep track of time (as in quartz wristwatches), to provide a stable clock signal for digital integrated circuits and to stabilize frequencies for radio transmitters and receivers. The most common type of piezoelectric resonator used is the quartz crystal, so oscillator circuits designed around them were called "crystal oscillators".

Piezoelectricity is the ability of some materials (notably crystals) and certain ceramics to generate an electric potential in response to applied mechanical stress. This may take the form of a separation of electric charge across the crystal lattice. If the material is not short-circuited, the applied charge induces a voltage across the material.

Specification
During the production process the inside of the crystal oscillator is evacuated and filled with inert nitrogen gas, to prevent electrodes from being oxidized or influenced by ambient temperature, then hermetically sealed. The leak tightness for the hermetically sealed crystal oscillator is performed using an helium mass spectrometer leak detector and 100% helium test gas.
A typical leak tightness specification is set the 10-9 atm.cc/sec range.

Benefits of using Adixen tracer gas HLD methods
• Very fast and very sensitive test method.
• Integral test with clear quantitative result - no operator interpretation.
• Fast, simple calibration and response time verification.

How to use a tracer gas method
Helium leak testing of sealed packages present some challenges as the component is sealed and cannot be evacuated. These obstacles have been resolved using the bombing test method.
The bombing test consists of placing the sealed package into a chamber that is pressurized with 100% helium for a defined time. Given the pressure differential between the outside pressure in the chamber and the inside of the sealed package, should the component leak, helium will be forced inside through the components flaw during the pressurization period.
The sealed package is then removed from the bombing chamber and placed inside a vacuum leak test chamber, the chamber is connected to a helium leak detector. In the case of a leak, the helium that was forced into the part from the pressure bombing will be driven out from the flaw in the component due to the lower pressure on the outside (vacuum). The helium leak detector will detect and quantify the leak rate from the sealed package and display the pass or reject status of the test based on the preset limit.

Tracer gas methods - the most sensitive and versatile NDT methods
An unrivalled sensitivity and wide dynamic range are the key characteristics for a tracer gas leak detection method.
Easy operation, high acceptance and regulation conformity have made tracer gas leak detection methods the optimum solution for quality control.

Information about leak rates ( typical)
The crystal oscillator has to be hermetically sealed: Fine leak test, leak rate less than 2x10-8 Atm.cc/sec. with 100% helium concentration.

Equipment used
Helium leak detector ASM 1002
The ASM 1002 can detect 10-9mbar.l/s leaks from sealed packages in less than 4 seconds (single or batch testing), which makes the ASM 1002 the fastest unit in its category on the market today.
The ASM 1002 is fully automated. As soon as the test chamber is closed , the cycle starts automatically, the powerfull pumping package evacuates the test chamber and allows the unit to perform the leak tests fast and accurately. At the end of the test, the ASM 1002 will display a Pass/Fail response to the operator.

For good parts, the test chamber will vent back to atmosphere and will be open automatically, optimizing the operator time. For leaking parts, the red (fail) indicator will illuminate, the test chamber will remain under vacuum until the operator acknowledges the result by pressing a button to vent the chamber back to atmospheric pressure.