Quartz crystal ageing

Quartz crystals used in filters and oscillators in electronic circuits are renowned for their performance, stability, frequency tolerance and their high Q. Yet they do change their frequency very slightly with time in a process known as ageing. Although the frequency variations are small by many standards, they are permanent and may have an effect in some applications where the frequency is of great importance. As a result manufacturing techniques take account of this to reduce the effects of ageing in these crystals as far as possible.

Ageing is caused by a number of interrelated factors. These include internal contamination, excessive drive level, surface change of the crystal, various thermal effects, wire fatigue and frictional wear. The level of ageing can be minimised in a number of ways. During manufacture they should be encapsulated in an inert gas environment, the ensuring should have a good seal so that other gases do not enter. Also the final stages of the preparation of the crystal blank must be prepared as finely as possible. Rather than lapping the blank to bring it to the right dimensions, chemical etching is used. In this way the minimum disruption is caused to the crystal lattice, and this reduces the ingress of contaminants over time that will cause ageing.

The design of the circuit in which the crystal will be used also has an effect. By keeping the drive levels low again the crystal ageing will be less.

As expected the rates of change of the crystal frequency vary with the time after manufacture. The maximum rate of change of frequency occurs immediately after manufacture and decays thereafter. As a guide it is found that it is fastest within the first 45 days of operation. Even so there is always some degree of ageing throughout the life of the crystal. In view of the fact that the greatest rate of change is immediately after manufacture, high tolerance items are run for some time before being shipped. In very high tolerance items this may extend to a few months of operation.

Once the ageing rate has settled it is found that typical figures can be quoted for many types. It is found that one of the main variations is the type of encapsulation that is used. The two most common methods of encapsulation for through-hole crystals are resistance weld and cold weld. These will typically give figures of around 5 parts per million (ppm) for a resistance weld sealed encapsulation, and 2 ppm for a cold weld sealed encapsulation using an HC43/U holder. These both move in a downward direction. Glass encapsulated crystals may also be found on some occasions. These tend to move in an upward direction, and may have a tolerance or slightly less than 5 ppm. Also there is a wide variety of surface mount crystals. A typical plastic package or a glass seam weld package may give around ±5 ppm while a metal seam weld package may give less than 3 ppm.

If the crystal is maintained in the same circuit and at the same temperature then the effects of ageing may stabilse after some years of operation. However if these are changed they may cause the ageing rate to change. It may even alter direction.

Cleanliness of the environment around the crystal is one of the main ways of reducing ageing. It is therefore esential to ensure that the crystal package or encapsulation is not damaged in anyway. The seal should not be damaged, nor the pins bent as this may break the seal.

While the effects of ageing may not be of importance in applications such as clock oscillators for running many digital circuits, they are important where high frequency stability is required. By choosing the right crystal, these effects can be kept within reasonable limits so that they do not cause any problems.