Analyze the aging of quartz crystal

The 'ageing' of a quartz crystal results in a small change offrequency over time and this effect may have to be takeninto account by the customer when designing their circuitdepending upon the overall specification that needs to beachieved. There are two main causes of ageing in quartzcrystals, one due to mass-transfer and the other due tostress.

Mass-Transfer

Any unwanted contamination inside the device package cantransfer material to or from the SMD CRYSTAL causing a change inthe mass of the quartz blank which will alter the frequencyof the device. For example, the conductive epoxy used tomount the quartz blank can produce 'out-gassing' whichcan create oxidising material within the otherwise inertatmosphere inside the sealed crystal package and so thisproduction process must be well controlled. Ideally themanufacturing method is as clean as possible to negate anyeffects and give good ageing results.

Stress

This can occur within various components of the crystalfrom the processing of the quartz blank, the curing of theepoxy mounting adhesive, the crystal mounting structureand the type of metal electrode material used in the device.Heating and cooling also causes stress due to differentexpansion coefficients. Stress in the system usuallychanges over time as the system relaxes and this cancause a change in frequency.

Ageing in practice

When looking at example ageing test results of crystals,it can be seen that the change in frequency is generallygreatest in the 1st year and decays away with time. It mustbe noted however that for example if a device is specifiedat ±5ppm max per year; it does not follow that the ageingafter 5 yrs will be ±5ppm x 5yrs, i.e. ±25ppm. In practice,the example ±5ppm ageing device may be only ±1ppm to±2ppm in the 1st year of operation and then reduces oversubsequent years. It is common to use a general 'guide-rule' for crystal ageing of ±10ppm max over 10 yearsalthough in reality it is usually much less than this. It isimpossible to predict the exact ageing of a device as evenparts made at the same time and from the same batch ofquartz will exhibit slightly different ageing characteristics.The production process must be consistent from part topart, from the manufacture of the quartz blank, the electrodesize and its placement, to the epoxy used to mount thequartz and its curing thermal profile, all have a slight affecton frequency. Devices can age negatively or positivelydepending upon the internal causes although parts fromone batch tend to follow similar results. Generally theageing effect is negative in over 90% of parts manufactured.

Accelerated ageing

It is common industry practice to use an accelerated ageingprocess to predict long term frequency movement bysoaking devices at elevated temperatures and measuringfrequency movement at relevant intervals. It is normal totest crystals using a passive test (i.e. non-powered). Thegeneral rule used is that soaking a crystal at +85℃ for 30 days is equivalent to 1 year of ageing at normal room temperature. If this test is extended for enough time then the recorded data can be plotted graphically to enable via extrapolation, the prediction of future long term ageing.

Frequency adjustment

Note that the ageing of quartz effectively changes the frequency tolerance of the crystal and does not directly influence the stability of the quartz over temperature to any great degree as this parameter is dictated by the 'cut-angle' of the quartz used. If using quartz oscillators that have a voltage-control function such as VCXOs, TCXOs or OCXOs, the output frequency can be adjusted back to its nominally specified value.

Design

The engineer designing a circuit using either a crystal or oscillator will generally know what overall stability figure their equipment must meet over a particular time period.

As the tolerance and/or stability of a device decreases then the more important ageing becomes. For example using a TCXO at ±1ppm stability over temperature will require ageing to be kept to relatively small values. However, if the total frequency movement allowance of a design is for example ±200ppm and a device with a rating of ±100ppm is used then a small amount of ageing can effectively be ignored.