IQD石英晶体LFXTAL002995BULK的老化介绍

IQD石英晶体LFXTAL002995BULK的老化介绍

自1973年以来，IQD一直是频率控制市场公认的领导者，并且是欧洲领先的无源元件供应商之一Würth Elektronik eiSos集团的一部分。IQD在80多个国家拥有活跃的客户，提供最全面的频率产品系列之一，从低成本商业级产品到高可靠性工业和汽车应用中使用的产品，包括：石英晶体，时钟振荡器，汽车的晶体 & 振荡器到AEC-Q200，快速制作振荡器，VCXOs，TCXOs和VCTCXOs，OCXOs，训练有素的OCXOs，铷振荡器等。

随着时间的推移，进口晶振石英晶体的“老化”会导致频率发生微小变化，客户在设计电路时可能需要根据需要达到的总体规格考虑这种影响。石英晶体老化的主要原因有两个，一个是由于传质，另一个是应力。

传质
器件封装内的任何不需要的污染都可能将材料转移到晶体或从晶体转移材料，从而导致石英坯料的质量变化，从而改变器件的频率。例如，用于安装石英坯料的导电环氧树脂可能会产生“排气”，这可能会在密封晶体封装内的惰性气氛中产生氧化材料，因此必须很好地控制这一生产过程。理想情况下，制造方法尽可能干净，以消除任何影响并产生良好的老化效果。

强调
这可能发生在晶体的各种成分中，包括石英坯料的加工、环氧树脂安装粘合剂的固化、石英晶振晶体安装结构和器件中使用的金属电极材料的类型。加热和冷却也会由于不同的膨胀系数而产生应力。当系统放松时，系统中的应力通常会随着时间的推移而变化，这可能会导致频率的变化

实践中的老龄化
当观察晶体的老化测试结果时，可以看出频率的变化通常在第一年最大，并随着时间的推移而衰减。然而，必须注意的是，例如，如果设备规定为每年最大±5ppm；这并不意味着5年后的老化将是±5ppm×5年，即±25ppm。在实践中，示例中的±5ppm老化装置在运行的第一年可能仅为±1ppm至±2ppm，然后在随后的几年中减少。通常使用一般的“指南”来衡量10年内最大±10ppm的晶体老化，尽管事实上通常远低于此。无法预测设备的确切老化，因为即使是在同一时间由同一批石英制成的零件也会表现出略有不同的老化特性。生产过程必须从一个零件到另一个零件保持一致，石英水晶振动子从石英坯料的制造、电极尺寸及其位置，到用于安装石英的环氧树脂及其固化热轮廓，所有这些都会对频率产生轻微影响。根据内部原因，设备可能会出现负老化或正老化，尽管同一批次的零件往往会出现类似的结果。一般来说，超过90%的制造零件的老化效应是负面的.

加速老化
通常的行业惯例是，通过在高温下浸泡设备并在相关时间间隔测量频率移动，使用加速老化过程来预测长期频率移动。使用被动测试（即无电源测试）测试晶体是正常的。使用的一般规则是，将晶体在+85°C下浸泡30天相当于在正常室温下老化1年。如果该测试延长足够的时间，则可以用图形绘制记录的数据，以便通过外推法预测未来的长期老化。

频率调整
请注意，石英的老化有效地改变了晶体的频率容差，并且不会在很大程度上直接影响石英在温度下的稳定性，因为该参数由所用石英的“切角”决定。如果使用具有电压控制功能的石英振荡器，如VCXO、TCXO或OCXO石英晶体振荡器，则可以将输出频率调整回其标称指定值。

设计
使用晶体或振荡器设计电路的工程师通常会知道他们的设备在特定时间段内必须达到的总体稳定性数字。随着器件的公差和/或稳定性降低，老化就变得更加重要。例如，使用温度稳定性为±1ppm的TCXO将需要将老化保持在相对较小的值。然而，如果设计的总频率移动容限为±200ppm，并且使用了额定值为±100ppm的设备，则可以有效地忽略少量老化。

IQD石英晶体LFXTAL002995BULK的老化介绍
The ‘ageing’ of a quartz crystal results in a small change of frequency over time and this effect may have to be taken into account by the customer when designing their circuit depending upon the overall specification that needs to be achieved. There are two main causes of ageing in quartz crystals, one due to mass-transfer and the other due to stress. Mass-Transfer Any unwanted contamination inside the device package can transfer material to or from the crystal causing a change in the mass of the quartz blank which will alter the frequency of the device. For example, the conductive epoxy used to mount the quartz blank can produce ‘out-gassing’ which can create oxidising material within the otherwise inert atmosphere inside the sealed crystal package and so this production process must be well controlled. Ideally the manufacturing method is as clean as possible to negate any effects and give good ageing results.The ‘ageing’ of a quartz crystal results in a small change of frequency over time and this effect may have to be taken into account by the customer when designing their circuit depending upon the overall specification that needs to be achieved. There are two main causes of ageing in quartz crystals, one due to mass-transfer and the other due to stress.

Mass-Transfer

Any unwanted contamination inside the device package can transfer material to or from the crystal causing a change in the mass of the quartz blank which will alter the frequency of the device. For example, the conductive epoxy used to mount the quartz blank can produce ‘out-gassing’ which can create oxidising material within the otherwise inert atmosphere inside the sealed crystal package and so this production process must be well controlled. Ideally the manufacturing method is as clean as possible to negate any effects and give good ageing results

Stress

This can occur within various components of the crystal from the processing of the quartz blank, the curing of the epoxy mounting adhesive, the crystal mounting structure and the type of metal electrode material used in the device. Heating and cooling also causes stress due to different expansion coefficients. Stress in the system usually changes over time as the system relaxes and this can cause 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 generally greatest in the 1st year and decays away with time. It must be noted however that for example if a device is specified at ±5ppm max per year; it does not follow that the ageing after 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 over subsequent years. It is common to use a general ‘guiderule’ for crystal ageing of ±10ppm max over 10 years although in reality it is usually much less than this. It is impossible to predict the exact ageing of a device as even parts made at the same time and from the same batch of quartz will exhibit slightly different ageing characteristics. The production process must be consistent from part to part, from the manufacture of the quartz blank, the electrode size and its placement, to the epoxy used to mount the quartz and its curing thermal profile, all have a slight affect on frequency. Devices can age negatively or positively depending upon the internal causes although parts from one batch tend to follow similar results. Generally the ageing effect is negative in over 90% of parts manufactured.

Accelerated ageing

It is common industry practice to use an accelerated ageing process to predict long term frequency movement by soaking devices at elevated temperatures and measuring frequency movement at relevant intervals. It is normal to test crystals using a passive test (i.e. non-powered). Thegeneral rule used is that soaking a crystal at +85°C 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 ‘cutangle’ 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.