If the chip is like a human brain for PCBA, then the crystal oscillator is the heart. Once it beats (vibrates) abnormally, such as when it jumps (vibrates), it does not jump (vibrate). The consequences can be imagined, not to mention this The "heart" completely stopped "beating".
The basic structural principle of the crystal oscillator is relatively simple. From the outside, it is the case plus the base, and the pins are under the base. The shrapnel in the base is fixed with a very thin crystal wafer with conductive glue, which is more fragile than glass. When the crystal is subjected to a current with sufficient excitation power, the wafer will vibrate regularly, which is the physical characteristic of crystal. Here, it is easy to understand the truth: the thinner the wafer, the higher the vibration frequency of the crystal. On the contrary, the lower the frequency of the crystal, the thicker the wafer. For example, the crystal of the 54MHZ crystal will be better than the 4MHZ crystal. The wafers are many times thinner, so the higher the probability of being damaged by physical impact. This is also the principle that we often say that the crystal oscillator should be "be careful not to use it when dropped".
In the SMT production line, the ultrasonic process is sometimes used. It is characterized by low cost and convenient operation, such as cleaning the PCBA after the completion and removing residual solder. Or in the encapsulation of certain products, such as the card reader, U disk, etc., to achieve the purpose of not using screws or glue, and reducing costs. However, it should be vigilant that ultrasonic waves are high-frequency oscillatory waves, while crystal oscillators are frequency components. Their commonality is to rely on high-frequency vibration to achieve their work goals.
Ultrasonic instruments generate high-frequency shock waves when working. If a resonance effect occurs with a crystal oscillator wafer, an extremely fragile wafer is likely to be shattered, causing the crystal oscillator to stop vibrating. On the other hand, the wafer is connected (fixed) with the elastic sheet on the base through conductive adhesive. Under the high-frequency oscillation of ultrasonic waves, the possibility of the conductive adhesive being cracked is greatly increased. Once there is a crack in the conductive glue, the crystal will appear to vibrate when it is working. The reason is very simple. When the device equipped with the PCBA is heated or shaken, the cracked conductive adhesive will be connected (conductive) due to thermal expansion and contraction or physical vibration, and it can still provide excitation current to the chip. When the device is cold or placed at rest, the crack of the conductive adhesive may open, and there is a disconnection between the chip and the base, no longer vibrating, that is, the pulse is gone, and the heart is dead. The chip that works as a brain can no longer capture the frequency signal emitted by the crystal oscillator, and the device will no longer work properly.
Nevertheless, in view of the cost advantages brought about by ultrasound, the ultrasound process is still very popular in some SMT production lines. This requires the SMT production line to clearly inform the crystal oscillator manufacturer in advance, otherwise the possibility of bad electronic products due to the destruction of the crystal oscillator will increase.
