Hysteresis effects after reflow soldering of surface mount crystal oscillators

After reflow soldering on the user´s p-c-board, surface mount crystal oscillators show a positive frequency shift up to several ppm, which decays with a time constant of several days. This effect is usually not described or specified in data sheets. It is however very significant and important for the users, particularly for oscillators with tight temperature tolerances, such as TCXOs and the upcoming SMT OCXOs. In modern equipment manufacturing lines, p-c-boards are often finally adjusted and tuned within a short time after the soldering process. If the board contains a SMT crystal oscillator, a strong negative frequency shift can be observed after approximately one week, the oscillator seems to have aged strongly, but this is due to a hysteresis effect connected with the thermal stress of the reflow soldering process. MIL-PRF-55310 distinguishes between “thermal hysteresis” and “retrace”. Retrace is the non-repeatability of the f vs. T characteristic (of an OCXO), at a fixed temperature, upon on-off cycling an oscillator under specified conditions, while hysteresis (of a TCXO) is the maximum value of the non-repeatability in the f vs. T characteristics during a temperature cycle. However the time-dependency of this effect is not considered. The reflow hysteresis, as defined within this paper, is caused by a single (or multiple) temperature peak(s), and the effect is described as frequency excursion over a period of time of several days. The highest temperature stress is during the liquidus time, which is between 10 sec and 40 sec at or above 215°C. The mechanisms that can produce it are “stress relief in the electrodes, and contamination transfer within the resonator enclosure”