Components heat resistance and moisture sensitive problems during lead-free soldering

First, components heat resistance

Ceramic resistors and special capacitors are very sensitive to the slope of the temperature curve (rate of temperature change), because the coefficient of thermal expansion (CTE) of the ceramic body and the PCB is quite different (3 ~ 5x10 ℃ for inner porcelain, and about 17x10 ℃ for PCB) , When the solder joint is cooled, it is easy to cause component body and solder joint crack. Component cracking is directly proportional to the difference in CTE, temperature, and component size.

0201, 0402, 0603 small components are rarely cracked, and large components above 1206 are more likely to have crack failures. As another example, electrolytic capacitors are extremely sensitive to temperature. Some tantalum capacitors can only withstand more than 183 ° C and cannot exceed 60s, 240 ° C

The skeleton of some small SMT surface mount transformers 1051 is not suitable for high temperature, and it is required that 220 ° C cannot exceed 10s: some leaded devices cannot exceed 106S at 230 ° C. Therefore, we need to pay attention to the selection of components, and try to reduce the peak temperature and the time by optimizing the temperature curve, so as to avoid the damage caused by the lead-free soldering temperature to the components.

Second, components moisture sensitivity problem

During the reflow soldering process, moisture-absorbing devices expand due to water vapor, which causes the water vapor pressure to rise with temperature, which may cause damage to devices that have been affected by the moisture. Connectors and other plastic packaging components (such as PBGA, QFP, etc.) have a significant increase in failure at high temperatures, mainly due to delamination and explosion rice flower, deformation and so on. The lead-free soldering temperature is high. According to rough statistics, when the soldering temperature is increased by 10 ° C, the humidity sensitivity level (MSL) of the humidity sensing device (MSD) is increased by one level. Reliability then drops by one level. The solution is to manage, store, and use temperature-sensitive components (MSDs). Minimize peak temperature. Dehumidify and bake sensitive components that have been subjected