2. In humid environments and under certain voltage conditions, electrochemical reactions may occur with the conductors, resulting in a reduction of surface insulation resistance (SIR).
3. If electromigration and dendritic crystal growth take place, short circuits between wires can happen, increasing the risk of electromigration, commonly referred to as “leakage.”
4. To ensure electrical reliability, it is crucial to evaluate the performance of different no-clean fluxes.
5. Whenever possible, the same PCB should utilize the same flux or be thoroughly cleaned after soldering.
1. Through the reliability analysis of solder joint mechanical strength, including factors like tin whiskers, voids, cracks, intermetallic compound cellularity, mechanical vibration failures, thermal cycle failures, and electrical reliability, failures are more likely to occur in solder joints exhibiting certain defects.
2. These defects include either an excessively thin or thick intermetallic compound after soldering; internal cavities or micro cracks within or at the solder joint interface; a small wetted area of the solder joint (where the component solder end and pad overlap has minimal size deviation); and a microstructure that is not dense, featuring large crystal particles and significant internal stress.
3. Some defects can be identified through visual inspection, AOI, or X-ray techniques, such as small solder joint overlap sizes, surface pores, and prominent cracks. However, hidden defects—such as microstructure issues, internal stress, voids, cracks, and particularly the thickness of intermetallic compounds—are not visible to the naked eye and cannot be detected through standard manual or automated inspections in SMT processes.
4. Consequently, a variety of reliability tests and analyses are required, including temperature cycling, vibration testing, drop tests, high-temperature storage tests, damp heat tests, electromigration (ECM) tests, highly accelerated life tests, and highly accelerated stress screening.
5. Following these tests, electrical and mechanical properties, such as solder joint shear strength and tensile strength, should be evaluated. Ultimately, judgments can be made through visual inspections, X-ray fluoroscopy, metallographic sections, scanning electron microscopy, and other analyses.
6. From the above analysis, it is evident that hidden defects introduce uncertain factors that compromise the long-term reliability of PCBA lead-free products. As a result, high-reliability products are now exempt; both visible and hidden defects arise due to lead-free high-tin content, elevated temperatures, narrow process windows, poor wettability, material compatibility issues, as well as design, process, and management factors.
7. Therefore, from the initial design phase of PCBA lead-free products, it is essential to consider the compatibility of lead-free materials, lead-free design, and lead-free processes. Additionally, heat dissipation must be thoroughly addressed, and careful selection of PCB substrates, solder mask surface layers, components, solder paste, and flux is necessary.
8. Furthermore, SMT process optimization and control should be more meticulous than with lead soldering, necessitating rigorous and detailed material management.
