How does the solderability of the base metal affect the soldering results of wave soldering?

The base metal refers to the PCB pads and component pins. Anyone who has practiced wave soldering will have a deep understanding of this, that is, as long as the base metal has good solderability, the sensitivity of other factors to the welding effect will appear to be very dull, that is, the process window is significantly widened. This is why people always consider the solderability of the base metal as the first factor affecting the welding effect. The PCB pads and component pins provided by the supplier must have good solderability and can withstand storage before soldering and multiple soldering temperatures without degradation. There are several ways to obtain PCB solderability coating (only lead-free PCB coating is introduced).

(1) Organic solderability protective coating (OSP)

This coating material is easy to handle, compatible with lead-free solder such as SAC, relatively free of ionic impurities, and has a smooth surface. The thickness of the OSP coating is usually 0.2-0.5 μm, and too high a preheating temperature (such as 130°C) will cause it to lose its protective effect. During the preheating process, since OSP is compatible with water-soluble flux, the activator and solvent in the flux will quickly dissolve the OSP coating, making it part of the flux, and then evaporate when the molten solder contacts the PCB.

This coating material has a short storage time, and even if stored in dry N2, it cannot exceed 12 months.

(2) Im-Sn coating

Im-Sn coating has good surface flatness and is easy to handle. When using this coating, it must be considered that at higher soldering temperatures, the base metal will diffuse outward and oxidize, thereby reducing solderability. In addition, the tin whisker problem is considered to be the main obstacle of Im-Sn.

(3) Im-Ag coating

Im-Ag coating is a silver coating directly applied on copper conductors. Its thickness is 0.127 to 0.254 μm. The silver coating is semi-bright silver, and its surface state and flatness are basically consistent with the copper surface state.

In order to evaluate the reliability of the silver coating, foreign scholars have conducted thermal cycle tests on the solder joint reliability of the 144 I/O narrow pitch (0.8 mm) and 156 I/O pitch (1 mm) area array components with silver coatings. The temperature variation range of -40 to 125 °C was used as a comparison with the results of the HASL solder coating. The temperature increase rate was 8 to 10 °C/min. The 144 I/O device went through 254 to 1264 cycles, and the 156 I/O device went through 546 to 1754 cycles. The test showed that the thermal cycle reliability of the Im-Ag layer is comparable to that of the HASL. Due to the low silver content, the Ag-Sn intermetallic compound formed on the solder joint interface does not have much effect on thermal cycle reliability. Because the amount of silver on the pad surface is small, it will not cause brittleness to the solder joint.

(4) ENIG-Ni/Au coating

The reliability of the second-level interconnection on the ENIG-Ni/Au surface is worse than that of the Im-Ag or HASL solder surface coating. In the experiment, LonChase found that the Sn layer adjacent to the Ni layer at the interface between the component and the pad contained gold through cross-sectional analysis of the solder joint. The lead-rich area in the Sn-Au layer will completely spread to the entire solder joint. The high concentration of Au-Sn intermetallic compounds on the interface indicates that the spread of Au on the solder joint is uneven.