Analysis of half-wetting phenomenon in the SMT process

The phenomenon of half-wetting, especially the similar behaviour of water on greasy surfaces observed during soldering, as shown in Figure 1-1, is indeed a complex and noteworthy problem.This phenomenon usually involves a number of aspects such as the solderability of the base metal, the presence of contaminants, the activity of the solder paste, and the formation of intermetallic compounds.The following is a detailed analysis of the causes of the formation of the half-wetting phenomenon:

Figure 1-1 Semi-wetting phenomenon

Poor and uneven solderability of the base metal:

Solderability is the ability to form a good metallurgical bond between the metal surface and the solder.If the solderability of the base metal is poor, then the wetting of the solder on its surface will be impaired.

Uneven solderability means that some areas of the metal surface bond to the solder more readily than others, which can result in the solder collecting in some areas and not wetting effectively in others.

Degradation of the solderability of the base metal:

Even if the base metal initially has good solderability, it may degrade over time due to a variety of factors (e.g., oxidation, moisture, contamination, corrosion, etc.).

Degraded solderability can significantly reduce the bond between the solder and the base metal, leading to half-wetting.

Contaminants on the base metal:

Contaminants may come from process residues, environmental dust, grease or other chemicals.

These contaminants can form a barrier on the surface of the base metal that prevents the solder from wetting, thus affecting the quality of the solder.

Contaminants hidden underneath coatings such as tin, tin-lead, silver or gold are especially difficult to detect and remove, and have a particularly significant impact on the soldering process.

Formation of intermetallic compounds:

During the soldering process, chemical reactions occur between the base metal and the solder to form intermetallic compounds.

These compounds often have different physical and chemical properties that may affect the wettability and bond strength of the solder.

If the layer of intermetallic compounds is too thick or unevenly distributed, half-wetting may result.

To summarise, the formation of half-wetting phenomenon is due to various reasons, including poor and uneven solderability of the base metal, degradation of solderability, contaminants on the base metal, and formation of intermetallic compounds.In order to solve this problem, a series of measures need to be taken to improve the solderability of the base metal, reduce the presence of contaminants and optimise the welding process parameters.At the same time, appropriate repair measures are required to ensure the quality and reliability of welded joints where half-wetting has already occurred.