1. Surface wetting
Surface wetting refers to the phenomenon that molten solder spreads and covers the surface of the metal to be welded during welding.
Wetting indicates that dissolution and diffusion occur between the liquid solder and the surface to be soldered, forming an intermetallic compound (IMC), which is a sign of good soldering.
When we immerse a solid metal piece into a liquid solder bath, there is contact between the metal piece and the liquid solder, but this does not mean that the metal piece has been wetted by the liquid solder, because there may be a barrier layer between them, only You can see if it's wet by pulling the small piece of metal out of the solder bath.
Wetting can only occur when the liquid solder is in close contact with the metal surface to be soldered, and sufficient attraction can only be guaranteed at that time. If there are any firmly attached contaminants on the surface to be welded, such as an oxide film, it will act as a barrier to the metal's connection and prevent wetting. On a contaminated surface, a drop of solder behaves the same as a drop of water on a greased plate, as shown in the figure below. It cannot spread and the contact angle θ is greater than 90°.
If the surface to be soldered is clean, then the position of their metal atoms is close to the interface, so wetting occurs and the solder will spread on the contact surface, as shown in the figure below. At this time, the solder and matrix atoms are very close, thus forming an alloy at the interface that attracts each other, ensuring good electrical contact and adhesion.
2. Weldability
Weldability refers to the ability of the base metal to be welded to be welded under specified time and temperature. It is related to the heat capacity, heating temperature and surface cleanliness of the base material (component or PCB pad) being welded.
Weldability is usually evaluated by the dipping method or the wetting balance method. These two methods are essentially the same, which is to see whether the base metal to be welded can be wetted under the specified time and temperature. Therefore, it can be said that solderability and wettability are closely related.
In the immersion test, one or more of the following phenomena can be observed on the surface of the sample taken out of the molten solder bath.
(1) Non-wetting: The surface becomes uncovered again, without any visible interaction with the solder, and the soldered surface maintains its original color. If the oxide film on the surface to be welded is too thick and cannot be removed by the flux within the effective welding time, non-wetting will occur.
(2) Wetting: After the molten solder is removed, a thin layer of solder still remains on the welded surface, proving that intermetallic interaction has occurred. Complete wetting means leaving a uniform, smooth, crack-free, and well-adhered layer of solder on the surface of the metal being welded.
(3) Partial wetting: Some parts of the welded surface are wetted, and some parts are not wetted.
(4) Weak wetting: The surface is wetted at first, but then the solder shrinks from part of the surface into droplets, leaving a thin layer of solder in the weakly wetted area.
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