1. Surface Soldering

 

Surface wetting refers to the phenomenon that molten solder spreads and covers the surface of the metal to be welded during welding.

 

Wetting means that the dissolution and diffusion between the liquid solder and the surface to be soldered has formed an intermetallic compound (IMC), which is a sign of good soldering.

 

 

When we immerse a solid metal sheet into the liquid solder tank, there is contact between the metal sheet and the liquid solder, but this does not mean that the metal sheet has been wetted by the liquid solder, because there may be a barrier between them, only Pull small pieces of metal out of the solder bath to see if they are wet.

 

Wetting only occurs when the liquid solder is in close contact with the surface of the metal to be soldered, and then sufficient attractiveness can be ensured. If there are any firmly attached contaminants on the surface to be welded, such as oxide film, it will become a metal connection barrier and prevent wetting. On the 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 be spread, and the contact angle θ is greater than 90°.

picture

 

If the surface to be soldered is clean, then their metal atoms are located close to the interface, so wetting occurs, and the solder will spread on the contacting surface, as shown in the figure below. At this time, the solder and the matrix atoms are very close, so an alloy is formed at the mutually attracted interface, ensuring good electrical contact and adhesion.

picture

 

2. Solderability

 

Solderability refers to the ability of the base material to be welded to be welded under a 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 soldered.

 

Solderability is usually evaluated by dipping method or wetting balance method. These two methods are essentially the same, that is, whether the base material to be welded can be wetted at the specified time and temperature. Therefore, it can be said that solderability and wettability are closely related.

 

In the dipping test, one or more of the following phenomena can be observed on the surface of the pattern 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 soldered is too thick, the flux cannot remove it within the effective soldering time, and non-wetting will occur at this time.

 

(2) Wetting: The molten solder is removed, and a thin layer of solder remains on the soldered surface, which proves that metal-to-metal interaction has occurred. Complete wetting refers to leaving a layer of uniform, smooth, crack-free, and well-adhered solder on the surface of the welded metal.

 

(3) Partial wetting: some parts of the welded surface appear to be wetting, and some parts are not to be wetted.

 

(4) Weak wetting: The surface is initially wetted, but afterwards the solder shrinks from part of the surface into droplets, leaving a thin layer of solder on the weakly wetted area.