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Principle of surface wettability and weldability in SMT processing plant
SMT processing plant surface wetting. Refers to the phenomenon of the molten solder spreading and covering the welded metal surface during welding.

Wetting indicates the dissolution and diffusion of liquid solder surfaces, forming the intermetallic compound (IMC), which is a good sign of soft soldering.

When we put a piece of solid metal into liquid solder groove, sheet metal, and leads to contact between liquid solder, but that doesn't mean sheet metal has been liquid solder wetting, because there may be a barrier between them, only the small pieces of metal from the solder bath take out to see whether the wetting.

The SMT plant's wetting occurs only when the liquid solder is in close contact with the surface of the metal being soldered, which ensures sufficient attraction. If there is any solid attachment contaminant on the soldered surface, such as oxide film, it will become a metal joint barrier layer, thus hindering wetting. A drop in the contaminated on the surface, the performance of the solder and stained with grease a drop of water on the tablet of performance is the same, as shown in figure 1-34, not spreading, contact Angle theta is greater than 90 °.
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If the soldered surface is clean, their metal atoms are positioned close to the interface, thus wetting occurs and the solder spreads over the contact surface, as shown in figure 1-35. At this point, the solder is very close to the basic atoms, so the alloy is formed on the interface of mutual absorption, which ensures good electrical contact and adhesion.  
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The weldability of SMT processing plant refers to the ability of the welded parent material to be welded at the specified time and temperature. It is related to the heat vessel, heating temperature and surface cleanliness of the welded substrates (components or PCB pads).

Weldability is usually evaluated by immersion method or wetting balance method. The two methods are essentially the same, that is, whether the welded parent material can be wetted at the specified time and temperature. Therefore, it can be said that weldability and wettability are closely related.

In the dip test, one or more of the following phenomena can be observed on the surface of the pattern taken out of the molten solder groove.

(1) non-wetting: the surface becomes uncoated again, without any visible interaction with the solder, and the welded surface retains its original color. If the oxide film on the welded surface is too thick, it cannot be removed by the flux within effective welding time, then non-wetting occurs.

(2) wetting: the molten solder is removed, and a thin layer of solder remains on the welded surface, proving that metal interaction has occurred. Complete wetting refers to leaving a layer of uniform, smooth, non-crack and adhesive solder on the welded metal surface.

(3) partial wetting: some parts of the welded surface are wet and some parts are not wet.

(4) weak wetting: the surface is wet at first, but after that, the solder will shrink from part of the surface into liquid drops, leaving a thin layer of solder in the place where the weak wetting has been.

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