Cleaning and activation before welding

The strength of welding depends on the effective bonding area and bonding strength between the solder and the welding base material. Oxidation and contamination on the surface of the welding base material and the solder can be factors that affect the effective bonding area and strength of each joint surface.

Potential causes of weld voids (reasons that affect weld strength)

1. Voids caused by solder not being wetted or low wetting

   
   

2. Solder oxidation can easily cause voids

   
   

3. Oxidation of the gold-plated layer of the welding base material can easily cause voids

   
   

4. Foreign matter physically embeds itself into the solder, causing voids.

   
   

5. The welding surface is not flat enough, and the gas in the depression cannot be completely released, causing voids.

   
   

6. The metallization surface is not dense and uniform, resulting in uneven solder flow and voids.

   
   

During welding, the molten liquid solder contacts the welding base material and must wet the surface well to fill the gap between the welding base materials. Therefore, factors such as surface cleanliness and roughness will affect the tension of each interface and the wetting effect.

Causes of welding void failure and measures to be taken

1. Poor solder wettability

The quality of wettability directly affects the solder flow performance and bonding strength. Good wettability reduces voids and improves bonding strength.

The poor wettability of solder can be considered that the contact area between solder and welding base metal is small, and on the other hand, it means that the interface tension on the edge of the wetting liquid is large, which hinders the spreading process of solder. Plasma cleaning of welding base metal can increase the contact area between solder and welding base metal, improve wettability, increase welding area and welding strength.

Comparison of wetting performance of copper-plated and silver-plated pad surfaces before and after plasma cleaning

2. Solder surface oxide layer

If the solder is stored for too long, a thick oxide layer will form on its surface. If there is no manual intervention during the welding process, the oxide layer is difficult to remove. The oxide film on the solder surface or the oxide film that is not completely removed will hinder the penetration of the solder and cannot fully wet the substrate. The oxide film formed after the solder melts will form voids after welding.

The solder should be vacuum treated for anti-oxidation and stored in a vacuum; plasma cleaning technology should be used to further clean the welding surface to remove impurities on the surface of the material and minimize the degree of oxidation of the surface to be welded. Hydrogen and argon plasma have excellent antioxidant effects.

3. Welding surface particles and contamination

During the welding process, if unclean welding base materials are used or the welding base materials are contaminated, the solder will not be able to diffuse completely during the welding process, forming voids and affecting the welding effect. Therefore, the welding base materials must be strictly processed before welding to remove the contamination caused by the material during the processing and transfer process. Ultrasonic and plasma cleaning can effectively remove particles and organic contamination on the surface of the material.

In summary: On the one hand, the use of plasma cleaning technology to further clean the surface of the welding material can remove the oxides and molecular-level pollution (organic matter, etc.) formed on the surface of the material, and the physical impact of plasma on the surface of the welding base material plays a cleaning and activation role. At the same time, the surface grains are broken and refined to obtain nano-sized grains. The finer-grained matrix base material structure is conducive to the mutual diffusion of atoms between the interfaces during welding; on the other hand, plasma cleaning improves the wettability and adhesion of the welding base material surface to a certain extent, ensures close contact between the atoms on the surface of the material and the atoms of the material to be attached, and improves the welding quality.