In PCB manufacturing，flux quality and selection

The flux is the key material in solder paste for purifying the soldering surface of PCB manufacturing, improving the wettability, preventing solder oxidation, and ensuring the quality and excellent process of the solder paste. During the heating process, the flux affects the PCB pads, component terminals, and the oxide layer on the surface of the lead which function as cleaning. At the same time, it activates the metal surface.

If the activity of the flux is good, sufficient activation energy can be obtained on the metal surface, so that the molten solder can be wetted and diffused on the metal surface purified by the flux. Because diffusion can only be performed on a clean metal surface, if the metal surface has oxides, the solder cannot wet the metal surface, and diffusion cannot occur. After reflow soldering, although it has accumulated solder between the component pins and the PCB manufacturing pads, but it is actually virtual soldering, there may be electrical connection when pressure is applied, and electrical connection will be lost when vibration or loosening. If the molten solder is locally wetted on the metal surface, then only the locally wetted places will produce a connection, which will also affect the strength of the PCB manufacturing solder joints.

Due to the high melting point and poor wettability of lead-free alloys, the activation temperature and activity of the flux in lead-free solder paste must be increased to match the melting point of the solder alloy: In addition, there may be chemical reactions between the solder and the surface of the solder alloy. Therefore, different fluxes must be selected for different alloy compositions. After determining the lead-free alloy, the key to solderability is the flux.

In leaded soldering, the active reaction of the flux is just before the melting point of the solder is 183℃  when cleaning the metal surface, the PCB manufacturing solder melts, the flux still maintains sufficient activity, which can reduce the viscosity and surface tension of the molten solder. The effect of improved wettability is conducive to diffusion and dissolution, forming an intermetallic alloy layer, but the melting point of lead-free soldering is 217 ℃, which is 34 ℃ higher than that of lead. If traditional flux is used, it will melt in lead-free alloy. Before the temperature of 217 ℃, the flux in the solder paste has been burned out. When the temperature rises to above 21 ℃, it will not only play a role in cleaning and activation, but may also cause the charring of the flux, which may cause PCB pads in severe cases, component pins and solder alloys re-oxidize at high temperatures and cause poor soldering. Therefore, the flux must be specially configured.

Selection of flux

First determine the activity of the solder paste based on the PCB and component storage time and the degree of surface oxidation. RMA grade is generally used; R grade can be selected for high-reliability products: PCB and components have a long storage time and the surface is seriously oxidized. RA grade should be used and cleaned after welding.

Matching of flux and solder

The choice of flux can not only consider the activation ability of the flux, it must be combined with the characteristics of the solder and the specific heating method. First of all, it is necessary to ensure that the active temperature range of the soldering flux for PCB manufacturing covers the entire soldering temperature, and secondly, the flow and spreading process of the soldering flux and the soldering material must be coordinated so that the melting of the brazing material and the active climax of the soldering flux are synchronized. When brazing, the brazing material is best to start melting within 5 ~ 6s after the flux is melted, so that the brazing material can be quickly spread out when it is melted.