The development trend of the electronics industry has higher and higher requirements for the assembly process of PCBA, and the reliability and quality of electronic products are mainly determined by the quality level of PCBA. In the process practice and the failure analysis of PCBA, we found that the residue on PCBA has a great influence on the reliability of PCBA. Today JRPanel will account for the types and sources of residues on PCBA.

Residues on PCBA mainly come from the assembly process, especially the soldering process, such as the flux residues used, the by-products of the reaction between the flux and the solder, adhesives, lubricants and other residues. Some other residues are relatively less harmful, such as pollutants and perspiration caused by the production and transportation of components and PCBs themselves.

These residues can generally be divided into three categories. One type is non-polar residues, mainly including rosin, resin, glue, lubricants, etc. These residues can only be removed with non-polar solvents for cleaning. The second category is polar residues, also called ionic residues, which mainly include active substances in the flux, such as halogen ions, and salts produced by various reactions. These residues need to be removed well, and polar residues must be used. Solvents, such as water, methanol, etc. There is also a type of residues that are weakly polar residues, mainly including organic acids and bases from the flux. For the removal of these substances to obtain good results, a complex solvent must be used. The following specifically introduces the basic categories of residues.

1. Residues of Rosin Flux

The flux containing rosin or modified resin is mainly composed of non-polar rosin resin, a small amount of halide, organic acid, and organic solvent carrier. The organic solvent will be volatilized and removed due to high temperature during the process. In welding, the complex chemical reaction process changes the structure of the residue. The product can be unreacted rosin, polymerized rosin, decomposed activating agent, and halides and other activating agents, and metal salts produced by the reaction with tin-lead. Unchanged rosin and active agents are easier to remove, but potentially hazardous reactants are more difficult to remove.

2. Organic Acid Flux Residue

Organic acid flux generally refers to the flux in which the solid part of the flux is mainly organic acid. The residues of this type of flux are mainly unreacted organic acids, such as oxalic acid, succinic acid, etc. and their metal salts. The vast majority of so-called colorless no-clean fluxes on the market are of this type. It is mainly composed of polybasic organic acids, including halogen-free ions at room temperature, and sometimes a very small amount of polar resin.

Among these residues, the most difficult to remove is the salts formed by organic acids and solder. They have strong adsorption properties, but very poor solubility. When the PCBA assembly process uses water-soluble flux, a larger amount of these residues and halide salts will be generated. However, due to timely water-based cleaning, such residues can be greatly reduced.

3. White Residue

White residue is a common contaminant on PCBA, and it is generally discovered after PCBA is cleaned or assembled for a period of time. Many processes of PCBA manufacturing can cause white residue.

The white pollutants of PCBA are generally by-products of flux. If the solder resist is too absorbent, it will increase the chance of white residue. Common white residues are polymerized rosin, unreacted activator, and reaction products of flux and solder. These substances expand in volume after absorbing moisture, and some of the substances also undergo hydration reaction with water, and the white residue will gradually become apparent. After these residues are adsorbed on the PCB, it is extremely difficult to remove them. Natural rosin is prone to a large number of polymerization reactions during the welding process. If the reaction process is high temperature for a long time, the problem is even more serious.

4. Adhesive and Oil Pollution

In the assembly process of PCBA, some yellow glue and red glue are often used. These glues are used to fix components. However, due to process testing, the electrical connection parts are often contaminated. The residue that is torn off after the pad protection tape will also seriously affect the electrical connection performance. In addition, some components, such as small potentiometers, are often coated with too much lubricating oil, which can also contaminate the PCBA board surface. The residue of this kind of pollution will mainly affect the electrical connection performance.