Does the residue of PCBA process affect the reliability of PCB?

The residue on PCBA mainly comes from the assembly process, especially the welding process. Such as flux residue used, reaction by-products between flux and solder, adhesive, lubricating oil and other residues. Other sources are relatively less harmful, such as pollutants and sweat stains caused by the production and transportation of components and PCB itself. These residues can generally be divided into three categories.

One is non-polar residue, mainly including rosin, resin, glue, lubricating oil and so on. These residues can only be well removed by cleaning with non-polar solvent.
The second category is polar residues, also called ionic residues, which mainly include active substances in flux, such as halogen ions and salts produced by various reactions. These residues need to be well removed and polar solvents such as water and methanol must be used.

There is also a kind of residue with weak polarity, mainly including organic acid and alkali from the flux. In order to get good results, compound solvents must be used.

The basic categories of residues are introduced in detail below:

1. Residue of rosin flux

The flux containing rosin or modified resin is mainly composed of nonpolar rosin resin, a small amount of halide, organic acid and organic solvent carrier, and the organic solvent will be volatilized and removed due to high temperature in the process. The active substances such as organic acid halide (such as adipic acid) mainly remove the oxide layer on the welded surface and improve the welding effect, but during welding, the complex chemical reaction process changes the structure of the residue. The products can be unreacted rosin, polymerized rosin, decomposed active agent and halide, etc. The metal salt produced by the reaction with tin and lead, unchanged rosin and active agent are easier to remove, but it is difficult to remove potentially harmful reactants.

2. Organic acid flux residue

Organic acid flux (OR) generally means that the solid part of flux is mainly organic acid, and the residues of this kind of flux are mainly unreacted organic acids, such as oxalic acid, succinic acid and its metal salts. At present, the vast majority of the so-called colorless cleaning-free flux on the market is this kind. It is mainly composed of multiple organic acids, including halogen-free ions at normal temperature, and compounds that can generate halogen ions when soldering at high temperature, and sometimes a very small amount of polar resins. Among these residues, the most difficult to remove is the salts formed by organic acids and solder, which have strong adsorption performance and poor solubility. When water-soluble flux is used in the assembly process of PCBA, a larger amount of such residues and halide salts will be produced, but due to timely water-based cleaning, such residues can be greatly reduced.

3. White residue

White residue is a common pollutant on PCB, which is usually found after PCBA is cleaned or assembled for some time. White residue can be caused in many aspects of the manufacturing process of PCBA.

The self-color pollutants of PCBA are generally by-products of flux, but the poor quality of PCB, such as the strong adsorption of solder resist paint, will increase the chance of white residue generation. Common white residues are polymerized rosin, unreacted activator, lead chloride or bromide, which is the reaction product of flux and solder. After absorbing moisture, these substances expand in volume, and some substances also hydrate with water, and white residues become increasingly obvious. , it is extremely difficult to remove these residues by adsorption on PCB. Natural rosin is easy to produce a large number of polymerization reactions in the welding process. If overheating or high temperature lasts for a long time, the problem will be more serious. The infrared spectrum analysis results of rosin and residues on PCB surface before and after soldering process confirm this process.

4. Adhesive and oil pollution

In the assembly process of PCBA, some yellow glue and red glue are often used to fix components. However, due to process inspection, the electrical connection parts are often stained. In addition, the residue torn off after bonding pad protection tape will seriously affect the electrical connection performance. In addition, some components, such as small potentiometers, are often coated with too much lubricating oil, which also pollutes the PCB surface. The residue of this kind of pollution is often insulated, which mainly affects the electrical connection performance and generally does not cause failure problems such as corrosion and leakage.