PCBA Tech - The principle of flux and the influence of residue on PCBA

The principle of flux and the influence of flux residue on PCBA

The flux can take on the role of soldering, which is the result of mutual diffusion, dissolution, wetting, etc. after the distance between the metal atoms is close. At this time, what hinders the interaction between atoms is the oxide film and pollutants on the metal surface, which are also more harmful substances that hinder infiltration.

For this reason, on the one hand, measures must be taken to prevent the generation of oxides on the metal surface, and on the other hand, various measures and treatment methods must be taken to remove pollution. However, due to the various front-end processes of PCBA production and even the process of component production, it is very difficult to completely avoid these oxidation and pollution. Therefore, some methods must be taken to remove the oxide film and contamination before the welding operation. The use of flux to remove the oxide film has the characteristics of no damage to the base material and high efficiency, so it can be widely used in the PCBA process.

From the perspective of the function of the flux and the control of the microelectronic assembly process, in addition to the activation performance of removing oxides and contaminants, the flux must also meet the requirements of non-corrosion, insulation, humidity resistance, stability, and non-toxicity., No pollution and other requirements. Generally speaking, its main components are active agents, film-forming substances, additives, solvents and so on.

In order to improve the soldering ability of the flux, active substances are often added to it, such as zinc chloride, ammonium chloride, organic acids and their halides, organic amines and their halides, hydrazines and their halides, urea amides Wait. They can remove the oxides and contaminants on the metal surface, so that the solder can infiltrate the metal surface of the base material. The activity of the active agent is related to its own structure. In particular, the organic active agent has a soft effect, short time, low corrosiveness, and good electrical insulation performance, so it can be widely used in the electronic assembly industry. Since the addition of the active agent will change the insulation resistance, dielectric loss, breakdown strength, anti-corrosion ability and other properties, usually the addition amount is about 2-10%.

Flux residues after soldering can form a tight organic film to protect solder joints and substrates, with certain anti-corrosion properties and electrical insulation properties. Generally, various resins are added as film-forming substances, and the addition amount is generally between 20% and 10%. The additives are added to the flux to make the flux have some special physical and chemical properties to meet the needs of the process and process environment.

In terms of the composition of the Hilti flux, the mass ratio of the solvent is relatively large. It dissolves various functional components together like a carrier, dissolves the solid component of the flux into a uniform liquid, and uses the diffusion and flow of the solvent to bring the dissolved flux active component into the micro gap between the welding parts to ensure welding Cleaning of metal microscopic surface. Most domestic liquid flux solvents use industrial pure ethanol, and some are also matched with turpentine, isoamyl acetate or acetone. In foreign countries, isopropanol is often used as a solvent. Isopropanol is used as a solvent, which has good solubility and is not easy to produce precipitation; its boiling point (82.5 degrees Celsius) is higher than that of ethanol (about 78 degrees Celsius), so the solder flux using isopropanol as a solvent has a longer service life.

After understanding the principle of the flux, you can know the problems caused by the residue of the flux residue on the PCBA, such as easy to cause the adsorption and condensation of moisture and debris; it is also easy to be affected by vibration and friction during production and life cycle to produce grinding effect ; It may also cause poor contact in the ICT test and affect the accuracy of the test results; at the same time, it affects the effect of the profile coating by changing the adhesion of the PCBA.

Especially for emerging optical communication systems, flux residues affect the absorption and reflection of optical signals, which can easily cause signal changes and termination. Therefore, cleanliness and coaxial adjustment have become two major challenges for optical device assembly automation. What's more serious is that the flux residues will dissociate under certain conditions. These free ions will chemically react to the metal conductors on the PCBA, causing oxidation and corrosion of the metal, resulting in a decrease in the mechanical strength of the metal and even components. The breakage of the pins and leads will eventually affect the normal realization of the PCBA function.

Under the action of high temperature and electric field, more importantly, the flux residue may be rearranged, directly causing short circuit or leakage. For high-frequency and high-speed circuits, even if the circuit is normal, the decomposition of the solid polymer will lead to the generation of leakage current, the change of the dielectric constant and the loss coefficient and other undesirable phenomena, which will lead to the weakening of signal integrity and power loss. Occurs, eventually leading to the failure of the product.

For many years, the control of flux residue and its cleaning have become the research focus of the electronic assembly industry. Whether it is OEM or CEM, whether it is flux manufacturers or end-user customers, they are improving flux performance and expanding flux process parameters. A lot of research work has been done in the aspects of controlling the influence of residues. However, there has been a lack of systematic research on the analysis of influencing factors of wave soldering flux residues, especially the relationship between wave soldering process parameters and flux residues. Of course, for the use of wave soldering flux, we can also pay special attention to the following characteristics when controlling the process:

1. Thermal stability:

After the flux removes the oxide film on the surface of the PCBA substrate, a protective film must be formed before contacting the tin wave to prevent it from re-oxidizing and improve the heat transfer efficiency. Therefore, the flux must be able to withstand high temperatures and will not decompose, evaporate, or sublime before soldering; after soldering, some active components will decompose and evaporate, leaving harmless substances.

2. Wetting ability and diffusivity:

The wetting ability can ensure that the flux is isolated from air during soldering, reduces the surface tension of the solder, and increases the spreading ability.

Hilley wave soldering flux can be divided into the following types according to the composition and processing technology: rosin flux (HX-801A), synthetic rosin flux (HX-801B), water-soluble rosin flux (HX-801C), low Residual halogen-free no-clean flux (XH-801).

3. The chemical activity of the flux at different temperatures:

The function of the flux is to remove the oxide film and present a clean surface. However, the required activity is different at different temperatures. For example, at room temperature, the flux is required to be weakly active to avoid unnecessary corrosion. The required activity is only activated at the welding operating temperature.