Take the widely used no-clean flux as an example, its activation composition is mainly fat dibasic acid and carboxylic acid derivatives. Factors affecting the acidity of carboxylic acid are molecular structure, solvent and temperature.
I. The formation process of flux residue
1. Molecular structure
Binary carboxylic acid molecule has two carboxyl groups (-COOH), carboxylic acid is an electron-absorbing group, has a strong electron-absorbing induced effect, so that it can be ionized out of two H +. The hydroxy acids in substituted carboxylic acids, on the other hand, exhibit the dual nature of carboxyl and carboxyl groups because they contain both hydroxyl (-OH) and carboxyl functional groups in their molecules. Compared to the carboxyl group, the hydroxyl group is a stronger electron-withdrawing group, which increases the degree of dissociation of the carboxyl group, so that the hydroxy acid is more acidic than the parent carboxylic acid, and thus the fluxing performance will be greatly improved.
2. Solvent
At room temperature the binary carboxylic acid mainly in the form of molecular H2R in the solvent, acidity is weak, less corrosive, but in the welding temperature, with the continuous evaporation of the solvent, the concentration of its flux becomes larger, a large number of H + in the acid is ionized, the acid becomes stronger, at this time, H + and the solder and the PCB metal surface of the oxidized film reaction, the formation of organic acid salts.
3. Temperature
In the soldering process, the activity of the flux not only depends on the analytical structure of the activator itself, but also has a close relationship with the boiling point and thermal stability of the activator.
A. Decomposition temperature
Flux in the activator is usually composed of decomposition temperature of a variety of different acid composite activator, so as to ensure that the flux at different temperatures under the activity.
Decomposition temperature of the activator can make the post-solder residue less, less corrosion. The high decomposition temperature of the activator if the preheating, soldering process is not sufficient heat will be difficult to completely decompose, so there will be residues left on the PCBA.
B. Solvent boiling point
Flux in the solvent is usually composed of a variety of different boiling points of alcohols and ethers, but the high boiling point of the co-solvent content can not be too much, otherwise it will cause the solvent evaporation rate slows down in the PCB through the preheating area, there will still be a large number of high-boiling point co-solvents remain in the PCB, and then into the soldering area, the same will be difficult to decompose some of them to stay in the PCBA as a residue.
II. Effectively reduce the residue measures and removal methods
1. Select the ideal flux
Ideal flux should have high activity, low corrosion, but the two are opposing indicators, often many fluxes in the pursuit of high activity while ignoring its corrosiveness. Therefore, in the face of many flux, it is necessary to carry out the actual welding process test to select a good performance, high reliability of the flux.
2. Do a good job of welding process control
Under the premise of ensuring the quality of welding, the welding process should be appropriate to increase the preheating temperature and welding temperature, to ensure that the necessary welding time, so that the active agent in the flux and the solvent as much as possible with the high temperature decomposition or volatilization, to reduce the post-weld residue.
3. Timely use of cleaning processes
For high reliability requirements of electronic products, after welding must undergo a rigorous cleaning process. In order to reduce the difficulty of cleaning, in the PCB after the completion of welding should be as soon as possible to enter the cleaning process, in the cleaning both for non-polar residues and polar residues, so you need to use polar and non-polar solvent mixture to clean in order to effectively remove the residue. Of course, the choice of those environmentally friendly cleaning agent is also an aspect to be considered.
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