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After the PCBA is cleaned, the board surface turns white, what should we do?

2023-02-09 17:51:24

After the circuit board is cleaned, the board surface turns white, what should we do?

Edited by Ming:

First, the board surface is whitish after cleaning:

In the electronic component manufacturing process, the PCBA circuit board is often over-wave soldered, and after the manual cleaning agent is used for cleaning, the board surface appears white (Figure 1).

Second, the reason for the whitish surface of the board after cleaning:

White residue is a common contaminant on PCBA and is generally a by-product of flux. Common white residues are polymerized rosin, unreacted activator, and lead metal chloride or bromide, which react with flux and solder. These substances expand in volume after moisture absorption, and some substances also hydrate with water. White residue is becoming more and more obvious. It is extremely difficult to remove these residues on the PCB. If the temperature is too hot or high, the problem is more serious. The infrared spectroscopy analysis of the rosin and residue on the PCB surface before and after the soldering process confirms the process. .

Regardless of whether the board has white residue after cleaning, or if the white material appears after the no-clean circuit board is stored, or the white substance on the solder joint found during the repair, there are four cases:

  1. Rosin in the flux: Most of the white matter produced after the cleaning is not clean, stored, and the solder joint fails, is the inherent rosin in the flux. Rosin is usually a transparent, hard and brittle solid material with no fixed shape, not a crystal. Rosin is thermodynamically unstable and has a tendency to crystallize. After the rosin crystallizes, the colorless transparent body becomes a white powder. If the cleaning is not clean, the white residue may be a crystalline powder formed by the rosin after the solvent is volatilized.

When the PCB is stored under high humidity conditions, when the absorbed moisture reaches a certain level, the rosin gradually changes from a colorless and transparent glass state to a crystalline state, and a white powder is formed from a viewing angle.

The essence is still rosin, but the shape is different, still has good insulation, and will not affect the performance of the board. The abietic acid and halide (if used) in the rosin are used together as an active agent. Synthetic resins generally do not react with metal oxides below 100 ° C, but react rapidly when the temperature is higher than 100 ° C. They volatilize and decompose faster, and have low solubility in water.

  1. Rosin denature: This is the substance produced by the reaction of rosin and flux during the welding process, and the solubility of this material is generally poor, it is not easy to be cleaned, and it stays on the board to form a white residue. But these white substances are all organic, still guarantee the reliability of the board.

  2. Organometallic salt: The principle of removing the oxide of the welding surface is that the organic acid reacts with the metal oxide to form a metal salt soluble in liquid rosin. After cooling, it forms a solid solution with the rosin, and is removed together with the rosin in the cleaning.

If the welding surface and parts are highly oxidized, the concentration of the product after welding will be high. When the degree of oxidation of the rosin is too high, it may remain on the board together with the undissolved rosin oxide. At this time, the reliability of the board will be reduced.

  1. Metallic inorganic salts: These may be metal oxides in solders and halogen-containing active agents in flux or solder paste, halide ions in PCB pads, halide ion residues in the surface coating of components, halogen-containing materials in FR4 materials. The substance formed by the reaction of the halide ion released at a high temperature generally has a small solubility in an organic solvent.

In the assembly process, it is highly probable that the halogen-containing flux is used for the electronic auxiliary materials (although the supplier provides environmentally friendly flux, but the halogen-free flux is still relatively small), and the surface of the board remains after welding. Halogen-based ions (F, Cl, Br, l). These ionic halogen residues, which are not themselves white, are not sufficient to cause whitening of the surface. These substances form strong acids when exposed to water or moisture. These strong acids begin to react with the oxide layer on the surface of the solder joints to form acid salts, which are white substances that are seen.

Third, after the circuit board is cleaned, the board surface is whitened:

1, the general solution:

  1. Washing method Note: When washing the PCB, the PCBA should be tilted. Do not lay it flat. You can place the paper in the washing station, so that most of the washed solution will flow down;

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