The Reasons Of White Residual On The PCB After Cleaning (2)
Ten reasons as follows:
1.6 Flux+ Halide surfactant residue
Halide surfactant among the flux used to improve the flux activity. However, in the welding process, and these active agent occur complex reactions with various components. The formation of the metal halide salts, usually tin, lead, copper chloride. If the board is not cleaned, cured rosin will firmly put these chlorides to wrap up. So that their activity is difficult to play. In addition to a certain extent, affect the appearance, they are mainly for highly insulating translucent material. It is not a big impact to the performance of the PCB. But once cleaned, after the rosin layer is damaged, it will show the performance of white plaques.
No matter alcohol solvent or water cleaning method,it can not be completely removed. And these chlorides which react further with air moisture and carbon dioxide to form carbonates, primarily white lead carbonate and lead hydroxide. This is a completely insoluble in water and alcohol, white substance. In order to avoid the generation of the white residue, using low activity flux. Or do not do any cleaning or do a thorough cleaning. The happening of the problem mainly caused due to incomplete cleaning.
1.7 Clean flux resulting white residues
Clean flux after soldering, there would have been a layer of transparent film wrapped the residue tightly, however, the cleaning destroy this layer of transparent material, so that the exposure of the residue appear in the form of undissolved white patches, the basic composition is similar with the residue composition .Solution as above, or do not do any cleaning or thoroughly clean.
1.8 Flux+Cleaning solvent residue solder
Fluorinated chlorinated solvents used to clean the soldering flux residue after welding. But much worse, these solvents is also the source of a variety of halide ions. These halide ions occur complex chemical reactions with various metal , the formation of insoluble compounds. The most common is still the lead chloride and lead carbonate. To avoid this problem occurs, it is best not using used and dirty cleaning solvents in the past. Because these used solvents, it is often a high concentration of chlorine ions. At the same time, we must avoid solvent to absorb moisture, so as not to produce insoluble carbonates.
1.9 laminates + flux residues
Generally used for the production of FR-4, epoxy glass fiber (epoxy board) is cured by a tetrabromobisphenol A and epichlorohydrin from heat, wherein bromine is added to it as a flame retardant. Due to various reasons, the epoxy resin may not be cured completely, and wherein the bromophenol occurs in the decomposition reaction when it is up to 135 degrees , the formation of free bromine ion.
Thus, the circuit board at soldering temperatures, uncured epoxy resin bromophenol rapid decomposition. The producing bromide ion will occur chemical reaction with the rich lead of the solder on the surface , a white lead bromide, insoluble to water and alcohol. And the white residue products often include chlorine ion flux chemical reaction with the solder. This white residue dissolved in dilute acid. Of course, in order to fundamentally solve the problem. Laminate manufacturer must control their process, to ensure complete curing of the epoxy resin.
1.10 Aqueous cleaning residual
Some white residual is unique in the water cleaning process. If the aqueous saponification agent is too concentrated or too strong activity in the water cleaning process, it will cause oxidation of the components on the circuit board solder surface. The solder surface is often almost completely covered in white tin oxide films. In the water cleaning process,if using the aluminum or zinc fixture (such as washing basket), or these fixtures are plated with a phosphating process, aluminum or zinc will first be oxidized to form a thin film of zinc oxide or aluminum oxide film. Or a mixture of zinc phosphate two films, these films will pollute solder surface of circuit board assembly.Aluminum jig will react saponification agent in the aqueous solution . Formed on the surface of aluminum white residue and will be transferred through the water to the surface of the circuit board assembly, causing pollution to the circuit board. And the water itself, if it is hard water, it will be rich in magnesium, iron or calcium. When the circuit board assembly process of drying after washing, after evaporation of water, the surface of the circuit board will be left a white residue. Normally, the water will be forced to blow away from the circuit board surface, in case the water-soluble white residue left on the board. However, this approach is often not so effective. Even soft water, after cleaning, they often have to stay in the white salt component surface.