Date：2019-09-23 Categories：Industry News Hits：403
In the electronic process, PCBA (circuit board or circuit board) often turns white after cleaning, and the white mark is scattered around the solder joint, which seriously affects the appearance acceptance and brings quality hidden trouble.
White residue risk factor: when considering whether the white residue will produce reliability risk, the key is to consider whether the residue is hygroscopic and ionized, and whether there will be potential corrosion in the presence of moisture and bias. White residues tend to be hygroscopic and conductive, which can potentially cause current leakage and stray voltage failure in sensitive circuits. Flux active substances, if they do not lose activity in the white residue and remain in the white residue, if there is moisture, they will separate and cause electrochemical migration.
The formation mechanism of white residue is as follows:Thermal oxidation: Rosin may undergo thermal oxidation when the temperature exceeds 200 ℃. Thermal oxidation of rosin reduces the unsaturated double bond of rosin acid. The decrease of unsaturated double bond leads to the formation of ethylene glycol, ketone and ester with different molecular weight. These residues gradually disappear on the surface and oxidize into the sticky white residue. The residue of coke is distributed around the flux and on the solder bumps. The flux film at both locations is thinner and easier to oxidize and zoom. Oxidation is common in the part of the veneer that absorbs the most heat. Multilayer boards with ground surfaces absorb heat away from circuit components, so a higher reflow temperature curve is required. Similar results are found in the solder face array components and wafer capacitors. Because the hot spot scorches the flux residue, the residue under these small components tends to oxidize in an irregular shape.
2. Polymerization: when the temperature exceeds 200 ℃, it will lead to the polymerization of rosin and resin structure. The polymerization is the result of heating. Metal salts act as catalysts to increase the rate of chemical reaction and form a three-dimensional network of polymer chains. Chain growing compounds connect double bonds and are added to the resin compound to form a repeating chain.
3. Solder mask absorption with low residue and no cleaning flux: when dry film solder mask and low residual flux are used, moisture absorption is very affected. Wave soldering flux and heat will decompose and expand the dry mask. This may be due to the adhesive curing and final curing during the veneer manufacturing. When the veneer passes through the preheating zone and solder crest, the pores on the dry film open and expand. The volatile solvent in the low residual flux is absorbed into the solder mask. The mask formed a white residue after wave soldering on the veneer surface. The white turbidity spots are removed by setting the temperature of hot air repair tool at 400 ℃ (752f). Temperature will activate the low residual flux and remove the white film.