Key words: circuit board cleaning, water-based cleaning, thermal oxidation, rosin, PCB component board
In the electronic process technology, PCBA (circuit board or circuit board) often whitish after cleaning, and the white mark is scattered around the solder joints, which seriously affects the appearance acceptance and brings quality hazards.
White Residue Risk Factor: When considering whether white residue poses a reliability risk, the key is to consider whether the residue is hygroscopic, ionized, and whether there is potential corrosion in the presence of moisture and bias. White residues tend to absorb moisture and conduct electricity, which can cause current leakage and stray voltage failure on sensitive circuits. Flux actives, if they are not deactivated in the white residue and are always present in white residue, they will separate if present in the presence of moisture, resulting in electrochemical migration.
The mechanism leading to the formation of white residues has the following factors:
1. Thermal oxidation: Rosin may undergo thermal oxidation at temperatures above 200 °C. Thermal oxidation of rosin reduces the unsaturated double bonds of rosin acid. The reduction of unsaturated double bonds results in the formation of ethylene glycol, ketones and esters of different molecular weights. These residues gradually disappear on the surface and oxidize into the sticky white residue. The residue of the coke is distributed around the flux and also spread over the solder bumps. The flux film at both locations is thinner and more susceptible to oxidation and zooming. Oxidation is common in the portion of the veneer that absorbs the most heat. Multilayer boards with ground planes absorb heat away from the circuit components and therefore require a higher reflow temperature profile. Similar results occur in the soldered surface array component and the wafer capacitance s. Residues underneath these small components tend to oxidize in the form of irregular shapes due to the scorch of flux residues in the hot spots.
2. Polymerization: When the temperature exceeds 200 ° C, it will cause polymerization of rosin and resin structure. The occurrence of polymerization is the result of heating, and the metal salt acts as a catalyst, increasing the rate of chemical reactions and forming a polymer chain of three-dimensional networks. The chain-growth compound is linked to a double bond and added to the resin compound to form a repeating chain.
3. Solder mask absorption using low-residue no-clean flux: When dry film solder mask and low residual flux are used, moisture absorption is very influential. The wave solder flux and heat are broken down and the dry film mask is expanded. This may be due to viscous curing and final curing during veneer manufacture. When the veneer passes through the preheating zone and the solder peak, the pores on the dry film expand and expand. The volatile solvent in the low residual flux is absorbed into the solder mask. After the surface of the veneer is wave soldered, the mask forms a white residue. The white turbid spots were removed by setting the temperature of the hot air rework tool at 400 ° C (752 F). The temperature activates the low residual flux and removes the white film.