PCB Fabrication

Basic knowledge of flux products that novices must know

by:A-TECH      2021-03-22
At present, flux has been widely used, so I will introduce the basic knowledge of flux for everyone here, hoping to be helpful to novices. 1. Requirements for fluxes for surface mounting have a certain chemical activity, good thermal stability, good wettability, promote the expansion of the solder, and the flux residue remaining on the substrate, which is non-corrosive to the substrate and has a good cleaning The content of natural chlorine is less than 0.2% (W/W). 2. The role of flux Welding process: preheating/solder starts to melt/solder alloy formation/solder joint formation/solder solidification effect: auxiliary heat transfer/removal of oxidation Description: The solvent evaporates/heats, and the flux covers the surface of the substrate and solder, so that the heat transfer is uniform/releases the activator to react with the ionic oxide on the surface of the substrate, and remove the oxide film/make The surface tension of the molten solder is small, the wetting is good/covering the surface of the high temperature solder, and the oxidation is controlled to improve the quality of the solder joint. 3. The physical properties of the flux The physical properties of the flux mainly refer to the melting point, boiling point, and softening point related to the soldering performance. , Glass transition temperature, vapor pressure, surface tension, viscosity, mixing, etc. 4. Defects and countermeasures of flux residues. Problems caused by flux residues: Corrosive to the substrate, reducing electrical conductivity, migration or short-circuiting Non-conductive solid objects such as intrusion into the contact parts of the components will cause excessive resin residues in poor bonding, adhesion dust and debris, and affect the reliability of the product. Reasons for use and countermeasures: Use a suitable flux with a moderate activator activity. The flux that forms the protective film is to use a flux that has no resin residue after soldering. Use a low-solid content no-clean flux to clean after soldering. 5. QQ-S-571E specifies the flux classification code code. Flux type S Moderate solid (no flux) R Rosin Flux RMA Weakly active rosin flux RA Active rosin or resin flux AC No rosin or resin flux American synthetic resin flux classification: SR inactive synthetic resin, rosin-like SMAR moderately active synthetic resin, rosin-like SAR active synthetic resin, rosin SSAR-like extremely active synthetic resin, rosin category 6. Flux spraying methods and process factors spraying methods are as follows: 1. Ultrasonic spraying: The oscillating electric energy with a frequency greater than 20KHz is converted into mechanical energy through a piezoelectric ceramic transducer, and the flux is converted into mechanical energy. Atomized and sprayed onto the PCB through a pressure nozzle. 2. Screen sealing method: The flux is sprayed out by a drum rotating air knife with a fine, high-density small-hole screen, and the resulting spray is sprayed onto the PCB. 3. Pressure nozzle Spraying: Directly use pressure and air with flux to spray from the nozzle. Spraying process factors: set the nozzle diameter, beacon volume, shape, nozzle spacing, avoid overlapping and affect the uniformity of spraying. Set the ultrasonic atomizer voltage to get normal The amount of atomization of the nozzle. The choice of the nozzle movement speed. The setting of the PCB conveyor speed. The solid content of the flux should be set steadily. The corresponding spraying width should be set steadily. The main characteristics of the no-clean flux are good solderability, full solder joints, no solder beads, Defects such as bridging are non-toxic and do not pollute the environment, and the operation is safe and the board surface is dry and non-corrosive after welding. Non-stick board, online test capability after welding, matching with SMD and PCB board, surface insulation resistance (SIR) after welding, suitable for welding process (dip soldering, foaming, spraying, coating, etc.)
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