First, the characteristics of SMT.
The assembly density is high, the size of the electronic product is small, and the weight is light. The volume and weight of the chip components are only about 1/10 of that of the conventional plug-in components. After the SMT is generally used, the volume of the electronic product is reduced by 40% to 60%, and the weight is reduced by 60%. 80%.
High reliability and strong anti-vibration ability. The solder joint defect rate is low.
High frequency characteristics are good. Reduced electromagnetic and radio frequency interference.
Easy to automate and increase productivity. Reduce costs by 30% to 50%. Save materials, energy, equipment, manpower, time, etc.
Second, why use surface mount technology (SMT)?
Electronic products are pursuing miniaturization, and the previously used perforated plug-in components cannot be reduced.
Electronic products are more complete, and the integrated circuits (ICs) used have no perforated components, especially large-scale, highly integrated ICs, which have to use surface mount components.
Product batching, production automation, the factory must produce high-quality products at low cost and high yield to meet customer needs and strengthen market competitiveness
Development of electronic components, development of integrated circuits (ICs), multiple applications of semiconductor materials.
The revolution of electronic technology is imperative, chasing the international trend.
Why use the no-clean process in surface mount technology?
The waste water discharged after the product is cleaned during the production process brings pollution of water quality, the earth and even animals and plants.
In addition to water cleaning, the organic solvent (CFC & HCFC) containing chlorofluorohydrogen is used for cleaning, which also pollutes and destroys air and atmosphere.
The residue of the cleaning agent on the board brings corrosion and seriously affects the quality of the product.
Reduce cleaning process operation and machine maintenance costs.
No-cleaning reduces the damage that the board (PCBA) can cause during movement and cleaning. Some components are still unclean.
The flux residue is controlled and can be used in conjunction with the appearance of the product to avoid visual inspection of the cleaning condition.
Residual flux has continually improved its electrical performance to avoid leakage of the finished product and any damage.
The no-clean process has passed many international safety tests to prove that the chemicals in the flux are stable and non-corrosive.
Fourth, reflow soldering defect analysis:
Solder Balls: Reasons: 1. The screen printing holes are not aligned with the pads, and the printing is not accurate, so that the solder paste stains the PCB. 2. The solder paste is exposed too much in the oxidizing environment and the moisture in the air is too much. 3. The heating is not accurate, too slow and not uniform. 4. The heating rate is too fast and the preheating interval is too long. 5. The solder paste dries too fast. 6. Flux activity is not enough. 7. Too much small tin powder. 8. Flux volatility is not appropriate during the reflow process. The process qualification standard for solder balls is: when the distance between the pads or the printed wires is 0.13 mm, the diameter of the solder balls cannot exceed 0.13 mm, or more than five tin beads cannot appear in the range of 600 mm square.
Bridging: Generally speaking, the cause of tin bridge is that the solder paste is too thin, including low content of metal or solid in solder paste, low solubility, easy to squeeze solder paste, too large solder paste particles, flux The surface tension is too small. Too much solder paste on the pad, the peak temperature of the reflow is too high.
Open: Reason: 1. The amount of solder paste is not enough. 2. The coplanarity of the component pins is not enough. 3, tin is not wet enough (not enough to melt, poor fluidity), tin paste is too thin to cause tin loss. 4, the pin is sucking tin (like the wick grass) or there is a connecting hole nearby. The coplanarity of the pins is especially important for dense pitch and ultra-dense pitch pin components. One solution is to pre-solder the pads. The pin suction can be prevented by slowing down the heating rate and heating the bottom surface less and heating less. It is also possible to use a flux that has a slower wetting rate, a higher activation temperature, or a different ratio of Sn/Pb to block the molten solder paste to reduce pin suction.