Basic Knowledge of SMT, a Very Practical Surface Mount Technology of Electronic Components

The full name of SMT is Surface mount (or mounting) technology. SMT is the abbreviation of Surface Mount Technology, also known as surface mounting or surface installation technology. It is currently the most popular technology and process in the electronic assembly industry. Generally speaking, the specified temperature in the SMT workshop is 25±3℃.

SMT Mounting Machine

Solder Paste: When printing solder paste, the materials and tools required for preparation are solder paste, stencil, scraper, wiping paper, clean paper, cleaning agent, and stirring knife. The components of solder paste include: metal powder, solvent, flux, anti-sag agent, and activator. Its main components are tin powder and flux, with a volume ratio of approximately and a weight ratio of approximately 9:1. The flux starts to volatilize in the constant temperature zone and performs chemical cleaning operations; the main functions of the flux in soldering are to remove oxides, break the surface tension of molten tin, and prevent reoxidation. Fluxes based on rosin can be divided into four types: R, RA, RSA, and RMA; the ratio of Sn/Pb in leaded solder is 63/37, and the melting point is 217℃. The ratio of Sn/Ag/Cu in lead-free solder is 96.5/3.0/0.5, and the melting point is also 217℃.
When the solder paste is taken out of the refrigerator and used after opening, it must go through two important processes: temperature recovery and stirring. The purpose of temperature recovery is to bring the temperature of the refrigerated solder paste back to room temperature for easy printing. If the temperature recovery is not carried out, tin beads are likely to be generated after the PCB enters the reflow soldering process. The principle of using solder paste is first in, first out. Currently, the solder paste sold on the market actually only has a tacky time of 4 hours.

SMT Production Workshop

Stencil and PCB: Common methods for making stencils are: etching, laser, and electroforming. The commonly used material for SMT stencils is stainless steel, with a thickness of 0.15mm (or 0.12mm). Generally, the openings of SMT stencils should be 4um smaller than the PCB pads to prevent the occurrence of tin ball defects. The commonly used PCB material is FR-4; the purpose of the vacuum packaging of PCB is to prevent dust and moisture. The warpage specification of the PCB should not exceed 0.7% of its diagonal length.
Components: Commonly used passive components include resistors, capacitors, inductors, diodes, etc., and these components are picked up by the nozzles of the mounting machine, simply referred to as nozzles. Active components include transistors, ICs, etc. The controlled relative temperature and humidity of the component drying oven is <10%. When the humidity on the humidity display card is greater than 30% after unpacking the IC, it indicates that the IC is affected by moisture and has absorbed moisture.

SMT High-Speed Mounting Machine

Heating Furnace: The most suitable maximum temperature of the reflow oven temperature curve is 215℃. When inspecting the soldering furnace, the temperature of the soldering furnace is more suitable at 245℃. The RSS curve is a curve of heating up → constant temperature → reflow → cooling; the ideal cooling zone curve has a mirror image relationship with the reflow zone curve.
SMT Solder Paste Adhesive: SMT solder paste adhesive is a kind of polyolefin compound. Different from solder paste, it solidifies when heated, and its solidification point temperature is 150℃. At this time, the solder paste adhesive directly changes from a paste state to a solid state. SMT solder paste adhesive has properties such as viscosity and fluidity, temperature characteristics, and wetting characteristics. According to this characteristic of the solder paste adhesive, in production, the purpose of using the solder paste adhesive is to firmly paste the components on the surface of the PCB to prevent them from falling off.
Usage on the Printer or Dispenser: To maintain the quality of the SMT adhesive, please store it in the refrigerator at a cold temperature (5±3℃); before taking it out of the refrigerator for use, let it recover to room temperature for 2 to 3 hours; you can use toluene or ethyl acetate to clean the glue tube.

Component Management in the SMT Workshop

Dispensing and Glue Scraping: Adding a back plug to the dispensing tube can obtain a more stable dispensing amount; the recommended dispensing temperature is 30-35℃; when dividing the dispensing tube, please use a special glue dispenser for division to prevent air bubbles from being mixed into the glue. The recommended glue scraping temperature is 30-35℃. Notes: After the solder paste adhesive is taken out of the refrigerated environment, it cannot be opened and used before reaching room temperature. To avoid contaminating the original products, do not pour any used SMT adhesive back into the original packaging.
Characteristics of SMT: High assembly density, small size and light weight of electronic products. The volume and weight of surface-mounted components are only 1/10 of those of traditional through-hole components. And with the progress of science and technology, the components will become smaller and smaller. Generally, after adopting SMT, the volume of electronic products is greatly reduced, and the weight is reduced by more than 60% to 80%. High reliability and strong seismic resistance. Low defect rate of solder joints. Good high-frequency characteristics. Reduced electromagnetic and radio frequency interference. Easy to achieve automation and improve production efficiency. Cost is reduced by 30% to 50%. Saves materials, energy, equipment, manpower, time, etc.