1. High-reliability products require high-quality solder paste, depending on the product's value and use.
2. The alloy composition of the solder paste is selected according to the assembly process of the product, the printed board and the components.
(1) Commonly used solder paste alloy components: Sn63Pb37 and Sn62Pb36Ag2.
(2) Components with poor solderability of gold or silver thick film terminals and pins should select silver-containing solder paste.
(3) Do not choose silver-containing solder paste for the water-gold board.
3. Solder paste is selected according to the cleanliness requirements of the product (printed board) and the different cleaning processes after soldering.
(1) When using the no-clean process, use a no-clean solder paste that does not contain halogens and strong corrosive compounds.
(2) When using a solvent cleaning process, a solvent cleaning type solder paste should be used.
(3) When using water cleaning process, water-soluble solder paste should be used.
(4) BGA and CSP generally need to use high-quality no-clean silver-containing solder paste.
4. The activity of the solder paste is selected according to the PCB and component storage time and the degree of surface oxidation.
(1) KJRMA grade is generally adopted.
(2) R-level can be selected for high-reliability products, aerospace and military products.
(3) PCBs and components are stored for a long time, and the surface is seriously oxidized, so RA grade should be used and cleaned after welding.
5. According to the assembly density of the PCB (with or without narrow spacing), the particle size of the alloy powder is selected. The alloy powder particle size of the commonly used solder paste is divided into four particle size grades. When the spacing is narrow, 20-45pm is generally selected.
6. The viscosity of the solder paste is selected according to the process of applying the solder paste and the assembly density. High-density printing requires high viscosity, and dispensing requires low viscosity.
Management and use of solder paste
1. Must be stored at 5-10°C.
2. It is required to take out the solder paste from the refrigerator one day before use (at least 2 hours in advance), and open the container lid after the solder paste reaches room temperature to prevent condensation. (15 minutes to room temperature when using a solder paste mixer).
3. Stir the solder paste well with a stainless steel stir bar before use.
4. No-clean solder paste should not be recycled. If the printing interval exceeds 1 hour, the solder paste must be wiped off the stencil and stored in the container used on the same day. .
5. Try to complete reflow soldering within 4 hours after printing.
6. Do not scrub with alcohol after no-clean solder paste repair.
7. Products that need to be cleaned should be cleaned after reflow soldering.
8. When printing solder paste and patch glue, it is required to hold the edge of the PCB or wear a finger cot to prevent contamination of the PCB.