Solder paste is an indispensable consumable material in SMT surface mount assembly. In the following sections, we will discuss the importance of solder paste in SMT surface mount assembly from three aspects: solder paste selection, proper use and storage of solder paste, and inspection.
1. Selection of Solder Paste
There are numerous types and specifications of solder paste, and even products from the same manufacturer may differ in alloy composition, particle size, viscosity, and other aspects. Selecting the appropriate solder paste for your product significantly impacts both product quality and cost.
2. Proper Use and Storage of Solder Paste
Solder paste is a thixotropic fluid. The printing performance of solder paste and the quality of solder paste patterns are closely related to its viscosity and thixotropic properties. The viscosity of solder paste is not only influenced by the percentage composition of the alloy, the particle size of the alloy powder, and the shape of the particles, but also by temperature. Changes in environmental temperature can cause fluctuations in viscosity. Therefore, it is best to control the environmental temperature at 23°C ± 3°C. Since solder paste printing is mostly done in the air, environmental humidity also affects solder paste quality. Generally, relative humidity should be controlled between 45% and 70%. Additionally, the solder paste printing work area should be kept clean, dust-free, and free of corrosive gases.
Currently, the density of PCBA processing and assembly is increasing, and the difficulty of printing is also increasing. It is essential to use and store solder paste correctly, with the following requirements:
1). It must be stored at a temperature of 2–10°C.
2). The solder paste must be removed from the refrigerator the day before use (at least 4 hours in advance) and allowed to reach room temperature before opening the container lid to prevent condensation.
3). Before use, mix the solder paste thoroughly using a stainless steel stirrer or an automatic mixer. When mixing by hand, stir in one direction. The mixing time for both manual and machine mixing should be 3–5 minutes.
4). After adding the solder paste, ensure the container lid is securely closed.
5). No-clean solder paste must not use recycled solder paste. If the printing interval exceeds 1 hour, the solder paste must be wiped off the stencil and returned to the container used that day.
6). Reflow soldering must be performed within 4 hours after printing.
7). When repairing boards using no-clean solder paste, if no flux is used, do not clean the solder joints with alcohol. However, if flux is used during repair, any residual flux outside the solder joints that has not been heated must be immediately wiped off, as unheated flux is corrosive.
8). For products requiring cleaning, cleaning must be completed on the same day after reflow soldering.
9). When printing solder paste and performing surface mount operations, hold the PCB by its edges or wear gloves to prevent contamination of the PCB.
3. Inspection
Since printing solder paste is a key process in ensuring SMT assembly quality, the quality of printed solder paste must be strictly controlled. Inspection methods mainly include visual inspection and SPI inspection. Visual inspection is performed using a 2-5x magnifying glass or a 3.5-20x microscope, while narrow spacing is inspected using SPI (solder paste inspection machine). Inspection standards are implemented in accordance with IPC standards.
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