(1PCB pad design
SMT assembly quality is directly related to PCB pad design.If the PCB pad is properly designed, a small amount of skewing during mounting can be corrected for surface tension during reflow soldering (known as self-aligning or self-correcting effects).On the contrary, if the PCB pad is not designed correctly, even if the mounting position is very accurate, the element position deviation, suspension bridge and other welding defects will appear after flow welding.
According to the analysis of welding spot structure of various components and components, in order to meet the reliability requirements of welding spot, PCB welding pad design should master the following key elements:
1. Symmetry -- both ends of solder pads must be symmetrical to ensure the balance of surface tension of molten solder.
2. Pad spacing - ensure that the component ends or pins fit properly with the pad.Welding defects can be caused by too much or too little spacing between the pads.
3. Remaining dimensions of the pad -- the remaining dimensions after the end of the component or the pin overlaps with the pad must ensure that the welding spot can form a curved surface.
4. Width of pad -- it should be basically consistent with the width of component end or pin.
For example, the rectangular plate is shown in FIG. 5, which is a schematic diagram of the welded plate. If the design requirements are violated, welding defects will be generated during the re-flow welding, and the problem of PCB welded plate design is difficult or even impossible to be solved in the production process.Such as:
1. When the spacing between the pads is too large or too small, it will cause suspension bridge and shift when flow welding occurs because the component welding end cannot be overlapped with the pads.
2. When the size of the pad is asymmetric (see figure 7), or the end of two components are designed on the same pad (see figure 8), the suspension bridge and displacement will also occur due to the asymmetric surface tension.
3. The conducting hole is designed on the welding pad, and the solder will flow out of the conducting hole, resulting in insufficient solder paste
(2) the quality of solder paste and the correct use of solder paste
The content of metal powder in solder paste, the oxygen content, viscosity and thixotropy of metal powder are required.
If the metal powder content of solder paste is high, the metal powder will splash with the evaporation of solvent and gas when the reflow welding heats up. If the oxygen content of metal powder is high, the splash will be intensified, forming tin beads.In addition, if the paste viscosity is too low or the paste conformability (thixotropy) is not good, the paste graph will collapse after printing and even cause adhesion, and solder defects such as tin beads and Bridges will be formed when re-flow welding.
Solder paste improper use, such as solder paste removed from a cryogenic tank used directly, because the temperature of the solder paste is lower than at room temperature, produce water vapor condensation, the solder paste to absorb moisture in the air, moisture after stirring mixing in the solder paste, reflow soldering temperature, moisture evaporates out of the metal powder, water vapor at high temperature can make the metal oxide powder, splash form tin beads, will produce the problem such as poor wetting.
(3) the quality of welding pads of the welding ends and pins of components and printed circuit substrates
When the solder ends and pins of components, the solder plates of printed circuit substrates are oxidized or polluted, or the printed circuit boards are affected by damp, the solder defects such as poor wetting, false soldering, tin beads and cavities will be produced when re-flow soldering.