Factors affecting the quality of SMT reflow soldering

Reflow soldering is one of the key processes of SMT, and the quality of surface assembly is directly reflected in the results of reflow soldering. Therefore, it is necessary to understand the factors affecting the quality of reflow soldering.

The welding quality problems in reflow soldering are not entirely caused by the reflow soldering process, because the reflow soldering quality is not only directly related to the welding temperature (temperature curve), but also closely related to the equipment conditions of the production line, PCB pad and producibility design, component solderability, solder paste quality, printed circuit board processing quality, as well as the process parameters of each process of SMT, and even the operation of operators.

The assembly quality of SMT patch has a direct and very important relationship with PCB pad design. If the PCB pad is designed correctly, a small amount of skew during mounting can be corrected due to the surface tension of molten solder during reflow soldering (called self positioning or self correction effect). On the contrary, if the PCB pad design is incorrect, even if the mounting position is very accurate, there will be welding defects such as component position offset and suspension bridge after reflow welding.

1. Key elements for PCB pad design:

According to the structural analysis of solder joints of various components, in order to meet the reliability requirements of solder joints, PCB pad design should master the following key elements:

(1) Symmetry - the pads at both ends must be symmetrical to ensure the balance of surface tension of molten solder.

(2) Pad spacing - ensure the overlap size between the component end or pin and the pad. Too large or too small pad spacing will cause welding defects.

(3) Residual pad size - the residual size after the component end or pin is overlapped with the pad must ensure that the solder joint can form a meniscus.

(4) Pad width - should be basically the same as the width of the component end or pin.

2. Defects easily produced during reflow welding:

If the design requirements are violated, there will be welding defects during reflow soldering, and the problem of PCB pad design is difficult or even impossible to solve in the production process. Take rectangular chip components as an example:

(1) When the pad spacing G is too large or too small, the suspension bridge and displacement will occur because the component welding end cannot overlap with the pad during reflow welding.

(2) When the size of the pad is asymmetric, or the ends of two components are designed on the same pad, the suspension bridge and displacement will also occur due to the asymmetric surface tension.

(3) The through hole is designed on the pad, and the solder will flow out of the through hole, resulting in insufficient solder paste.