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Quality Influencing Factors Of SMT Reflow Soldering?
Mar 16, 2018

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


   The welding quality problems that occur in reflow soldering are not completely caused by the reflow process because the reflow soldering quality is not only directly related to the soldering temperature (temperature profile), but also to the production line equipment conditions, PCB pads, and producibility design, and the yuan. The solderability of the device, the quality of the solder paste, the processing quality of the printed circuit board, and the process parameters of each process of SMT are even closely related to the operation of the operator.


The assembly quality of the SMT patch has a direct and very important relationship with the PCB pad design. If the PCB pad design is correct, a small amount of skew during placement can be corrected during solder reflow due to the surface tension of molten solder (known as self-aligning or self-correcting effects). On the contrary, if the PCB pad design is not correct, even if the placement position is very accurate, after the reflow, there will be component misalignment, suspension bridges, and other welding defects.


1, PCB pad design should grasp the key elements:


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


(1) Symmetry - Both ends of the pad must be symmetrical to ensure that the molten solder surface tension balance.


(2) Pad Spacing—Make sure that the size of the component's tip or pin is the same as that of the pad's paddle. Too large or too small pad spacing can cause soldering defects.


(3) Residual pad size - The remaining dimensions after the component ends or pins are lapped with the pad must ensure that the pad can form a meniscus.


(4) Pad Width - Should be substantially the same as the width of the element tip or pin.


2, the reflow process easy to produce defects:


If the design requirements are violated, soldering defects will occur during reflow and PCB pad design issues will be difficult or even impossible to resolve in the production process. Take rectangular chip components as an example:


(1) When the pad pitch G is too large or too small, the soldering end of the component cannot overlap with the pad during reflow, resulting in a suspension bridge and displacement.


(2) When the pad size is asymmetrical, or the tip of the two components is designed on the same pad, the surface tension is asymmetric, and suspension bridges and shifts are also generated.


 (3) The via hole is designed on the pad. Solder will flow out from the via hole, which will result in insufficient solder paste.