Talking about the Factors Affecting the Quality of SMT Reflow

Reflow soldering is one of the key processes of SMT, and the quality of surface assembly is directly reflected in the reflow results.

The following briefly describes the factors that affect the quality of reflow soldering.


The following factors affect the welding quality problems that occur in reflow soldering:

1. Reflow soldering process

2. Directly related to the welding temperature (temperature curve)

3. Production line equipment conditions, PCB pads and manufacturability design, component solderability, solder paste quality, printed circuit board processing quality, and process parameters of each process of SMT.

4. Operator's operation


The assembly quality of SMT patches has a direct and very important relationship with PCB pad design. If the PCB pad is designed correctly, a small amount of skew during placement can be corrected during reflow soldering due to the surface tension of the molten solder (called self-positioning or self-correcting effect). Conversely, if the PCB pad design is not correct, even if the SMT position is very accurate, there will be soldering defects such as component position shift and suspension bridge after reflow soldering.


1.The key elements of PCB pad design should be mastered:


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


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


(2) Pad distance—ensuring the overlap size of the component terminals or pins to the pad. Excessive or too small pad spacing can cause soldering defects.


(3) Remaining dimensions of the pad—The remaining dimensions of the component terminals or pins after bonding to the pads must ensure that the solder joints can form a meniscus.

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


2. Defects easily generated during the reflow process:


If the design requirements are violated, solder defects will occur during reflow and the 

PCB pad design problem is difficult or even impossible to be solved in the production process. Take a rectangular chip component as an example:


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


(2) When the pad size is asymmetrical, or when the ends of the two components are designed on the same pad, the surface tension is asymmetrical, and the suspension bridge and displacement are also generated.


(3) The via hole is designed on the pad, and the solder will flow out from the via hole, which may cause insufficient solder paste.