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Talking About The Relationship Between Assembly Quality Of SMT And PCB Pad Design First, PCB Pad Design
Jun 07, 2018

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. Pad Spacing—Make sure the proper size of the component header or pin and pad. Too large or too small pad spacing can cause soldering defects.

 

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

 

3. Pad Width - Should be substantially the same as the width of the component header or pin.

 

Second, the quality of solder paste and the correct use of solder paste

 

The metal powder content of the solder paste, the oxygen content of the metal powder, the viscosity, and the thixotropy of the solder paste have certain requirements.

 

If the content of the metal powder in the solder paste is high, the metal powder will splash with the evaporation of solvent and gas when the temperature is increased by reflow. If the metal powder has high oxygen content, it will also aggravate the splash and form the solder ball. In addition, if the viscosity of the solder paste is too low or the shape retention (thixotropy) of the solder paste is not good, the solder paste pattern may collapse after printing and even cause sticking. Solder defects such as solder balls and bridges may also be formed during reflow.

 

Improper use of solder paste, such as direct removal of solder paste from low temperature cabinets, due to lower solder paste temperature than room temperature, resulting in condensation of water vapour, that is, the solder paste absorbs moisture from the air, mixes the water vapour in the solder paste after stirring, and reflow heats up. At that time, water vapor evaporates and brings out the metal powder. At high temperatures, water vapor will oxidize the metal powder, splashing and forming tin beads, and problems such as poor wetting.

 

4. Residual pad size - The remaining dimensions after the component header or pin overlaps the pad must ensure that the pad can form a meniscus.

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