An Analysis of SMT Solder Paste Printing Defects

In SMT PCB production, solder paste printing is a critical step. Since the solder paste is used to directly form the soldering joint, the quality of solder paste printing affects the performance and reliability of the surface mount assembly. Quality solder paste printing guarantees a quality solder joint and final product. Statistics demonstrate that 60% to 90% of soldering defects are related to solder paste printing defects. So it is very important to understand what causes defects in solder paste printing. 

The following table lists an analysis of solder paste printing defects:

Item	Factors		Analysis
1	Solder Paste	Powder formation	The irregular shape of solder powder will easily clog stencil apertures. This will cause a big slump after printing. It can also cause solder ball and short bridge defects after reflow. A spherical shape is best, especially for fine-pitch QFP printing.
		Particle Size	If the particle size is too small, the results will be poor paste adhesion. It will have a high oxygen content and cause a solder ball after reflow. The particle size should be controlled to about 25 ~ 45 ?m in order to meet the requirements for fine-pitch QFP soldering, If the partical size desired is 25 to 30 ?m, it should applied with less than 20 ?m solder paste for an ultra fine-pitch IC.
		Flux	Flux contains a thixotropic agent, which allows the solder paste to have pseudoplastic flow characteristics. Since the viscosity decreases when the paste passes through the stencil apertures, the paste can be applied to the PCB pads rapidly. When the external force stops, the viscosity will recover to ensure no deformation occurs. The flux in the solder paste should be controlled to between 8 and 15 percent. A lower flux content will result in excess amount of solder paste applied. Conversely, a high flux content will result in an insufficient amount of solder applied.
2	Stencil	Thickness	A stencil that is too thick will cause a solder bridge short. A stencil that is too thin will cause an insufficient solder to be applied.
		Aperture size	When the stencil aperature size is too big, a solder bridge short can occur. When the stencil aperature size is too small, and insufficient solder paste will be applied.
		Aperture shape	It is best to use a circular-shaped stencil aperture design. Its size should be slightly smaller than the PCB pad size, preventing a bridging defect during reflow.
3	Printing parameters	Blade Angle Speed & Pressure	The blade angle affects the vertical force applied on the solder paste. If the angle is too small, the solder paste will not be squeezed into the stencil apertures. The best blade angle should be set around 45 to 60 degrees. A higher the printing speed means that less time will be spent in applying the solder paste through the stencil aperture surface. A higher printing speed will cause insufficient solder to be applied. The speed should be controlled to around 20 ~ 40 mm/s. When the blade pressure is too small, it will prevent the solder paste from being cleanly applied to the stencil. When the blade pressure is too high, it will result in more paste leakage. The blade pressure is typically set at about 5N ~ 15N / 25mm.
4	Printing process control	PCB moisture	If the PCB moisture is too high, the water under the solder paste will quickly evaporate, causing the solder to splash and creating solder balls. Dry the PCB if it was fabricated over 6 months ago. The recommended drying temperature is 125 degrees for 4 hours.
		Paste storage	If the solder paste is applied without a temperature recovery period, the water vapour in the surrounding environment will condense and penetrate the solder paste; this will cause the solder to splash. Solder paste should be stored in a refrigerator at 0 to 5 degrees. Two to fours hours before use, place the paste in a normal temperature environment.

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