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The solder paste volume will make the difference betweena conforming and a non-conforming solder joint Written by HC AUTOMATOIN / Read 293 times

  The solder paste volume will make the difference betweena conforming and a non-conforming solder joint. Insufficient solder and excesssolder are just the 2 extremes of the defects than can have the paste volume asthe root cause. And of course these 2 non-conformities can be caused by solderpaste alignment. Another common defect, the solder bridge, can be caused by thevolume of paste, the paste alignment and upstream in the process (solder pasteviscosity and stencil exposure) and downstream in the process on the pick andplace equipment and reflow. Some of the deviations in the paste deposition (volume orX, Y alignment) will be, to some extent, "compensated" later depending on thesolder paste type and the reflow profile settings that will determine how muchthe surface tension in liquid stage will "pull the solder towards the hotmetallic surfaces (pads and component leads). However, it is not recommended tobet on that, especially with the component dimensions that we have to deal withnow and in the future. Most of the printers have fiducial mark recognition asthe way to align the PCB with the stencil – the higher acceptance criterion forthe fiducial mark, the higher paste deposition accuracy you will get. So,again, it is critical to have a close to perfect print alignment combined witha consistent solder paste release through the stencil. For a consistent solder paste volume, there is a list ofparameters to look into:

Stencilthickness and material

· Aperturedesign/opening in combination with the paste type for the application (type 3 ,4 , etc.). The type of paste to use isusually dictated by the smallest aperture / smallest pad on the PCB

· Stencilcleaning cycles and method (dry, wet, vacuum or combination of the 3)

· Stencilenhancements (e.g. nano-coating)

· PCBflatness and machine supports

· Bladetype, size, angle, pressure

· Numberof strokes

· Separationspeed