Stencil defect

Assembly requirements also place constraints on the circuit board layout. A very high part density requires a large number of apertures in the solder paste stencil, which can cause the stencil to become locally too flimsy to control the solder paste deposit. A surface-mount circuit board with a very wide range of component sizes and package configurations may require multi-thickness stencils to properly control the paste deposit. Solder paste printing quality is a determining factor in solder-joint defects observed after the reflow process. 

By and large, the greatest mrniber of solder-joint defects can be traced to the solderdispensing process and, specifically, the stencil or screen printing step. Therefore, process control is critical here. An absence of the control of any one of these attributes can significantly impact process yields. Important factors include the following ... 

In the solder paste printing process, defects typically are caused by poor alignment between the substrate and stencil, incorrect material selection (substrate, paste type and stencil design), or variations in the amount of paste deposited. Defect elimination relies on the engineer and operator to address these variables and monitor the process. 

Mechanical Stability. The printer s ability to provide accuracy and repeatability requires a robust design. Structurally, the machine must be rigid enough to prevent relative motion between any axis and the rest of the machine. This can cause variations in the system s response — resulting in assembly defects. Rigidity also is required to ensure that stencil movement in relation to the board is accomplished in planes that are parallel within narrow limits. 

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