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The Solder Paste Print Step
December 31, 1969 |Estimated reading time: 2 minutes
Surface mount technology (SMT) is widely used in electronics assembly.1Solder paste, which serves primarily as the attachment medium between the device interconnection features and the PCB itself, is deposited (usually by printing) on the attachment sites (pads) of the PCB. Solder paste deposition is one of the most critical steps of SMT assembly. It’s normally accepted in the industry to consider that 60% of process defects can originate in the solder paste printing step. Therefore, appropriate controls must be in place to improve the chances of successful paste transfer between stencil and PCB.
Solder paste is transferred in patterns that mimic the land patterns on the circuit board, usually by masking via stencil the areas of the PCB surface where no paste is required. Stencils are manufactured by referencing PCB design artwork from Gerber files. The stencil aperture definition layer is referred to as the solder paste layer.
Once the paste is deposited in the land patterns,2 the device interconnections are accurately positioned (at pick-and-place) over the deposited solder paste, and subassembly solder is reflowed to produce a well-formed, continuous fillet at each solder joint. Some of the critical parameters for high-quality solder paste printing include print speed, print pressure, separation speed/distance, and printer alignment. Paste rheological properties given by the flux chemistry and mesh type of the solder paste are also factors. Excellent operator training is imperative since solder printing is a sensitive and delicate process.
The solder paste printing environment influences finished PCB assembly reliability. Defects such as solder bridging and poor wetting can be caused by dust particles or microscopic fibers in the air that end up on the PCB or stencil. Quick drying of the solder paste, on the other hand, can be caused by high ambient temperature or the presence of air draft that accelerates solvent evaporation. Solder paste viscosity will also be difficult to keep under control in an environment with fluctuating ambient temperature and humidity.
Figure 1. Inadequate prints (incomplete, residues, shorts, etc.)
Aperture design contributes to paste deposit quality, as do stencil material and the thickness in which apertures are tooled. The aperture geometry is in great part dependent on the component pitch and land pattern used on the PCB. Pitch often dictates what stencil thickness is required for adequate paste release. This can be the differentiator between a good and a bad paste print (Figure 1).
This excerpt is taken from Laser-cut Electropolish and Laser-cut Nanocoat Stencils: A Comparison of Finish Performance for Complex Designs, by Miguel A. Lara, Celestica. Read it in the SMT Printing center.
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