Solder paste printing process

Solder paste printing is the most important process related to PCB assembly yield. The solder paste thickness, pad coverage, and alignment with the pad must be checked.

Selecting solder paste: Solder pastes of type 3 (25 to 45 micron solder ball size) or 4 (20 to 38 micron) should be used, depending on the size of the template opening. The use of low-halide content (<100 PPM) and no-clean ROL0/REL0 resin flux specified by J-STD-00 is recommended, eliminating the need for post-assembly cleaning.

The production of a template: the use of laser cutting stainless steel foil electropolishing or nickel metal electroforming process. Although the nickel electroforming process is relatively expensive, the process for depositing solder paste from ultra-small openings is the most reproducible. This method also has the advantage that it can be formed to any thickness desired by the user. A templated opening with a trapezoidal cross-section contributes to the release of the solder paste.

Solder stencil opening design: Use aperture ratio of 0.75 for laser-cut SS; use aperture of 0.66 for nickel metal electroforming, square-round (25-micron-diameter) openings help repeat solder paste deposition Sex. The aperture aspect ratio is defined as the aperture opening area divided by the aperture edge area. It is also possible to use X and Y axes that deviate from the pad of the substrate, so that the solder paste has the highest degree of deposition and the influence between the solder joints is minimized.

Solder stencil thickness: The thickness of the solder stencil should not exceed the height of the solder joint and must meet the aspect ratio requirements of the actual aperture design. In PCB assembly using hybrid technology, if these stencil requirements conflict with the requirements of other SMT components, a lower-level stencil process or dual-print stencil process that conforms to the IPC-7525 design standard can be used.

Source:: Electronic Equipment Network