The design of stencil opening is one of the main means to optimize the SMT process, and it is also an important factor affecting the quality of the SMT process. The opening design of the stencil mainly includes the shape of the hole, the size of the hole, and the stencil thicknesses. The following SYS Technology will introduce you to the specifications and precautions of stencil opening in SMT processing.
1. Stencil thickness
For 0.4mm QFP, 0201 chip, the suitable stencil thickness is 0.1mm; for 0.4mm CSP devices, the suitable stencil thickness is 0.08mm, which is the benchmark for different thicknesses of stencil design. If a step stencil is used, the appropriate maximum thickness is 0.08mm.
2. Stencil Hole size design
Except for the following cases, the principle of 1:1 with the pad can be used to hole design.
1). On the soldering surface (wetted surface) at the bottom of the lead-free component, the opening of the Stencil must be shrunk inwards to eliminate the bridge or tin ball phenomenon, for example, shrink 0.8 mm in the heat sink PAD of QFN, and for capacitor resistance, the holes on stencil need to form an inner chamfer.
2). For components with poor coplanarity, the stencil opening generally needs to be expanded by 0.5-1.5mm to the outside, so as to make up for a lack of poor coplanarity. In the process of opening and enlarging the stencil, attention should be paid to whether the enlarging will affect the shifting of the components.
3. For large area pads, grid holes or wire holes must be opened to avoid scratching during solder paste printing, or lifting parts during soldering, which will make other pins pseudo solder.
4. The keyboard PCB should avoid the design that the stencil opening is longer than the pad.
5. No solder paste should be found on the non-wet surface of the packaged components, otherwise, there will be solder ball problems after reflow soldering in the SMT process.