Field process is essential for the fabrication of PCBA product. Field process engineers should identify the manufacturing difficulty of the product before the tooling begins and control the potential design defects, so as to ensure the product could be produced in an economical way. As the electronic products are becoming light, thin and small, the soldering terminal of components and PCB pads are becoming smaller and smaller as well. In the meantime, the field process review is becoming more and more important.
1. Solder paste printing is the core of SMT, and the quality of solder paste printing determines the quality of soldering. Therefore, the stencil opening is crucially important. The design of stencil should be conducted with the consideration of PCB pad, solder mask, silk screen, characteristics of component soldering terminal, layout of adjacent component and adjacent exposed via, etc.
2. It is relatively more difficult for printing, surface mounting and soldering of fine-pitch component, and problems are prone to occur in these processes, therefore, to some extent, the soldering quality of fine-pitch components determines the quality of final product. For the fine-pitch components, the stability of paste printing, solder bridging issue after printing and mounting as well as cold solder due to micro displacement of SMD are necessary to be considered. Besides, it cannot be denied that the core of solder paste printing process is to eliminate the small printing gap of fine-pitch components.
3. If SMD solder paste printing pad is too close to the exposed via, it’s likely to cause cold solder and insufficient solder due to solder paste loss. In such case, avoiding holes, adjusting aperture width and fine tuning the location of aperture could help to prevent the happening of these issues.
4. The fine-pitch SMD solder paste printing pads is likely to cause solder bridging during printing or after surface mounting. In such case, adjusting aperture width and fine tuning the location of aperture are effective to prevent bridging.
5. Small and fine-pitch SMD pads would make stencil opening quite difficult. Then aperture area ratio, safe space between apertures, micro printing gap and melting solder paste overflow risk of SMD (Solder mask defined) pad are important factors to be considered.
6. SMD solder paste printing pad with Via is poor design, especially for BTC device. When soldering the via plugged by solder mask at the bottom of pad, the bubble would be out of control; the exposed via would make the solder paste run off, causing cold solder and insufficient solder. Therefore, avoiding holes is necessary.
The following presents some common printing issues of small pads as examples, sketching the process difficulties, improvements and efficient reviewing methods.
Process difficulty: difficult for printing of small pads, possibility of solder paste leak/insufficient solder
Root cause: printing gap exists between the micro pads and stencil
1. Pad design: to increase the diameter of pads (from 0.27 to 0.31) and decrease the area of pit around the pads, making the opening area locate on the copper foil of pad and reducing the gap between opening area and stencil bottom.
2. Thickness of solder mask: to decrease the thickness of PCB solder mask, reducing the impact of relatively higher solder mask on the circuit near the pad. The thickness of PCB solder mask is recommended to be less than 25um.
3. To adopt new PH stencil, so as to eliminate the printing clearance to the utmost.
Note: in this case, the root cause is the printing gap caused by solder mask, which is likely to be ignored. Eliminating the printing gap of micro pad especially the printing gap caused by PCB routing and resistance welding is essential for addressing such issues.
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