On-site craftsmanship is the backbone of the product manufacturing process of PCBA factories. On-site craftsmen must identify product manufacturing difficulties in advance, prevent and control various potential design defects, and ensure that products are produced in the most economical way. This is the core work of on-site craftsmanship. As electronic products become thinner and smaller, component welding terminals and PCB pads are also getting smaller, and on-site process review is becoming more and more important and more difficult.
What are the technological difficulties of SMT?
- Printed solder paste is the core of SMT. The quality of solder paste printing determines the quality of welding, so the steel mesh opening is the key. The steel mesh opening design should be combined with PCB pads, solder mask, silk screen, device welding terminal characteristics, adjacent device distribution, adjacent exposed copper distribution, pads in large copper foil area and other comprehensive considerations.
- Printing, SMT and soldering of fine pitch devices are relatively difficult and prone to problems, so the soldering quality of fine pitch devices determines the quality of the product. Fine pitch devices must not only consider the stability of tin printing, but also consider the tin connection after printing and SMT, and false soldering caused by slight deviation of the SMT. Eliminating the tiny printing gap of fine pitch devices cannot be ignored, which is the core of the printing solder paste process (Teacher Jia Zhongzhong calls this the process in the SMT process, which shows its significant impact).
- The close distance between the SMD tin-printed pad and the exposed through-hole can easily cause solder loss, resulting in false soldering and insufficient tin. Avoiding holes, reducing the opening, and fine-tuning the opening position can avoid these problems.
- The small spacing between SMD tin pads makes it easy for tin connection during printing and after patching to occur. It is necessary to reduce the opening and fine-tune the opening position to avoid this problem.
- SMD pads with small area and close spacing will make it difficult to open the steel mesh. At this time, we must not only consider the opening area ratio and the safe spacing between the openings, but also the tiny printing gap and the risk of solder overflow when the solder of the pad defined by the solder mask melts. The tiny pads defined by the solder mask are prone to solder overflow and cause bridging.
- It is a bad design to have via holes on SMD printed solder pads, especially for BTC devices. When soldering via holes blocked with solder mask at the bottom of the pad, bubbles often get out of control; exposed via holes will cause solder to flow out, resulting in false soldering and insufficient soldering, which will cause countless poor soldering, so it is necessary to avoid via holes.
The following takes the common problem of printing tiny pads as an example to briefly describe its process difficulties, improvement methods, and efficient review methods.
Process difficulties: tiny pads, difficult printing, potential tin leakage /tin shortage issues
Root cause of the problem: There is a printing gap between the tiny pad and the steel mesh
Improvement methods:
- Pad design: increase the pad diameter ( change 0.27 to 0.31), reduce the area of the deep pit around the pad, make the opening area originally on the deep pit become on the pad copper foil, and reduce the gap between the opening area originally on the deep pit and the bottom of the steel mesh (verified OK).
- Solder mask thickness: Reduce the thickness of PCB solder mask to reduce the impact of the high solder mask layer on the circuit near the pad. It is recommended that the PCB solder mask thickness be less than 25um.
- Adopt new PH steel mesh to eliminate printing gaps to the greatest extent.
Note: In this case, the printing gap caused by solder mask is the root cause. This type of problem is most easily overlooked. Eliminating the printing gap of tiny pads, especially eliminatingthe printing gap caused by PCB routing and solder mask, is the key to solving this type of problem.
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