As the functionality of PCBs (Printed Circuit Boards) continues to evolve, a variety of products with specialized requirements have emerged to meet customers' needs for specific product characteristics.
This article will discuss the key factors and solutions for precisely controlling line resistance on PCBs to achieve a specific routing pattern as required by the customer.
1. Theoretical Calculations
First, the customer provided the design layout for the resistor control board, which features long resistor traces on the BOT side, with a total length of 6,413 mm and a trace width of approximately 0.2 mm. Based on these parameters, we performed theoretical calculations for the resistors.
Theoretical calculations show that when the copper thickness is 35 μm, a wire 0.2 mm wide and 6,413 mm long is required to achieve the ideal resistance value of 16 Ω. This theoretical value provides a reference point, but other factors must also be considered during actual fabrication.
2. Process Control
2.1. Copper on the Back of the Circuit Board
During the manufacturing process, the copper plating thickness on the board was 8 μm. Actual measurements showed good uniformity in the copper thickness, with a Cpk of 1.4—well above the acceptance criterion (Cpk > 1.33). This indicates that the plating uniformity is good, which aids in resistance control.
2.2. Thickness of the Finished Copper Layer After Electroplating
Measurements of the copper thickness on the holes and the surface were taken from cross-sections; the measured values fell within the acceptable range, indicating that, under process control, both the hole copper and surface copper thicknesses are within the required range.
2.3. Testing the Resistance of Finished Products
Following final product testing, all resistors passed inspection. However, it should be noted that, compared to the etched semi-finished products, the resistance of the finished resistors decreased by approximately 1.5 Ω. This suggests that we should account for a margin of error when measuring resistance during the etching process, as the resistance of PCBs after etching varies depending on factors such as the design, solder mask, and surface treatment processes.
3. Conclusion
Based on the comprehensive test results, we can draw the following conclusions:
- The thickness of the copper plating on the board is controlled within the range of 21 ± 3 μm.
- The average thickness of the finished copper is controlled to be between 35 μm and 40 μm.
- The line width should be between 0.19 mm and 0.21 mm.
- All finished resistors met the specifications.
These results indicate that precise control of line resistance can be achieved through appropriate process control. At the same time, it should be noted that the resistance after etching is related to design and process variations; therefore, these factors should be taken into account during the manufacturing process.
These insights can help PCB manufacturers better meet their customers' resistance requirements and ensure product quality and reliability.
Frequently Asked Questions (FAQs)
1. What is PCB trace resistance control?
PCB trace resistance control refers to the process of ensuring that the resistance of traces remains within a specific range during the manufacture of printed circuit boards, in order to meet design and performance requirements.
2. Why is resistance control so important in PCB manufacturing?
Resistor control is a key factor in ensuring PCB performance and reliability. Substandard resistors may cause circuit instability or malfunction.
3. What is the Cpk value?
The Cpk value is a metric used to measure process stability and consistency, and it is used to evaluate process quality. The higher the Cpk value, the more stable the process.
4. Why does the resistance of a finished resistor decrease?
A decrease in the resistance of the finished product may be related to factors such as the etching process, solder mask, and surface treatment. This requires careful control and adjustment during the manufacturing process.
5. How can we ensure the accuracy of PCB trace resistance control?
To ensure precise control of PCB trace resistance, appropriate design and process control are required, including the selection of appropriate copper thickness, trace width control, and process monitoring.
By implementing the measures outlined above, you can effectively address the issue of controlling PCB trace resistance and improve the quality and reliability of your products. If you have any other questions or need more information, please feel free to contact us.
