Key Considerations for PCB Design from a Soldering Perspective

Hello everyone, I am Jide Cheng.
Today, I want to discuss the important points to consider when designing PCBs from a soldering perspective.
1. Factors Affecting PCB Soldering Quality
From PCB design to the completion of all component soldering, creating a high-quality circuit board requires the control of various stages including PCB engineers, soldering processes, and soldering workers.
The main factors affecting quality include: PCB drawings, the quality of the circuit board, the quality of the components, the oxidation level of the component pins, the quality of the solder paste, the printing quality of the solder paste, the precision of the programming for the pick-and-place machine, the placement quality of the pick-and-place machine, the temperature curve of the reflow oven, and so on.
The stage that the soldering factory cannot surpass is the PCB drawing.
People who design circuits rarely solder circuit boards and cannot gain rich soldering experience, while workers in soldering factories do not understand drawing boards and only focus on completing production tasks, lacking the time and ability to analyze the causes of poor soldering.
These two types of talent each have their own roles, making it difficult to combine them organically.
2. Recommendations for PCB Drawing
Here are some suggestions to avoid various poor drawing methods that affect soldering quality.
1. Regarding Positioning Holes
The four corners of the PCB should have four holes (minimum diameter 2.5mm) for positioning the circuit board when printing solder paste. The centers along the X-axis or Y-axis should be aligned as shown in the figure below:
　　
Key Considerations for PCB Design from a Soldering Perspective
2. Regarding MARK Points
Mark points should be labeled on the PCB for positioning by the pick-and-place machine.
Specific location: on the diagonal corners of the board, it can be circular or square pads, and should not be mixed with the pads of other components. If there are components on both sides, both sides must be marked.
Consider adding MARK points on the panel as well.
3. When Designing PCBs, Please Note the Following:
a. The shape of the Mark point should refer to the pattern in the figure below, either vertically or horizontally symmetrical.
　　
Key Considerations for PCB Design from a Soldering Perspective
b. The size of A should be 2.0mm.
c. Within a range of 2.0mm from the outer edge of the Mark point, there should not be any shapes or color variations that could cause misidentification (pads, solder paste).
d. The color of the Mark point should differ in brightness from the surrounding PCB color.
e. To ensure recognition accuracy, the surface of the Mark point should be plated with copper or tin to prevent reflection. For markings that only consist of lines, light spots cannot be recognized.
As shown in the figure below:
Key Considerations for PCB Design from a Soldering Perspective
4. Regarding Leaving a 5mm Edge
When drawing PCBs, at least 3mm of edge should be left on the long side for the pick-and-place machine to transport the circuit board; within this range, the pick-and-place machine cannot place components. Do not place surface mount components within this range.
As shown in the figure:　
Key Considerations for PCB Design from a Soldering Perspective
For double-sided components on the circuit board, consider issues such as component wear and pad wear during secondary reflow soldering.
Key Considerations for PCB Design from a Soldering Perspective
It is recommended not to place surface mount components within 5mm of the long edge on the side with fewer chips. If the PCB area is indeed limited, processing can be done on the long edge.
5. Do Not Drill Holes Directly on Pads
Drilling holes on pads can cause solder paste to flow into the holes during reflow soldering, leading to insufficient solder on the component pads and causing cold solder joints, as shown in the figure below.
Key Considerations for PCB Design from a Soldering Perspective
6. Regarding Polarity Marking of Diodes and Tantalum Capacitors
The polarity marking of diodes and tantalum capacitors should comply with industry standards to prevent workers from soldering in the wrong direction based on experience. As shown in the figure:
Key Considerations for PCB Design from a Soldering Perspective
7. Regarding Silkscreen and Marking
Please hide the component model, especially on high-density circuit boards. Otherwise, the clutter will affect the identification of soldering positions. As shown in the figure:
　　
Key Considerations for PCB Design from a Soldering Perspective
Do not only label the model without marking the number. As shown in the figure, this can cause issues during programming for the pick-and-place machine.
　　
Key Considerations for PCB Design from a Soldering Perspective
The font size of the silkscreen characters should not be too small, and the placement of characters should be offset from the holes to avoid misreading.
Key Considerations for PCB Design from a Soldering Perspective
8. Regarding IC Pad Length
For ICs with SOP, PLCC, QFP packages, the pad length on the PCB should equal the length of the IC pins multiplied by 1.5 for ease of manual soldering with a soldering iron, ensuring the chip pins, PCB pads, and solder melt together. As shown in the figure:
Key Considerations for PCB Design from a Soldering Perspective
9. Regarding IC Pad Width
When designing PCBs for ICs with SOP, PLCC, QFP packages, pay attention to the width of the pads; the width of the pads on the PCB should equal the width of the IC pins (Nom. value in the datasheet). It is not recommended to widen the pads, ensuring there is sufficient space between the two pads to avoid solder bridging. As shown in the figure:
　　
Key Considerations for PCB Design from a Soldering Perspective
10. Do Not Rotate Components at Arbitrary Angles
Since the pick-and-place machine cannot rotate components at arbitrary angles, it can only rotate at 90°, 180°, 270°, or 360°.
If a component is rotated 1°, the pins of the component will be misaligned with the pads on the circuit board after placement, affecting the soldering quality, as shown in the figure below:
　　
Key Considerations for PCB Design from a Soldering Perspective
11. Issues to Note When Shorting Adjacent Pins
The shorting method in Figure a is not conducive for workers to identify, and the soldering outcome may not be aesthetically pleasing.
If shorting is done using the methods in Figures b and c, and solder mask is applied, the soldering results will be different.
As long as each pin is not connected, there will be no short circuit on that chip, and the appearance will also be aesthetically pleasing.
　　
Key Considerations for PCB Design from a Soldering Perspective
12. Regarding Central Pads Under Chips
For chips with a belly, it is recommended to reduce the size of the central pad to increase the distance from surrounding pads, thus reducing the chances of short circuits. As shown in the figure:
　　
Key Considerations for PCB Design from a Soldering Perspective
13. Do Not Place Two Thick Components Too Close Together
As shown in the figure, such a layout may cause the pick-and-place machine to encounter the previously placed component when placing the second component, leading to a detection of danger and automatic shutdown of the machine.
　　
Key Considerations for PCB Design from a Soldering Perspective
14. Regarding BGA
Due to the special nature of BGA packages, their pads are located underneath the chip, making it impossible to see the soldering results.
To facilitate rework, it is recommended to drill two Hole Sizes: 30mil positioning holes on the PCB for accurate positioning of the stencil during rework.
Tip: The size of the positioning holes should not be too large or too small; they should allow the pins to fit snugly without falling out or wobbling, ensuring accurate positioning. As shown in the figure:
　　
Key Considerations for PCB Design from a Soldering Perspective
15. Regarding PCB Color
It is recommended not to use red. Red circuit boards appear white under the red light source of the pick-and-place machine’s camera, making programming impossible and complicating soldering.
16. Regarding Small Components Below Large Components
Some people prefer to place small components underneath large components on the same layer, such as placing resistors under a digital tube, as shown in the figure:
　　
Key Considerations for PCB Design from a Soldering Perspective
Such a layout will create difficulties during rework, as the digital tube must be removed first, which may damage the digital tube. It is recommended to place the resistors on the bottom side instead, as shown in the figure:
　　
Key Considerations for PCB Design from a Soldering Perspective
17. Regarding Copper Coating and Pad Connection Affecting Solder Melting
Since copper coating absorbs a lot of heat, it can make it difficult for solder to fully melt, leading to cold solder joints, as shown in the figure:
　　
Key Considerations for PCB Design from a Soldering Perspective
In Figure a, the device pad is directly connected to the copper coating; in Figure b, although the 50-pin connector is not directly connected to the copper coating, the two inner layers of the four-layer board are large areas of copper coating, so both Figures a and b will cause solder paste not to melt fully due to the copper coating absorbing a lot of heat.
In Figure b, the body of the 50-pin connector is made of high-temperature intolerant plastic; if the temperature is set too high, the connector will melt or deform. If the temperature is set too low, the copper coating will absorb a lot of heat, preventing the solder paste from melting fully. Therefore, it is recommended to isolate the pads from large areas of copper coating, as shown in the figure:
　　
Key Considerations for PCB Design from a Soldering Perspective
18. Recommendations for Panelization and Processing Edges
Key Considerations for PCB Design from a Soldering Perspective
Key Considerations for PCB Design from a Soldering Perspective
Conclusion
Nowadays, more and more engineers are able to use software to draw, design, and route PCBs, and upon completion, they can significantly improve soldering efficiency. The author believes that the above factors should be given special attention.
Developing good drawing habits and effectively communicating with processing factories is something every engineer should consider.

This article is reprinted from the public account ‘Artificial Intelligence Science and Technology’.

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Key Considerations for PCB Design from a Soldering Perspective

Key Considerations for PCB Design from a Soldering Perspective

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