In PCB fabrication, pseudo-exposed copper is a critical issue that affects both the performance and appearance of printed circuit boards. Understanding its root causes and implementing effective solutions can significantly enhance PCB quality.
With over five years of expertise in PCB prototyping and manufacturing, PCBYES is dedicated to delivering high-quality PCBs and PCBA turnkey services. Our experienced engineers ensure that every board meets the highest industry standards.
I. Causes of Pseudo-Exposed Copper
Insufficient Squeegee Pressure
During the screen-printing process, inadequate squeegee pressure prevents solder mask ink from fully transferring through the screen to the PCB surface. This results in a thinner ink layer and may cause pseudo-exposed copper. Manual printing operations, in particular, are prone to this issue if the operator fails to apply consistent pressure.
Excessive Printing Speed
If the squeegee moves too quickly, the ink does not have enough time to fill the screen openings and transfer onto the PCB properly. This can lead to an insufficient solder mask thickness, creating the appearance of exposed copper. In automated production lines, improper speed settings can exacerbate this problem.
Shallow Squeegee Depth
If the squeegee does not engage the screen deeply enough, the ink transfer is compromised. A shallow squeegee depth means less ink is deposited on the PCB surface, leading to an incomplete solder mask coverage and pseudo-exposed copper.
Improper Squeegee Angle
The angle of the squeegee affects ink transfer efficiency. The ink may not be distributed evenly across the PCB surface if the angle is too large or too small. A larger angle can cause ink to accumulate in front of the squeegee rather than properly passing through the screen.
Insufficient Squeegee Length
A squeegee that is too short to span the entire width of the screen may result in uneven ink distribution, leaving certain areas with inadequate solder mask coverage. This localized deficiency creates pseudo-exposed copper on the PCB.
High Ink Viscosity
When the solder mask ink is too thick, its poor fluidity makes it difficult to pass through the screen openings. Even if the squeegee is correctly adjusted, high-viscosity ink may still result in insufficient coverage, leading to pseudo-exposed copper.
Excessive Screen Distance
If the gap between the screen and the PCB surface is too large, the ink transfer process encounters more resistance, preventing a uniform application. This increased distance can lead to a thinner solder mask layer and pseudo-exposed copper defects.
II. Solutions to Prevent Pseudo-Exposed Copper
Optimize Squeegee Pressure
The squeegee pressure should be carefully calibrated based on the screen type, ink properties, and PCB requirements. Conducting pre-production tests with different pressure settings can help determine the optimal value that ensures complete ink transfer without damaging the screen.
Adjust Printing Speed
The printing speed should be balanced with the ink's drying time and the screen’s mesh opening size. Slower speeds allow the ink to transfer more effectively, reducing the risk of pseudo-exposed copper. Testing different speeds will help identify the most suitable setting.
Modify Squeegee Depth and Angle
The depth of the squeegee should be adjusted according to the screen thickness and ink characteristics to ensure adequate ink transfer. Generally, a squeegee angle between 45° and 60° is optimal for uniform ink distribution.
Ensure Squeegee Length is Sufficient
The squeegee should be long enough to cover the entire width of the screen. Before printing, verify that the squeegee is correctly installed and evenly aligned to prevent uneven ink application.
Regulate Ink Viscosity
If the ink is too thick, adding an appropriate amount of thinner can improve its flow characteristics, making it easier to pass through the screen openings. After adjusting the viscosity, test prints should be conducted to ensure proper coverage.
Optimize Screen Distance
The gap between the screen and the PCB must be finely adjusted to facilitate smooth ink transfer. Typically, a spacing of 0.5 to 2 mm is ideal. Reducing this distance can improve ink application while preventing screen damage.
By addressing these root causes and implementing the appropriate corrective measures, PCB manufacturers can effectively minimize pseudo-exposed copper issues. This leads to improved PCB reliability, performance, and aesthetics, ensuring a higher quality final product.
At PCBYES, we continuously refine our manufacturing techniques to enhance PCB quality and meet the most stringent industry demands. Partnering with us guarantees top-tier PCBs that exceed expectations in every aspect.
