In PCB design, the solder mask opening (exposed copper) or solder mask coverage (green oil) for laser blind vias directly impacts signal integrity, reliability, and cost. Different applications require tailored approaches. Below are the key considerations:
I. Solder Mask Coverage Design
For Consumer Electronics, Blind vias typically employ a solder mask coverage (via filling) design to prevent copper oxidation. In high-density routing areas (e.g., under BGA), this reduces short-circuit risks. Applications in harsh environments such as automotive electronics and industrial equipment, where resistance to salt spray and vibrations is needed, also benefit from this design.
Key Considerations: Ensure complete coverage to avoid air bubbles inside the via. Simulate and verify how the solder mask coverage affects impedance for impedance-controlled PCBs.
II. Solder Mask Opening Design
For High-Frequency & High-Speed Applications: In products like graphics cards and 5G base station RF PCBs, blind vias utilize an open solder mask design to minimize signal loss and facilitate chip packaging.
Key Considerations: When opening a solder mask for blind vias, it is best to design the vias as filled to prevent solder leakage during assembly. Techniques such as electroplated via filling ensure the via is filled with copper before applying the solder mask opening.
Benefits of Filled Blind Vias: They enhance electrical performance by mitigating signal reflection and transmission delay, crucial in high-frequency circuits. Additionally, they reinforce the PCBs' mechanical strength, reducing the risk of line breakage or delamination caused by external forces.
III. Case Studies of PCB Blind Via & Solder Mask Design
1. Smartphone Motherboards
Blind vias enable high-density routing, connecting chips with internal layers, using laser-drilled vias of 0.15-0.2mm in diameter.
Solder Mask Opening: This is used for soldered blind vias, such as those connecting BGA-packaged chips, to ensure reliable electrical connections. Since BGA pitch is typically 0.65mm or less, NSMD (Non-Solder Mask Defined) pads are preferred, with solder mask opening dimensions exceeding pad size by 0.05-0.1mm.
Solder Mask Coverage: Applied to non-soldered blind vias at board edges or areas not involved in electrical connections, preventing leakage and environmental contamination.
2. Graphics Card PCBs
Blind vias ensure high-speed signal transmission by linking GPU chips with inner power and signal layers.
Solder Mask Opening: Used for test points on blind vias to ensure accurate probe contact, with opening tolerance controlled within ±0.05mm. Key signal vias also adopt this design to ensure soldering quality and reliable transmission.
Solder Mask Coverage: Applied to blind vias under chips or inaccessible regions that do not require soldering or testing, preventing solder ingress and improving insulation.
With more than 5 years of expertise, PCBYES specializes in PCB prototyping and fabrication, ensuring the highest quality solutions for blind via design. Our team of experienced engineers offers PCBA turnkey services tailored to your needs, making us a trusted partner in the electronics industry.
