Nowadays, the demand for smaller, faster, and more efficient electronic devices is driving PCB technology beyond traditional design methods. One such advancement is the Via in Pad technique, which involves placing vias directly beneath component pads rather than routing them to a separate location. This method is particularly crucial for high-density interconnect (HDI) designs, fine-pitch components, high-speed circuits, and thermal management applications.
Understanding the importance of Via in Pad, its necessity, and the process of its implementation is essential for engineers and students working with PCB design.
PCB design has improved a lot over the years, and older methods like dog-bone via fanout are no longer effective for modern circuits. Earlier, vias were placed outside the component pads, requiring extra routing to connect them. While this worked for older, low-density PCBs, today’s smaller and faster electronic devices need a more efficient solution.
Better Space Utilization in Compact PCBs : With electronics becoming smaller, traditional routing methods take up too much space. Via in Pad allows direct vertical connections, eliminating extra wiring and making PCB layouts more compact.
Improved Signal Quality for High-Speed Circuits: Longer circuit traces create unwanted electrical effects that slow down high-speed signals. Placing vias directly under pads shortens the path, reducing interference and improving signal integrity.
Better Heat Dissipation in Power Components: In power electronics, LEDs, and RF circuits, heat can damage components if not managed well. Via in Pad helps heat escape quickly through the vias, keeping the circuit cooler and more reliable.
Replaces Outdated Techniques: Older dog-bone fanout designs took up more space and slowed down signals (Fig.50). Traditional thermal vias placed around power components were not as effective as vias directly under pads.
Drilling and Via Formation: The process begins with drilling the via, which can be done using mechanical CNC drilling for standard vias or laser drilling for microvias in HDI designs. Depth-controlled drilling is required for blind and buried vias, ensuring proper layer-to-layer connectivity.
Via Filling and Planarization: One of the most critical steps in Via in Pad technology is via filling. The vias must be completely filled to prevent issues such as solder wicking, which can lead to poor solder joints. Common filling materials include:
- Conductive epoxy (for improved electrical performance).
- Non-conductive epoxy (for mechanical strength and solderability).
- Copper filling (for high-power and thermal applications).
Once filled, the surface undergoes planarization, where excess material is polished off to ensure a completely flat and smooth pad. This step is essential for proper solder paste application and assembly.
Plating and Surface Finish: After planarization, the filled via is plated with copper to restore conductivity. Finally, the PCB undergoes surface finishing using ENIG (Electroless Nickel Immersion Gold), ENEPIG (Electroless Nickel Electroless Palladium Immersion Gold), or Hard Gold Plating, ensuring better solderability and oxidation resistance.
Proper implementation of these steps is crucial to maintaining PCB reliability and manufacturing yield.
Advantages of Via in Pad
- Space Optimization for High-Density Designs
- Enhanced Signal Integrity for High-Speed Circuits
- Superior Thermal Performance
- Reliable Solder Joints and Assembly
- Essential for Fine-Pitch Components
Challenges and Disadvantages of Via in Pad
- Higher Manufacturing Cost
- Complex Fabrication Process
- Risk of Solder Voiding
- Not Suitable for All Designs