PCB vias, generally fall into three categories: basic protection (tented via), fill protection (via plugging), and high-performance filling (resin pluging or via-in-pad plating).
The primary differences are performance and cost, they are the key factors and replying on the product’s specific requirements and market positioning.
1. Via Tenting (Tented Via)
This is the most basic and common choice, involving the complete coverage of the via opening with solder mask ink (typically green).
Advantages:
– Lowest cost, mature process, and fast production speed.
– Protect the vias from oxidation and moisture, protecting the internal copper layer from corrosion effectively.
– Prevents short circuits, and prevent solder flowing through the via to the opposite side of the PCB board, during wave soldering, which could cause bridging shorts.
Disadvantages:
– Prone to trap air inside: During high-temperature like reflow-soldering process, air trapped inside the vias might expand and may rupture the surface ink, causing “ink popping” or “outgassing,” which affects appearance and reliability.
– Poor resistance for mechanical stress: The coverage is merely a thin layer of ink, any physical pressure from component pins or test probes could crack the ink easily.
– Uneven surface: Leaves a slight depression on the surface, making it unapplicable for BGA (Ball Grid Array) pads.
2. Via Plugging (Ink Plugging)
This type of vias are completely filled by special plugging ink — using a screen printing mesh or aluminum sheet — followed by curing and planarization (leveling).
Advantages:
– Excellent sealing: The ink completely fills the via, eliminating issues such as trapped air or flux seepage, resulting in higher reliability than via tenting.
– Prevents solder beads: Prevent solder paste from seeping out of the via during reflow soldering process and not to form solder beads, it is a common approach for improving yield rates.
– Moderate cost: Cheaper than resin via filling, makes it suitable for mass production.
Disadvantages:
– Surface prone to unevenness: The ink shrinks during curing, causing a depression in the center of the via opening, or sometimes even a slight protrusion (known in the industry as “oil bleeding”), which affects coplanarity during SMT assembly.
– Moderate resistance for high-temperature: Standard via-fill inks may discolor or crack at high temperatures and are unapplicable for long-term high-temperature environments.
– Prone to air bubbles: If via-filling pressure or ink viscosity is not well-controlled, micro-bubbles may remain inside the via, appearing as voids if inspect by X-ray.
3. Resin Plugging
High-Tg (glass transition temperature) epoxy resin is commonly applied to fill the vias, followed by a grinding process, to ensure a perfectly flat surface.
Advantages:
– Extremely flat surface: After grinding, the resin is flush with the board surface, free of depressions or protrusions, it is the only mainstream process applied nowadays for BGA pads (Via-in-Pad).
– Resistance for high-temperature and aging: The resin’s resistance for heat and chemical is significant, exceeds other processings, enhancing it to withstand the high temperatures of lead-free soldering and long-term thermal cycling.
– High mechanical strength: Once it is cured, the resin reinforces the via walls, reducing the risk of wall fracture which caused by drill impact or external forces.
Disadvantages:
– Complex process and expensive: Requires an additional grinding process, both the equipment and laborforce are expensive than other approaches, and the price is several times higher than via-tenting.
– Rigorous process control: Excessive work of grinding could cause accidently damage to adjacent circuits, and incomplete grinding will lead to contamination on PCB surface, affecting subsequent SMT process negatively.
– Plating adhesion risks: Improper surface treatment of the resin can lead to plating delamination, during subsequent plating steps.
4. Plated Via Fill
This is a high-end process, the via is completely filled with copper during the plating process, resulting in a solid copper core.
Advantages:
– Superior performance: Copper offers excellent thermal and electrical conductivity, serving as an ideal channel for heat dissipation from the underside of high-power components (such as CPUs and power amplifiers).
– Superior signal integrity: Features an all-copper structure within the via, ensuring optimal impedance continuity, minimizing signal loss for high-frequency (>10GHz) or high-speed signals, and effectively reducing signal reflection.
– Highest long-term reliability: The solid copper structure has a coefficient of thermal expansion matched to the substrate, offering exceptional resistance to shock and thermal cycling; suitable for ultra-high-reliability applications such as military and aerospace sectors.
Disadvantages:
– Highest cost: The process is extremely time-consuming and requires specialized pulse plating equipment and chemical solutions, resulting in high prices.
– Extremely difficult process control: Prone to “protrusions” or “depressions”—where the copper layer at the via opening is excessively thick and raised, or the center sinks (creating a void)—requiring precise in-line monitoring.
– Demanding equipment requirements: Difficult to implement in standard PCB factories; requires dedicated production lines and often entails longer lead times.
The form below provides a clear comparison of the characteristics of each type of vias:
| Process Type | Key Advantages | Main Limitations / Costs | Typical Applications |
|---|---|---|---|
| Via Tenting | • Lowest cost and simplest process• Helps prevent oxidation and moisture ingress• Reduces solder bridging during assembly | • Via cavity remains, which may cause “oil popping” due to gas expansion at high temperature• Moderate sealing performance• Not suitable for vias on BGA pads | • Cost-sensitive consumer electronics, e.g. TVs and home routers• Standard signal vias outside BGA areas |
| Ink Via Plugging | • Relatively low cost• Prevents solder-ball residue and flux contamination• More reliable protection than simple tenting | • Surface may be uneven or recessed• Limited high-temperature resistance of some plugging inks• Solder-paste plugs may develop voids | • Standard signal vias with low surface-flatness requirements• Cost-sensitive power circuits |
| Resin Via Plugging | • Flat surface suitable for Via-in-Pad• Excellent sealing performance• High-temperature resistance• High mechanical strength• Suitable for high-reliability applications | • More complex manufacturing process• Moderate-to-high cost• Approx. RMB 0.05–0.10/via | • Vias on BGA pads / Via-in-Pad• Automotive electronics• Medical equipment• Areas beneath power components requiring thermal dissipation |
| Copper-Filled Vias (Plated-Filled Vias) | • Highest overall performance• Completely flat surface• Excellent thermal and electrical conductivity• Very high mechanical strength• Supports maximum signal-integrity performance | • Highest cost, approx. RMB 0.15–0.30/via• Complex manufacturing process• Long plating time | • High-power thermal management applications (>5 W)• High-frequency/high-speed signals (>10 Gbps)• Military and aerospace electronics |
Summary and Selection Recommendations
The choice of via types primarily depends on the product’s cost budget and working environment:
– Cost-effective and hassle-free: For standard designs without BGA on PCB, where cost and lead time are critical, then Via Tenting (tented via, Solder Mask Covering) is the best.
– High-density routing: If the design has extensive BGA packaging, traces and vias, and requires placing vias directly on pads (Via-in-Pad), then resin plugging is ideal, it ensures a flat pad surface, preventing poor solder joints (cold joints) during assembly of BGA.
– Ultimate performance: For high-frequency PCB/high-speed PCB and high-power PCB applications — such as 5G communications and servers, then Copper-Filled Vias is the best choice, it’s minimized signal loss and heat generation, ensuring long-term reliability.
Selection can be quickly determined with product positioning:
– Consumer electronics, household appliances (Cost-sensitive): use via tenting. (and ensure vias are not placed on BGA areas).
– Standard BGA, industrial control (Value-oriented): use solder mask plugging, it meets most requirements for preventing solder beads in non-high-density BGA designs.
– High-density PCB and BGA (Via-in-Pad), automotive electronics (Reliability-oriented): use resin plugging, it is mandatory to ensure high BGA soldering yields.
– High-frequency/high-speed, high-power, military (Performance-oriented): use copper-filled vias. Performance and safety are prior than cost and lead time.
Kyrid has been manufacturing PCB for over 1 decade and very good at the manufacturing processes of various vias. If you want to learn more acknowledges of PCB fabrication, please read other articles and Kyrid will proceed to post more good articles!
