Tented Vias in PCB

PCB surfaces often feature numerous vias, playing crucial roles in connections. Manufacturers drill holes on the surface according to design files, then coat the hole walls with a layer of metal (usually copper) through electroplating processes. Finally, component pins are inserted into the holes for assembly. However, these holes may be susceptible to external influences like dust and moisture, necessitating protective measures such as using tented vias.

Tenting, also known as capping or covering, is a technique aimed at covering the area around PCB vias with a protective material to form a “curtain-like” protective layer. Through this technique, vias can be effectively isolated and protected from the influence of external substances.

Exposed vias VS Tented vias

Benefits of Using Tented Vias

Let’s first discuss what happens when vias are left uncovered. When the metal around vias is exposed without any shielding or insulation, it forms an open antenna-like structure, which is highly detrimental to signal integrity. When powered, electromagnetic waves may radiate due to the electric field between the vias and adjacent signal traces, leading to signal crosstalk. For applications requiring high-speed PCB designs to cope with high-frequency environments, it’s strongly recommended to use tented vias, as such projects demand higher signal integrity.

Moreover, when electrical signals pass through uncovered areas, impedance mismatches need to be considered. This refers to abrupt changes in impedance along signal paths on the PCB. In such cases, some signals may reflect due to impedance mismatches, potentially causing signals to return to the original signal source or propagate along other paths, resulting in signal distortion or potential risks of data errors during transmission.

Additionally, tented vias are also advantageous for PCB assembly processes. In modern electronic assembly,  SMT technology is used in over 70% of cases. However, there’s a risk of solder paste migration from SMD pads when vias are too close. Adopting tented via technology can reduce the probability of this risk.

Considerations for Designing Tented Vias

The manufacturing quality of tented vias is related to various factors, such as via size and spacing. Tented vias are suitable for designs with via sizes smaller than 12 mil, while they may fail when the size exceeds 15 mil. In such cases, using filling instead of tenting is recommended. As for spacing, it’s essential to always adhere to IPC standards to determine the minimum spacing between vias.

Manufacturing Process of Tented Vias

The main materials for manufacturing tented vias include solder resist ink and dry film.

Solder resist ink is the material used to create the solder mask, so the manufacturing process for both is similar and more common. In the process, solder resist ink is first applied to the via surface, and then the PCB is placed in a UV exposure machine to cure and form a protective layer. While this material is more compatible with traditional processes, it may not be a wise choice for more miniature holes, as it could lead to covering difficulties.

On the other hand, dry film is similar to a semi-finished product; it is directly pressed onto the board, then exposed to UV light in an exposure machine to form via openings, followed by copper plating to create electrical connections. Using dry film as an alternative can compensate for the shortcomings of solder resist ink, but the manufacturing process is more complex.

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