Vias are an integral part of any multi-layer PCBA, as they allow for interplane routing that would otherwise only be possible (and generally unfeasible) on the outer layers. Protecting the quality of these vias is paramount to the long-term reliability of the device; vias are common mechanical failure points of the board for many reasons. One method fabricators can use to protect vias is tenting. Tented vias are the most rudimentary method of via encapsulation, yet they are highly suitable for many applications at a cost-efficient price.
| Types of Vias by IPC-4761 | |
|---|---|
| I – Tented | A mask material bridges the via hole opening and is absent within the via barrel. |
| II – Tented and Covered | Like Type I, but with a secondary covering over the base masking material. |
| III – Plugged | A partial filling of the via barrel leaves an air gap in the center. |
| IV – Plugged and Covered | Like Type III, but with an additional cover material over the plug. |
| V – Filled | A complete filling of the via barrel. |
| VI – Filled and Covered | Like Type V, but with an additional cover material over the plug. |
| VII – Filled and Capped | Like Type V, but with an additional metallization step over the plug. |
Are Tented Vias Necessary?
Vias are one of the most essential structures of a PCB, offering interconnection between conductive layers. While they are indispensable, their inclusion typically involves some reliability tradeoffs in the stackup: vias expand at different thermal rates compared to the surrounding substrate, their hole size requires calibration to the board thickness, and the cavity formed during drilling can trap process fluids, moisture, and other debris. To enhance reliability, product teams take precautions to protect the via during operation by closing the hole opening and extending the solder mask over the board’s surface without interrupting the connection’s continuity. Some advantages of via protection include:
- Prevents solder bridging in densely drilled board areas
- Prevents solder upflow via capillary action for wave solder process
- Prevents epoxy resin from flowing into holes (contrasts with an alternate via protection method known as filled vias)
- Prevents trapped etchant from attacking via barrel that endangers via integrity/reliability
Historically, via tenting was the primary method of protecting vias from debris ingress that could potentially harm the reliability of the via. Fabricators would extend the dry-film solder mask process to cover the via holes. This relatively simple application method precludes any additional surface finishing but, on the upside, does not require any additional aperture openings on the stencil. Due to its highly flexible and straightforward procedure, via tenting is a suitable option for cost-conscious boards, e.g., high-volume lots without reliability concerns.
However, the standard practice for via tenting changed in response to other developments in PCB manufacturing technology. Finer copper features make dry film solder masks less appealing due to their imprecise nature for application. Today, dry film solder masks are mainly a legacy option for board manufacturing, yet via tenting is still the preferred low-cost option for via protection.
| Analyzing Dry Film Solder Mask | |
|---|---|
| Advantages | Disadvantages |
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Solder Mask Applications and Via Tenting Impact
The modern standard for solder mask application is a liquid photo-imagable solder mask (LPISM). There are three application methods:
- Curtain coating – A curtain of low-viscosity ink falls through a narrow slot tent via holes equal to or less than 0.3 mm. The process is rapid, and material loss is low. However, results can be unreliable.
- Spray coating – A thin LPISM mixes with decompressed air, producing atomized resist droplets that coat the PCB’s surface. Overall, the solder mask is uniform, yet thicker strips can form where nozzle sprays overlap, slightly varying board thickness and dielectric constant. Spray coating is ill-suited for via tenting.
- Screen coating – The dominant LPISM application technique uses a squeegee and customized stencil to apply the solder mask evenly to the board’s surface. Care is necessary with the leading edge of the solder mask to ensure coverage remains consistent for the dielectric constant. Tenting via screen coating has some constraints, primarily the hole size limit and surface tension of the solder mask. Adding apertures in the solder mask layer can account for larger via holes.
Note that tenting, like other via protection types, should always be double-sided. Single-side tenting can exacerbate entrapment issues more so than an untented via. If the etchant solution enters the opening of a half-tented via, it can produce a solid crystalline structure that continually eats away at the via barrel.
Your Contract Manufacturer Builds Reliability Into Each Board
Tented vias are the most cost-efficient option for protecting vias (and the overall reliability of your boards), but more resilient options are also available. Given the importance of vias to the overall design and their tendency to fail first during operation, via protection is crucial to reliability, especially for devices where downtime is unacceptable. Fortunately, the engineering team at VSE can walk you through the process of via protection. We’re committed to building electronics for our customers, including DFM analysis for service life optimization. We’ve been realizing life-changing and life-saving designs with our valued manufacturing partners for over forty years.
