As high-density interconnect boards continue to proliferate to meet modern devices’ dual high-performance and functionality requirements, board real estate is becoming more precious. Couple this development with a general tendency for consumer electronics toward portability, and designers must face the all-too-common problem of doing more with less. Historically, one of the more intractable board features was that of the through-hole: designs had to have a way to connect all of the boards’ layers while integrating the dominant component packaging of the time. However, the rise of surface mount devices (a separate effort at board minimization) often reduced the component need for large through-holes; microvias perform similarly from the perspective of pure routing. Blind and buried vias – microvia classes defined by their stackup location – minimize board space allotted to interlayer connectivity, allowing for greater component placement and routing density.
Manufacturing Process Comparison of Via Types |
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| Four-layer through-hole | Four-layer blind vias | Four-layer blind and buried vias |
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How Blind and Buried Vias Support HDI
Designers are familiar with through-hole connections, allowing electrical connections to all the board layers. While some through-holes support assembly integration of through-hole packages, most through-holes will be vias. Although they may seem trivial, vias are the basis for double-sided assemblies and multilayer boards that have opened the door to extensive design density reflective of modern electronics. Ironically, the feature most essential to HDI designs can often constrain it. Given the prevalence of vias (which can number in the hundreds or sometimes thousands), the minimum hole/pad size requires considerable board area.
Minimum via hole sizes are relatively fixed by manufacturability and reliability concerns to a hole diameter no less than a tenth of the board thickness. Smaller diameter openings experience incomplete plating action along the length of the via barrel, which is more liable to fail during high temperatures experienced during later manufacturing processes or in-field operation. Since the board thickness has to accommodate the enclosure’s or system assembly’s needs, it may seem like minimum through-hole sizes have no recourse. The way around this is microvias – drilled holes that only span portions of the board’s stackup. For example, a microvia may cross layers 11-12 in a 16-layer board.
According to IPC, a microvia has two essential qualities: a depth of no more than 150 microns (~6 mils) and an aspect ratio of no less than 1:1. While this is technically a more constrained aspect ratio than a through-hole, there is a multitude of dielectric thickness options available for designers that can shrink the microvia diameter below reliable through-hole sizes. Note that the dielectric thickness defines the microvia hole diameter in this scheme. Depending on the location within the stackup, microvias have two distinct categorizations:
- Blind – These microvias start at an outer layer but cannot “see” through the board, i.e., they terminate on an internal layer (usually the adjacent layer to the outer layer).
- Buried – These vias span only internal layers.
While blind and buried vias are complementary in design, it’s worthwhile to understand that blind are independent via structures and buried depend on the former. However, blind vias only require a depth-controlled drill (the same technology used to backdrill vias for signal integrity) to contact the necessary layer pair.
In combination, blind and buried vias allow designers to realize the same interlayer connectivity as a through-hole via for a fraction of the planar area. Microvia arrangements are either stacked or staggered, with the former configuration denoting a vertical column of blind and buried vias that superficially resemble a through-hole and the latter indicating that the microvias have different x- and y-positions throughout the plane. In either of these Microvias require a more considerable lamination process known as sequential lamination, and overall, their inclusion adds significant cost to bare board manufacturing and some reliability concerns compared to standard through-hole technology.
How do Microvias Affect Manufacturability?
For all PCBs, the interface between the via’s copper pad and the surrounding epoxy-glass fiber substrate matrix is liable to stress accumulation due to the difference in the materials’ coefficients of thermal expansion (CTE). This stress is a defect mode for through-hole vias, but the effect is typically negligible in performance and reliability as the pad pushes against air. Blind and buried vias, however, have at least one opening of the via barrel immersed in the substrate material, meaning the accumulated stress cannot easily exit the system. Because the glass fibers cannot relieve the surrounding stress, a high-pressure area around the microvia opening occurs, which can lead to delamination.
To minimize this failure mechanism, designers will want to collaborate with their manufacturers on material selection, with the primary goal being a larger area to distribute the stress or more fracture-resistant materials. Consider the following attributes:
- CTE – Low z-axis CTE materials will produce less stress during lamination, soldering, and other high-temperature processes.
- Resin content – There needs to be a Goldilocks zone: reducing the resin content limits z-axis expansion, while higher resin content increases the distribution area of the stress.
The designer can assist this process by monitoring the copper distribution around the inner layer openings of the microvia. Retaining the maximum copper content on the layer provides ample surface for the liquified resin to flow and bond during lamination.
Your Contract Manufacturer Opens New Stackup Possibilities
Blind and buried vias give designers additional flexibility during PCB design. While they incur a higher cost on the manufacturing end due to the need for sequential lamination and greater precision of microvias, they are often the only solution for modern HDI boards. To reap the maximum benefits of these via structures, the designer and manufacturer must collaborate early on design requirements and feasible rules. Here at VSE, we’re a team of engineers committed to building electronics for our customers; alongside our valued manufacturing partners, we’ve been realizing life-saving and life-changing devices for over forty years.
