Design area is precious in PCB layout. Designers often must get creative towards the end of the layout when component placement on the outer layers and planes and signals on the inner layers occupy most of the routable area. Smaller components—when available—are a possibility but may be limited by either supply or package constraints. For trace widths, fabrication below four mils will drive up bare board production costs, but more importantly, the circuit’s impedance requirements will hinder width reduction.
What’s left is the via: typically a mechanically drilled and plated through-hole that spans the board from one outer layer to the other. Manufacturing practices ensure that the diameter of the drilled hole is within a magnitude of the depth, but microvias can subvert this restriction. The significant reduction in via diameter improves high-density interconnect (HDI) design and is one of the fabrication technologies that supports fine-pitch ball grid arrays (BGAs) for an even more compact layout.
Comparison of Microvias and Through-hole vias
The HDI Design Incentive for Microvias
The standard mechanically drilled and plated via adheres to the aspect ratio, defined as the ratio of the drill depth to the via diameter. Board thickness is unlikely to vary after the earliest stages of design due to the impact of impedance and system integration within the enclosure, locking the minimum through-hole diameter of the board to a tenth of the board’s thickness (the upper limit).
Via diameters that exceed a 10:1 aspect ratio are likely to encounter reliability issues due to incomplete barrel plating towards the center of the barrel:
- The longer the barrel is relative to the diameter, the more likely the copper ions plate to the surface of the barrel closer to the hole opening.
- As the via diameter shrinks, the flow rate of the plating solution restricts.
Poor plating in the barrel compromises the structural integrity of the via. As the board undergoes heating cycles during operation, materials expansion in the z-axis (that of the board’s thickness) causes a stress-strain matrix to develop due to a mismatch between the copper plating and surrounding (most commonly) epoxy-fiberglass substrate. An aspect ratio above 10:1 inhibits proper plating along the length of the barrel and the via is liable to experience an earlier-than-expected failure during its service life.
Designers are in a quandary: how can the via diameter shrink if the board thickness constrains its lower boundary? The answer is to pass up through-hole interlayer connections entirely with a microvia.
A microvia, as defined by IPC, is a drilled hole with an aspect ratio of 1:1 and a depth no deeper than ten mils (.25 mm). Microvias usually only span two, and very occasionally, three layers in a board’s stackup; “through-hole” connections for microvias require chaining consecutive microvias together in one of two arrangements:
- Stacked microvias are similar in function to a traditional through-hole via, with the hole center of the microvias aligned in the z-axis. While more space-efficient, stacked microvias encounter more reliability issues.
- Staggered microvias can be distributed anywhere throughout the planes of the board, provided there is an electrical connection (e.g., a trace) between them.
The Effect of Microvia Fabrication on the Overall PCBA Production
Microvias are just one type of via structure and can coexist alongside through-hole vias, but the fabrication process is more involved. Whereas through-hole drilling and plating are individual, consecutive steps that follow the fusion of the board’s layers during lamination, microvias require multiple iterations of drilling and plating.
Microvia construction forces lamination to become a multi-stage procedure where microvias are drilled in the appropriate layer pairs before final lamination completes the physical stackup of the board. Microvias are further definable by their location in the finished layup:
- Blind microvias have one opening on an outer layer of the board.
- Buried microvias have no openings on either outer layer of the board, i.e., entirely buried within internal layers.
The drilling process itself also accommodates multiple fabrication modes. While mechanical drilling can still apply to microvia fabrication through controlled-depth drilling, laser drilling offers many benefits, primarily greater throughput on high-volume lots and smaller diameters.
Moreover, since laser ablation is a non-contact drilling method, it is exempt from some of the defects common to mechanical drillings, such as drill wander, vibration, smear, and bit breakage, which can severely impact the final quality of the microvia.
Do I Need Microvias? Analyzing Their Cost-Benefit
For HDI boards, a designer should ask their manufacturer, “Are microvias the correct choice for my design?” There is some nuance here, but if a design cannot otherwise free up the space required for routing within the board’s dimensions, microvias are practically unavoidable.
Layout designers should start constructing a rough board placement to understand routability and whether through-hole vias are suitable. An excellent starting point is assuaging the finest pitch component – typically a BGA – to determine the smallest possible via and what additional fabrication processes may be required (i.e., microvia or via-in-pad).
Microvias are a cost adder to the fabrication process and create new reliability issues. Designers will want to assist manufacturing by considering some DFM constraints:
- Design stackups containing microvias that require four or fewer lamination cycles. Additional lamination steps complicate fabrication and introduce more high-temperature/pressure stress that can undermine long-term reliability.
- Each lamination step must plug buried microvias with epoxy to support reliability.
- Multiple laminations reduce the registration of the drilled microvias, resulting in designs that drift in accuracy from their design documentation. While tolerances allow for some misregistration, an excessive amount is enough to disrupt board performance and functionality, leading to scrap.
No Production Is Too Small for Your Contract Manufacturer
As boards, printed circuits, and component packages continue to shrink, microvias are here to stay, making them an indispensable tool for the designer and manufacturer. Due to the additional challenges inherent to their fabrication that can increase cost, reduce yield, and curb long-term reliability, the decision on microvias should be a joint effort between the designer and manufacturer to optimize the board’s performance and production timeline.
At the same time, designers will want to integrate microvias early in the design process to reap their full benefits. Whatever your PCBA requirements, VSE’s engineers are committed to building electronics for our customers. Alongside our valued manufacturing partners, we can meet design intent with a company-wide dedication to quality and reliability.