Standard PCB vs HDI PCB: Comparing Process Capability

Two Capability Sets, Not Two Quality Levels

The difference between a conventional multilayer board and an HDI board is not quality, it is the set of process limits the design can use. A standard board is built with mechanical drilling, conventional imaging and a minimum line width that is comfortable for a mature process. An HDI board adds laser drilled microvias, finer imaging, thinner dielectrics and the sequential lamination steps needed to build them into the stack. The HDI route buys density: more connections in less area, which usually means fewer layers, a smaller board or both. It costs money in process steps, and it imposes new design constraints that a standard board does not have.

Feature Size and Imaging

A conventional process typically holds 0.1 mm line width and spacing in production, with 0.075 mm available at lower yield. An HDI process moves to 0.05 to 0.075 mm as a routine capability and reaches below 0.05 mm in the most advanced shops, using thinner copper and finer dry film or laser direct imaging. The consequence for the designer is that routing channels and pad pitches shrink, which allows fine pitch devices to be fanned out without extra layers. It also means the etching tolerance becomes a larger fraction of the feature size, so impedance control has to be approached more carefully and the trace width tolerance is quoted in absolute rather than percentage terms.

Via Technology and Stack Structure

The defining feature of HDI is the microvia: a laser drilled hole of 0.1 to 0.15 mm diameter that connects only adjacent layers. Microvias are formed in thin dielectric layers, they are usually filled and plated over to give a flat surface for the next layer, and they can be stacked or staggered from layer to layer. Stacked microvias require a reliable fill and a very flat surface, which is why the number of stacked levels is limited by the process. The alternative to microvias in a standard board is the mechanical through via, which passes through the whole stack and consumes routing area at every layer. That single difference is usually what forces a design to more layers: where an HDI board can fan out on the outer layers and drop down with a microvia, a standard board has to use a through via or a buried via, and the anti pads around it cost space.

Layer Count, Thickness and Aspect Ratio

HDI is normally built with thinner cores and prepregs, which allows a given number of layers in a thinner board and makes the drilling and plating easier. Aspect ratio is the board thickness divided by the hole diameter, and it is the parameter that most often limits a design: a standard board is comfortable to about eight to one and can reach twelve to one at a premium, while an HDI microvia has an aspect ratio of roughly one because the dielectric layer is thin. A thick backplane with a small via is therefore a difficult standard board, whereas the same connection density on a thin HDI board may be routine.

Materials, Registration and Yield

HDI work usually uses thinner copper and often lower loss or higher performance laminates, both of which raise the material cost. Registration becomes more critical because the same absolute error is a larger fraction of a thin dielectric layer, and the sequential lamination builds in more opportunities for movement. Yields are lower at first, particularly for stacked microvias, which is one of the reasons an HDI quotation looks high on the first build and improves once the process is stable. Testing also becomes more demanding, since the finished product cannot be probed everywhere and the coupon design has to prove the impedance of layers that are buried under subsequent lamination.

Where the Cost Steps Are

The price difference between a standard board and an HDI board is not a single multiplier; it arrives in steps. The first step is fine line imaging. The second is laser drilling and the associated cleaning and desmear for the microvia. The third is via filling and planarisation. The fourth is each additional lamination cycle, which for a two level stack means at least two extra press operations. The fifth is the reduction in yield as the stack gets more complex. A single level HDI board with staggered microvias costs noticeably more than a standard board of the same size, and a stacked or any layer design costs considerably more again. At the same time, the extra cost frequently buys back a layer or two and a reduction in board size, so the comparison should be made on total cost per function rather than per square metre.

How to Decide

Start with the component pitch and the number of nets that have to escape from each device. If the fanout fits in the available routing channels with a standard minimum line width and through vias, the standard process is the answer, because it is cheaper and more robust. If the fanout does not fit, or if the layer count needed to make it fit pushes the stack beyond what is practical, then HDI becomes the economic choice rather than the premium one. Confirm the achievable aspect ratio for the intended thickness, decide whether the microvias will be staggered or stacked, and ask for the yield assumption behind the quotation, because the difference between a comfortable design and a marginal one is often only visible at that level.

FAQ

What line width does an HDI process allow? Around 0.05 to 0.075 mm is routine and below 0.05 mm is available in advanced shops, compared with about 0.1 mm for a conventional production process.

What is a microvia? A laser drilled hole of roughly 0.1 to 0.15 mm diameter connecting two adjacent layers, typically filled and plated over so the next layer can be built on a flat surface.

Is HDI more reliable? Not inherently. It is more dense. Reliability depends on the design, the fill quality of the microvias and the number of stacked levels, all of which have to be controlled.

Why does HDI cost more? Fine line imaging, laser drilling, via filling, each additional lamination cycle and a lower initial yield, arriving as separate cost steps rather than as one multiplier.

When should I use HDI? When the fanout of fine pitch devices does not fit within conventional line width and through via rules, or when the layer count needed to fit it makes the stack impractical.

Conclusion

Standard and HDI are two capability sets, and the decision between them is driven by fanout density, aspect ratio and layer count rather than by ambition. Compare the total cost per function rather than the price per square metre, confirm the achievable aspect ratio at the thickness you need, and decide up front whether the microvias will be staggered or stacked. The feature sizes, aspect ratios and stack options that a supplier can build are listed in PCB capabilities, the fanout and stackup planning belongs in PCB design and layout, and the laser drilling and sequential lamination steps are described in PCB manufacturing. A prototype PCB assembly build is the cheapest way to confirm the stack before a volume commitment in 2026.