Introduction
HDI stands for High Density Interconnect — a class of printed circuit board that achieves significantly higher wiring density per unit area than conventional PCBs. HDI boards accomplish this through a combination of finer lines and spaces, smaller vias, and multiple layers of copper connected through microvias rather than through-holes that span the entire board thickness.
As electronic products get smaller and more capable — smartphones, wearables, medical implants, automotive radar modules, 5G base stations — the PCBs inside them have had to follow. HDI technology makes it possible to fit more circuitry into less board real estate without sacrificing signal integrity or reliability. At Linked Electronics Co., Limited, HDI PCB manufacturing is one of our core capabilities, and we routinely produce 1st, 2nd, and 3rd order HDI boards for customers across the industrial, medical, consumer, and automotive sectors.
This guide explains what HDI PCB is, how it differs from standard PCB, the different via types and stackup options, when you should consider HDI for your next design, and what to expect when ordering HDI PCBs from a China-based manufacturer.
HDI PCB vs. Standard PCB: The Key Differences
A standard multilayer PCB uses through-hole vias — drilled holes that pass through every layer of the board, from top copper to bottom copper. Through-hole vias are simple and low-cost, but they consume pad space on every layer they pass through, even layers where no connection is needed. On a dense 8-layer board with hundreds of vias, this via-in-pad waste can consume a significant fraction of the available routing space.
HDI boards replace some or all through-hole vias with smaller, more targeted interconnects:
- Microvias — laser-drilled vias typically 0.1 mm or smaller in diameter, connecting only adjacent layers. They consume very little pad space and can be placed directly under component pads (via-in-pad).
- Blind vias — connect an outer layer to one or more inner layers without going all the way through the board.
- Buried vias — connect two or more inner layers without appearing on either outer surface.
The result is a board where routing channels open up dramatically. Component pitch that would be unroutable on a standard board becomes straightforward on HDI. BGAs with 0.4 mm or 0.3 mm pitch — impossible to fan out conventionally — become manageable with microvias placed directly in the BGA pads.
HDI Via Types Explained
Microvias
Microvias are the defining feature of HDI technology. They are formed by laser drilling rather than mechanical drilling, which allows diameters as small as 0.1 mm — compared to the 0.2 mm minimum practical for mechanical drills. Microvias only connect adjacent layers (for example, Layer 1 to Layer 2, or Layer 3 to Layer 4), which means they do not interfere with routing on other layers.
Because microvias are so small, they can be placed directly beneath surface-mount component pads — a technique called via-in-pad. This is essential for ultra-fine-pitch BGAs, where there is simply no room to route a via outside the component footprint.
At Linked Electronics Co., Limited, our standard microvia diameter is 0.1 mm with a laser drill, and we support via-in-pad with copper fill and planarization to ensure a flat, solderable surface.
Blind Vias
A blind via starts on an outer layer and terminates at a specific inner layer — it is "blind" because you can only see it from one side of the board. Blind vias free up routing space on layers they do not pass through, and they are less wasteful than through-hole vias on multi-layer boards where a connection is only needed between a surface layer and one specific inner layer.
Blind vias can be laser-drilled microvias (connecting Layer 1 to Layer 2, for example) or mechanically drilled for larger diameters connecting an outer layer to a deeper inner layer.
Buried Vias
A buried via connects two inner layers without touching either outer surface. From the outside, the board looks like a solid surface — the buried via is invisible. Buried vias are used when inner-layer connections are needed but no outer-layer routing space should be consumed. They are drilled and plated in a sub-laminate before the final outer layers are pressed on.
Buried vias add process steps and cost, but they are often the only way to achieve the required routing density in very complex, high-layer-count HDI designs.
HDI Stackup Orders: 1st, 2nd, and 3rd Order
HDI boards are classified by the number of sequential lamination cycles used to build up the microvia layers. Each order adds manufacturing complexity and cost, but also increases routing density.
1st Order HDI
The simplest HDI structure. One layer of microvias connects the outer layers to the first inner layers. A typical 1st order HDI stackup is: microvia layer — core — microvia layer. 1st order HDI is sufficient for most smartphones, tablets, industrial controllers, and consumer wearables. It provides a significant density improvement over standard PCBs at a moderate cost premium.
Linked Electronics Co., Limited's standard HDI capability starts at 1st order, and it is our most commonly requested HDI configuration.
2nd Order HDI
Two sequential lamination cycles create two layers of microvias. 2nd order HDI can include stacked microvias (a microvia on top of another microvia) or staggered microvias (microvias offset from one layer to the next). Stacked microvias require copper-filling of the lower microvia before the upper one is drilled — adding process steps but allowing a smaller footprint.
2nd order HDI is used in advanced smartphones, high-performance computing modules, complex medical devices, and automotive radar units. Linked Electronics Co., Limited manufactures 2nd order HDI with both stacked and staggered via configurations, with minimum laser drill diameter of 0.1 mm and registration tolerance of ±0.05 mm.
3rd Order HDI (and beyond)
Three or more sequential lamination cycles. 3rd order HDI is found in the most demanding applications — flagship smartphone main boards, high-speed networking ASICs, aerospace computing modules, and advanced medical imaging equipment. The manufacturing process is complex and requires tight process control at every lamination step.
At Linked Electronics Co., Limited, we produce 3rd order HDI boards for customers in the industrial and communications sectors. Our engineering team reviews 3rd order designs closely during DFM to ensure the via stackup and pad sizes are achievable within our process capabilities.
When Should You Use HDI PCB?
HDI is not always the right choice — it adds cost and manufacturing lead time compared to standard multilayer PCBs. Consider HDI when one or more of the following conditions apply to your design:
- BGA components with fine pitch (≤0.5 mm) — Standard through-hole vias cannot be routed out from between 0.4 mm or 0.3 mm pitch BGA balls. Microvias are the practical solution.
- High component density with limited board area — If you are constrained to a small form factor (wearable, implant, handheld device) and cannot increase board size, HDI lets you route more circuitry in less space.
- High-speed signals requiring controlled impedance and short via stubs — Through-hole via stubs create signal reflections at high frequencies. Blind microvias eliminate the stub, improving signal integrity.
- Reducing layer count without reducing functionality — HDI often allows an 8-layer standard board to be redesigned as a 6-layer HDI board, reducing cost and thickness while maintaining the same functionality.
- Weight-sensitive applications — Fewer layers and smaller board area mean a lighter board, important in aerospace and portable medical devices.
If your design uses only through-hole components or standard-pitch SMT parts, and board real estate is not constrained, standard multilayer PCB is almost certainly more cost-effective than HDI.
HDI PCB Design Considerations
Moving to HDI requires attention to several design rules that differ from standard PCB design:
- Microvia diameter and capture pad size — Our standard is 0.1 mm laser drill with a 0.35 mm capture pad. Smaller capture pads are possible for very dense designs but require discussion with our engineering team.
- Via-in-pad rules — Vias placed directly under component pads must be copper-filled and planarized before solder mask application. Specify this requirement explicitly in your stackup notes.
- Stacked vs. staggered microvias — Stacked microvias (directly above each other) require copper-filled lower vias; staggered microvias (offset by at least one via diameter) are easier to manufacture. For 2nd order and above, confirm your preference during DFM review.
- Aspect ratio — Microvia depth-to-diameter ratio should not exceed 1:1 for laser-drilled vias. Our engineering team checks this during DFM.
- Impedance controlled layers — HDI boards often include impedance-controlled traces for high-speed signals. Include target impedance values and reference layer information in your stackup documentation.
- Copper fill on buried vias — Specify whether buried vias require copper fill (adds cost but ensures reliable plating in subsequent lamination steps).
HDI PCB Manufacturing Process at Linked Electronics Co., Limited
Manufacturing an HDI PCB is fundamentally different from standard multilayer PCB production. The process involves multiple lamination and laser drilling cycles rather than a single drilling and lamination step.
For a 1st order HDI board, the general process at Linked Electronics Co., Limited is:
- Inner core fabrication — imaging, etching, and AOI of the inner core layers.
- First lamination — pressing the inner core layers with prepreg.
- Laser drilling — forming microvias with a CO2 or UV laser.
- Microvia cleaning and desmear — removing drilling debris from the via walls.
- Copper electroplating — plating the microvia walls (and filling if specified).
- Outer layer imaging and etching — forming the surface copper pattern.
- Second lamination (for 2nd order and above) — repeating the laser drill and plating steps for additional microvia layers.
- Surface finish — ENIG is most common for HDI boards due to its flat surface, critical for fine-pitch BGA soldering.
- Electrical testing, AOI, and final inspection.
Linked Electronics Co., Limited performs in-house laser drilling, copper plating, and all inspection steps. Our minimum trace and space is 2.5/2.5 mil (0.064 mm) on HDI layers, and we maintain registration accuracy of ±0.05 mm across lamination cycles.
Frequently Asked Questions About HDI PCB
What is the minimum via size for HDI PCB?
At Linked Electronics Co., Limited, our standard minimum laser-drilled microvia diameter is 0.1 mm. The corresponding minimum capture pad diameter is 0.35 mm. Mechanical blind vias can be drilled to 0.2 mm minimum diameter.
How much more expensive is HDI PCB compared to standard multilayer?
Cost depends heavily on the order complexity and layer count. A 1st order HDI board typically costs 30–50% more than an equivalent standard multilayer board, primarily due to the additional laser drilling and lamination cycle. 2nd and 3rd order HDI adds further cost increments. However, HDI often allows a reduction in layer count, which partially offsets the per-layer cost increase. Our engineering team can provide a detailed cost comparison during DFM review.
What surface finish do you recommend for HDI PCB?
ENIG (Electroless Nickel Immersion Gold) is strongly recommended for HDI boards. ENIG provides a flat, solderable surface that is critical for fine-pitch BGA assembly, and it is compatible with via-in-pad designs. HASL is not recommended for HDI due to its uneven surface profile.
Can you manufacture HDI PCBs with impedance control?
Yes. Linked Electronics Co., Limited manufactures impedance-controlled HDI boards with tolerance of ±10% (standard) or ±5% (tight tolerance on request). Please include your target impedance values and reference layer assignment in your Gerber documentation.
What is the lead time for HDI PCB?
Standard HDI lead time at Linked Electronics Co., Limited is 10–15 business days for 1st order HDI. 2nd order HDI typically requires 15–20 business days. 3rd order HDI is quoted on a project basis. Expedited options are available — contact our engineering team to discuss your schedule requirements.
Why Choose Linked Electronics Co., Limited for HDI PCB
- In-house laser drilling capability — 0.1 mm minimum microvia diameter, CO2 and UV laser
- 1st, 2nd, and 3rd order HDI — single source for all complexity levels
- Via-in-pad with copper fill and planarization — essential for fine-pitch BGA assembly
- Minimum trace and space 2.5/2.5 mil on HDI layers
- Registration accuracy ±0.05 mm across lamination cycles
- IATF 16949 quality system, RoHS and REACH compliant
- 24-hour DFM review and quoting on submitted Gerber files
- NDA-protected design files — your Gerber data is never shared
- Prototype to mass production — same process and quality controls at all volumes
Ready to Quote Your HDI PCB?
Send your Gerber files, stackup requirements, and impedance targets to [email protected] for a 24-hour HDI PCB quote from Linked Electronics Co., Limited.