HDI circuit boards demand high precision in tight space, with little tolerance for error. They appear in smartphones, aerospace systems and other compact high-density products, and getting them right depends on two things: following the relevant IPC standards and using design techniques that hold up in production. This guide covers the four IPC standards that govern HDI, followed by blind via and stackup practices that relieve routing pressure. If you are new to core printed circuit board fundamentals and basic terminology, start with our foundational overview: Printed Circuit Board(PCB)
The IPC Standards That Govern HDI Design
Designing HDI outside the IPC standards tends to end one of two ways: the board does not pass production, or it fails in the field. IPC (Association Connecting Electronics Industries) publishes four standards specific to HDI, and each answers a different question.
1. IPC/JPCA-2315
This standard works as a density check. It is not deeply technical, but it provides a clear model for estimating design density at the start of a project. Enter the requirements and it shows whether the HDI layout is feasible before routing begins, which stops time being spent on a build that cannot be manufactured.
2. IPC-2226
Every HDI design uses micropores, and IPC-2226 covers them in detail. It does more than list rules: it describes how to form micropores, how to set wiring density and design rules, and how to choose materials and interconnection structures. The aim is a common basis for micropore technology, so designs stay consistent and manufacturable. It also settles the question of micropore size, because the acceptable range is stated rather than assumed.
3. IPC-4104
IPC-4104 covers materials. HDI needs thin, specialized laminates, and this standard defines which are acceptable. It uses a slash notation and sorts those thin materials into three categories, which saves working through individual datasheets:
- Dielectric Insulator (IN): the non-conductive material that keeps layers separate
- Conductor (CD): the metal traces and pads that carry signals
- Conductor and Insulator (CI): composite materials that do both
A wrong material choice shows up later as signal integrity loss, so this check comes before material selection. (For common substrate materials, see our guide on What is FR-4? The PCB Industry Standard & Its Limitations and the comparison between aluminum substrate PCB and FR-4 for context on material choices.)
4. IPC-6016
IPC-6016 answers the question of whether a finished HDI board is good. It sets the performance requirements and certification criteria for HDI structures: electrical performance, mechanical durability, and behavior in harsh environments. A board that meets this standard is ready to ship.
Blind Vias: How to Recover Routing Space
Blind vias are what make HDI practical. They let more traces pass through a tight area, which matters most around high-I/O components such as BGAs. The techniques below are the ones that hold up in production.
1. Move Blind Vias to Free Up Space
Blind vias do not have to sit in a fixed position. They can be shifted, or swung, in the XY plane or at an angle to create extra routing room. Placing them on inner layers where the stackup allows, sometimes called 3D placement, opens further breakout space for BGAs.
Adjusting the center distance of inner-layer blind vias works the same way. A small change can free enough room for one more trace, and in a dense design that single trace is often the difference.
Applied near the primary side of the board, these techniques free space under the secondary-side BGA or for decoupling capacitors. That space is usually what prevents a design respin later.
2. Blind Vias for BGA Fan-Out
BGA routing is difficult, and blind vias are what make it manageable. Placing blind vias between through-holes roughly doubles the routing space available on inner layers.
A 1.0mm BGA fits two traces between through-holes on the surface. Add blind vias and that number rises to six, which makes a real difference on high-I/O devices.
Blind vias can also be arranged in patterns, L-shaped or diagonal, depending on where the power and ground pins sit. On FPGAs, reprogramming the power and ground pin positions makes those patterns more effective.
3. Micropore Placement on BGA Pads
Micropores used for BGA fan-out can sit outside the pad, partially on it (partial VIP), or fully on it (VIP). The recurring mistake is placing a through-hole dead center in a BGA pad.
Left unfilled, that hole traps air during reflow: the solder melts, the BGA ball drops, and the air has nowhere to escape.
Placing the through-hole off-center, or eccentric, lets air escape as solder flows into the micropore, which avoids the resulting solder defects.
HDI Stackup Design: Keep It Simple
Stackup design is often overcomplicated. Good performance does not require an exotic build; a few changes in layer placement are usually enough.
The Stackup Change That Cuts EMI
Moving the GND plane, normally the second layer, to the surface removes the GND through-holes that consume valuable space. Moving the PWR plane to layer 2 then allows those through-holes to be replaced with blind vias.
Compared with a traditional microstrip stack, this change brings four advantages:
- Fewer manufacturing issues: no thin lines on the surface to plate or etch
- Lower EMI: a continuous GND pour on the surface acts as a Faraday cage
- Better signal integrity: the PWR layer sits closer to the GND layer, which raises planar capacitance and lowers PDN inductance
- Fewer decoupling capacitors: planar capacitance stores energy close to the components
The Three Standard IPC Stackups
IPC defines three standard stackups, Type I, II and III, all widely used to reduce through-hole counts. The dielectric between the first and second layers can be conventional prepreg, laser-drillable prepreg, RCC (Resin-Coated Copper), reinforced RCC or BC core, depending on the project and the budget.
Where cost matters, the IPC-Type III structure can be replaced by a skip from layer 1 to layer 3. Even with thin dielectrics below 0.005 inch (0.125mm), that build keeps PS impedance low and reduces resonance and noise.
Where HDI Boards Are Used
Design detail tends to crowd out the reason for it. HDI boards serve high-precision, space-constrained products: smartphones, aerospace systems, medical diagnostic tools, industrial robots and high-frequency communication equipment.
What makes them indispensable is the combination of high routing density, small form factor and stable performance, which conventional boards cannot deliver at the same scale. The practices above apply across these applications. For a fuller treatment of HDI fundamentals, technology architecture and industry use cases, see our dedicated guide: HDI PCB: Core Technology for Miniaturized High-Performance Electronic Devices
Key Takeaways
The essentials:
- Use the four IPC standards: IPC/JPCA-2315, IPC-2226, IPC-4104 and IPC-6016.
- Use blind vias deliberately: shift them, place them on inner layers and apply them to BGA fan-out.
- Adjust the stackup: move GND to the surface, PWR to layer 2 and keep dielectrics thin.
- Never center a via in a BGA pad: place it eccentric.
For questions on BGA fan-out, stackup choices or material selection, the engineering team at OPCBA can review the design and advise on the build.
