IPC-2226 Explained: HDI PCB Design Standard & Microvia Rules

Standard PCB design rules fall apart the moment you need to route a 0.5mm pitch BGA. Suddenly, your familiar 8/8 mil trace and space becomes impossible. Via-in-pad becomes mandatory. And those comfortable through-hole vias? They’re blocking escape routes you desperately need. This is where high-density interconnect technology—and IPC-2226—becomes essential.

IPC-2226 is the sectional design standard that governs High Density Interconnect (HDI) printed boards. Officially titled “Sectional Design Standard for High Density Interconnect (HDI) Printed Boards,” this document establishes the requirements for microvia design, HDI stackup configurations, fine-line routing, and the classification system that defines HDI board types. If you’re designing boards with microvias, fine pitch components, or sequential lamination, IPC-2226 is your reference.

What Is IPC-2226?

IPC-2226 establishes requirements and considerations for designing organic and inorganic HDI printed boards. Unlike standard PCB design where through-hole vias connect all layers, HDI technology uses microvias—laser-drilled holes typically ≤150μm (6 mils) in diameter—that connect only adjacent layers or span limited layer distances.

The standard covers microvia formation, HDI construction types, feature size recommendations, thermal management, and the unique design considerations that separate HDI from conventional PCB technology.

IPC-2226 Standard Overview

IPC-2226A Updates from Original

How IPC-2226 Works with Other Standards

IPC-2226 doesn’t stand alone—it’s part of the IPC-2220 family and requires IPC-2221 as its foundation. When your HDI design uses a rigid core, IPC-2222 requirements apply to that core. For flex-core HDI, reference IPC-2223.

IPC-2220 Family Hierarchy for HDI

When designing an HDI board, start with IPC-2221 for baseline requirements, then apply IPC-2226 for HDI-specific guidance, and reference IPC-2222 or IPC-2223 depending on your core construction.

When Do You Need HDI and IPC-2226?

Not every design requires HDI. The technology adds cost and complexity, so understanding when HDI becomes necessary helps you make informed decisions.

HDI Triggers

HDI vs Standard PCB Comparison

IPC-2226 HDI Type Classifications

The heart of IPC-2226 is its classification system for HDI constructions. The standard defines six types based on microvia configuration, buried via presence, and core construction.

HDI Type I Construction

Type I is the simplest HDI construction—a single microvia layer on one or both sides of a conventional core.

HDI Type II Construction

Type II adds buried vias in the core, increasing routing density beyond Type I capability.

HDI Type III Construction

Type III features two or more microvia layers on at least one side, enabling stacked or staggered via configurations.

HDI Types IV, V, and VI

Type III is the most common advanced HDI construction. Types IV-VI address specialized applications including coreless substrates used in advanced semiconductor packaging.

HDI Type Summary Table

HDI Stackup Notation System

IPC-2226 uses a notation system to describe HDI constructions concisely. Understanding this notation helps you communicate designs clearly with fabricators.

Notation Format Explained

The format X+N+Y describes:

• X = Number of microvia/buildup layers on top
• N = Core layers (or C for core, P for passive)
• Y = Number of microvia/buildup layers on bottom

Common HDI Stackup Notations

Stackup Selection by BGA Pitch

Microvia Design Requirements per IPC-2226

Microvias are the defining feature of HDI technology. IPC-2226 establishes the design rules that ensure reliable microvia fabrication and performance.

Microvia Definition

Per IPC-2226, a microvia is:

• Diameter ≤150μm (6 mils) finished hole size
• Spans only one dielectric layer (typically)
• Formed by laser drilling (primarily)
• Aspect ratio ≤1:1

Microvia Feature Sizes

Microvia Aspect Ratio

The aspect ratio determines manufacturability:

Aspect Ratio = Dielectric Thickness / Hole Diameter

Most fabricators prefer 0.75:1 or better (lower ratio). Higher aspect ratios create plating challenges in the via barrel.

Read more IPC Standards:

MIL-PRF-38534: Hybrid Microcircuits Military Specification Guide for Engineers

MIL-PRF-31032: The Definitive Guide to Military-Grade PCB Requirements

MIL-PRF-19500: Military Semiconductor Device Specification Explained

MIL-PRF-123: High-Reliability Ceramic Capacitor Military Standard

MIL-I-46058CMIL-I-46058C: Military Conformal Coating Specification Complete Guide

MIL-HDBK-263: ESD Control Handbook & Implementation Guide

J-STD-075 Guide: PSL Classification for Passive Components & Assembly Process Limits

J-STD-035 Guide: Acoustic Microscopy for IC Package Inspection & Defect Detection

J-STD-033 Guide: MSD Handling, Floor Life, Baking & Dry Storage Requirements

J-STD-032 Explained: Ball Grid Array Ball Construction & Performance Specs

J-STD-027 Guide: Mechanical Outline Standard for Flip Chip & CSP Packages

J-STD-020 Guide: MSL Classification, Reflow Profiles & Moisture Sensitivity Testing

IPC/WHMA-A-620 Explained: Complete Guide to Cable & Wire Harness Standards

IPC-SG-141: Complete Guide to S-Glass Fabric Specifications for PCB Laminates

IPC-QF-143: Complete Guide to Quartz Fabric Specifications for RF/Microwave PCB Laminates

Stacked vs Staggered Microvias

Stacked microvias require copper filling of the lower via before drilling the next level. This adds process steps and cost but saves board area.

Stacked Microvia Requirements

Fine Line and Space Requirements

HDI enables finer traces than conventional PCB technology. IPC-2226 provides guidance on achievable feature sizes.

IPC-2226 Feature Size Guidelines

Design Rule Recommendations

HDI Material Selection

HDI fabrication requires materials compatible with laser drilling and sequential lamination. IPC-2226 references IPC-4104 for HDI material specifications.

HDI-Compatible Materials

Laser Drilling Compatibility

IPC-2226 Thermal Management

HDI boards concentrate more circuitry in less space, intensifying thermal challenges. IPC-2226 addresses thermal design considerations.

Thermal Via Design in HDI

Thermal Resistance Comparison

Tools and Resources for IPC-2226

Official Documentation

Related IPC Standards

Frequently Asked Questions About IPC-2226

What’s the difference between IPC-2226 and IPC-2221?

IPC-2221 is the generic design standard covering all printed boards—conductor spacing, material selection, performance classes. IPC-2226 is a sectional standard specifically for HDI boards that supplements IPC-2221 with microvia design rules, HDI type classifications, and fine-feature requirements. You need both: IPC-2221 for baseline requirements and IPC-2226 for HDI-specific guidance.

When should I use HDI instead of standard PCB technology?

Consider HDI when your design includes BGAs with pitch below 0.8mm, requires via-in-pad for fine-pitch components, needs line/space below 100/100μm (4/4 mil), or when standard through-hole vias consume too much routing area. The cost premium (typically 1.5-3×) is justified when HDI enables a smaller board, fewer layers, or makes an otherwise impossible design manufacturable.

What’s the maximum aspect ratio for microvias per IPC-2226?

IPC-2226 specifies microvia aspect ratios of 1:1 maximum, with 0.75:1 preferred for optimal plating quality. For a 75μm dielectric, this means minimum hole diameters of 75μm (at 1:1) or 100μm (at 0.75:1). Higher aspect ratios create plating voids and reliability issues. Always confirm your fabricator’s capability—some achieve 1:1 reliably, others prefer 0.6:1 or better.

How many microvias can I stack?

IPC-2226 Type III constructions allow stacked microvias, but reliability data suggests limiting stacks to 2-3 vias maximum. Each stacked via must be copper-filled and planarized before the next layer is drilled. Staggered vias (offset position) are more reliable and don’t require filling, so use stacking only where space constraints demand it.

What materials work best for HDI fabrication?

Standard FR-4 works for basic HDI but may show inconsistent laser drilling due to glass fiber reinforcement. RCC (Resin Coated Copper) and ABF (Ajinomoto Build-up Film) are optimized for HDI—no glass reinforcement means cleaner laser drilling and finer features. For high-speed designs, low-Dk versions of these materials maintain signal integrity. Reference IPC-4104 for detailed material specifications.

Designing Reliable HDI Boards with IPC-2226

IPC-2226 provides the framework for designing manufacturable, reliable HDI boards. The standard’s classification system—Types I through VI—helps you select the right construction for your density requirements. The microvia design rules ensure your vias will plate properly and survive thermal cycling. The feature size guidelines keep your design within manufacturing capability.

Start by determining your HDI type based on component pitch and routing density needs. For 0.65-0.8mm pitch BGAs, Type I (1+N+1) usually suffices. For 0.4-0.5mm pitch, plan for Type III (2+N+2) with stacked or staggered microvias. Specify materials appropriate for laser drilling—RCC or ABF for finest features. Maintain microvia aspect ratios at 0.75:1 or better for reliable plating.

The combination of IPC-2221 for generic requirements and IPC-2226 for HDI specifics gives you a complete design framework. Follow these standards, work closely with your fabricator on capability verification, and your HDI designs will be manufacturable and reliable.