PCB - Hdi

Description

HDI PCBs are printed circuit boards that achieve extremely high wiring density using **finer line widths/spacing and microvias** than conventional PCBs. They are optimized for products requiring miniaturization, high performance, and high-speed signal processing. HDI PCBs are a next-generation printed circuit board that implements ultra-high-density, high-performance circuits through fine patterns and microvias.

Specifications

Layer Structure
• Rigid Area: 2-N Layers (FR-4-based)
• Flex Area: 1-4 Layers (PI-based)
• Overall Structure: Rigid + Flex + Rigid (Multi-Foldable)

Selection Criteria
• Simple Folding/Connection → Flex 1-2 Layers
• High-Speed/High-Density → Flex Multi-Layer + Rigid Multi-Layer

Base Material
Rigid Area
• FR-4 / High-Tg FR-4
• Provides mechanical rigidity and component mounting stability

Flex Area
• Polyimide (PI, Kapton)
• Adhesiveless construction recommended (excellent heat resistance and durability)

Selection Criteria
• Repeated bending and high reliability → Adhesiveless PI
• Cost-conscious and static bending → Adhesive PI

Board Thickness
• Rigid area: 0.8 ~ 1.6 mm (typical)
• Flex area: 0.05 ~ 0.2 mm
• Total thickness depends on stack-up

Thinner for better bending performance, thicker for better rigidity

Copper Type & Weight
• RA Copper (Rolled Copper): Required for Flex Areas, Recommended
• ED Copper (Electrolytic Copper): General for Rigid Areas
• Thickness: 0.5 to 1 oz (Flex), Up to 2 oz (Rigid)

RA Copper is required for repetitive bending areas

Lines & Spaces
• Standard: 75–100 μm
• Minimum: 50 μm (HDI/laser process)

The finer the material, the greater the impact on cost and yield.

Via Specifications
• Through Via (PTH)
• Blind/Buried Via
• Micro Via (Laser Via)

Micro Via is used for high-density, miniaturized designs.

Stack-up design
• Separate signal, power, and ground layers
• Symmetrical structure minimizes warpage
• Impedance control (differential, high-speed signaling)

Critical impact on SI/PI/EMI performance

Protection Layers
• Rigid Area: Solder Mask
• Flex Area: Coverlay (PI + Adhesive)

Coverlay is required for bending sections.

Stiffener
• Materials: FR-4, PI, SUS
• Usage: Connector fastening, ensuring rigidity of component mounting areas

Surface Finish
• ENIG: Excellent flatness, solderability, and reliability (recommended)
• OSP / Im Tin / Im Silver: Select based on application

Bending Radius
• Static bending: 6 to 10 times the thickness
• Dynamic bending: 10 to 20 times the thickness

Risk of cracks and short circuits if radius is not observed

Electrical Test
• 100% BBT inspection
• Open/Short circuit confirmation
• Required for high-reliability products

Technical Data

Item	General Specifications
Structure	Rigid + Flex Integration
Rigid material	FR-4 / High-Tg
Flex material	PI (Kapton)
Copper Foil	RA / ED, 0.5~1 oz
Line WIdth/Pitch	75~100 μm
Via	PTH / Blind / Micro
Protect	Solder Mask / Coverlay
Surface Finish	ENIG recommended
Test	BBT transfer