Design Rules for High-Density Interconnects in Multilayer Boards

　　High-density interconnect (HDI) technology enables miniaturization and performance enhancement in multilayer PCBs by allowing finer traces, smaller vias, and tighter component packing. Designing HDI boards requires strict adherence to rules that balance electrical performance, manufacturability, and reliability.

　　1. Trace and Space Geometry

　　Minimum Line Width/Spacing:

　　For conventional HDI (1–3 μm copper), 30–50 μm lines/spaces are typical. In advanced HDI (e.g., 5G modules), sub-20 μm features (e.g., 15/15 μm) are achievable with laser direct imaging (LDI) and electroplating.

　　Impedance Control: Microstrip impedance (e.g., 50 Ω ±10%) is critical for HS signals. Use controlled-depth etching (±5% thickness tolerance) and dielectric thickness uniformity (<±2%).

　　Aspect Ratio: For vias, a ratio of 3:1 (via depth to diameter) is ideal for reliable filling. In 10+ layer boards, sequential lamination with blind vias reduces aspect ratio to 2:1 or lower.

　　2. Via Structures

　　Blind/Buried Vias:

　　Blind vias (connecting outer layers to inner layers) enable surface-mount component fan-out without exposing inner traces. Typical sizes: 60–100 μm diameter, buried in 50–100 μm dielectric.

　　Buried vias (connecting inner layers) reduce via density on mid-layers. Laser-drilled vias (30–50 μm) are common for HDI, with copper plating thickness ≥25 μm to prevent cracking.

　　Via-in-Pad (VIP): Allows vias beneath BGA pads, but requires thermal relief pads and anti-pad clearances (e.g., 100 μm pad with 60 μm via, 80 μm anti-pad) to avoid solder bridging.

　　3. Layer Stackup Strategy

　　Signal Layer Separation: Separate high-speed signals (e.g., PCIe, HDMI) from power/ground planes using ≥50 μm dielectric layers to reduce crosstalk. Embedded capacitance (e.g., 1 nF/mm²) in power planes can be achieved with ultra-thin dielectrics (25–50 μm).

　　Core and Prepreg Balance: Symmetrical stackups (e.g., Layer 1- Core -Layer 2- Core -Layer 3) minimize warpage. For asymmetric designs (e.g., heavy top-layer copper), add counterbalance layers with dummy copper.

　　4. Thermal Management

　　Thermal Vias: Arrayed vias (100–150 μm) with thermal vias (filled with copper or solder) connect power components to ground planes. Aim for ≥10 vias/mm² in high-power areas (e.g., voltage regulators).

　　Heat Spreader Layers: Integrate aluminum or graphite sheets between cores to dissipate heat. In LED driver boards, thermal vias connecting to a 2 oz copper plane can reduce hotspot temperatures by 15–20°C.

　　5. Manufacturability and Testing

　　Drill and Etch Tolerances:

　　Laser drilling (±5 μm) is preferred for sub-100 μm vias; mechanical drilling (±25 μm) is used for larger vias.

　　Etch factor (vertical/horizontal etch ratio) should be ≥1.2:1 to prevent trace necking. Use dry film photoresist for fine features instead of liquid ink.

　　Electrical Testing: Flying probe tests for HDI boards with >10,000 nodes, using 50 μm tips for contact. Automated optical inspection (AOI) checks for shorts/opens in sub-50 μm traces.

　　6. EMI/RFI Mitigation

　　Shielding Layers: Add a grounded copper layer between analog and digital sections, connected via stitching vias (500–1,000 vias/m²) to create a Faraday cage.

　　Differential Pairs: Route HS signals (e.g., USB4) as differential pairs with 100 Ω impedance, spaced 3× trace width from adjacent traces to reduce EMI.

　　Case Study: A 20-layer HDI board for a smartphone application uses 40 μm traces, 80 μm blind vias, and sequential lamination. By following IPC-2226 guidelines for HDI, the board achieves a component density of 500 components/in² with <0.5% via failure rate, meeting both performance and reliability targets.