Thermal Management PCB Design for AI Accelerator Cards: PDN, Via Arrays, and Material Selection

The Thermal Challenge: 700W in a 50mm Package

Modern AI accelerators push 300-700W through a single BGA package mounted on a PCB. The board must simultaneously:

1. Conduct heat away from the die (thermal via arrays to bottom heatsink)
2. Deliver 500-800A at sub-1V to the core (power delivery network)
3. Route 5,000+ high-speed signals at 112G PAM4 (signal integrity)
4. Survive 5-10 years of thermal cycling (125C junction, 40C ambient)

These requirements conflict — thicker copper improves thermal/power performance but degrades signal integrity. This guide covers how to balance them.

Thermal Via Array Design

Geometry and Placement

For a GPU/ASIC with exposed thermal pad:

Thermal Resistance Calculation

Single via thermal resistance:

• Via diameter: 0.3mm
• Board thickness: 2.4mm (typical for 16L+ server board)
• Copper wall thickness: 25 um (1 mil)
• Thermal resistance per via: approximately 70-90 C/W

For 500W die with 400 vias:

• Parallel resistance: 90/400 = 0.225 C/W
• Temperature delta (via contribution): 500W x 0.225 = 112C

This is too high. Supplementary thermal paths are essential:

• Bottom-side heatsink with thermal interface material (TIM)
• Internal copper planes for lateral heat spreading
• Thicker via plating (35-50 um) reduces per-via resistance to 50-60 C/W

HIGH-POWER PCB DESIGN

Up to 30 Layers with 5oz Copper

AtlasPCB fabricates AI server boards with filled thermal vias, heavy copper planes, and via-in-pad for BGA breakout. Min drill 0.15mm mechanical, 0.075mm laser.

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Power Delivery Network (PDN) for 500A+ Loads

The Problem: Transient Response

An AI accelerator switching between idle (100W) and compute (700W) in microseconds demands:

• Less than 3% voltage droop (at 0.85V core = max 25mV droop)
• Sub-nanosecond current delivery from decoupling caps
• Ultra-low loop inductance (< 50 pH for bulk capacitor loops)

PCB PDN Design Rules

Copper weight allocation:

Via farm for power:

• VRM output to GPU: Use 100+ power vias (0.3mm) in parallel
• Each 0.3mm via with 25 um wall at 2.4mm length: approximately 0.5 mohm
• 100 vias in parallel: 5 uohm contribution (acceptable)
• Ground return needs equal via count

Decoupling Strategy

Critical: Via-in-pad for decoupling caps under BGA

Caps placed under the BGA (bottom side or in BGA field) need via-in-pad connections to power/ground planes. These vias must be:

• Filled and planarized (same as thermal vias)
• Connected to the nearest power/ground plane (not routed to distant planes)
• Placed within 2mm of the BGA power ball they serve

HDI CAPABILITY

Via-in-Pad with Copper Fill — Standard Process

We process filled and planarized vias as a standard offering. Stacked microvias up to 5+N+5 for high-density BGA breakout.

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Material Selection for AI Boards

Thermal Cycling Drives Material Choice

A GPU running at 95C junction temperature with fan cycling creates thermal excursions of 60-80C every few minutes. Over 5 years, the PCB sees 100,000+ thermal cycles. This drives material requirements:

Recommended Materials by Tier

Tier 1: High-performance (data center, HPC)

• Panasonic Megtron 6 (R-5775K): Dk 3.71, Df 0.004, Tg 200C
• Isola I-Speed: Dk 3.6, Df 0.004, Tg 200C
• Nelco N7000-2 HT: Dk 3.6, Df 0.005, Tg 200C

Tier 2: Mid-range (edge AI, inference servers)

• Isola 370HR: Dk 4.04, Df 0.009, Tg 180C
• Shengyi S1000-2M: Dk 4.0, Df 0.008, Tg 175C
• TUC TU-872 SLK: Dk 3.9, Df 0.008, Tg 200C

Tier 3: Budget (development boards, eval kits)

• High-Tg FR-4 (Tg 170): Dk 4.2-4.5, Df 0.015-0.020
• Adequate for prototypes where thermal cycling is limited
• Not suitable for production data center deployments

Thermal Conductivity Limits of FR-4

Even premium FR-4-based laminates conduct only 0.3-0.4 W/mK (vs copper at 400 W/mK). The board is fundamentally a thermal insulator. Heat removal strategies must rely on:

• Copper (planes and vias) for thermal conduction
• Thermal interface materials connecting to external heatsinks
• Forced airflow over board surface

For extreme cases (700W+), consider:

• Metal-core PCB for the VRM section (aluminum or copper substrate)
• Embedded copper coin under the die (provides direct thermal path)
• Thermal paste-filled through-holes (experimental, some shops offer this)

MATERIAL SELECTION

Megtron 6, I-Speed, 370HR — All In Stock

We stock high-performance laminates for AI server boards. Get material-specific pricing when you upload your design.

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Stackup Architecture: Balancing All Requirements

A typical 16-layer AI accelerator board stackup:

L1  — Top signal (BGA breakout, high-speed) — 1oz
     Prepreg 1080x2 (3.5 mil)
L2  — GND (reference for L1 impedance) — 2oz  
     Core (5 mil)
L3  — Signal (inner routing) — 1oz
     Prepreg 1080x1 (3.5 mil)
L4  — VCore power — 2oz
     Core (4 mil)
L5  — GND — 2oz
     Prepreg 2116x1 (4.5 mil)
L6  — Signal — 1oz
     Core (4 mil)
L7  — GND — 2oz
     Prepreg 1080x1 (3.5 mil)
L8  — Signal — 1oz
     Core (4 mil)
L9  — Signal — 1oz
     Prepreg 1080x1 (3.5 mil)
L10 — GND — 2oz
     Core (4 mil)
L11 — Signal — 1oz
     Prepreg 2116x1 (4.5 mil)
L12 — GND — 2oz
     Core (4 mil)
L13 — VCore power — 2oz
     Prepreg 1080x1 (3.5 mil)
L14 — Signal (inner routing) — 1oz
     Core (5 mil)
L15 — GND (reference for L16 impedance) — 2oz
     Prepreg 1080x2 (3.5 mil)
L16 — Bottom signal (decap, VRM routing) — 1oz

Key design choices:

• Power planes (L4, L13) sandwiched between ground planes for low-inductance PDN
• 2oz on all GND/PWR planes for thermal and DC resistance
• Symmetric construction for warp-free assembly
• Total thickness approximately 2.4mm (standard for server boards)

Reliability: Surviving 5 Years at 125C Junction

Via Reliability Under Thermal Stress

The most failure-prone element is the plated through-hole via. At 16+ layers and 2.4mm board thickness:

• Aspect ratio: 2.4mm / 0.3mm = 8:1 (at our process limit)
• Each thermal cycle stresses the copper barrel
• CTE mismatch between copper (17 ppm/C) and FR-4 z-axis (50-70 ppm/C) creates tensile stress

Mitigation:

• Use IPC Class 3 copper plating thickness (minimum 25 um average, 20 um minimum)
• Specify IST (Interconnect Stress Test) qualification to 1000 cycles
• Consider via-in-pad fills with conductive paste for additional thermal path
• Backdrilling unused via stubs (also improves signal integrity at 56+ Gbps)

AtlasPCB performs IST and thermal cycling qualification per IPC-TM-650 for all server-class builds.

ATLASPCB

Building AI Hardware? Let's Talk Stackup.

We fabricate boards for GPU/ASIC cards with thermal via arrays, 2-5oz copper, backdrilling, and via-in-pad. Upload your design for engineering review.

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Related Reading:

• HDI PCB Stackup Design: Sequential Lamination and Any-Layer HDI
• PCB Via Reliability Testing: IST, Thermal Cycling, IPC-6012
• High Layer Count PCB Challenges: 20 to 68 Layer Boards
• PCB Trace Width and Current Capacity: IPC-2152 Guide