PCBMASTER shares with you ----Thermal Management of High-Power PCBs From Material Selection to Structural Design

Author: Jack Wang

Failure analysis of an IGBT module used in a new energy vehicle shows that 80% of the failures are caused by solder joint fatigue due to thermal cycling. When the local temperature rise of the PCB exceeds 85°C, the device life is reduced by 50% for every 10°C increase. Thermal management has become a crucial aspect in high-power applications such as electric vehicles and photovoltaic inverters.

Thermodynamic Considerations in Material Selection

The Trap of Substrate Thermal Conductivity

The thermal conductivity of traditional FR-4 is only 0.3W/mK, while that of aluminum-based printed circuit boards (MCPCBs) can reach 2.0W/mK. However, actual measurements reveal:

1.For a 1.5mm-thick aluminum-based PCB under a heat flux density of 150W/cm², there is still a lateral temperature difference of 22°C.

2.By using a ceramic-filled resin substrate (such as Laird Tflex HD900 with a thermal conductivity of 9W/mK), the temperature difference can be reduced to 8°C.

Actual measurement data of a 5G base station PA module:

Substrate Type	Junction Temperature (°C)	Thermal Resistance (°C/W)
FR-4	128	18.7
Aluminum-Based PCB	95	6.2
Ceramic-Filled Substrate	78	3.8

The Hidden Value of Copper Thickness Design

In the case of a 48-layer server motherboard, when the copper thickness of the power layer is increased from 2oz to 3oz:

The current-carrying capacity is increased by 30%.

However, the thermal coupling effect causes the temperature of the adjacent signal layer to rise by 15°C.

The balanced solution: Use local thick copper (3oz+ in the target area and 1oz in other areas) and cooperate with a heat dissipation via array with a 2mm pitch.

Breakthroughs in Structural Design

1. Practical Application of Metal Embedding Technology

A military radar power module embeds a 0.6mm-thick copper block inside the PCB:

The temperature of the heat source drops by 41°C (from 127°C to 86°C).

However, be alert to the CTE mismatch: The expansion difference between the copper block (17ppm/°C) and FR-4 (14ppm/°C) can cause a 0.15mm deformation during the -40~125°C cycle.

Solution: Set a 0.3mm buffer groove at the edge of the copper block and fill it with highly elastic silicone rubber (Shore A hardness of 40).

2. Innovation in Three-Dimensional Heat Dissipation Architecture

Case of an electric vehicle controller:

Traditional solution: Bottom heat sink + thermal pad (thermal resistance of 0.8°C/W).

Improved solution: Create 3×3mm² heat dissipation channels inside the PCB and directly pour in liquid metal (gallium-indium alloy).

Actual measurement comparison:

Parameters	Traditional Solution	Liquid Metal Solution
Peak Temperature	142°C	103°C
Temperature Uniformity	±18°C	±5°C
Performance after Vibration Test	12% Attenuation	Less than 2% Attenuation

Bridging the Gap between Simulation and Reality

①A photovoltaic inverter project shows that when using Flotherm for simulation:The error under steady-state conditions is less than 5%.

②The error under transient shock conditions can reach 30% (because the creep characteristics of the solder are not considered).

Correction methods:

Import the actual reflow soldering curve data into ANSYS Icepak.

Write the creep model (Norton Power Law) of SAC305 solder into the material library.

Set the non-linear contact thermal resistance (in the dynamic range of 0.05 - 0.15°C·cm²/W).

Actual measurement verification: After model correction, the transient temperature prediction error is reduced to less than 8%, and the efficiency of device layout optimization is increased by 60%.

If you want to know more, please feel free to contact us at www.pcbmaster.com

Author: Jack Wang

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