Heat Collapse in Dense Electronics

Modern electronics are shrinking while their power consumption grows. A smartphone processor dissipates 5–10 watts in a package the size of a fingernail. A server CPU dissipates 200–400 watts. An electric vehicle inverter dissipates 1–2 kilowatts. This heat must be removed, or the semiconductor junctions will exceed their maximum operating temperature-typically 125°C for silicon and 175°C for silicon carbide.

When a junction overheats, several things happen. First, the leakage current increases, causing more heat generation-a positive feedback loop. Second, the carrier mobility decreases, reducing the switching speed and increasing the on-resistance. Third, the thermal expansion of the package creates mechanical stress, leading to solder fatigue and delamination. Eventually, the device fails catastrophically.

Traditional PCB materials are poor thermal conductors. FR-4, the most common laminate, has a thermal conductivity of about 0.3 W/m·K-roughly the same as wood. A 1.6 mm thick FR-4 board has a thermal resistance of about 50°C/W per square centimeter. This means a 1 W heat source raises the board temperature by 50°C above ambient. For a 10 W source, the temperature rise is 500°C-impossible to sustain.

The solution is to provide a low-resistance thermal path from the heat source to the ambient environment. Copper, with its thermal conductivity of 390 W/m·K, is the ideal material for this purpose. It conducts heat 1,300 times better than FR-4.

Heat Transfer Principle

Heat transfer occurs through three mechanisms: conduction, convection, and radiation. In electronics, conduction is the primary mechanism for moving heat from the junction to the heatsink. Convection and radiation then transfer heat from the heatsink to the ambient air.

Copper plays a critical role in the conduction path. A copper thermal pad or spreader is placed between the heat source and the heatsink. It spreads the heat over a larger area, reducing the heat flux and allowing the heatsink to work more efficiently.

The effectiveness of a copper spreader depends on its thickness and area. A thicker spreader conducts heat more effectively in the lateral direction, spreading it over a larger area. A larger spreader provides more surface area for convection. However, there is a point of diminishing returns-beyond a certain thickness, the additional copper provides little benefit.

The thermal resistance of a copper spreader is given by the equation: R = L / (k × A), where L is the thickness, k is the thermal conductivity, and A is the cross-sectional area. For a 3 mm thick, 20 × 20 mm spreader, the thermal resistance is about 0.02°C/W-negligible compared to the junction-to-case resistance of a typical power device.

The interface between the copper spreader and the heat source is equally important. Even a microscopic air gap-just 10 µm-creates a thermal resistance of about 0.1°C·cm²/W. This is why thermal interface materials (TIMs) are used. A good TIM fills the gaps and provides a continuous thermal path.

Component Types

• Cold Plates with Micro-Channels.​ These are copper blocks with internal channels through which coolant flows. The coolant absorbs heat from the copper and carries it away. Micro-channels-channels with a hydraulic diameter of 100–500 µm-provide a high surface area-to-volume ratio, enabling heat transfer coefficients of 10,000–50,000 W/m²·K. This is 10–50 times better than a conventional heatsink.
• Press-Fit Heat Blocks.​ These are copper blocks that are pressed into a PCB to provide a thermal path from a component on the top side to a heatsink on the bottom side. The block is machined to a precise height and press-fit into a plated through-hole. The interference fit ensures good thermal contact.
• Copper Core PCB Backing Bars.​ These are copper bars that are bonded to the back of a PCB to spread heat from multiple components. They are typically 2–5 mm thick and are attached with thermally conductive adhesive or solder.
• Evaporator-Style Finned Links.​ These are copper structures with fins that increase the surface area for convection. They are used in applications where a heatsink is not practical, such as in sealed enclosures. The fins are machined or stamped from copper sheet and are attached to the heat source.

Design Execution

• Step 1: Calculate the heat load.​ Determine the total power dissipation of the heat source. This is the starting point for all thermal design.
• Step 2: Define the maximum temperature.​ Determine the maximum allowable junction temperature and the maximum ambient temperature. The difference is the allowable temperature rise.
• Step 3: Calculate the thermal resistance budget.​ Divide the allowable temperature rise by the heat load to get the maximum allowable thermal resistance from junction to ambient.
• Step 4: Allocate the budget.​ The total thermal resistance is the sum of the junction-to-case resistance, the TIM resistance, the spreader resistance, and the heatsink resistance. Allocate the budget among these components.
• Step 5: Design the copper spreader.​ Choose the thickness and area based on the allocated budget. Use the thermal resistance equation to verify the design.
• Step 6: Select the TIM.​ Choose a TIM with a thermal conductivity appropriate for the application. Apply it in a thin, uniform layer.
• Step 7: Verify the design.​ Use thermal simulation software to verify the design before production. This is cheaper than building and testing prototypes.

Practical Experience

A power electronics manufacturer designed a liquid-cooled copper cold plate for a 400 W IGBT module. The initial design used a brazed construction, with the internal channels formed by brazing two copper plates together. The brazed joints leaked under pressure testing.

The redesign used a CNC-machined construction. The channels were milled into a solid copper block, and a flat cover plate was welded on top. The welded construction eliminated the leak problem. The cold plate achieved a thermal resistance of 0.02°C/W, maintaining the IGBT junction temperature below 100°C at a coolant flow rate of 2 L/min.

The lesson: the manufacturing process is as important as the design. Brazed joints are a weak point in liquid-cooled systems. Machined channels with welded covers are more reliable.

Insight

Thermal resistance is cumulative: copper helps only if interfaces are flat. A perfectly designed copper spreader is useless if the interface to the heat source is poor. The TIM must be applied correctly, the surfaces must be clean, and the clamping force must be sufficient to minimize the gap.

For OEM buyers, the recommendation is to specify the entire thermal path, not just the copper part. Include the TIM, the clamping mechanism, and the heatsink in the specification. This ensures the complete system performs as intended.

Future Path

• Additive Manufacturing of Conformal Copper Cooling Skins.​ 3D printing enables the creation of copper structures that conform to the shape of the PCB or the component. These conformal cooling skins provide intimate thermal contact and can be optimized for the specific heat distribution of the application.
• Embedded Cooling Channels.​ Instead of attaching a separate cold plate, the cooling channels can be embedded directly into the PCB. This requires copper-filled vias and channels, which are still experimental but show promise.
• Two-Phase Cooling.​ Using a refrigerant that boils at the operating temperature, two-phase cooling can absorb large amounts of heat through the latent heat of vaporization. Copper is the ideal material for the evaporator section of such systems.