Thermal management for Printed Circuit Boards

PCB heat management

Due to the trend towards miniaturization, the heat output per unit area increases, which means that more and more heat is transferred to a smaller and smaller surface. Often this is the case for LED applications, which convert the greater part of the current into heat rather than light.

Therefore, not only in high-performance applications is sufficient heat management of central importance. This should be designed individually for each specific application and the environmental conditions of the entire system. 

Basic considerations for thermal management

As a carrier of the components, the printed circuit board is the main component for achieving coherent thermal management. Inadequate optimization must be compensated later by possible cost-increasing measures.

In advance, the following basic considerations should be made:

  • Which amount of heat (power loss) must be removed where?
  • What are the dimensions and performance data of the components?
  • Where are the heat sources (such as LEDs) on the PCB?
  • Available space and mounting peripherals?
  • Application temperatures and ambient temperatures?
  • What mechanisms are used to cool the system? (Temperature drops)
  • How should heat be conducted to the heat sinks?
  • Are there certain reliability requirements (e.g. cycle stability)?

Due to the low thermal conductivity of FR4 (0.2 - 0.5 W/mK), pure FR4 printed circuit boards are usually not suitable for heat-intensive applications (eg high-power LEDs).

The following options for thermal management on printed circuit boards are available:

  • Design / construction of printed circuit board
  • Thick copper PCBs
  • Thermal Via Array
  • Metal core – IMS

Design / construction of printed circuit boards

The following options can help to limit heat generation or dissipate heat better:

  • Installation of the printed circuit board upright
  • black solder stop (better heat dissipation)
  • local separation of high power and signal conductors
  • wide tracks for heavy currents
  • additional copper surfaces and higher copper thicknesses
  • additional thermal vias (see below)

A thermal simulation or prototype measurement can help to better locate hotspots.

Thick copper PCBs

Thick copper circuit boards (eg 70μm, 105μm, max 400μm) improve the heat dissipation and thus allow heat distribution over larger surfaces. The heat distribution occurs in the copper layer on the PCB surface and inner layers.

Thick copper circuit boards are primarily used for high power (high current) applications and for cooling components with high thermal dissipation.

Please note the maximum conductor width depending on the copper thickness.

Thermal Via Array

[Translate to English:] Beispiel: Leiterplatte Via in Pad

Thin, double-sided or multilayer printed circuit boards with a array of thermal vias (Via-in-Pad), which improve the heat conduction. The thickness of the circuit board is typically 0.3 - 1.0mm.

Two types of thermal vias are possible:

Filled & capped vias (IPC-4761 type VII) have the important advantage that they can be placed directly under a thermal solder pad of LEDs and thus derive heat directly. The copper filling also prevents solder flow (wicking) and uncontrolled cavities in the via during reflow soldering.

In many cases, however, simple thermal vias are sufficient to achieve a significant reduction in thermal resistance to the target value.

The extent of the resulting thermal resistance is affected by the number and position of the vias. The closer the vias are to the heat source, the better and faster heat can dissipate and the lower the thermal resistance.

Thermal Via Array design example for the Texas Instruments DRV8312:

Thermal Via Footprint

Source: Texas Instruments DRV8312 datasheet

Metal core – IMS

Metal core (also called IMS - Insulated Metal Substrate) PCBs are used to dissipate heat quickly and effectively from components. At Multi-CB you get IMS circuit boards with aluminum core.

Due to its significantly higher thermal conductivity compared to FR4 technology, IMS is widely used in LED applications, especially high power LEDs. In most applications, IMS circuit boards, along with a thermal interface material (e.g. thermally conductive paste ), are attached to a heat sink by screws.

When using an IMS board with ceramic LEDs, it should be noted that the difference in thermal expansion coefficients (CTE-z) between the LED ceramic material and the IMS board creates stress on the solder joint.

Typical construction of a one-sided metal core PCB:

Metal core PCB - typical build-up

You can find all metal core capabilites here: Metal core (IMS) circuit boards.Metal core PCBs

Thermal comparison of PCB types (Osram)

 

Based on data from the OSRAM OSTAR Compact high-performance LED (company Osram) and a predefined heat sink, an exemplary thermal comparison of specific printed circuit board concepts was carried out by thermal simulation.

Typical characteristics and material parameters were used for the various PCB concepts.

Here is the comparison between FR4 (with thermal vias) and metal core:

[Translate to English:] Metallkern vs. FR4

Due to the good thermal distribution and mechanical stability, IMS technology with improved dielectrics is preferable for many heat-intensive applications (e.g. LED), especially for higher power classes.

In the medium to high power range, the focus is on FR4 technology with thermally optimized layouts. With simple measures such as additional copper surfaces, higher copper thicknesses and thermal vias, the overall system can be positively influenced.