While planning backpacking trips, I tend to focus intensely on details and trail markers to reassure myself I’m following the correct path. In areas that are not well-marked and with no internet, it’s comforting to know that when I’ve passed from A to B, from B to C, until I’m back to my car. Similarly, conduction paths are important in PCB design — it is crucial to know electricity will flow as expected! Analogous to the electrical conduction is the thermal conduction. The heat from high current or high voltage signals (typically from power circuitry) needs to have a design that allows it to flow from areas where it is being generated. PCB thermal conductivity influences all design aspects, beginning with stack-up and material choices and continuing through the layout. External heatsinks can be included in the design further to reduce heat buildup on or within the board.
PCB Thermal Conductivity: Using Copper Features and Vias to Control Heat Flow
Thermal considerations are a crucial design constraint in any board, but they can quickly move into the forefront when designing copper and layout around high voltage and high current pathways. From a thermal standpoint, designing power circuitry involves many of the same problems and, fortunately, solutions as electrical conductivity.
Begin by examining the package — most components will have an exaggerated metal surface area for improved thermal and electrical conductivity. A typical package might be a TO-220 for a MOSFET, but other more compact designs such as a D2PAK or PQFNs exist. In any of these cases, an oversized metal surface helps provide ample surface area for current flow to spread. By effectively reducing the current density, the package assists the design from creating hotspots that could lead to malfunction or failure. An added effect of the increased surface area is improved cooling by air conduction.
For QFN-styled packages, a central thermal pad may be used to dissipate excess heat; when designing land patterns, thermal pads should be set to maximum tolerances provided by manufacturers’ datasheet for improved thermal performance. Vias will also provide a conduction path away from the component — a good rule of thumb is a via per half-amp of standard operating current.
The Role of Thermal Conductivity in Material Selection
While thermal conductivity has a guiding hand in placement, via fanout, and copper feature design, its influence on the final board begins much earlier. The stack-up and material selection will have an outsized effect on how well the board can dissipate heat. Consider that FR-4, an exceedingly popular choice for PCB substrate material, possesses poor thermal characteristics. There are a few ways to counteract this. As some thermal properties are extrinsic rather than intrinsic to the material, a thinner board may be able to alleviate some issues. However, other factors may restrict board thickness, such as a fixture or interfacing with other boards. Furthermore, even if the thickness can be reduced, it comes with a reduction of mechanical strength that may lead to failure conditions of its own.
Heatsinks Can Provide Additional Relief
The biggest effect on thermal performance will arise from passive and active cooling elements integrated into the design. Heat sinks are often utilized more than other methods due to their flexible functionality. As a passive cooling element, heat sinks can be used in environments and constrained systems that would not allow for bulkier cooling solutions. Additionally, the lack of power draw means that boards employed in the field with limited accessibility or intended for long deployment periods between maintenance have fewer active components that could fail.
The operation of the heatsink is fairly straightforward. Forming a conductive path between the board electronics and the ambient air can provide more effective cooling with a large, exposed surface area. To attach the board to the heatsink and form a conductive pathway with a minimal air gap, a thermal interface material can serve as a bridge between the board and the heat sink. Without continuous contact, heat instead will dissipate in the space between the board and the TIM, reducing the effectiveness of the conduction path and potentially a cause for board failure. Two potential methods to join the PCB with the TIM:
- Phase shift TIM: Strong thermal conductivity design must be joined together under compression with mounting screws. It is important both for the integrity of the conductive path and the material strength of the board itself that mounting holes are placed equidistantly.
- Adhesive TIM: A solution for designs where equidistant mounting holes or other design restraints such as board thickness may inhibit a large compressive force. While not as effective as a heat-conducting solution, it can be used on some boards where the phase shift is not an option.
Let a Contract Manager Take Care of Your Thermal Design
PCB thermal conductivity can be straightforward in application, but balancing other broad design parameters like electrical and mechanical needs can complicate layout and manufacturing. In any system, heat is the enemy — poor cooling or unsuitable conduction pathways will degrade the material.
To avoid heat-related malfunction or failure in your design, you want a CM and partners that can tackle thermal issues from various angles. At VSE, we build electronics for customers by engineers to ensure all aspects of your board are equipped to handle the rigors of operation in a wide variety of environments.
