Why heat poses such a detrimental effect on a board.
How designers can design thermal circuits for analysis.
The role of heat sinks alongside other fundamental thermal management features.
A heat sink for SMD components may come in many shapes and sizes, but all perform the same crucial task of drawing heat away from the board.
If an ounce of prevention is worth a pound of cure, a heat sink for SMD components pays for itself several times over a board’s field usage. Acting analogous to a thermal shunt, heat sinks are arguably the most effective passive heat dissipators evaluated on a per unit basis (area, weight, etc.), though they need to operate in tandem with thermal vias and copper pours to maximize thermal flux. Not every design requires the usage of a separate heat sink (for some components, heat sinks may be part of the package), and tight designs may inhibit their placement and performance. However, nearly all designs can benefit from improving thermal routing.
A Heat Sink for SMD Components Extends Service Life
While there may be slight deviations due to function and environment, heat is almost universally accepted as the primary failure route or cause of a reduced lifespan for a board. Mechanically, heat wears down materials over long periods, either by regular heating/cooling cycles, extended operation at elevated temperatures, or thermal shock experienced by entering into a temperature range beyond a component’s rating. The combination of these different thermomechanical failure mechanisms ultimately results in embrittlement of material, cracking, and crack propagation in the long term, while temperatures above maximum ratings can cause much more immediate component failure. In any case, the degradation of the material due to Joule heating is an irreversible process, making its mitigation key to preventing early board maintenance or replacement.
Although boards are shrinking, feature and performance demands climb ever higher. High-density assemblies inhibit airflow, and active cooling solutions often take up exceptionally large amounts of space within enclosures that may not be available due to prior established constraints. With less area for heat to dissipate, the relative effects become more pronounced. For boards and enclosures that allow it, heat sinks are an excellent option. By providing a conductive thermal path from heat-generating components to a large surface area for improved convective and radiative cooling, heat sinks enhance the passive cooling effects of any associated components.
Thermal Solutions Extend Beyond Heat Sinks
Thermal relief technology includes more than just heat sinks, however. Depending on package constructions and technologies, there are two common SMD solutions for excess heat generation:
- Heat sinks, as mentioned, form a conductive path with components and offer improved radiation and convection cooling by increasing the effective air-SMD surface area.
- Thermal-enhanced leadframes (TEL) also draw heat away from a package using metal bridges that connect pins to the leadframe but direct it towards an internal plane of the board without any additional devices.
The latter of the two methods, though technically outside the scope of this article’s focus, is a useful manufacturer’s tool in thermal routing and effectively acts as a heatsink via an exposed contact on the underside of the package. The difference between similar packages with and without TEL can be in the range of 20℃+ at the high end of operating temperatures.
Thermal Circuit Theory Provides Guidelines for Heat Sink Implementation
Heat sinks utilize thermal routing design principles to transport heat away from its source to an area or element where it is more easily dissipative and less likely to cause heat-related aging issues. Much like electrical routing, thermal routing relies on conductive materials and sufficient trace width, pour areas, and vias to adequately handle the heat flux flowing out of its points of generation. Tracking this heat transit becomes slightly more involved for SMD component heat sinks rather than those operating at the board level: consider that heat has to travel from the chip, onto the chip carrier, out the pin, through the solder, into the footprint, and then through the substrate materials either with or without the aid of a via.
The Issue of Linearity in Modeling
To better model thermal resistance in a circuit, an analogous relationship to Ohm’s Law can be made using thermal properties. Substituting power dissipated, thermal resistance, and temperature for current, ohmic resistance, and voltage, respectively, allows engineers to devise a linear relationship between heat and power. After plotting this information, designers can determine the maximum dissipation for any temperature that falls within the operating range.
Just like electronic circuits, however, linearity is not a guarantee; in non-linear cases, the thermal equivalent of Ohm’s Law is no longer applicable. It’s possible instead to measure the total amount of heat absorbed (or emitted) using the thermal heat capacity of the material in question. This is a rule-of-thumb approach by hand (it can be expanded upon with finite element analysis and other sophisticated modeling techniques) due to the different intrinsic and extrinsic properties of the materials composing the device. Crudely, the mass can be calculated using different package dimensions and the density of the material; the product of the mass of a particular element (case, pins, die, etc.) and its specific heat capacity indicates the thermal capacity, the device's total of which can be summed from the discrete elements.
It is far more likely for the manufacturer to include any relevant thermal information, usually including graphs of thermal impedance for continuous and pulsed power modes. However, these techniques provide a ballpark estimation to determine the necessity of a heat sink or other thermal management features.
Designing for Thermal Considerations Vis-à-Vis Simulation
A heat sink for SMD components can take on many forms, but whether that’s a thermal pad, a tab, or a device all its own, the goal is the same: draw heat away from its generating source to prevent the early aging of materials and associated degradation. Combined with other passive thermal design elements like thermal vias and increased copper areas for improved heat flux capabilities, they represent the most space and energy-efficient choices for thermal management in board designs. In severe cases, active cooling solutions such as fans, heat exchangers, and more may be utilized to maintain optimal operating conditions for boards generating significant amounts of heat.
Simulating the effects of thermal sources and sinks on designs is a cinch with the powerful library of Cadence’s PCB design and analysis software. Designers can integrate the results of these models into OrCAD PCB Designer for an expedited layout process with a focus on thermal design for manufacturing.
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