Basic Guide To Transistor Selection
Step 3 – Mechanical, Package, and Thermal Characteristics
Mechanical considerations are rarely the first thing engineers want to think about, but with transistors, they matter more than many people expect. In practice, package choice is often constrained rather than freely selected, as once you fix the technology and the electrical ratings, the available packages narrow quickly, especially as power dissipation increases.
Low Power Level Packages
At low power levels, package choice is largely more about convenience and assembly methods. You will usually have both through-hole and surface-mount options, and neither presents serious mechanical challenges. Transistors are generally forgiving in terms of size and placement, with most SMD packages being thin enough to avoid clearance issues, and through-hole devices often allowing to be mounted vertically if board space is tight. Compared to connectors, displays, or large magnetics, transistors are rarely the component that breaks a mechanical design.
High Power Level Packages
This flexibility in package selection disappears as power goes up. High current and high dissipation devices require packages that can physically move heat away from the silicon die (often integrating large thermal pads and tabs). At that point, the package is no longer a secondary concern, but becomes part of the thermal design, whether you planned for it or not.
Package Thermal Characteristics
The thermal characteristics of a package are critical during the transistor selection process (the thermal analysis of transistors and their packages could fill an entire chapter on its own). To keep things simple, transistor datasheets tightly couple thermal performance to the package geometry and materials, and one of the first parameters to look at is the junction-to-case thermal resistance. This value describes how effectively heat can travel from the transistor’s silicon junction to the outside of the package, where a lower number means heat escapes more easily, directly improving reliability.
If the application requires a heatsink, then the next factor to consider is the case-to-heatsink thermal resistance, and this highly depends on the mounting method, interface materials, and surface finish used. A perfectly rated transistor can still overheat if the thermal path beyond the package is poorly designed.
Through Hole Transistor Technologies (THT)
For small-signal and low-power applications, the available package choices is extremely generous. Common through-hole packages such as TO-92 and TO-220 are mechanically robust and easy to work with, tolerating manual assembly, rework, and harsh handling better than most SMD parts. Their physical size also makes them ideal for prototyping, education, and low-volume production. Despite their simplicity, packages like TO-220 can handle power levels in the tens of watts when properly heatsinked (making them common with voltage regulators).
SMD Packages
When dealing with surface-mount transistor packages, engineers are spoiled for choice, but sadly such packages are not always as standardized as one might hope (compared to those like TO-92). At lower power levels, small-outline transistor packages such as SOT-23, SOT-89, and SOT-323 are extremely common, and are compact, inexpensive, and well suited to automated assembly. However, naming conventions and footprints can vary between manufacturers, which makes careful footprint verification extremely essential (some manufacturers, who shall not be named, like to create their own custom packages which can quickly lock designs).
As power requirements increase, SMD packages grow accordingly, with packages such as DPAK, TO-263, and TO-277 providing larger thermal pads and lower thermal resistance paths into the PCB. In these cases, the circuit board itself becomes part of the heatsinking strategy, with copper area, layer count, and thermal vias playing a major role in heat removal.
For very high power applications, even soldered packages reach their limits. At this end of the scale, transistors may use press-fit pins, large exposed thermal bases, or direct mounting to metal substrates with screw terminals. These types of transistors stop being simple components, and start becoming mechanical assemblies that demand deliberate thermal and structural design.
Transistor package selection is where electrical theory meets physical reality. A transistor that looks perfect on paper can be rendered useless by a poor thermal path or an unsuitable package. Treat the package as part of the system, not an afterthought, and many reliability problems simply never appear.
