Basic Guide To Resistor Selection
Step 2 - Mechanical and Installation Characteristics of Resistors
Once the electrical characteristics of a resistor are understood, the next step is selecting one that both fits the application physically and can be assembled with ease. Despite mechanical and installation characteristics often being treated as secondary concerns, in practice, they strongly influence manufacturability, cost, reliability, and even long-term performance. A resistor that is electrically perfect but mechanically inappropriate is still a bad resistor to choose .
Package (Through-Hole vs. SMD)
Resistors are available in a wide range of physical formats and mounting technologies, with the most familiar being the through-hole variants, commonly referred to as Through-Hole Technology (THT). By far most resistors of this type are the axial leaded resistors that pass through drilled holes in a PCB and are soldered on the opposite side with just two leads (but there exist other packages, such as SIL containing multiple resistors). For simple projects, prototypes, and low-volume builds, through-hole resistors remain popular because they are inexpensive, robust, and very easy to handle. They work perfectly with breadboards, stripboards, and hand-assembled PCBs, which also makes them ideal for experimentation and education.
Through Hole Technology (THT)
Through-hole resistors are usually identified by color bands that encode resistance value and tolerance check out our 4 color band resistor calculator here. While not the easiest method to read and decode, this system has endured the test of time because of its simplicity in manufacturing and requires no additional markings on small components. The physical size of these resistors is almost always linked to their power rating, with 1/4 W parts being the most common.
However, the downside of through-hole resistors quickly becomes obvious in modern manufacturing. Drilling holes adds cost, manual insertion is slow, and even if automation equipment is used, it is far less efficient than automation systems used in surface mount technology. Soldering through-hole parts on PCBs can be done rapidly using techniques such as wave soldering, but such techniques can be incompatible with surface mount devices (especially for double sided designs). Thus, for high-volume production, most modern designs rely on surface mount resistors instead.
Surface Mount Technology (SMT)
Surface mount devices, or SMD resistors, are available in standardized package sizes such as 0805, 0603, and 0402, where these numbers describe the physical dimensions of the component (often in inches), and while they may look cryptic at first, they are merely shorthand for length and width (for example, 0603 has a dimension of 0.06" by 0.03"). Being as small as they are, SMD resistors are supplied on reels, designed for automated placement using pick-and-place machines. This makes them perfect for mass production, storage, and consistent assembly.
Just as with through-hole parts, package size directly affects power dissipation, as larger SMD packages can dissipate more heat due to their increased surface area and better thermal coupling to the PCB. Designs involving higher currents or continuous power dissipation generally require larger packages such as 0805 or above, even if the resistance value itself seems trivial.
Mechanical Size
Mechanical size, an often overlooked factor, also greatly affects how a resistor behaves under stress. Very small packages are more sensitive to solder joint fatigue, board flex, and thermal cycling, all of which can rapidly destroy a circuit. In applications exposed to vibration or mechanical shock, using slightly larger resistors than needed can provide better long-term reliability. This trade-off between size and robustness is typically considered during the PCB design stage.
Sadly, in space-constrained designs, minimizing component size is often unavoidable. Here, smaller packages are needed despite their lower power ratings, meaning that engineers need to pay close attention to derating and PCB thermals. Vertical height can occasionally matter, particularly in enclosures with tight clearances, although resistors are usually thin enough that height constraints are never the primary concern.
Mechanical and installation characteristics determine how a resistor fits into the real world rather than the schematic. Choosing the right mounting technology, package size, and physical format ensures that the resistor can be assembled efficiently, operate reliably, and survive the conditions it will be exposed to over its lifetime.
