If you’ve ever picked up an old radio, a vintage gaming console, or even a space heater from the 1990s, the odds are high that you were looking at through-hole technology (THT) in action. As a PCB supplier who’s been in this industry for 12 years, I’ve watched THT dominate the landscape for decades—my first project in the mid-2010s was assembling a batch of industrial control boards entirely with through-hole components, and I still have the hand-soldering tip I used back then, stained with decades of rosin flux. But over the last decade, the shift to surface-mount technology (SMT) has accelerated dramatically, and it’s not just because it’s trendy. Every time a customer calls us with a new project, we talk through the pros and cons of THT, and more often than not, they’re leaning toward SMT now—because they’ve hit the same limitations of through-hole that we’ve seen play out time and again. Let’s break down what those limitations actually are, because it’s not just a matter of “SMT is smaller”; it’s tangible, real-world challenges that affect production, cost, reliability, and even design flexibility. Printed Circuit board
First, let’s ground this in what THT actually is, for anyone who might be new to PCB manufacturing. Through-hole components have leads that are inserted into holes drilled through the printed circuit board, then soldered on the opposite side to connect to the board’s copper traces. It’s a method that’s been around since the 1950s, so it’s got a lot of history, and it’s still useful for some specific use cases—like high-power connectors or large electrolytic capacitors. But its core design is a major constraint, and that’s physical size and board density. When I started, a typical consumer PCB might have been 4×6 inches, and most of that space was wasted on the bulky bodies of through-hole parts and the gaps between them. A standard through-hole resistor, for example, is about a quarter of an inch long from lead to lead, and you need at least an eighth of an inch of clearance between adjacent parts to avoid solder bridges during hand-soldering. Compare that to a surface-mount resistor, which is barely 0.02 inches long, and can be placed as close as 0.005 inches apart—enough to fit 10 times more resistors in the same space. This isn’t just a minor detail; it’s a game-changer for industries like smartphones, wearables, and IoT sensors, where every cubic millimeter counts. A few years ago, we had a startup client come to us with a smart thermostat project. They’d initially designed it for THT, and the board was 30% larger than their target size, which meant the thermostat would have been too bulky to fit inside most standard wall plates. Switching to SMT cut the board size by almost half, and they were able to hit their launch deadline without reworking their entire enclosure design. THT’s size limitations aren’t just about small consumer devices, either—they affect industrial equipment too. A robotics client we worked with last year had to shrink their control module to fit inside the arm of a collaborative robot, and through-hole was non-negotiable for some high-current parts, but without SMT, they would have had to expand the arm by 2 inches, which would have made the robot too big to work on a standard factory floor. It’s not that THT is useless for size-sensitive projects; it’s that its inherent bulk makes it a poor fit for anything that demands compact, space-efficient designs.
Next up, let’s talk about production efficiency and cost at scale. Back in the day, THT assembly was mostly done by hand. When I was first learning the trade, I spent weeks practicing hand-soldering through-hole connections, because that’s what most small batch jobs required. Even today, if you run a small electronics shop with 100 or 200 units a month, hand-soldering THT parts is manageable. But when you’re talking about high-volume production—like 10,000 units a month or more—THT becomes incredibly slow and expensive. SMT uses automated pick-and-place machines that can accurately position hundreds of tiny components per hour, then solder them in a reflow oven in a single pass. THT, on the other hand, requires inserting each component by hand (or with slow, specialized through-hole insertion machines that cost a fortune, and are hard to find these days) then soldering them, either by hand or with a wave solder machine. Wave solder works by passing the bottom of the board over a molten solder wave, but even then, you have to mask off parts of the board that don’t need to be soldered, and you still get a lot of solder defects. We had a client a few years ago who was producing 50,000 units of a portable power bank with THT. They thought THT would be more reliable for the power connectors, but after a month of production, their defect rate was 8.2%—most of which were solder bridges or cold joints from manual soldering. They switched to a hybrid THT/SMT design, and their defect rate dropped to 1.1%, and their production time per unit was cut by 40%. The cost difference adds up fast, too. For high-volume orders, SMT assembly costs about 20-30% less per unit than THT, because of the speed and lower labor required. When you’re making 100,000 units, that’s tens of thousands of dollars in savings. The other catch with THT production is lead times. Finding parts for THT, especially newer, specialized components, is getting harder. Most component manufacturers are phasing out through-hole lines to focus on SMT, so if you need a custom through-hole part, you might have to wait 12 weeks or more, compared to 2 weeks for a surface-mount equivalent. We had a client working on a medical device that required a rare through-hole connector, and the manufacturer quoted a 16-week lead time, which would have delayed their FDA approval. Switching to a surface-mount variant cut the lead time to 3 weeks, which kept their project on track.
Then there’s reliability, or more accurately, the reliability drawbacks of THT in modern applications. It’s a common misconception that THT is more reliable than SMT, because the leads go through the board and are soldered on both sides. But that was true back when SMT was new, and solder joint technology was less advanced. Today, SMT joints are just as reliable, if not more so, for most applications. The key issue with THT reliability is thermal stress and vibration. Through-hole components have longer leads, which act like little antennas for heat and mechanical stress. When a device heats up and cools down repeatedly—like a server or a car’s engine control unit—the copper traces and the component leads expand and contract at different rates. That causes stress on the solder joint, and over time, it can crack. For parts that are moving, like components in a washing machine or a drone, the vibration makes this even worse. We had a automotive client a few years ago who was testing a THT-based control module for electric car batteries. After 1,000 thermal cycles, 12% of the solder joints on the through-hole capacitors had cracked, causing the module to fail. They switched to surface-mount capacitors, and the failure rate dropped to less than 0.5% after the same number of cycles. Another reliability issue with THT is moisture sensitivity. Through-hole boards have more exposed surface area and gaps between components, which can trap moisture during manufacturing, especially when you’re using wave soldering. If moisture gets trapped, it can boil during high-temperature soldering, causing tiny gaps in the solder joints, or even damage the component itself. SMT boards have more uniform surface areas, so moisture is easier to control during the soldering process, leading to fewer defects. It’s also worth noting that THT is more prone to human error during assembly, especially when done by hand. A solder joint that’s too cold, a lead that’s not fully inserted, or a part that’s placed backwards can all cause failures. Even with the best quality control, it’s hard to catch every single defect in high-volume production, whereas automated SMT inspection systems can pick up even the tiniest solder joint flaw.
Another big limitation is design flexibility and compatibility. When you use THT, your design is constrained by the size and placement of through-hole components, which limits your ability to innovate. For example, if you want to add a wireless charging coil to a smartphone, that’s a large, flat surface-mount component, which fits perfectly with SMT. A through-hole wireless coil would be bulky, would take up half the inside of the phone, and would make the phone too thick to compete in the market. Even for non-consumer projects, like aerospace or defense, THT’s design constraints are a problem. Those industries need to pack more functionality into smaller, lighter boards to meet weight and size requirements for aircraft and satellites. THT’s bulk adds unnecessary weight, which increases fuel costs for aircraft and reduces the payload capacity of satellites. We worked with an aerospace client a few years ago who was designing a satellite sensor module. The original THT design was 2 pounds, and switching to SMT cut that to 0.8 pounds, which saved them $12,000 in launch costs (launching anything to space costs about $10,000 per pound). That’s a huge difference, and it’s all because of THT’s inherent size and weight limitations. There’s also the issue of component availability for long product lifecycles. If you’re manufacturing a product that needs to be supported for 10 or 15 years—like a medical device or industrial control system—you need to be able to source components over that time. As component manufacturers phase out THT parts, it’s getting harder to find long-term supply for through-hole components. SMT parts are still being produced in high volumes, so they’re more likely to be available for decades. We had a client in the medical device industry that was manufacturing a diagnostic machine for hospitals. They needed to support the machine for 15 years, and their original THT design had a connector that was being phased out by the manufacturer. We helped them switch to a surface-mount connector that was still in production, and they were able to meet their 15-year support requirement without reengineering the entire machine.
I want to be clear here: this isn’t to say THT is obsolete. There are still very valid use cases for through-hole technology. High-power components, like large electrolytic capacitors or power connectors, often benefit from through-hole mounting because the through-hole connection can handle higher current and heat better than surface-mount. Some repair and prototyping work also uses THT, because it’s easier to replace a through-hole component if it’s damaged. But for most high-volume, size-sensitive, or long-lifecycle projects, the limitations of THT make it a less viable option than SMT. As a PCB supplier, we’ve seen clients struggle with these limitations for years, and our job is to help them navigate that. We don’t push SMT just because it’s more profitable—we present the facts: what their size, volume, reliability, and timeline requirements are, and weigh the pros and cons of THT and SMT accordingly.
If you’re working on a new PCB design, or you’re having issues with production delays, defects, or size constraints on a current project, our team has the experience to help you find the right solution. We’ve helped dozens of clients transition from THT to hybrid THT/SMT designs that balance the reliability of through-hole parts with the efficiency of surface-mount assembly. Whether you need a small prototype run, a high-volume production order, or help troubleshooting a current issue, we’re here to walk through your project with you. Reach out to our team to discuss your PCB needs and find the best approach for your application.
Metal Based Board References
- Harper, C. A. (2003). Printed Circuits Handbook (5th ed.). McGraw-Hill.
- Tummala, R. R. (2017). Fundamentals of Microsystems Packaging (2nd ed.). McGraw-Hill.
- National Center for Manufacturing Sciences. (2020). Through-Hole vs. Surface-Mount Technology: Cost, Efficiency, and Reliability Comparison. NCMS Technical Report Series.
- IPC Association Connecting Electronics Industries. (2019). Design Guide for High-Reliability Printed Circuit Boards. IPC-2221.
- Aerospace Industries Association. (2021). Spacecraft Electronic Component and Packaging Requirements. AIA-2021-001.
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