A funny thing about physicists (or physics students) is they can describe the complex phenomena occurring within modern electronics but don’t always trust them to fill out your BOM. Case in point: while building out a prototype for a tachometer project with a fellow student, we made the mistake of purchasing several surface mount components instead of through holes. At this point in my education, I knew better too, but I didn’t review the order in time before it was placed.
While SMT is typically renowned for its small package size, this was a detriment to our level of design; we could fit numerous chips within the opening of a breadboard’s node. Though it may be a problem that is unlikely to affect most designers, this story underlines that through-hole vs. surface mount technology is not an entirely open-and-shut case.
Through-Hole: An Older, But Not Antiquated Technology
The first time working with components for hobbyists and students is almost certain to be through-hole technology. Though they may initially be used in an educational prototype like a breadboard, they are equally compatible with a fabricated board as an end-use product. The leads of through-hole components are placed through drilled holes in the PCB board. Then a soldering process, either automated or by hand, forms a joint between the through-hole pad and the lead at the reverse side of the board.
Though its size is a double-edged sword in many ways, certain through-hole packages are more compact than others, dependent on the lead placement relative to the package. Axial leads run parallel to the length of the body, while radial leads extend downward from the package body; the classic example is a resistor and ceramic capacitor, respectively. For less impacted designs, the advantage gained by switching from axial to radial lead components may be minimal. Still, it is an optimal tool for integrating through-hole components in dense assemblies.
Through-hole components, in general, possess several advantages over SMTs related to their human manipulability and other aspects related to their larger dimensions:
- Proof of concept/prototyping: Design is not a linear progression. Instead, products go through many revisions as teams hone in on the best expression of features that captures the design intent while maximizing performance, reducing cost, and improving manufacturability. Some designs may be locked into solely SMT from the outset due to the density of the build. However, through-hole components are often used as DNIs to allow test engineers to add/remove components of different values rapidly.
- Assembly re-work: Soldering/desoldering through hole components is a task that a technician can perform without needing expensive equipment. Through-hole components’ solder bonds are mechanically stronger than those of SMT, making them a preferred choice in specific high-reliability applications.
- Extensive properties: Some of the most important component functionality comes from the amount of material present to perform some function. Through-hole resistors have a much larger capability for dissipating power than SMT, so a special class of resistors (known as power resistors) are used primarily for power dissipation. Additionally, smaller power resistors can be used as current sensors, typically found in conjunction with switched-mode power supplies. High voltage/capacitance capacitors tend towards through-holes for additional mechanical support due to increased package mass.
SMT: The New Kid On The Board
SMT features heavily in mass-produced boards for a good reason: dense designs cannot afford to use through-hole components in the literal and figurative sense. HDI boards or similarly styled designs usually boast a high net count and a corresponding amount of components to interface with. Regarding package density, it’s no contest: literal dozens (and sometimes more) of SMT components can take up the same amount of space as equivalent through-hole packages. Because SMT is the more efficient package design and drives the market, ICs that take the place of several individual components may not even be found in an equivalent through-hole package. Some additional considerations in favor of SMT:
- Size: This is the most significant advantage SMT possesses over through hole, and if you’ll excuse the pun, it’s a big one. Not only are package sizes of similar SMT components minuscule relative to through holes, but there is no inherent need for drilling due to the package alone. For HDI boards, this saves precious space by reducing the layout amount that is dedicated to a through-hole pad instead of a surface mount. Features like microvias, which improve ease of routing by decreasing hole size and allowing more flexibility in hole placement between layers, are incompatible with through-hole routing.
- Manufacturability: Dense assemblies and mass production are better served by SMT components than through-hole. Automated placement rates using SMT can perform at speeds with factors of magnitude greater than those of through holes. In terms of scalability, solder reflow lends itself much better to high volumes than wave soldering, which, similar to its package technology, is contained more to prototyping and other low-volume lots.
- Reliability: Due to their low profile and minimal clearance from the board when soldered properly, SMT is less likely to experience failure modes due to vibration or shock experienced by the board or installation. However, consistent mechanical stresses induced by connector mating cycles are better served by through-hole connections than SMT.
Your Contract Manufacturer Can Brainstorm Through Hole vs. Surface Mount Assembly Solutions
Is there a clear-cut winner in the bout of through hole vs. surface mount? Hardly; both technologies are likely to be used in different design stages. Even though SMT has a marked advantage in mass production, through-hole components may still end up a part of the final assembly for a multitude of reasons. It’s far more likely that many designs, absent additional constraints, are likely to include some mixture of SMT and through-hole technology due to the relative advantages of each package style. If you need support in optimizing your BOM for performance or production, VSE can help. Here at VSE, we’re a team of engineers that builds electronics for our customers. Alongside our valued manufacturing partners, we’re committed to producing a board that meets the modern demands of industries.