Do you prefer powerboating or sailing? There are more debates about this than there are stars in the sky. On one hand, you could say that the true measure of a sailor is someone that can bend the wind to their will in order to navigate to where they want. Those with the opposing view might counter with the argument that it is awfully hard to pull a water skier using only a mainsail and a jib for propulsion.
The reality is that there isn’t a clear winner in this contest. Sailboats perform one function while powerboats perform another, and the truth is that ‘boating’ includes both powerboats and sailboats.
In the same way, there really isn’t a contest between surface mount parts and through hole parts on a circuit board. Although some passive components may share values, for the most part, each type of component serves a different function and the majority of PCBs will end up with a mixture of both types. Worth noting are some manufacturing differences between the two, though. Let’s take a closer look at the differences between SMT vs through hole components, and find out which one may be better than the other.
SMT vs Through Hole Parts: Why the Difference?
When printed circuit boards first became a staple in the production of electronics, through hole parts were the only components that were available for use. As time went on, however, surface mount technology (SMT) parts slowly became more popular until they eventually became the dominant component package style used on PCBs today. There are a number of reasons for the popularity of SMT parts now including:
- Size: Without the need for a lead to poke down through a drilled hole, a SMT part is by default a smaller part. This makes it much more attractive to designers who are trying to fit more circuitry onto smaller board sizes in today’s electronics.
- Expense: Since a SMT part is by default a smaller part, it is also less expensive to manufacture. This makes SMT parts more cost effective than their through hole counterparts.
- Availability: With SMT parts being smaller and less expensive, they have mostly replaced their through hole counterparts. This is especially true with passive parts like resistors and capacitors where the SMT component package is often the only option anymore.
- Electrical Performance: Smaller parts have less distance for electrical signals to travel through which lowers the signals flight time. This makes SMT components the better performers electrically than their through hole counterparts.
With these reasons it would be tempting to think that all PCB components should be surface mount parts. There are however, some very good reasons why through hole parts are still used in the assembly of circuit boards:
- Power: For components that are used in high power circuits, SMT packages are not a good option. High power parts often have more metal in them making it more difficult for surface mount soldering techniques to achieve a good solder joint. Additionally, the larger sizes of power parts often require the more robust mechanical connection of a through hole for high voltages, thermal, and mechanical stability.
- Strength: Components such as connectors, switches, or other interface parts need the strength that is provided from the leads being soldered into a drilled hole. The constant physical stress that the parts go through from normal use could eventually break an SMT solder joint.
- Availability: Some components, especially those larger parts used in high power applications, simply aren’t available yet in a true SMT equivalent.
These are the major reasons why components are divided between SMT and through hole packages. These two packages also affect how a printed circuit board is manufactured.
Manufacturing Pros and Cons
There are two different ways that printed circuit boards have their components soldered onto them; wave solder and solder reflow. Although there can be exceptions, through hole components are usually wave soldered and SMT components are usually solder reflowed. After the through hole components are inserted into their drilled holes, the circuit board is run through a molten wave of solder that wicks up into the hole around the lead and forms a solid solder joint.
For SMT parts, solder paste is applied to the pads of the parts allowing the leads of the component to be held in place. The circuit board is then run through an oven which causes the solder paste to reflow and form a solid solder joint between the leads and the pads. For circuit boards with both through hole and SMT on them, a mixture of the two methods is required. In some cases, portions of the board are masked off to go through the wave solder while in other cases the through hole parts may be manually soldered.
The SMT assembly line is typically both faster and less expensive. The automated pick and place machines can reliably handle a variety of components during one run, while the through hole parts are often manually inserted by hand. The reflow ovens simply reflow the solder paste that has already been applied to the SMT pads, while the through hole parts have to be run over a wave of solder. This wave has to be continually supplied from a temperature-regulated source of molten solder, which is a more costly process than the SMT parts going through solder reflow.
Who Is the Winner?
Based on the manufacturing processes, it would be easy to say that SMT components are the best choice to use on your printed circuit board design. But that simply isn’t the case. The truth is that both of these component package styles are important. Although most passive and integrated circuits will come in an SMT package, it is still important to use standard through hole parts for your connectors, switches, and other mechanical or power parts. Additionally, there are legacy boards that are still being produced that have to have older through hole parts used in order to satisfy the copy exactly requirements.
Although SMT components are less expensive, smaller, and better performers, the world of circuit board design and manufacturing has not seen the end of through hole components. These robust parts are going to be with us for a long time, and for very good reasons. The important part for you is to choose the right part for your specific application. Your next choice is just as important: you need to use a PCBA manufacturer that has the experience and facilities to expertly manufacture both types of component packages. You need to know that all of the components on your board are soldered into place with good solid solder joints to ensure the greatest level of quality for your board.
