What's a through hole?

A hole of many names--clearance hole, thru-hole, via, plated thru-hole, etc--, a through hole (TH) is a hole in a PCB that is either drilled, reamed, or milled all the way through. Which means if you guessed the hole in the center of the image was the TH, you’re right. The hole on the left is a blind hole and on the right, is an interrupted hole.

A not so thorough history of the through hole

Through hole technology once dominated as the electronics assembly technique from the 1950’s until the 1980’s when SMT (surface-mount technology) rose to prominence. During that period, every component on a PCB was a through hole component. 

Since then, SMT’s have enjoyed wide adoption for their advantages over TH components and changing technology requirements. However, THs are still a common method used to deliver electrical connections from one layer to another (think breakout boards and header pins).

PCBs originally had tracing on one side, then on both sides, later multi-layers were added. Vias, or plated through holes (PTH), were then designed to make contact with conductive layers.

   Through hole components

   There are two types of components that belong to the TH category: axial and radial. Axial components have wire leads at each end. Radial, on the other hand, have wire leads that protrude from the bottom of the device.

   These types of components are especially useful during testing and prototyping phases where manual adjustments and replacements are commonplace. Each type can be converted into the other by bending the leads to mimic the other’s form

TH components can either be mounted manually or automatically with insertion mount machines. Both methods solder the two leads onto pads on opposite sides of the PCB. This process will then complete the circuit.

To thru-hole or not to thru-hole?

SMT has many reasons for overtaking TH, however, each method still has their pros and cons. Let’s take a look at TH’s pros and cons.

TH is the more convenient selection for hobbyists and engineers during the development stage since they are compatible with breadboard pin-holes and are easily swappable. The components and circuitry can be tested on breadboards before upgrading to a PCB. Although they eat away at tracing space below the top layer, engineers often prefer larger through holes while prototyping because of its ease of use with breadboards.

Through hole mounting (THM) also offers a certain amount of reliability when undergoing environmental stress. Threading components through the board increases the strength of the physical connections. They can withstand more strain than SMD and continue to operate efficiently. In fact, THM is widely used in aerospace and military products that require boards and components to endure accelerations, collisions, and extreme temperatures. 

THM’s ability to withstand mechanical stress is also good for prototyping. And for those who must shell out hundreds of dollars on prototype boards, the relative affordability of TH components is a small win for engineers and makers. Sometimes you don’t even have a choice whether you use TH or SMD, as not every component you need comes in an SMD form.

However, there is a reason why SMT overtook TH technology—multiple reasons really. Unfortunately, as previously mentioned TH take up more board real-estate and requires drilling which can increase the costs of PCBs. Drilling can also increase the time needed to fab. Mounting TH components by hand is not as reliable as mounting SMT components, not an ideal situation for prototyping. 

Compact, high-speed, and high-frequency designs have phased TH out of its heyday in favor of SMT which was designed to meet the fast-changing and highly demanding needs of technology. For many good reasons, SMT isn’t going anywhere, but then, neither is TH technology. For certain situations, there are more advantages than costs in choosing to use TH and TH components.

Do you try to avoid through-holes and their components at all costs or can you not get enough of them? Let us know in the comments below!