In this article, we introduce the two main types of component packing: through-hole and surface mount device (SMD).

Electronic components come in all shapes and sizes but the right package for your project may not be as obvious as you think.

Some applications may heavily benefit from older technologies such as through-hole packages while others may be reliant on surface mount devices (SMD). Choosing the right one can be a difficult decision at times, so in this article, we will look at both technologies, how they compare, and when each type should be used.

Main Characteristics of Through-Hole and SMD Components

Through-hole devices have longer legs, called leads, which are inserted into holes through the top of the PCB and then get soldered into place on the underside of the PCB. The holes used on a PCB for through-hole leads are referred to as pads and are almost always plated through.

Surface mount devices are directly mounted onto a PCB and do not have protruding leads. Instead, the pads on the PCB are lined up with the contact on the SMD components and the pads and contacts are soldered together, forming an electrical contact. SMD components come in an incredibly wide variety of contact types and sizes. Some SMD parts have small legs that touch the PCB and others only have tiny connections on the underside of the IC. 

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Prototyping

Prototyping is arguably the most important deciding factor especially for hobbyists who may not have access to custom PCBs.

Since through-hole devices have leads, they can be easily inserted onto breadboards and sockets, allowing them to be easily moved from the prototype and inserted into the final design, or to be reused in completely new designs. This makes through-hole parts very cost effective for expensive devices such as microcontrollers, sensors, and memory ICs.

SMDs, meanwhile, have to be soldered onto something before they can be used—even in prototypes. There are adaptor boards that can convert an SMD component into a through-hole part. However, they still require the user to solder the part to the adaptor which means they cannot be removed without difficulty.

Some SMD parts (such as BGA, QFN, and MFL), are incredibly difficult to solder without appropriate tools such as hot air soldering irons which many hobbyists do not own. One advantage of using simple SMD components like resistors and capacitors is that they’re very cheap, but again, they can be , to solder (even if they have legs). 

Mechanical Strength

Through-hole parts use top and bottom pads that are electrically connected to each other via a copper plating which is either tin or gold plated.

When a through-hole device is soldered onto a PCB, the solder will often travel into the plated through-hole, coating both top and bottom pads with some extra solder. This results in an incredibly strong connection that is nearly impossible to break (assuming the solder has been applied properly), and provides one of the best mechanical fittings for a component.

SMD components are often very small and their connection involves two pieces of metal touching with a small solder joint between them. This makes SMD components somewhat fragile, so the strength of the SMD relies on quality PCBs to prevent traces from being pulled off should an SMD component be accidentally knocked.

Cost Comparisons

Through-hole is not often used in the industry due to its cost when compared to surface mount devices. SMDs are packaged in very small housings which use significantly less material than their through-hole counterparts and when combined with the huge demand and rapid production the cost of many SMD parts are laughable.

While the price difference between through-hole and SMD components may not be that large during prototyping, the cost difference can be phenomenal on mass produced parts.

Mass-produced circuits that use through-hole parts most often require humans to insert components and solder them in place! While wave soldering can be used to solder through-hole parts automatically, this technique is not always appropriate—especially in situations where a PCB has SMD components on the underside. Therefore, a human operator is required to solder the parts in place which is considerably more expensive than pick and place and reflow soldering.  

The Growing Scarcity of Through-Hole Packaging

While through-hole technology was the preferred manufacturing technique 20 years ago, SMD technology has now dramatically overtaken the market.

Some manufacturers such as Microchip do sell their microcontrollers in through-hole packages (such as DIP16 and DIP40) but through hole options are becoming scarcer. Most modern components, such as ARM microcontrollers, are found only in SMD packages.

Modern designs that rely on through-hole parts may potentially suffer from manufacturing problems in the future should a supplier decide to stop producing a through-hole part. This is more likely to happen with through-hole devices due to their lessening popularity whereas SMD components are insanely popular with designers. 

When to Use Each Package Type

Through-hole technology is useful when an expensive part needs to be reused in later projects or when equipment such as a hot air soldering iron is not available. Through-hole parts can be incredibly valuable to hobbyists who use solderless breadboards or only have access to strip board that uses the conventional 2.54mm pitch.

Often, through-hole technology is able to handle higher currents and power making them useful in power and prototyping applications.

Surface mount devices should be avoided if possible during prototyping stages, but this is not always possible. However, circuits for mass production should use SMD technology as much as possible because the cost savings could be the difference between a successful and a failed product. 

Robin Mitchell
Graduated from the University Of Warwick in Electronics with a BEng 2:1 and currently runs MitchElectronics.

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