SMD vs. SMT vs. THT: Comparing PCB Mounting Technologies

This article is a comparison of SMD vs. SMT. vs. THT.

If you have researched the PCB assembly processes, you must have encountered acronyms like SMD, SMT, and THT. 

These terms can be confusing, especially considering there are several more, such as QFN, BGA, QFP, etc. But let’s first focus on comparing SMD vs. SMT. vs. THT in this article. Read on to learn more about them.

What Is SMD?

SMD refers to Surface-Mount Devices, components mounted on PCB pads (on the board surface). In comparison, through-hole devices contain pins that go through the boards for soldering on the other side.

So the ideal term to compare with SMD in PCBs is THC (Through-Hole Components).

Most modern electronics have PCBs with SMDs because these components are tiny and occupy little real estate.

Therefore, the electronic product size will be more compact.

On top of that, these electronic devices are cheaper to produce than THCs and allow for assembly automation.

Their assembly process uses automated equipment like placement machines and reflow ovens.

The machines are quick, and eliminating human interaction cuts labor costs, making it cheaper.

So assembling SMDs is suitable for PCB mass production.

A surface-mount device assembling machine

But since the components are tiny, hand-assembling SMDs is challenging. You will need more skills, practice, and equipment than when dealing with THCs.

Surface Mount Component Types and Packages

Generally, electronic components are active, passive, or electromechanical, which applies to through-hole and SMD components.

But the miniaturized SMT components have tiny packages like 0201 and 01005 for passive components like capacitors and resistors.

On the other hand, active components like transistors have the popular SOT package, the most typical one being SOT-23.

But perhaps the most common surface mount components are chips (integrated circuits).

These can have different surface-mount packages depending on the application. They include the following.

• SOIC (Small Outline Integrated Circuit) for standard chip packages
• SOP (Small Outline Package) for analog amplifiers and digital logic chips
• QFP (Quad Flat Package) and QFN (Quad Flat No-leads) for microcontrollers
• PLCC (Plastic Leaded Chip Carrier) for prototype PCBs
• BGA (Ball Grid Array) for processors and Field Gate Programmable Arrays
• POP (Package On a Package) for microprocessors and high-density interconnect designs

A QFP chip soldered on a circuit board

These SMDs generally have a finer pitch than their THC counterparts, such as SIP and DIP, making them ideal for high-density circuits.

Also, they have shorter lead lengths that reduce the electrical path between the board and the chip, improving the electrical performance.

What Is SMT?

SMT (Surface-Mount Technology) refers to the technique of soldering SMDs on PCBs. As stated earlier, SMD components can have coplanar tails on either side (usually for passive devices) or leads/pins for active devices and chips.

In either case, these components sit on flat, exposed pads on the circuit board to make contact with other component types.

Holes take up a lot more space on circuit boards than pads. So the lack of them when using surface mount technology makes the PCBs smaller. And the components are tiny, as well.

So this mounting technology allows you to make HDI circuit boards that are tinier and even multilayered. Additionally, it enables you to mount the components on both sides of the printed circuit board.

Also, the lack of drilled plated-through holes for solder attachment makes blank boards for SMT assembly cheaper to fabricate.

But the technology is not without its downsides. Although the assembly process is cheaper than THT, SMT machines are more expensive than their through-hole counterparts.

So if investing in a PCB assembly line, you’ll need a higher capital investment to acquire automated pick-and-place machines, reflow soldering ovens, stencil printers, etc.

The SMT process requires stencil printers to apply solder paste, placement machines to lay parts above the paste, and reflow ovens to heat and melt the applied solder paste to weld the components in place.

Machines in a surface-mount technology assembly line

Also, you’ll need skilled design engineers to develop intricate SMT board designs and an advanced production system from manufacturing to assembly and inspection.

But after the high initial investment and acquiring skilled personnel, you will most likely get a higher return on investment due to mass production. Combining efficient automation and high throughput leads to good profits.

SMT Pros

• Easy to automate the assembly process
• SMT boards are smaller because solder pads occupy little space
• Ideal for HDI PCBs
• Low blank PCB manufacturing cost

SMT Cons

• Harder to hand assemble
• Requires high initial capital investment for the assembly line
• Solder joints are small and not as strong as their THT counterparts.

When To Use SMT

• To mass-produce circuit boards
• If you want to mount chips that don’t come in PTH packages
• To build tiny printed circuit board designs

What Is THT?

THT stands for Through-Hole Technology. It refers to the technique of mounting PCB components via plated-through holes.

So another name for through-hole components is plated-through-hole components.

These parts must have long leads or pins that go into the drilled holes for soldering either by hand or using the wave soldering process.

Compared to reflow soldering, wave soldering requires a pool of molten solder to attach the components to the plated-through holes. SMT devices require solder paste instead of molten solder.

A vintage circuit board with through-hole components

Applying solder paste in the holes will be wasteful because the material will melt and flow out of the hole when placed in a reflow oven.

But through holes creates a sturdier mechanical bond because the solder will have more surface area to attach the component lead to the PCB. So the board will be able to withstand intense environmental stress.

And speaking of leads, THT parts can either have axial leads or radial leads. Axial lead components have pins placed on either side. But radial lead components have pins on one side.

However, THT components are more sizable than their SMD counterparts.

So they are not ideal for building compact electronics because the board size must be larger to accommodate the holes and large parts. Plus, these parts have lower pin counts.

Also, the THT assembly process has some manual elements; assemblers usually insert the components by hand before wave soldering.

So mass production using the through-hole method is challenging.

And the cost of production of blank boards for through-hole assembly is higher because they are larger and require drilling plus plating.

So the initial investment cost is low, but manufacturing costs will be higher than with SMT.

THT Pros

• Create stronger mechanical bonds with the board
• Usable directly without soldering in breadboards
• Requires low initial capital investment 

THT Cons

• Manual/labor-intensive assembly process (not ideal for mass production)
• High assembly cost
• Sizable electrical components (not suitable for HDI boards)
• High blank board costs
• Unsuitable for multilayer boards

A blank PCB for mounting THCs

When To Use THT

• If using breadboards for prototyping
• For low-scale production
• When the application requires strong mechanical strength to withstand mechanical stress

What About Mixed Technology

When choosing between SMT and THT, you will most likely realize that you cannot use either as the sole attachment method. You need to blend the two, which introduces the mixed assembly technology.

A mixed-assembly PCB 

The combination gives you the best of both worlds. You can use THT for critical components that require high mechanical strength and SMT for the rest to reduce the PCB size.

Differences Between SMT and THT

Wrap Up

Although the three terms relate to components and their mounting technologies, comparing SMT vs. THT and SMD vs. THC makes more sense. And we’ve done that for you above. 

We hope the article has been insightful. Contact us or comment below to let us know your thoughts or comments. We appreciate your feedback.