There have several types of PCB assembly service in the market. Choosing the right mounting technology is essential for the design of any electronic device. So, should you use surface mount technology (SMT) or through-hole technology (THT)?
What Is Surface Mount Technology (SMT), and How Does It Work?
Surface mount technology (SMT) is the process of mounting surface mount devices (SMDs) on printed circuit boards (PCBs). SMT involves the use of SMT component placement systems, which are commonly called pick-and-place machines, or PNP machines.
SMT assembly includes three main steps:
- Applying soldering paste,
- Placing SMD components, and
- Soldering components to the PCB.
Manufacturers typically use a silk-screen printing method to apply soldering paste. A stainless-steel stencil is made specifically for the project. The stencil allows for the precise application of solder paste where the SMD components need to go.
The boards are then inspected to ensure that the paste is evenly applied. Next, the boards are prepared for use in the PNP machine. One by one, the machine transfers each component to the board. Some machines are capable of placing up to 25,000 SMDs per hour.
Soldering the components to the PCB may involve one of two methods—reflow-soldering or vapor-phase soldering.
Reflow-soldering involves heating the PCB in a special chamber with a nitrogen-purged atmosphere. The components and soldering paste gradually heat until the paste melts. As the paste hardens, the SMDs become affixed to the PCB.
Vapor-phase soldering involves heating the boards until the solder paste reaches its melting point. It requires a lower temperature compared to reflow soldering, which is useful for connecting more sensitive SMD components.
Pros
- Faster manufacturing process
- Ideal for high-volume production
- Helps create smaller electronics
Cons
- Requires more of an upfront investment
- Prototyping requires new solder stencils for each revision
What Is a Surface-Mounted Device (SMD), and How Does It Work?
A surface-mounted device (SMD) is a component mounted to the surface of a PCB. SMT assembly includes the use of SMDs instead of through-hole components.
Using SMDs allows for more efficient use of the space on a PCB. You can create smaller, slimmer PCBs.
The typical pitch, which is the distance between the lead balls or pads on the bottom of the component, is often between 0.5 mm and 1.5 mm. Through-hole components often have a pitch size of 2.54 mm.
The smaller pitch size means that you can place components closer together. You use less space, which helps reduce material costs.
SMD components also have a couple of drawbacks. First, the components are more susceptible to damage compared to through-hole components.
The components are mounted to the surface of the board, which means that they rely on the strength of the solder joint to remain in place. Dropping a PCB with SMD components may break or loosen one or more solder joints, which can impact the function of the device.
SMD components are also more difficult to inspect due to their smaller size. In some cases, the solder joints are not easily visible, making it more difficult to inspect the quality of the connection.
Pros
- More efficient use of space on the PCB
- Components often have shorter profiles
- Helps reduce material cost
Cons
- SMD components are more prone to damage
- SMD components are more difficult to inspect
What Is Through-Hole Mounting, and How Does It Work?
Through-hole technology (THT) refers to a mounting process that involves inserting longer leads through holes in the PCB.
Through-hole technology is also occasionally spelled as “thru-hole” technology. Holes are drilled wherever a component needs to be mounted. Metal solder pads are added to one side of the holes. Through-hole components include long wire leads that pass through the holes and are soldered on the other side.
The strength of the design also makes THT a better choice for use in high temperatures. The resiliency of through-hole components is ideal for PCBs that may be exposed to extreme conditions, such as PCBs for use in automobiles and other outdoor equipment.
THT is also often used for PCB prototyping. Developers can revise the design and add or remove components without needing to entirely redesign the PCB each time, which can save time and money during the prototype stage.
THT also has a few drawbacks, as it is a more time-consuming process compared to SMT. While machines are available to automate parts of the process, THT is typically completed by hand. It is a slower process, which increases the timeframe and cost for PCB manufacturing.
THT also uses more space on the board. As mentioned, 2.54 mm is the average pitch size for through-hole components, which is considerably more space compared to the spacing on SMDs.
The larger integrated components (ICs) take up more space. Using more space can increase the cost of materials and the size of the final design.
Pros
- Less prone to breakage compared to SMD boards
- Increased protection against heat and extreme conditions
- Faster prototyping compared to SMT
Cons
- Slower manufacturing process
- The components require more spacing
What’s the Difference Between SMD, SMT, and THT?
Surface mount technology (SMT) is a process used to mount surface mount devices (SMDs) on circuit boards. THT is a process used to mount through-hole components.
The main difference between SMT and THT is that one involves mounting components to the surface of a PCB while the other involves passing leads through holes in the PCB.
Through-hole components include leads that pass through drilled holes. As the leads pass through the board, THT designs are less prone to breakage compared to SMD boards. The surface-mounted devices may break from the board if dropped or exposed to extreme acceleration.
SMT boards are more prone to breakage but offer several advantages. SMT with surface mount devices (SMDs) allows for the design of smaller electronics compared to THT. The components can be placed closer to each other on the board, which helps keep the design as small as possible.
SMT can be more costly compared to THT, as it requires more of an upfront investment. You need to create stencils for the application of solder, and this requires the use of high-end equipment, such as pick-and-place machines.
Each time that you make a change to the design, you need to create a new set of solder stencils, which adds to the cost of production. Yet, SMT is still more efficient for high-volume production.
With SMT, the assembly process is automated. With THT, the assembly process is typically completed manually. You may want to consider using THT for prototyping. It can simplify the design process when making changes to the board design. You can even remove and add components manually using a soldering gun.
The simplicity of through-hole soldering also makes it the preferred choice for hobbyists. You do not need much experience to solder through-hole components and achieve quality solder joints.
SMT and THT are two of the most common processes for PCB manufacturing. SMT has mostly replaced the use of THT in commercial and industrial applications. Yet, THT is still an ideal solution for specific projects or stages of product development. You may want SMT for designing high-end electronics in small packages and THT for prototyping and hobby projects.