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What is SMT PCB

SMT PCB is a method of placing electronic components and circuits on printed circuit boards (PCBs). SMT is a subcategory of Electronic Hardware Assembly.

Features of SMT PCB:

  • SMT is used for large production runs.
  • The components are placed on the PCB using a pick and place machine. The components are placed on the PCB one at a time.
  • The process is automated which makes it faster and production more cost effective.
  • The surface mount components can be placed in any direction.
  • If the circuit board is properly designed, it can have any number of holes on it. The holes can be crowded together due to automated placement of SMT components. However, SMD components cannot have as many holes as they are assembled in groups rather than individually.
  • The size of the surface mount component or ICs is usually small compared to through-hole components which are usually larger and easier to handle.
  • SMT has a high failure rate due to improper placement of components and improper soldering. This makes it more expensive compared to SMD.
  • SMT components are usually smaller and thinner compared to SMD components. This makes them more fragile and susceptible to damage during assembly, storage or transit. The lack of space on PCBs due to the small size of SMT components can also make the PCB less reliable resulting in a shorter life of the product.
  • The overall design of a circuit board is usually very simple and compact when using SMT method of assembly compared to using through-hole technology (THT). The PCB design is usually quite complex when using the SMD method of assembly.

What actually is surface mount technology?

SMT PCB

SMT is the process of placing electronic components (like transistors, diodes, capacitors etc) on PCBs. The most common method of SMT assembly is placing the component on the PCB using a pick and place machine which picks up a component from its storage container and places it on the PCB. This is done one at a time.

The reason why SMT is called surface mount technology is because the components are placed on the surface of a printed circuit board. The components are then soldered to make connections between them.

This approach has several advantages compared to the older technique of through-hole technology (THT):

SMT components are usually much smaller and thinner than through-hole components. This makes it possible to place more electronics in a given space. SMT electronics also use less energy compared to traditional electronics due to their small size.

Multi-layered PCBs can be made with SMT process. This is not possible with THT.

The design of the circuit board can be kept very simple and compact which reduces the manufacturing cost of SMT electronics. THT circuit boards may have complex layouts and this makes them expensive to produce. However, there are some disadvantages of Surface Mount Technology which may make it unsuitable for some applications:

SMT components are much more fragile and susceptible to damage during assembly, storage or transit compared to through-hole components. Proper handling is required to avoid damage to SMT components.

The size of SMT components is usually small and this makes them harder to handle compared to through-hole components. The smaller size also makes them difficult to find among other components in storage bins.

SMD components are not as reliable as THT electronics due to less space on the PCB. They also have a shorter life span compared to THT electronics due to the small size which allows heat dissipation from the chip being small too.

SMD electronics are usually costlier than THT electronics due to a more complex manufacturing process. SMD electronics are more expensive to produce than SMT electronics.

SMT technology can be used for both through-hole type components and surface mount type components. However, SMD is used only for surface mount components.

SMT technology is best suited for high volume production of electronic products while SMD is best suited for small batch production of electronics. For example, a large consumer electronics company may use SMT in their factories while a small garage company using microcontrollers may use SMD in their manufacturing.

How does surface mount technology work?

Surface mounted transistors, diodes, capacitors etc are placed on PCBs using pick and place machines which pick the component from its storage container and place it on the PCB.

Some surface mount components come with solder paste already applied on them. These components do not require additional paste to be added to the PCB and can be placed directly on the PCB.

The components are then soldered to make connections between them. Soldering is usually done by placing a hot soldering iron on top of the component which melts the solder and makes a connection between it and the PCB.

A small piece of tape is usually attached to the bottom of each SMT component to make sure that it stays in place during storage, transit or assembly process. This also helps keep components from falling off the PCB during the assembly process (especially when using pick and place machines). The tape is removed before soldering.

Features of SMD:

  • SMD is used for small production runs.
  • The components are placed on the PCB using a pick and place machine. The components are placed in groups rather than one at a time like SMT.
  • The process is automated which makes it faster and production more cost effective.
  • The surface mount components can be placed in any direction.
  • SMD components can have more holes on a PCB as they are assembled in groups rather than individually. This makes the design of the printed circuit board much more complex. The PCB designers usually take advantage of this by increasing the number of layers to increase the functionality and reliability of the product. While this is possible with SMT, it is usually not required as the small size of the SMT components makes it very rare for them to short circuit.
  • The size of the surface mount component is usually larger compared to SMT components. This makes them easier to handle.
  • SMD has a lower failure rate due to improper placement of components and improper soldering. This makes it less expensive compared to SMT.
  • SMD components have space between them which usually results in a more reliable PCB design. However, this is not the case when they are placed on an inner layer where there is no space between them.
  • The overall design of a circuit board is usually quite complex using the SMD method of assembly compared to using through-hole technology (THT). The PCB design is usually very simple when using the SMT method of assembly.

How do surface mount devices work?

SMD circuit board

The surface mount device used in the PCB assembly is usually a multilayer printed circuit board. It has traces of copper on the top and bottom layers of the board. The traces are usually interconnected to form a circuit. The components are then soldered to the PCB using a stencil method.

The top layer of the PCB is made up of an insulating material which makes it resistant to heat so that when solder paste is applied, it does not damage any trace on the PCB.

The components with solder tabs are placed on top of this insulating layer, and then they are placed in a stencil machine where they are attached to the top layer using solder paste that is placed in holes on this layer.

After filling the holes with solder paste, the PCB is placed in an oven to melt the solder paste and adhere it to the surface mount components.

While this is happening, the stencil is moved down so that it does not block the holes where the solder paste was previously placed. Then, the solder paste is melted and any excess solder left on the surface of each hole is removed using a vacuum pump.

Then, after a few seconds, an adhesive tape called a “pick-and-place” tape is applied over each hole. This allows for easy handling and placement of components from a pick-and-place machine onto the surface mount device. The device places each component in its respective position on the PCB using its fingers which is usually controlled by a computer.

Once the surface mount device is placed on the PCB, another solder paste is applied using stencils to secure the components. This is done using a hot air tool which heats up the solder paste and ensures that any misshapen parts are heated and corrects them.

Once this is done, any excess solder on the surface of each hole is removed using a vacuum pump. The device can then be tested through an in-circuit test (ICT) machine which performs tests on each component placed on the board. This also helps in measuring how much heat each component gets from this process.

After each component is tested, the entire surface mount device is tested through an ICT machine. This makes sure that every component on the board works properly.

This process is repeated for each layer of the multilayer printed circuit board to ensure that all layers function correctly.

Once this is done, the PCB assembly can be shipped to the customer where it can be used as a product or a part in another product.

What is the difference between SMT and SMD:

SMT stands for Surface Mount Technology and SMD stands for Surface Mount Device. They are both used in the manufacturing of PCBs. SMT is a method of placing electronic components on PCBs while SMD is a component which is placed on PCB using SMT.

The main difference is that SMT is a method of assembly and SMD is what is being assembled.

SMT components are placed on the PCB one at a time, while SMD components are placed on the PCB in groups. SMT and SMD components both use paste for mounting. While SMD uses paste for soldering, it does not require paste for mounting. The assembly process can be automated which makes production faster. Due to automation the process can also be cost-effective if done by an experienced team.

In summary, both SMT and SMD are used in PCB fabrication. But what is important to note is that they are two different processes and SMD is a subset of SMT.