THT vs SMT

Hello everybody! Previously we had a look at the THT and SMT component mounting methods in the PCB boards. it was a very interesting class and today we want to focus on the differences between the two methods. Let us get started without wasting any time.

Difference b/w Through Hole Technology and Surface Mount Technology

Although THT and SMT are component mounting methods that are employed in the design and manufacturing of printed circuit boards, it is very important to note that the methods have got quite a number of differences that a designer and a manufacturer must be well equipped with. These differences are very important because they form the basis on which components should be given priority in the design process.

Let us know have a look at these differences and get to know the way forward when it comes to the process of component selection.

Definition – THT vs SMT

Both for THT and SMT, there is a difference in their meaning and definition.

• THT stands for through-hole technology.

From the word through-hole technology, we coil the meaning of the THT method. here components are mounted by the use of holes that are systematically drilled through the printed circuit boards. the components have long leads that are properly fitted in the holes and soldered.

• SMT method stands for the Surface Mount Technology.

From the statement surface mount technology, it is evidence that in this method, the components are mounted on the surface of the printed circuit board. This is the most recent method that has been employed in the design and manufacturing of printed circuit boards.

Component mounting

The component can be mounted on the board by either use of the lead wire terminals or the use of the pads.

Lead wire terminals;

As you can see from the image, the two silvery wires are what we call lead wire terminals. They are very important coz they play a great role when it comes to the connection of the components on the board.

Pads;

This is an exposed metal piece on the board where the leads of the electrical components are lied.

• The through-hole technology components has no pads mounting surfaces since they are mounted through the holes drilled on the PCB boards.
• For the surface mount devices, they employ the use of the pads in their mounting process. The components have flat terminals that are laid on the pads and soldered to form a joint.

Method of component mounting

• For the through-hole technology, this method involves the use of holes in the mounting of the components. Holes are drilled through the PCB boards, the components are fitted by the use of their long leads and then proper soldering is done to ensure that they are firmly fitted on the surfaces.

• When it comes to the surface mount process, things are different here. Components are placed on the surface of the PCB board on a systematic manner that is determined by the designer, then solder is used to mount them on the surface.

Process of component mounting

When we address the issue of the mounting process, we want to classify it into two methods. This involves the processes involved in the production or even manufacturing of the boards.

Manual mounting

The manual mounting method involves the use of human labor in most areas of this process. No use of advanced systems in the process.

Automatic mounting

Here the automatic process involves the use of automated machines in ensuring that the process is made successful.

• The through-hole technology is in most cases done manually. There is no great advancement in this area on matters of technology utilization in the manufacturing and production processes.
• The surface mount process involves the use of sophisticated systems to ensure that the process is very successful. Some of the automated systems are the pick and place machines and the stencils.

The stencils

They are used by present-day manufacturers to do the printing of the print solder paste. This paste forms the deposits on the board that is used to hold the components in place.

• The through-hole technology does not involve the use of the stencils since the components are mounted by the use of the hole technology.
• The surface mount technology of course must employ the use of stencils for the process to be successful. This is simply because the components are surface mounted and the only way this is possible is by the use of the print solder paste that can only be done by the use of the stencils.

Vias

Vias are used to electrically join pads, traces and the polygons on different layers that make up the PCB. They are formed of small tiny insulated holes drilled on the board for electrically connectivity purposes.

• For the through-hole technology, vias are not that necessary since the components are connected through the wholes to the other side of the boards.
• The surface-mount devices employ the use of the vias to connect between layers of the board. Since components are mounted on the board, it is very necessary to employ the use of the vias to interconnect between the layers of the PVB boards.

Prevalence of use

The surface-mount devices are the most preferred in the industry because they are cheap, light and very small. They are manufactured very fast hence saving on the manufacturing time. They are also very cheap to make since the process is purely automated.

The through-hole technology is not preferred due to bulkiness and is very expensive to produce them due to manual production.

Reworking and repairs

• Through-hole technology mounting components and boards are easily reworked or repaired in case of the fault. That is why they are used when a prototype or testing is required.
• The surface-mount technology components are not easily reworked or repaired since components are very tiny for that process.

Soldering type

There are two methods used in the soldering of the board components;

Wave soldering

This is an in-line soldering process that treats the solder with flux which after that preheats the solder and immerse the board on the solder.

Reflow soldering.

Here, a solder paste is used to mount components temporarily on the board by the use of the pads after which the entire design is preheated to complete the process.

• Through-hole technology employs the use of the wave soldering method.
• The surface mount technology employs the use of the reflow soldering method.

Area

The through-hole technology components are large hence they have a very big area.

For the surface mount devices, they are very tiny and hence they have a small area

Component types

The through-hole technology involves the use of the lead capacitors and resistors, DIP components and the GPA components.

The surface mount technology involves the use of the LCCCP, SPOIC, QFP, BGA, SOT, LCC, chip resistors/capacitors.

Assembly density

The through-hole technology is used when a low density of the board is required.

The surface mount technology is involved where a high density of components is required.

Solder joint

The solder joint is done differently for both the mounting methods;

• For the through-hole technology, the solder joint connects the component leads to the board through a hole drilled on the board. The solder is done on the lower side of the board and then the upper side is filled with the flux.
• For the surface mount devices, the board is first spread with the solder according to the designer’s specifications, then components that have pads on them are placed accordingly on the soldered parts and then the solder is melted to fix the components on the board.

Cost of production

The cost of production for the two methods varies. This is determined by how the manufacturing process is done and the size of the components used.

• Through technology proves to be the most expensive when it comes to initial production. This is simply because it involves drilling holes to implement the method and also since the method is done manually, there will be a lot of involvement of human labor which comes with an additional cost. But when it comes to routine maintenance of the THT boards, the cost is relatively low since it is very easy and fast to replace aby components on the boards. THT is easily repairable.
• The surface-mount devices are cheaper when it comes to initial manufacturing. This is simply due to the involvement of the new technology in this area. The process is purely automated hence saving oh human labor costs. Also, the process does not involve the use of holes in the mounting of the components hence also reducing such costs. The boards are produced in bulk through the process of PCB panelization, hence maximum utilization of the machines. This also lowers the cost of production of the boards. Now, there is one challenge with this type of board, the maintenance cost is too demanding. You will need a lot of expertise to change any faulty components. Some of them cannot be changed since they are so tiny. Therefore, in most cases, if the board has a problem, you might end up buying a new one hence it is very expensive when it comes to maintenance and repair.

Use

These boards have various applications. There are well areas where you can use the boards. let us see;

• THT mounted boards are used in areas where there is no limitation of space and this is true due to the fact that they are very large in size. They are also used in areas where strong mechanical strength is required such as in military equipment and equipment with high vibrations. This is because their soldered joints are mechanically strong.
• SMT boards are very tiny and they are used in all areas including areas that are space limited. These boards cannot be used in areas that require strong mechanical connections since their mounting is not that strong and is prone to melting off and therefore, cannot offer the required strong mechanical strength to overcome such environmental challenges.

So, that was all for today. I hope you have enjoyed today’s lecture, let me know if you need any help with it. Thanks for reading.

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