What is Surface Mount Soldering?

We could describe it as the process of soldering Surface Mount Devices (SMDs) on the PCB (Printed Circuit Board – could be linked to your own page: https://raypcb.com/a-detailed-introduction-of-pcba/) , but we should first about two different assembly technologies.

Surface Mount Technology (SMT) and Trough-Hole Technology (THT)

THT was the first assembly technology in PCBs and was the industry standard in 1940s, since 1980s the PCB electronic industry has two assembly technologies: SMT and THT. The difference among them is very intuitive. We use the word surface in SMT because component leads remains on the surface of the PCB instead of breaking through the board. SMT uses Surface Mount Devices (SMD) and THT uses Trough-Hole Assembly components. You can see this difference very clear in the pictures below:



So in SMT the connection lead-PCB is made on the surface but in THT connection is usually made on the back PCB layer. The back layer of the PCB would be like this:

The back layer of the PCB

The picture on the right exemplifies a manual soldering process on a THT PCB. This process can also be done by an automatic assembly machine. Pick and place machines are used in SMT and THT as well. These machines get components from an input buffer and put components in their position before soldering them.



THT and SMT comparison

When the SMT appeared in the 80s, many believed that the THT would gradually disappear due to surface mounting it is more cost effective than the through-hole assembly. However, the manufacture of THT not only survived the arrival of surface mount technology, but also evolved during these years. This was driven because THT offers some advantages compared to surface mounting processes.



Through-Hole Technology (THT)

As usual, we will find some benefits of using this technology along with some disadvantages of using it.



1. Strong mechanical bonding: That makes it more resistant to stress environments. When it comes to design systems that would be exposed to mechanical and environmental stress or high heat, this technology is the best choice. Due that leads run through the board instead of simply being secured to the board’s surface, THT can better withstand the stress environment than SMT.

2. Higher heat tolerance and more durability in extreme accelerations and collisions. This aspects plus the one mentioned before makes THT the preferred chosen technology by for military and aerospace industries.

3. Easier prototyping and more reliable

4. Easier component replacing

5. More power handling capacity: THT offers a stronger solder joint so that more voltage and more power can be handled. When it comes to high power or high voltage or both, THT is the natural choice.



1. More expensive and more time demanded: Since PCBs needs to be drilled before soldering; the process needs more time and money.

2. Components restricted to only one side: This limits the routing area on multilayer boards because holes must cross all the PCB’s layers

3. Less reliability and repeatability: Either we are using hand or wave welding, these two characteristics will be affected, therefore the efficiency of the manufacturing process will decrease.



Surface Mount Technology (SMT)

We will now look at the benefits and downsides of choosing this technology.



1. Lower production costs: Since no pre-drilled of PCB is required the consumed time is lower than in THT but also the production cost; less process is required. SMT components can be placed up to ten times faster than THT components.

2. More reliable solder: By using SMD soldering paste the outcome is more reliable. Components are placed usually by a machine into the SMT soldering paste, a stencil (could be linked to https://raypcb.com/what-is-smt-stencil/) is used to cover the exposed areas and then the PCB is heated to reflow the paste. This type of soldering has proven to be more reliable and more resistant to shock vibrations.

3. Smaller PCBs and devices: Components are placed on both sides of the board, allowing higher densities. This gives us smaller PCBs.

4. Lower resistance and inductance connection: Uses a smaller connection area along with smaller components that generates lower radiated emission as well.



1. Biggest downside: unreliable in mechanical and environmental stressor high heat. When exposed to this environment and compared to THT, the mechanical connection will be weaker as we explained above.

2. Not recommended for high-voltage components: Since the melted SMD soldering paste tension is what sustain the component to the PCB, we can expect that this is not the best option for assembling large components to the PCB. Therefore high-voltage components will not be available in this technology and we should look for THT options.

3. More expertise will be required: Generally speaking in order to design in SMT more knowledge, design level and also more advanced technology will be use.



What to use? THT or SMT? Use them together: Mixed Assembly

We already discuss about this two technologies and which one use depending on our needs. But you should also know that these technologies coexist with each other and can be combined for mass production in the same PCB design.


Originally, through-hole technology designs were based on copper paths across the surface of the PCB combined with holes drilled in it so that devices could be welded. These holes are called "not plated trough hole". Once the industry evolved, the need for an automated process that would allow the use of solder paste also in this assembly technology led to the development of an improvement. "Plated through holes" system was implemented. The great benefit of this improvement is also that you can combine THT and SMT in the same design for mass production.


PTHIn Plated Through Holes (PTH) a thin copper layer is placed after drilling the hole, this provides conductivity from both sides of the PCB. This also shows less resistance and more mechanical resistance. Welding process here is the following:  once the soldering paste is applied in the hole, the component lead is pushed through the paste. Then the PCB is heated to reflow this solder paste. This process is called pin-in-paste welding.



Mixed assembly combines both assembly technologies. SMDs and THDs are placed on the PCB, and then the soldering paste is placed on the PCB so it can be heated, the result is to combine all the advantages for each type of technology.