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SMD vs SMT vs THT: 2021 Ultimate Guide to PCB Assembly Terms

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Wed, Nov 24, 2021

When you plan a PCB assembly project, SMD, SMT, and THT are three terms you can't miss. The following is what this article reveals:

Part 1: What is SMD

SMD stands for surface mounted device. SMDs are electronic components that are picked and placed on PCBs during the SMT assembly, and there are the passive SMDs and active SMDs.

passive SMD

Passive SMDs: electronic components that do not consume or convert electric energy, such as resistors, capacitors, inductors, wave filters, couplers, antennas. Passive SMDs do not change the signal features but only let them pass. And they work only if there comes signal input, with no need for an external power supply.

active SMDs such as IC

Active SMDs: electronic components that consume and convert electric energy, such as ICs, transistors, silicon controlled rectifiers, diodes, triodes, valves. Active SMDs generate signals in analog and digital circuits and can change signal features. And they work with signal input and external power supply.

SMDs have very short pins, and their sizes are about only 1/10 of the through-hole components. Packaging sizes of SMDs include 01005, 0201, 0402, 0603, 0805, 1206, 1210, 1812, 2225.

SMD vs SMT

SMT is a PCB assembly technology, while SMDs are PCB parts mounted via SMT technology.

Please see details about SMT below.

Part 2: What is SMT

SMT surface mounted technology

SMT stands for surface mounted technology. SMT is the main technology for PCB assembly, and it is automatic. SMT assembly can meet fine-pitch requirements, while the other technology THT can't be fine-pitch.

Currently, almost all electronic components can be surface mounted, especially for high-dense circuits. As electronic products are smaller and smaller, SMT is the trend of PCB assembly.

The steps of SMT assembly are:

Step 1. PCB manufacturing and SMT stencil manufacturing

Step 2. Solder paste printing on PCB pads via SMT stencils

Step 3. Solder paste inspection to check solder paste thickness and shapes

Step 4. High-speed SMT assembly (high-speed SMT machine picks and places SMDs of 0805 and below)

Step 5. Function SMT assembly (functional SMT machine picks and places SMDs of 0805 and above)

Step 6. (optional) X-ray test if there are BGAs (ball grid array, a surface-mounted packaging) on the PCBA

Step 7. Reflow soldering to melt and cure solder paste

Step 8. AOI test (automatic optical inspection) to make sure the SMDs are soldered correctly on the surface

For batch PCB assembly, the PCBA manufacturer PCBONLINE picks one of the first five PCB assembly pieces to go through the first article inspection (FAI). And then an FAI report is generated and sent to customers to review. Only when customers approve it, the batch PCB assembly shall start.

Is SMT assembly steps the same in all PCB assembly factories?

Yes.

Then why do we bother to compare different PCB assembly service providers in SMT manufacturing levels?

Because they have different SMT assembly capacities.

And the comparison in SMT manufacturing is necessary.

The main considerations to choosing SMT assembly companies are below:

  • Lead-free assembly capability of reflow soldering ovens
  • RoHS standards require the PCB assembly to be lead-free. The lead-free reflow soldering temperature range is 240°C - 270°C, while tin-lead reflow soldering temperature range is 210°C to 245°C.
  • Fine pitch assembly
  • Pitch is the center-to-center distance between adjacent PCB pads, balls, or IC pins. Before you place your order, you need to ask the PCB assembly company the smallest fine-pitch it can deal with.
  • PCBA tests and conformal coating
  • You never want to spend extra time looking for test companies before your products enter the market. That's why you'd better work with a one-stop assembly company that completes all tests for your products.
    And if you want the circuit boards to have a longer life span, find a PCB assembly company that can apply the conformal coating.

One-Stop Lead-Free Fine-Pitch SMT Assembly Services Provider | PCBONLINE

PCBONLINE is a leading one-stop PCB assembly services provider with customers around the world. It has competitiveness in contract electronic manufacturing for these advantages:

  • 0.35mm fine-pitch SMT assembly capability with 01005 smallest SMD packaging
  • Lead-free PCB assembly and the whole electronic manufacturing process RoHS certified
  • PCB assembly following IPC-A-610 Class 2/3standards
  • Complete PCBA tests such as thermal aging, functional testing, four-terminal sensing
  • IC programming, conformal coating, and end-product assembly are available
  • Fair PCB assembly prices. The more quantity, the lower PCBA per price

Please feel free to get a quote for your PCBA project online or by email at info@pcbonline.com. It is fast and free.

Part 3. What is THT

through hole technology

THT stands for through-hole technology. THT is a PCB assembly technology used less than SMT, done by component pins plug-in pre-set holes.

THT soldering has stronger mechanical stability than SMT soldering, but it can't meet the fine-pitch requirements.

THT components are usually for plug needs, such as USB, tap-C, connectors. Capacitors of large capacitance can't be surface mounted, and they are usually also THT components.

The THT components area on the circuit board is much larger than SMDs'. And THT components have long pins so that they can plug into the pre-drilled holes on the PCB.

Here is the THT assembly process:

Step 1. Let THT component pins through-hole manually

Step 2. Wave soldering to let tin fill the gaps between THT component pins and through-hole walls

Step 3. Trim THT component pins to be within 2mm

Step 4. Clean the extra flux on the PCBA surface

Step 5. (optional) IC programming, functional testing, thermal aging, end-product assembly

Part 4: SMT vs THT, SMD vs THT Components

In PCB assembly factories, both SMT lines and THT lines are necessary. We can't say which is better because SMT and THT serve different purposes. But you can see the differences between SMT and THT from the comparison table.

SMT

THT or PTHT

Necessity for PCB assembly

Necessary

Unnecessary if PCB has no plug needs

Method

Automatic

Usually manual

Complexity

Advanced

Simple

Density

High

Low

Soldering

Reflow soldering

Wave soldering

Speed

Fast

Slower than SMT

Precedence

Before THT

After SMT

Through holes

No

Yes

Applications

Batch PCB assembly, high-density products

Prototype assembly, simple circuit boards

SMDs and THT components are only different in size and packaging.

PCB Components

SMDs

THT components

Area on PCB

Small

Large

Weight

Light

Heavier than SMDs

Pins

Very short

Long

Fine-pitch

Can be fine-pitch

No

Prices

Normal

Lower than SMDs

Typical devices

Resistors, capacitors, inductors, ICs

Components for plug needs such as USB

At the one-stop PCB assembly factory of PCBONLINE, both SMT assembly and THT assembly are available. If you order PCB assembly services from us, we can complete PCBA in the next day (IPC-A-610 Class 2/3 standards) (at fastest). And the shipping usually takes 2 to 3 days.

Conclusion

This article gives an introduction to SMD, SMT, THT one by one, and it reveals the differences between SMT and THT, SMD and THT components.

We can conclude that SMT is the mainstream PCB assembly technology, and THT is mainly used when there are plug needs. SMDs and THT components are mainly different in size and packaging. If you are looking for PCB assembly services, do not miss the 22-year-old turnkey PCBA source factory PCBONLINE.