SMD vs SMT vs PTH: Ultimate Guide to PCB Assembly Terms

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

Part 1: What is SMD

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

Passive SMDs: electronic components that do not consume or convert electric energy, such as resistors, capacitors, inductors, wave filters, couplers, and 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: electronic components that consume and convert electric energy, such as ICs, transistors, silicon-controlled rectifiers, diodes, triodes, and 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 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 PTH 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). Then an FAI report is generated and sent to customers for review. Only when customers approve it, the batch PCB assembly start.

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

Yes.

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

Because they have different SMT assembly capacities.

And the comparison in SMT manufacturing is necessary.

The main considerations for 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 the 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 for 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:

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 PTH

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

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

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

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

Here is the PTH assembly process:

Step 1. Let through-hole component pins through-hole manually

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

Step 3. Trim PTH 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 PTH, SMD vs PTH Components

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

SMDs and through-hole components are only different in size and packaging.

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

Conclusion

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

We can conclude that SMT is the mainstream PCB assembly technology, and PTH is mainly used when there are plug needs. SMDs and PTH 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.