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Most Clear and Fullest Guide on PCB SMT Assembly

SMT assembly
pcbonline PCBONLINE Team Thur, Jul 28, 2022

When you receive bare PCBs from your PCB manufacturer, you still have to solder electronic components on the boards so that they become PCBAs to function. SMT assembly is necessary for PCBA manufacturing. If you have any questions about SMT assembly, check out this article first!

Part 1: Terms about PCB SMT Assembly

If you are not familiar with PCB assembly services and haven't had a chance to visit a PCB assembly factory, please check out the below terms about SMT assembly first.


PCB is a printed circuit board, a board made from copper, prepreg, glass fiber or aluminum, ceramics, PTFE, or polyimide in layers. Electronic components mount on the PCB to function, and the PCB transfers heat from components to the external heat dissipation system.

PCB pads:

Pads are bare copper areas that are specially treated with surface finishes so that they are easier to solder and conduct electricity. We can also call them joints. SMDs are soldered on PCB pads.


SMT assembly line

SMT is a surface mount technology, a mainstream PCB assembly process. It is automatic, efficient, and precise to pick and place electronic components on PCB pads. Please check the SMT assembly process in Part 2.


SMD is a surface mount device, an electronic component mounted on the PCB. Currently, all electronic components can be made into SMDs. That's to say, PCBs can be without through-holes and assembled only in SMT.

Solder paste:

Solder paste is a conductive semi-liquid made of alloy powder (usually, tin/silver/copper/bismuth) and flux. It is "printed" at the PCB pads or joints to connect components and the PCB. In the reflow oven, solder paste melts and cures so that it bonds the PCB and components.

SMT stencil:

SMT stencil

SMT stencil or PCB stencil is a stainless sheet with openings used for SMT assembly. It can be framed or frameless. In the SMT process, it is placed in an SMT printing machine. Solder paste goes through the openings and "prints" on the PCB pads.


reflow soldering

Reflow or reflow soldering is the process to preheat, melt, and cure solder paste. The temperatures of reflow soldering are set up according to the PCB materials. A reflow oven has 6 to 10 temperature zones, where temperatures increase and reduce at a relatively stable speed.

Part 2: What is the Process of SMT Assembly?

The SMT assembly steps are almost the same for all PCBs, except that flex circuit assembly requires additional jigs to fix the boards. SMT assembly to PCBs with double component-mounting sides is different from single-sided PCB assembly only in that when the first side is mounted, the boards return to the SMT line to mount the other side.

So how is the SMT assembly process? Please check bulky SMT assembly to single component-mounting-side rigid PCBs below.

Step 1. Silkscreen printing solder paste on PCB pads

SMT solder paste printing

The beginning of an SMT line is a feeder and an SMT printing machine. The feeder automatically sends PCB panels one by one to the SMT printing machine, and solder paste is pressed to go through SMT stencil openings on PCB pads.

Step 2. Solder paste inspection (SPI)

solder paste inspection

On the SMT line, now PCBs "printed" with solder paste pass the SPI machine one by one to ensure the solder paste shape, size, and thickness are qualified for PCB assembly.

Step 3. High-speed SMT mounting

high-speed SMD mounting

SMDs in small sizes are automatically sent from SMD trays set into the high-speed SMT mounting machine, and the machine automatically picks and places SMDs on the PCBs panel by panel. The speed of the mechanical arm is fast. These SMDs are resistors, capacitors, transistors, LEDs, etc.

Step 4. Functional SMT mounting

functional SMD mounting

In a functional SMT machine, SMDs in larger sizes are also automatically picked and placed on PCB pads. But the speed is slower. These SMDs are often ICs, BGAs, sockets, connectors, etc.

Step 5. X-ray inspection

x-ray inspection

Please note that this step happens only for PCBAs mounted with BGAs. As we know, BGAs don't have pins but solder balls. When BGAs mount on PCBs, both visual eyes and infrared cameras can't check the balls between the BGA and PCB. So we use x-rays to check the solder balls to ensure they are qualified for soldering.

Step 6. Reflow soldering

Before reflow, our technicians will visually check the boards to ensure no scratches, contamination, or missing components. On the SMT line, PCBA panels go into the reflow oven and pass its temperature zones one by one. After reflow soldering, solder paste cures and bonds the SMDs and PCB.

Step 7. Automatic optical inspection (AOI)

automatic optical inspection

At the end of the SMT line is an AOI machine. The PCBAs pass the infrared camera one by one, and the scanned images are compared with the reference. At PCBONLINE, not only the top surface but also the sides of the PCBA are inspected.

Bonus: Free Complete PCBA Sample for Bulky SMT Assembly

For bulky PCB assembly, the one-stop PCBA manufacturer PCBONLINE provides a free PCBA (a PCB + a set of electronic components + PCB assembly + value-added services) sample for the client. Besides, the functional testing of bulky PCBAs is also free.

PCB assembly services of PCBONLINE

Founded in 1999, PCBONLINE has two large advanced PCB manufacturing bases and one PCB assembly factory. We provide PCB fabrication, SMT assembly, THT assembly, and value-added services, including box-built. If you want PCB design, our CAM engineer with more than 20 years of experience can also design for your project.

So what are the reasons to work with PCBONLINE for SMT assembly?

  • SMT assembly following IPC-A-610 Class 2/3 standards, ISO 9001:2015, IATF 16949, UL, REACH, and RoHS certified.
  • At PCBONLINE, reflow temperatures are programmable from 120°C to 260°C, and our reflow ovens are always used for lead-free.
  • 0.35mm finest pitch SMT assembly capability. (pitch: the center-to-center distance between adjacent SMDs, pins, and solder balls)
  • 3D AOI is on the SMT line, and after PCB assembly, we have functional and aging testing.
  • Besides SMT and THT assembly, we also provide conformal coating, IC programming, mechanical boxes, and box-built services.
  • We accept PCB assembly orders without a quantity limit, from prototyping to bulky manufacturing.

If you have any PCB assembly needs, you are welcome to send your BOM (bill of material), Gerber, and pick-and-place file to PCBONLINE by email at info@pcbonline.com to get a quote and consultancy.

Part 3: FAQs about SMT Assembly

Question 1. How does PCBONLINE ensure PCBA quality before delivery?

For electronic components, we source from original factories and audited suppliers. Before PCB assembly, we conduct the IC counterfeit testing to ensure the ICs are real. During SMT assembly, the SPI, AOI, and visual inspections are on the SMT line. After PCB assembly, we conduct functional testing and aging testing. You can check out PCB assembly inspections from PCBONLINE.

Question 2. The price of electronic components for bulky PCB assembly manufacturing is too high. Can your company help reduce the costs so that we can stay within our budget?

Yes, we notice that ICs made in the USA take up the most portion of your PCBA cost. If you agree, we can use local ICs (made in China) to replace them. If possible, we can use local components for all electronic components for your PCBA. The product quality is the same, and the price of electronic components is greatly reduced.

Question 3. What is special about SMT assembly for flexible PCBs?

Flexible PCB assembly requires jigs to fix the flex boards during SMT assembly. And before assembly, we have to bake the boards. Depending on the flexible PCB quantity for SMT assembly, there are two jig solutions to fix the boards. For details of the jigs, you can check the article about flexible PCB assembly jigs.


This article gives a clear introduction to SMT assembly, including explaining related terms, illustrating the SMT assembly process step by step, and frequently asked questions about PCB assembly. Besides, we provide a bonus of a free complete PCBA sample for bulky PCB assembly. If you work with PCBONLINE for SMT assembly, the one-stop PCBA manufacturer won't let you down.

© This article is an original work of the PCBONLINE team. Please indicate the author PCBONLINE if you reprint. If the article is reproduced without permission or indicating the author's source, PCBONLINE reserves the right to investigate the infringement.


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