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What is Going on in PCBA Manufacturing? - A Guide in 2021

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Thur, Feb 25, 2021

PCBA manufacturing is, in fact, the process of having bare printed circuit boards mounted with components. You can get an insight into what's going on in PCBA manufacturing at the PCB  factory from the two parts. Please note that the below steps are just the basic steps for PCBA manufacturing because different circuit boards have different assembly requirements.

Knowledge Preparation: SMT and THT in PCBA Manufacturing

PCBA means "PCB assembly" or "assembled PCB". When a printed circuit board is mounted with required electronic components, we call it a PCBA. It is semi-finished electrical equipment or part of an electrical device and it achieves the main functions of the device.


A complete PCBA manufacturing process needs two assembly procedures - an SMT (surface mount technology) assembly and a THT (through-hole technology) assembly. During and after PCB assembly, a series of tests happen for ensuring PCBA quality.

Most components are SMD components such as deductors, resistors, compactors, ICs, etc. They can be soldered on the pads of the board using solder paste with SMT stencils. A reflow soldering oven melts the solder paste and connects the SMT components with the board tightly.

Some components having long legs have to insert the board’s drill holes and need to be assembled after the SMT assembly. Technicians use hands or machines to insert the component's long legs and then the assemblies go through a wave soldering oven to connect the THT components with the board. This process is the THT assembly.


Complete Process of PCB Manufacturing in PCBONLINE

When printed circuit boards are ready, you need to have them assembled with components. Usually, a PCB company doesn't produce components but sources from their trusted suppliers. You can buy all components on your own and send them to the PCB company to assemble, or provide special components and source basic components from the PCB company. At PCBONLINE, all basic  components are stored in their warehouse and ready to use at once, and they also source special custom-made components. When circuit boards and components are available, let's start to manufacture PCBAs.

  • Step 1. Preparation - Checking Bill of Materials

Bill of Materials (BOM) is an excel containing all your components' information. When you order from a PCB assembly company, you need to provide the BOM. Not all assembly companies check your BOM and this may lead to failures. But PCBONLINE does, and it is free. An experienced engineer checks your board design, the component samples, and your BOM carefully. If everything is okay, the assembly starts. If your components include BGAs or ICs, 

bill of materials

  • Step 2. Preparing PCB Stencils and Components for PCB Assembly

PCB stencils are also known as SMT stencils are used for surface mounting. In fact, the PCB stencils are made when the PCBs are manufactured. Standard solder paste thickness ranges from 0.09mm to 0.15mm, and the SMT stencil thickness has to be within the tolerance of ±0.03mm thicker than the solder paste thickness. How many stencils are manufactured depends on how many SMT lines are open to assemble your boards in a PCB assembly factory. 

PCB assemblers usually do not manufacture but source electronic components. Clients can also provide all the components or parts of them. This is flexible.

SMT stencil

  • Step 3. Manufacturing a PCBA Sample for First Article Test

The first article test (FAT) is necessary for mass PCBA production because if minor mistakes happen, the whole large batch of PCBA becomes a failure. So PCBA manufacturers assemble the first piece of board to test its specifications and generate a FAT report for customers to review. If the PCBA is good and customers approve, then the PCBA manufacturer starts batch production. The PCBA sample and FAT are free of charge for customers.

At PCBONLINE, if the order amount is at or above $5000, the complete PCBA sample is free of charge.

first article inspection

  • Step 4. Begin PCBA Batch Production - Solder Paste Printing

Functional testing of electronic components is conducted right before the assembly, and printed circuit boards are sending on the autoloader. When technicians make sure that the assembly line is ready, all machines are powered on. The boards are sending from the autoloader to the solder paste printer. A blade carrying the solder paste presses the PCB stencil, and the solder paste filters the stencil holes and prints on the pads of the PCB. when the board is pasted, it leaves the printer and the next board is printed.

solder paster printing

  • Step 5. Mounting Small Components

Small components mean the SMD components small in size, like deductors, resistors, capacitors, diodes, etc. These components’ package sizes are 0201, 0402, 0603. When the board is sent into the high-speed mounter, these components are quickly picked and placed on their desired PCB pads. Then the board mounted with small components leaves to the functional mounter and the next board comes in.

high speed mounter

  • Step 6. Mounting Large Components

Large components are the SMD components large in size, like BGAs, FPGAs, ICs, USB, connectors, etc. These components’ package size is larger than 0895. Like in the high-speed mounter, larger components in the functional-mounter are also picked and placed on the required pads, but the machine operation is much slower and more accurate. When the board leaves the functional mounter, it has to pass the solder paste inspection (SPI) to make sure the solder paste thickness and shape on the pads are qualified.

functional mounter

  • Step 7. Reflow Soldering

In this step, the boards pass the reflow oven and the solder paste melts and solidifies. In this way, the SMD components and the board are connected tightly electrically and mechanically. When the boards leave the oven, they have to go through an automatic optical inspector to check whether the mounting is qualified. If the assemblies are qualified, they head to the next line.

reflow soldering oven

  • Step 8. Through-Hole (THT) Assembly

The above is the SMT assembly, and here is the THT assembly line. SMD components can be "printed" on the circuit board, but components with long legs can’t and they have to insert the through-holes. This operation can be completed either by machines or by hand as it is quite simple.

THT assembly

  • Step 9. Wave Soldering

THT assembly only inserts the component legs through holes loosely and the purpose of this step is to connect the THT components tightly to the PCBA. The assemblies go through a pre-heater to warm up and then go through the wave soldering oven. In the oven, the liquid tin fill the gaps between the THT component legs and the board. When the PCBA leaves the oven, the tin solidifies and then the THT components are connected tightly to the assembly electrically and mechanically. Then technicians trim the THT legs from 1mm to 2mm long.

wave soldering machine

  • Step 10. PCBA Functional Testing

During PCBA manufacturing, PCBA testings are necessary in case of soldering defects to avoid economic losses due to re-assembly. At PCBONLINE, functional testing is free for customers. Technicians use a functional tester to test the current, voltage, and so on while the PCBA is powered on. If the PCBA works well, then technicians conduct the thermal aging test. Then it is assembled with the outer shell and packaged for delivery.

functional test

  • Step 11. Thermal Aging Test

The PCBAs are put in a thermal chamber to power on for about 250 hours without interruption. The temperatures are elevated to a certain level. All the PCBAs should follow the ECIA - EIA-364-17 standard.

thermal aging test

In fact, the above steps are just the basic operations for PCB assembly. For the PCBs used for aerospace, defense, automotive, medical, and communications, the four-terminal sensing is also required for PCBA manufacturing. Besides, the flying probe test, in-circuit test, IC programming, repairing if there's any problem, and so on may also occur depending on the practical needs.

Why PCBONLINE is a Reliable PCBA Manufacturer

If you’re looking for batch PCBA manufacturing, you need to work with a reliable PCB assembler. PCBONLINE, a leading one-stop PCB manufacturer for medical, aerospace, defense, automotive, industrial, communications, and consumer electronics, is recommended for PCBA manufacturing. All PCBAs at PCBONLINE follow the IPC-A-610 Class 3 standards. You can register for free and get a quote online, or contact info@pcbonline.com for a quotation.


This article tells what is going on in PCBA manufacturing. In fact, PCBA manufacturing is the PCB assembly process. Different PCBs have different requirements for assembly. The above is just the basic process. To get to know more, you may learn from your own PCB projects.