PCB assembly process is the most critical step in developing any electronic product, so here we would like to talk about the all the PCB assembly process steps.
Following the PCBA process will ensure that the printed circuit board assembly process runs smoothly and produces an end product of the highest quality.
Preparations before PCB Assembly Process Steps
- DFM inspection
Before carrying out the actual assembly process, the documents of PCB design should go through a thorough check, having a sufficient understanding of the circuit board’s function and manufacturability. This stage helps inspect design mistakes and allows the designer to clear all defects, leading to successful production.
- Incoming Quality Control ( IQC )
According to the part number and quantity of the BOM list, manufacturers have to check if the incoming components are correct and if the shape is normal ( if they have transshape, broken pin, oxidize etc. ), especially for IC chips or other complex and expansive components. Also, tools such as test fixtures, multimeters etc. will be used to spot test or full test for the incoming components. If defects or differences with the BOM list are found, all those components will be returned to component suppliers or customers.
- Machine Data Programming
After receiving the bare PCB board and all components, the next step is setting and programming all machines needed during the PCBA process flow. SMT machine, AOI (automatic optic inspection) machine, and other machines need to set up a program. For example, an SMT mounter needs to be programmed before it can accurately mount components, this program is better generated by CAD data, but it usually can’t work. Gerber files are always used because this is the file required for manufacturing bare PCB.
PCBA Process Flow – A Step by Step Guide
1. Solder Paste Printing
Solder paste printing is the first step in the printed circuit board assembly process, involving the use of a stencil which precisely applies the required amount of solder paste onto the PCB’s surface so that the components can be soldered on the boards properly.
The stencil must be the exact dimensions of the circuit board, and the solder paste must be the correct type for the soldered components. The solder paste is then spread on the PCB at the designated pads using a squeegee or a knife edge to ensure an even coating.
2. SMT Assembly
After the surface of the PCB is printed with solder paste, the next step is component placement. In the past, the picking and placement of parts (such as surface mount components) were once done manually.
Now, the machine will pick and accurately place components in the pre-planned area of the circuit board. Then, each component will be taken out by a vacuum nozzle or fixture nozzle from the component package, checked by the system and placed in the programmed position at high speed.
3. Reflow Soldering
After verifying that there is no error, the reflow soldering will be performed. In reflow soldering, the solder paste is first applied to the board. Next, the components are placed in the correct positions on the PCB, and the assembly is then heated with infrared radiation and convection. As the PCB is heated, the solder paste melts and reflows around the component leads and joints, creating a secure mechanical and electrical connection.
4. Inspections after Reflow Soldering
- Automatic Optical Inspection ( AOI ) after Reflow Soldering
After the reflow process, we need to carry out Automatic Optical Inspection (AOI) to check if there is a poor quality connection, misplaced components caused by the PCB’s continuous movement during the reflow soldering. Those methods are manual inspection, AOI, manual inspection x radiographic testing ( AXI ) etc.
- X-ray Inspection for BGAs
X-rays can penetrate the surface of a BGA package, providing an internal view of the solder balls, pad connections, and other features. X-ray inspections provide detailed images, enabling production engineers to make informed decisions about the reliability and performance of their components.
5. THT Assembly
Some types of components need to be inserted into the PCB board. This step is typically finished by manual operation.
6. Wave Soldering
After the PTH components are inserted on the PCBs and inspected, we will solder the components by wave soldering.
Wave soldering is an efficient and cost-effective method of attaching PTH components to PCBs. This process uses a wave of molten solder that is applied to the PCB by passing it through a bath of hot liquid solder. Then hot will will run up into the PTH holes, and components pins will be soldered on the PCB after cooling down.
7. Visual/AOI Inspections
Inspections after wave soldering involve visual inspections, AOI inspection and tests to measure electrical characteristics such as continuity, voltage drop, and resistance etc. By doing these checks regularly, manufacturers can be sure their products will function correctly once in use.
8. Conformal Coating
Some customers will need a conformal coating for the finished PCBA. Conformal coating is a protective and non-conductive dielectric layer on the printed circuit board assembly. The function is to protect the boards and prevent components from being eroded by pollution, salt spray, humidity, fungi, dust and harsh or extreme environment.
Conformal coating is a crucial step in the PCB assembly process as it helps protect the board from environmental elements and static electricity. In addition, utilizing the correct coating material during the PCB assembly process steps helps ensure the final product’s quality and reliability, saving time and money in the long run.
9. ICT Test and Function Test
ICT test may needed to make sure the components are soldered correctly by electronic connection testing. And if the customer asks for a function test, test method, test software and tools are usually provided by customers, we also can make test fixtures according to the customer’s requirement.
10. Clean and Drying
The production process is filthy. Solder paste will remain with some scaling powder. So before sending the boards to the customer, you must completely clean the oil and dirt on the surface. First, clean them with polychlorinated biphenyls in deionized water or ultrasonic washing. After cleaning, the use of compressed air will completely dry the PCB.
Above are almost printed circuit board assembly process, in actual assembly, it will be more complicated, and you will encounter a lot of problems, but as an experienced PCB assembly supplier, all problems are growth for us.
