PCB Assembly Process

The majority of the surface mounting technology (SMT) parts on a printed circuit board (PCB) will be assembled with automated pick-and-place machines. To start PCB assembly, firstly require to prepare all things, stencil, solder paste, SMT carrier, pick-and-place program, bare PCB, components, reflow oven profile for production. The preparation is in fact the PCB assembly process. These begins with both being validated by the contract manufacturer’s inspection teams. Once that is complete, the PCBs will then be prepared for assembly using below PCB assembly process:

• Preparing Pick-and-Place Program: Pick-and-place machines can only work when they are pre-programed with the information about component positions so that they where to place the components. This program is normally developed from the printed circuit board (PCB) design -centroids file, and the panelized Gerber files if necessary.

 

• Solder Paste Printing: Using a laser stencil that is designed from the same CAD output files used to build the bare PCB, solder paste is applied to the SMT pads on the board. This paste is kept in a controlled cool environment, maybe a refrigerator, and once it is applied to the board after warming up and mixture, there is a time limit on how long the boards can wait for assembly.

 

• Solder Paste Inspection: Once the solder paste is applied to the SMT pads through the windows openings of the stencil, the boards are conveyed to run through solder paste inspection (SPI), which is similar to automated optical inspection (AOI) tool used after assembly, the SPI evaluates the paste location, the physical volume of paste applied, and other key parameters.

 

• Repeat Process for Secondary Side: If the board has SMT parts on both sides (double-sided assembly), the process will be repeated for the secondary side. At this point, the board is also being tracked for the amount of time the solder paste has been exposed to room temperature.

 

• PCB and Components Baking -Moisture Removal: Bare PCBs and any other moisture sensitive components, especially in today’s higher temperature lead-free assembly processes need baking before usage in a baking oven in a certain temperature and time.
• Uploading Components into Feeders: While the PCB board is being prepared for assembly, the parts on reel and tape, tube and tray, are prepared and uploaded into the feeders of SMD placement machines at relevant stations.

 

• Reflow Profile Setting: The New Product Introduction (NPI) engineer will usually start the process by selecting an reflow oven profile they have used before and that is stored on their system. Over time, as an EMS provider, MADPCB developed a range of baseline profiles, which can then be selected depending on the PCBA they are working on. In the golden first article assembly, we will adjust the best profile on each specific project.

 

Once the placement machines are loaded with correct parts, the pick-and-place machine is programmed with the placement data, and the boards with applied solder paste, baseline profile are ready, the SMT assembly line is now ready to begin to work.

 

SMD Components that Don’t Fit into the Automated Assembly Process

 

Even though most components will be placed using the automated pick-and-place machines, some will not. This could be for a variety of reasons need to consider when customize a PCB assembly process for an unique PCB design.

• Too Light: Some components aren’t heavy enough and, therefore, have a poor mass-to-adherence ratio for the automated placement equipment requiring manual soldering.

 

• Thermal Stress: Other components may have too much heat sensitivity and are not good candidates for going through the solder reflow oven. These components must be assembled after the normal assembly process to protect them.

 

• Robust Solder Joints: Some components, such as connector leads, need a more robust solder joint. These parts will be manually soldered to accommodate this.

 

• Limited Quantity ICs in Prototype PCB Assembly: The IC quantities maybe limited because of expensive or hard-to-find. If lost one piece, the assembly cannot go through or shipping in enough PCB assemblies. This kind parts will be placed by manual and go through reflow oven.

 

• Panel Separation: Some components may be damaged when the individual PCB assemblies are separated out of their assembly panels. In these cases, the components will be assembled after the boards have been broken out of the PCBA panel.

 

To handle the majority of these manual assembly needs, it is not uncommon for a contract manufacturer to have a manual rework station built right into the automated assembly lines. This allows for immediate inspection and correction of parts that may be in jeopardy of Design for Manufacturability (DFM) before they go through the solder reflow oven.

Additionally, some boards get assembled in low volumes that do not have their components available in shipping containers optimized for automated assembly. Whether these parts are unusually configured or are unique and expensive, they both must be manually placed before the board can go through solder reflow.

Once the parts are placed, the board can proceed into the solder reflow oven to complete the assembly process.

 

How Will Your PCB Assembly Benefit from MADPCB?

 

At MADPCB, we understand prototype, low-volume, medium- to high-volume PCB assembly builds and the need to create golden first-article boards. If there are any problems during assembly, we will catch and correct those problems with our inspection points by using any or multiple methods of SPI, Visual Inspection, AOI and X-ray before it gets through the next step in production.

In addition, the processes we’ve put into place and the equipment we use are all designed for parallel activity. For example, the next contract PCB assembly project can set up in the pick-and-place machine while the current project is still running. This can help reduce the amount of time and expense other PCB contract manufacturers will charge you to get your PCB assembled.

MADPCB is a one-stop PCB services provider, PCB and assembly manufacturer in China, our services can cover all your needs from components sourcing, PCB fabrication, SMT tooling (stencil, SMT carrier, depaneling pallet, testing jig), assembly to PCBA functional testing. We’ll help you with each step of the way and provide you with comprehensive expertise and quality assurance.

For more details of PCB Assembly Process arrangement for your project, feel free to contact us.