PCB Assembly (PCBA) Process

Geospace Technologies Contract & Manufacturing Division

January 21, 2022

We all own electronics; they have become an essential part of our lives. Whether our phones, TV, or cars, all electronics have a printed circuit board, also called PCB. The PCB is the heart of all electronics. The PCB assembly process is necessary for any electronic device to work.

An assembled PCB is one with all the necessary components mounted. The assembled PCB has multiple features, including running signals between connectors and components, allowing the circuit board to perform different functions ranging from the simplest to the most complex.

The PCB assembly process is simple and consists of multiple manual and automated steps. Today, I will take you through the PCB assembly process and the different PCB assembly methods we use to create a fully functioning printed circuit board.

Understanding PCB Terminology

Before delving into the whole process, let us first define some terms. This will help you know what the words mean once we start the assembly process.

  • Substrate- refers to the base material of the circuit board. The substrate is what makes the PCB rigid.
  • Copper- this layer consists of thin conductive copper foil added to all PCB functional sides. If only one side is functional, known as a single-sided PCB, only one layer is covered, and both sides are covered with double-sided PCBs.
  • Solder mask- this is responsible for the green color of PCBs and is what keeps everything where it belongs. The solder paste is applied, which acts as an insulation that prevents the copper traces from touching other conductive materials, which could cause a short.
  • Silkscreen- this is the final layer on a PCB board. It uses symbols and characters to add labels to the PCB to indicate the function of each component.
  • Pin-through paste- a method used to solder Through Holes parts during the SMT process. This method only applies to parts that can withstand reflow temperatures.

 Next, let us look at the main types of PCB. They include:

  •  Rigid PCB- is the most common type of PCB base and is the major part of the assembly process. It has a solid core that makes the board thick and rigid. It can be made using different materials such as fiberglass (FR4). There are also cheaper PCBs that use materials like phenolics and epoxies; these are, however, less durable than FR4.
  • Flexible PCB- these are more pliable than rigid PCBs. They are made using more flexible, high-temperature plastic, such as Kapton.
  • Metalcore PCB- is an alternative to the usual FR4 board; they use metal cores to make them rigid and are more efficient at spreading heat. The efficient heat transfer protects the more heat-sensitive components and helps dissipate heat more efficiently.

 Types of PCB Assemblies

There are three main types of PCB assembly processes.

Surface Mount Technology (SMT)

This is when sensitive PCB parts are automatically mounted on the board’s surface features, including diodes or resistors. SMT is applicable on integrated circuits (ICs) and small-size components. It is popular because of its ability to place the board components on both sides of the board in high density, allowing for more compact and more powerful board designs.

Thru-Hole Technology

This assembly-type works best with large components subjected to mechanical stress and high power and voltage, such as coils and capacitors. Components with wires or leads are mounted on board by plugging them through pre-drilled holes on the board.

Mixed Assembly

This process combines the best parts of SMT and Thru-hole technology. The boards go through the SMT process and then the thru-hole technology process.

PCB Assembly Process

Before Assembling

At Geospace, we review the PCB design, called a design for manufacturability test or DFM check. The review helps us know if the PCB file has issues affecting its manufacturability or functionality. We review all products for manufacturing at no cost to you.

The check helps us look for any redundant, missing, or problematic features that help us know what needs to be fixed. While the DFM check is done at no charge, verifying all customer information and how they fit together, plus a full DFX service, are done at an affordable price.

The Assembly Process

Once the review is complete, the assembly team takes the PCB design. We use the mixed assembly process, which uses both the SMT and THT assembly processes. The process flow is as follows:

  • SMT
  • PTH
  • Hand soldering
  • Washing
  • Testing
  • Coat
  • Mechanical
  • Packing

Step 1: Surface Mount Technology

Paste Print

Applying solder paste to the board is the first step in assembly. This is done by placing a thin, stainless steel stencil over the PCB. The paste print process applies the solder paste to certain parts of the PCB where the components will sit once it is finished.

For more complex PCB designs, a more detailed solder paste is used. We used the 3D paste measurements for complex design as it is more precise and allows a small margin of error. It ensures that only the parts that will have components placed have the paste.

Pick and Place

This is the next step after applying the solder paste print. Here, a machine places the surface mount components on PCB. Machines are used because they offer error-free, precise, and quick processes. The device picks up the board takes it to the pick and place station, where it then applies the SMTs to the board. The components are placed on the solder paste on the already-assigned locations on the board.

Reflow Soldering

Once the components are placed, the board passes through a furnace or an oven. It has a temperature of 500𝇈 F. the high heat liquefies the solder paste, ensuring the SMD components are firmly glued to the board.

Inspection

After the reflow soldering, the board now moves to the inspection. We use different types of inspection, including 3D automatic optical inspection (AOI), which helps with bigger batches. This inspects if any low-quality solders need to be redone.

Step 2: PTH

Component Placement

Since we do mixed assembly, the board comes from the SMT process to THT, where plated through-hole components (PTH components) are added. A through-hole plated component is plated through the PCB. The process requires highly-skilled engineers who follow the regulations and set standards to produce a high-quality end product.

Inspection

After placing the components, the board is placed in a frame to check whether the components are correctly set. The inspection used can be manual, AOI, or x-ray inspection.

Step 3: Hand Soldering

After inspection, the PCB is moved to the soldering station. Here, one person is tasked with manually inserting one component on a specified PTH. The board moves from station to station, with workers at each station inserting their designated PTH until the whole board is fully outfitted.

Wave Soldering

Manually placing each PTH on the board can be a lengthy and physically taxing process. Some companies opt to use wave soldering, where the board is passed through a conveyor belt after a PTH component is inserted. The belt then goes through an oven where molten solder washes over the board, ensuring all pins at the board’s bottom are securely soldered.

Step 4: Washing

The board is then moved to a stainless steel high-pressure machine for cleaning. The machine uses deionized water to remove any residue from the PCB. The board then passes through a quick-drying cycle that uses compressed air where it is dried.

Step 5: Testing

Once the final soldering is done, the PCB then undergoes an inspection to test its functionality. The PCB runs through different tests that simulate the circumstances it will operate in, and any anomalies are noted. The PCB should pass all tests, hitting outside the predetermined range with no test records.

If the board fails the test, it can be scrapped or recycled as it can no longer be used. Regularly testing and inspecting the PCB throughout the assembly process is necessary as it will help prevent any failures from occurring. 

Step 6: Coat

Step 7: Mechanical

Step 8: Packing

Once the PCB is confirmed to be fully functioning, washed, and dried, it can now be packed and ready for shipment.

Why Choose Geospace

At Geospace, we work with you to follow your specifications. We can use any PCB assembly process, as per your directions. We also offer our services to different industries, including

  • Healthcare
  • Consumer electronics
  • Manufacturing
  • Automotive
  • Telecommunications
  • Military and defense
  • Home safety and security

Working with us ensures you are part of the process, from the beginning to the end, making it cost-effective and ensuring you get the product you want. With Geospace, we offer quality assurance, have technological capabilities, and are able to reduce lead times with automation. Contact us today to learn more about our services.

713-986-4444