The ins and outs of printed circuit board assembly (PCBA)

September 18, 2023

TABLE OF CONTENTS

The meticulous attention to detail within PCB manufacturing, especially the crafting and eventual electronic component placement, can be extremely daunting, especially without the proper guidelines.

Join us as we dive deeply into printed circuit board assembly (PCBA), including the manufacturing process, inspection, benefits, and tips that can save your company time and money and provide a foundation for all future developments.

What is printed circuit board assembly (PCBA)?

As the name suggests, PCBA is the assembly of various electronic components mounted on a printed circuit board (PCB).

A PCB is constructed using a flat panel made of non-conductive material. Popular components are crafted using fiberglass or epoxy, and pathways are printed or etched on the surface.

The soldering process integrates resistors, diodes, integrated circuits, capacitors, and other essential functions and components onto the circuit board to create a robust PCBA.

This is done through two main methods, Surface Mount Technology (SMT) and Through-Hole Technology (THT), which we’ll touch on in more detail below.

Documentation requirements for PCBs

A successful PCB assembly process requires in-depth documentation that many requirements must be met.

Although it may be lengthy, including the following in your PCB design and fabrication is necessary to comply with regulators and craft an effective PCB.

  • Bill of materials (BOM): The BOM is a complete list of everything required for PCB assembly. Part numbers, quantities, and reference designators are necessary for inventory management and procurement.
  • GERBER files: Vector image files, created by PCB design software incorporate all the information required to manufacture the PCB. They typically include details about vias, pads, traces, and design elements and the placement of the components for integrity..
  • Schematics: A schematic diagram shows the electrical connections between components on a PCB. They are created during  PCB design, and show every detail, particularly routing information between components. 
  • Functional diagrams for components: These diagrams show the high-level design of the PCB and its functionality. Stakeholders can use them to understand the purpose of the PCB, whereas electrical engineers rely on schematics for more detail.

Printed circuit board assembly methods

As we alluded to earlier, there are two primary methods for PCB assembly. When reading through this section, remember the benefits and unique advantages each one offers when selecting a PCB assembly service.

The main distinctions have to do with mounting onto the PCB board.

Surface mount technology (SMT)

SMT can be thought of as a “direct approach.” Components have small metal pads or leads mounted directly onto your circuit board’s surface.

SMT components are generally lighter, smaller, and fused directly onto the solder pads. This process is best for miniature devices.

Through-hole technology (THT)

THT, sometimes also known as PTH (plated through-hole soldering), is an older technique still used for various electronic devices.

The main difference is the mounting technique, which is placed through the holes drilled into the PCB. The elements and components on the board pass through the PCB and are soldered opposite the side to create a secure electrical and mechanical connection.

Essential materials for PCBA

Circuit board manufacturing is a detailed process with many critical pieces that must work together to produce great results.

  • Substrate: The substrate is the base of the PCB, providing a solid fiberglass-reinforced laminate foundation that gives you the strength and insulation necessary to support other components and ensures proper connectivity.
  • Copper: Copper is an excellent conducive metal used in many electrical devices. In a PCB, it’s integrated as a foil onto the substrate, and then the excess is cut to the desired length and position.
  • Solder mask: Safety is an integral part of the PCB. A solder mask — a protective layer — is applied over the entire PCB surface (except where components are fused). This enhances the reliability of your soldering machine and protects the copper.
  • SilkscreenSilkscreen is an ink-based coating (non-conducive) used to indicate and label directions, polarity, and components.
  • Vias: A small, drilled hole into the PCB that connects the board and various copper layers. The most common are through-hole vias which are used in the THT technique.

How the printed circuit board assembly process works

The PCBA process involves integrating electronic components onto a printed circuit board to create a functional electronic device. The process typically includes the following stages:

PCB component procurement

Although this may seem straightforward, it sets the tone for the end-to-end process. You cannot craft a functional PCB without all the correct components. Sometimes, it’s also good to keep duplicates or redundancies, especially if this is a new technique or order.

A BOM is necessary in this situation. It provides a comprehensive list of everything needed to move forward in the assembly process. Keep track of everything you need, including quantities, references, and part numbers for each component by using a PLM platform for BOM management.

Solder paste application

A jiggling component, a dangling coil, and a finicky loose screw can affect your board’s functionality, so it’s important to make sure they’re well secured with solder paste. Solder paste (a mixture of flux and particles) is a temporary adhesive used during the soldering process that holds the surface mount components in place until the solder is applied.

Positioning components with pick and place machine

While there’s a time and place for an intricate human touch, automation is the way to go during the assembly process. A pick-and-place machine uses mechanical or even vacuum-based movements to take a component from a feeder and put it into the correct position to be soldered, reducing costs and human error during at-scale production.

Soldering

Once all components have been gathered and placed in the correct positions on the circuit board, it’s time for soldering. Soldering melts a metal alloy at high temperatures to affix the components to the board, ensuring strength, durability, and longevity.

Reflow soldering

Reflow soldering is usually used with SMT assembly to provide a reliable and precise solution. Once the PCB is assembled, it passes through a reflow oven which causes the solder paste to melt through a controlled heating process.

The ensuing solder joints offer a permanent bond for the components to stay in place on the circuit board.

Wave soldering

This method leverages the through-holes created in the THT or PTH technique to solidify the exposed leads of the components.

To wave solder, you must first create solder joints that flow through the through-holes by passing it over a molten solder wave.

The technique makes contact with the leads of the components and is bonded through the solder joints. You can also use wave soldering for the SMT technique, but you may have issues with copper dissolution.

Inspection and quality control

Once the parts and components are attached to the circuit board, it’s considered the first official PCB prototype.

The PCB then needs to go through a quality control process that ensures overall quality and that the soldering connections are up to par.

There are various inspection techniques that can be employed in this step, including AOI (automated optical inspection) and X-ray inspection, to detect anomalies and defects such as incorrect placement and solder bridging.

The AOI technique employs an autonomous camera that scans the device and alerts it to anything incorrectly placed. You can better allocate human resources, such as leveraging management or subject matter experts for final inspections.

Through-hole component insertion

When using the THT technique to assemble the parts onto the board, you must verify that the components were correctly inserted through the through-hole. This process is manual, which means you’ll have to insert the leads into the pre-drilled holes to ensure it’s secure and in proper alignment.

(You’d skip this process if you used the SMT method to mount the parts onto the PCB.)

Final inspection and testing

Testers will give the final product a thorough inspection to confirm that it meets the functionality and performance specifications outlined in the design. Failure to perform a proper final inspection and testing process could lead to faulty products being released to the market, which could damage brand reputation.

Managing PCB documents in a PLM system

PCBA requires attention to detail and various reviews to ensure compliance and quality. The best way to do this is to document all the components and techniques in a centralized platform.

A product lifecycle management (PLM) system helps to establish a structured approach and simplifies the import of designs from CAD applications, such as Autodesk Eagle, or Altium Designer.

Component hierarchy

The component hierarchy shows you a structured and visual representation of your components, including, electrical your BOM (EBOM) and sub-assemblies. The PLM system will allow you to view all components (including connectors and passives) specified and organized.

Independent revisioning

PCB and BOM components should be reviewed and revisioned independently. PCB layout changes, such as rotating footprints and re-routing races, need to be updated with new schematics and Gerber files — regardless of what happened with the BOM components.

BOM components should also be reviewed. For example, let’s assume there are no PCB layout changes. You may need to substitute a resistor for another that is pin-compatible. In this case, only your EBOM needs to be updated.

Streamline your PCBA with Duro

Procuring all components, reviewing them, and tracking changes are all crucial to ensure your PCBA process is effective.

With Duro’s PLM solution, you can integrate all documents and revisioning of your PCBA process, in one place. Collaborate, manage documents, control revisions, track changes, and integrate with popular electrical CAD applications for a smooth experience.

Request a demo today to experience the benefits of our PLM solution firsthand and unlock a better, more streamlined PCBA management process.

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