PCB Assembly, PCB Manufacturing

PCB Manufacturing and Assembly: A Comprehensive Guide

The electronic devices that we use so conveniently today may look simple, sleek, and straightforward. If you try to break into these devices, you will see the complicated tiny parts and boards that make them work and fulfil their functions. These boards are called Printed Circuit Boards (PCBs) which have gone through rigorous design and manufacturing steps. In this article, we will elaborate on the processes and details to widen your understanding of PCB manufacturing and assembly.

  • Types of PCBs
  • PCB Manufacturing and Assembly Process Flow
  • Process Control, Inspection and Testing
  • Quality Standards in PCB Manufacturing and Assembly

Types of PCBs

Printed circuit boards are dielectric substrate materials with circuitry assembled on the substrate. There are three major classifications for PCB namely: Single-Sided, Double-Sided and Multi-Layer PCB.

  • Single-Sided PCB

Single-sided boards have circuitry on one side only. The arrangement of a single-side PCB can be visualized as a board with a “component side” and a “solder side” where the circuit pattern is located. This board type has the simplest and least-cost PCB manufacturing and assembly among all types.

  • Double-Sided PCB

Double-sided boards have circuitry on top and bottom, and electrical connections are made by drilling holes through the substrate. This type of PCB board can be grouped into a double-sided PTH board, where “PTH” stands for Plated-Through Hole connection.

  • Multi-layered PCB

The most complex one, the multilayer PCB, has more than two dielectric substrate layers that have a connection from one side to its inner circuitry by drilling holes and plating copper. This type of PCB is required for high-density connections. The different layers are interconnected by means of plated through holes.

The substrate is commonly made of fiberglass sheets laminated with copper foil through an epoxy resin called prepreg. Other types of PCB can be classified based on the type of substrate used: rigid or flexible or even a combination of both types.

PCB Manufacturing and Assembly Process Flow

To give you an understanding of how PCBs are made, below are the major processes in PCB manufacturing and assembly.

  • Documentation and Material Preparation

The manufacturing process starts by receiving the lot traveller from the Planning team which usually contains the materials and assembly steps to be followed, including the detailed requirements for each step. Laminate is then blanked and prepared for the process. The drilling tape is programmed and a working film is generated.

  • Drilling

The panels are stacked together depending on panel thickness and loaded into the drilling equipment. The drilling program is fed into the machine which dictates the hole location and bit size to be drilled into the panels.

  • Plating

The panels are deburred to remove the excess burrs from the drilling, then move to a plating process where copper is deposited into the holes. This process is called through-hole plating and acts as the electrical connection between the panel sides.

  • Circuit Patterning

After the plating process, the circuit pattern is imaged using silk screening or photoresist methods. In silk screening, the pattern is made with an ink plating resist. On the other hand, photoresist is done by coating a layer of photosensitive film and exposed to UV light. The unexposed photoresist is removed during development while the exposed photoresist is cured to a hard resist. This image transfer process is also called “photolithography”.

  • Electroplating

The circuit pattern is electroplated with copper to a desired thickness and plated over again by tin-lead, nickel, or tin-nickel. The second coating of metal serves to protect copper from oxidation during moisture exposure.

  • Copper Etching

The dry film photoresist is stripped away by a solvent. This etching process will leave the plated circuit on the fiberglass by removing the unwanted copper foil. The etchants used can be sulfuric acid, cupric chloride, and hydrogen peroxide.

  • Solder Mask Printing

The next step is to apply the solder mask and label to the PCBs. The solder mask is applied from both sides of the panels. It serves as a barrier to prevent bridging or shorting during soldering operations. Solder mask is formed through silk screening and curing.

  • Components Notations Printing

When solder masks are done, we need to apply a silkscreen layer on the PCB surface, which is used to mark out the functions of each pad. It is very useful in the PCB assembly process, and also helpful for end users to know each component if needed.

  • Surface Finish

We need to cover the PADs with a thin metal layer, to prevent the pads from oxidation, and also helpful in the PCB assembly process. This is a very critical step to get a good PCB fabrication and assembly since any minor defect may cause serious bad soldering issues.

  • Solder Paste Printing

Now we started the PCB assembly process. Firstly, we need to apply a layer of solder paste on the PCB surface with SMT stencil. Usually, we will use an automatic solder paste printing machine. But manually applying solder paste is possible for very simple prototypes.

  • Components Placements

When solder paste is applied properly, a pick-and-place machine is used to put the components on the PCB surface. Very tiny components can be placed accurately, such as BGA components and 0201 pcakges components.

  • Reflow Sodering

The next step is to melt the solder past by running the PCBs with components through a high-temperature reflow oven. In this process, solder paste will be melted and components will be soldered on the PCBs permanently.

  • PTH Components Insertion

On some PCBs, there are PTH components. Firstly, we need to insert the components into the PCB drills according to the BOM list correctly.

  • Waving Soldering

After all PTH components are inserted into PTH holes, we will run the PCBs through the waving soldering machine, it looks like running through boiling tin. The pins will be soldered on the PCB when cooled down.

  • Manual Soldering

Sometimes, manual soldering is still needed. Such as a very simple prototype with quick delivery, manual soldering is a good choice, or any component cannot withstand high temperatures.

PCB Production and Assembly

PCB Production and Assembly

Process Control, Inspection and Testing

When PCB manufacturing and assembly are done, the board is then inspected for any defects and tested per product requirements. The acceptability of the PCB will depend on the conditions and criteria established by the manufacturer and customer. PCB production and assembly involve many steps to ensure the quality of the products. Below are the general inspection and test processes for PCBs.

  • In-process Inspection

Inspection during the early steps of PCB assembly is being done to check for any broken traces and short circuits. This will give an immediate heads-up to the production line if there are any manufacturing issues so that correction and prevention are both provided. Both AOI inspection and visual inspections will be done.

  • Final Inspection

The PCB undergoes final inspection with sampling size based on AQL (Acceptable Quality Level). The AQL is a tabulated sampling plan which is usually based on the lot size to determine the reliable sampling size to ensure good parts are only shipped. The inspections will be done according to commonly accepted standards for PCB fabrication and assembly, such as IPC-A-600 and IPC-A-610. But the most important certirai is the customer’s standard.

  • Electrical Testing

A significant step in PCB manufacturing and assembly is the electrical testing of the boards. Note that testing is done both on bare board and on loaded board with electronic components. To determine the electrical performance of the bare PCB, the board is mounted on a test fixture and tested for any short or open circuits. Loaded board test includes the detection of manufacturing defects through in-circuit and functional testing. As the visual inspection method can be inaccurate, especially for boards with hundreds to thousands of components, electrical testing will confirm if the boards have functional issues.

  • Quality Standards in PCB Manufacturing and Assembly

The design, fabrication and assembly of PCB is a complex series of activities which may be vulnerable to variation. To standardize and establish a common ground for all PCB manufacturers, critical standards were developed to produce quality circuit boards. Below are some of the key standards widely recognized for PCB production and assembly.

  • Institute of Printed Circuits (IPC)

The Institute of Printed Circuits, more commonly known as “IPC” is a known association that develops standards through the identification of industry challenges and collaboration with its members in generating solutions. IPC has guidelines at every stage of PCB life from design to manufacturing and assembly. For instance, PCB acceptability standards are outlined in IPC-A-600 while printed board quality and reliability standards are specified in IPC 6000 series.

  • International Organization for Standardization (ISO)

The International Organization for Standardization (ISO) is the standard for quality management systems, usually used not just by PCB manufacturers but by almost every other industry. ISO define standards as documented agreements with technical criteria and guidelines to ensure that products and services are aligned with the intended purpose. ISO standards make use of PDCA (Plan-Do-Check-Act) as a framework for continuous improvement.

  • Underwriters Laboratories (UL)

PCB manufacturing and assembly should also be compliant with safety standards. Underwriters Laboratories or UL provides safety testing and certification which helps increase confidence in the product’s reliability and electrical performance. UL’s testing capability includes chemical, electrical, flammability and physical tests.

Summary

We have discussed important topics in PCB manufacturing and assembly. By now, you have a glimpse of what printed circuit boards are, how they are manufactured and how to ensure their quality. This article gives us a comprehensive guide to PCB fabrication and assembly. It will be helpful for us to know more in the PCB industry, which will benefit us. As technology advances in this modern world, PCB technology will also continue to evolve.

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