PCB Basics - PCB Manufacturing, PCB Terminology

PCB Basics - PCB Manufacturing, PCB Terminology

Printed circuit boards are a crucial part of electronic devices. When many people hear this word, they feel that they have touched a knowledge blind spot and know nothing about PCB. This article will briefly describe PCB for everyone so that everyone can understand printed circuit boards and some PCB professional terms.

What is a PCB?

PCB can also be called printed wiring board (PWB). In professional terms, PCB refers to the carrier for the electrical connection of electronic components and is also the support of electronic components. In simple terms, PCB is a place to put electronic components so that these electronic components are connected to form a circuit.

Classification of PCB

Printed circuit boards can be divided into various PCBs according to different characteristics.

According to the strength of the substrate, it can be divided into rigid PCB (printed circuit board), flexible circuit board, rigid-flex circuit board, etc.

According to the base material, it can be divided into Metal core PCB and FR4 PCB.

According to the number of layers, it can be divided into single-layer PCB, double-layer PCB, multi-layer PCB, and high-density interconnect board (HDI).

1) Single-layer PCB

Single-layer boards are the simplest boards. One side is used for routing on the circuit board, and the other is used to install electronic components and print logos.

Single-layer boards are suitable for simple circuit connections because it is difficult to wire only one side. The best feature of single-layer boards is their low price and simple manufacturing process.

2) Double-layer PCB

Double-layer boards can be routed on both sides of an insulating board, with one side as the top layer and the other as the bottom layer. The top layer and the bottom layer are electrical connections through vias.

Generally, electronic components are installed on the top layer. However, to reduce the size of the circuit board, components can also be placed on both layers.

The characteristics of double-layer boards are moderate prices and easy wiring. It is the most commonly used circuit board type at present.

3) Multi-layer PCB

Circuit boards with more than two layers.

4) High-Density Interconnect Board

Circuit board with high density using micro blind buried via technology. HDI is a compact product designed for small-capacity users.

Structural composition of PCB

PCB mainly consists of copper-clad laminates (CCL), prepreg (PP sheet), copper foil, and solder masks. At the same time, to protect the exposed copper foil on the surface and ensure the welding effect, the PCB needs to be surface-treated and marked with characters (also known as silk screen printing).

1) Copper-clad laminate

Copper-clad laminate (CCL) is the basal material for manufacturing printed circuit boards. It is a composite material composed of a dielectric layer (resin, glass fiber) and a high-purity conductor (copper foil). FR4 is the most commonly used in the market.

2) Prepreg

Prepreg, also known as PP sheet, is one of the primary materials produced by multi-layer boards. It is mainly composed of resin and reinforcing materials. Prepreg will soften under heating and pressure and solidify after cooling.

3) Copper foil

Copper foil is a thin, continuous metal foil on the base layer of the circuit board. It is the conductive source of the PCB. It is readily bonded to the insulating layer and etched to form a circuit pattern.

4) Solder mask

Printing ink on the panel surface protects the copper. The ink is usually green, so the PCB boards that we see are green. Of course, we also can print other colors of ink, such as red, black, blue, etc.

5) Silk screen printing

It is located on the top layer of the PCB board and is used to print text notes, etc. It is equivalent to making a PCB label.

What are the steps in PCB manufacturing?

PCB fabrication starts with blueprint design and goes through multiple stages to get a complete PCB product. Each step is significant because they are all interrelated.

1) Design PCB

PCB designers usually use a software called Extended Gerber, which not only helps with the design but also encodes information about the number of copper layers, solder mask layers, and other component symbols.

Our PCB manufacturer doesn't need to do PCB design. Because the PCB design (Gerber files ) is provided by customers, Before production, the factory will conduct a secondary defect inspection, a process called manufacturing design inspection. After confirmation, the next step will be carried out: design review and engineering issues.

2) Design review and engineering issues

This step is to check the design for omissions and errors. The PCB can be printed after the engineer confirms the design has no defects or potential errors.

3) Print PCB design

The printing of the PCB design uses a particular plotter. This printer prints the PCB design as something like film. The PCB's inner and outer layers are represented by two ink colors.

• Black ink

The inner layers represent the copper traces and circuits of the PCB; Black ink on the outer layers of the PCB represents areas where copper has been eliminated.

• Transparent ink

Used on the inner layer for non-conductive areas; on the outer layer to indicate copper vias.

Each PCB layer has a solder mask. The PCB has four layers, which need eight sheets of black film. At the same time, the film needs a punched hole with a puncher to position it.

4) Printing copper

This step marks the official start of PCB fabrication. The PCB design prints onto a laminate with copper foil pre-bonded on each side. Then, the copper is etched away to reveal the blueprint of the original design.

Next, clean the copper. To ensure the printed panel has no dust on the surface. Otherwise, it may cause a short or open circuit in the PCB.

A light-sensitive film, called photoresist, is applied to the clean board. Photoresist contains a chemical that hardens when exposed to ultraviolet light.

Next, the film and circuit board are aligned with the positioning holes and placed under ultraviolet light to harden the photoresist. Finally, the unhardened areas of the circuit board are cleaned with an alkaline solution and then dried.

5) Remove excess copper

Similarly, we use an alkaline solution to dissolve the excess copper in the inner layer, and the required copper will not be affected under the photoresist hardening layer. The time required for dissolution and the solvent are in positive feedback with the weight and size of the circuit board.

6) Alignment of inner and outer layers

After cleaning, align the inner layer with the outer layer. Usually, the technician places the circuit board on a punching machine called an optical puncher, and the machine aligns the inner layer with the outer layer by passing pins through the positioning holes.

7) Automatic optical inspection

This step is very significant. Once the layers are assembled,  the question cannot be corrected in the inner layers. To ensure the PCB isn't defective, technicians will use AOI inspection. The AOI machine will compare the PCB board with the original design. Some problems will be displayed on the display. Failure circuit boards will be disused at this stage.

8)Lamination PCB layer

At this step, the PCB board is formed. The layers without defects are pressed together. There are the following steps:

• Riveting

Rivet multiple inner core boards and films (prepregs) together to avoid deviation during subsequent processing.

• Lamination

Pre-laminated boards are stacked in the form of multi-layer boards to be pressed in preparation for pressing.

Under vacuum conditions, the stacked boards are pressed into multi-layer boards by heating.

• Post-processing

The laminated boards go through grinding, targeting, and milling for the initial profile treatment.

9) Drilling

PCB drilling requires high precision because a small error can lead to heavy economic loss. Use an X-ray machine to locate the drill holes to ensure that you find the correct drill location.

Before drilling, workers need to put buffer material under the drilling target to ensure the cleanness of the drilling. The positioning hole is to fix the PCB. When drilling, the computer will control the drill bit to determine the appropriate drilling position with the Gerber design file as the target. Once drilling is complete, remove the remaining copper on the edge.

10) PCB plating

After a series of chemical reactions, a thin layer of copper is deposited on the wall of the drilled hole.

11) External imaging

Apply photoresist and use yellow light for illumination, as photoresist hardens under UV light. Use pins to fix the black transparent film to prevent it from being misaligned with the panel.

As in step 4, carried out in a sterile environment, we also use ultraviolet light on the outer layer to harden the photoresist. The panel enters the machine to remove the unhardened photoresist protected by the black ink.

12) Outer layer etching

The photoresist is dissolved and cleaned to remove the excess copper. An alkaline solvent dissolves excess copper foil in preparation for AOI inspection and soldering.

13) Outer layer AOI inspection

Like the inner layer, the outer layer also needs to undergo automatic optical inspection to ensure that the process of this layer meets the design requirements. It can also check whether excess copper has been removed from the previous layer.

14) Solder mask

First, clean the circuit board to remove impurities or excess copper.

The board surface is applied with ink and a solder mask. The PCB is exposed to ultraviolet light to harden the photosensitive film and remove the unhardened parts.

The board then goes into an oven to cure the solder mask, forming a film that will protect the copper from corrosion and oxidation.

15) Silk screen printing

Silk screen printing is a business card or label attached to the circuit board. The information is printed directly on the circuit board using an inkjet printer. The information includes company ID, symbol, component number, etc.

16) PCB electrical testing

PCB testing detects the integrity of the circuit board's routing and through-hole interconnection to ensure no short or open circuits on the PCB. There are several testing methods:

• Online Test (ICT)

In-circuit testing, also known as bed of nails testing, is currently the most reliable method. But it is expensive.

ICT excels at ascertaining various PCB component failures, including circuit opens, shorts, and other defects, while providing clear guidance on handling defective PCBs.

• Automated Optical Inspection (AOI)

This method is suitable for high-volume PCB inspection. The automatic optical inspection machine uses a high-performance camera to observe and detect whether there are any problems with the welding from various angles.

• X-ray inspection

Use infrared rays to check potential problems in the inner layers of the PCB.

17) Analysis

Analysis is the last step in the PCB production process. Before this step, the printed circuit board is a constructed panel that needs to be cut into individual boards compared to the original design file. There are several methods of cutting：

• V-groove

In this method, the CNC machine makes a V-shaped cut along the side of the PCB board.

• Laser cutting

This method is suitable for PCBs with complex shapes and high precision.

• Mechanical cutting

PCBs are cut using cutters, knives, etc. This method is suitable for large quantities of PCBs.

Whichever method you use, you can easily break the PCB board after analysis.

PCB Terminology

During the PCB production and assembly process, many professional terms will appear. Let's take a look!

Substrate - the most basic material of PCB, made of glass fiber. Glass fiber lays the foundation for the rigidity of PCB and has the function of anti-breakage. It is equivalent to the skeleton of our human beings.

Copper foil - A thin layer of copper foil is attached to each functional surface of the PCB. Copper foil is a cationic electrolyte material. It is the conductor of the PCB and for electrical information transmission. It is similar to our central nervous system transmitting movement commands through the spinal cord and peripheral nerves, transmitting the brain's commands to the muscles.

Slots - This refers to any hole in the PCB that is not round. Slots can be plated or unplated. Slots can sometimes increase the cost of the board because they require extra cutting time.

Angle ring - The copper ring around the metalized hole in the PCB.

Drilling hit --This indicates the deviation between the required drilling position in the design and the actual drilling position. Nowadays, computers control the drilling bit to ensure the accuracy of the drilling position.

Pad - A portion of metal exposed on the surface of a PCB used to mount soldered components.

Trace - refers to the continuous copper path on the circuit board.

Panelization - A large circuit board consisting of many small divisible circuit boards. When high-volume PCBs assembly, the use of panelization technology can improve assembly efficiency.

Picking - the process of placing electronic components on a circuit board. There are usually machine and manual picking.

Wave soldering - A method of soldering components. The printed circuit board is passed through a molten solder furnace at a constant speed to produce a stable wave crest, and the solder wave crest will solder the component pins and exposed pads together.

Surface mount - a method of PCB assembly where the machine only needs to place electronic components on the circuit board and then firmly fix the components on the pads through steps such as solder paste printing and reflow soldering.

Reflow soldering - melts the solder paste to connect the pads (SMD) and device pins.

Solder bridging - -Two connected pins on a component are connected, which is a printing error.