How to Check the Quality of a PCB？
Printed Circuit Boards (PCBs) are the backbone of electronic devices. They have become ubiquitous in today’s modern technology, powering everything from smartphones to computers. But how do you know if the PCB is of good quality?
PCBs are essential components in electronic devices, and their quality directly impacts the performance and reliability of the device. A poor quality PCB can cause device failure, compromising its functionality and compromising user safety.
To ensure that you are purchasing a reliable PCB, you need to check its quality before finalizing your purchase. In this article, we will discuss how to check the quality of a PCB and provide you with the necessary knowledge to make a sound decision when purchasing a PCB for your electronic device.
Methods to Check PCB Quality
The simplest way to check PCB quality is with a manual visual inspection (MVI). This involves visually inspecting the PCB, including the traces and placement of the components, to verify that it follows certain best practices in PCB design. Here is a closer look at the steps involved.
1. PCB Surface
The PCB surface should be free of dents, scratches, pits, and other types of defects. It should also include a suitable coating.
Many PCBs feature conformal coatings to protect the components and traces from damage. The conformal coating should be evenly applied and firm to the touch. It should also be fully cured instead of sticky or tacky.
Also Read: How to Remove Conformal Coating from PCB？
You can also visually inspect the board for signs of warpage and proper thickness. A warped board may not properly fit its intended housing. A board that is too thin may crack easily.
2. Component Placement
Components need the proper placement to decrease the risk of interference and improve heat dissipation. In many cases, components are grouped to shorten and simplify the trace routes. However, high-power components may need to be separated.
Components that produce more heat can create a large hotspot when grouped together. Spreading them out across the board can help keep temperatures down.
Analog and digital components are also typically split into different areas of the PCB. Separating the analog and digital components helps decrease electromagnetic interference (EMI). EMI can impact the quality or stability of connections between components.
3. Trace Width & Routing
The right trace width and routing are needed to maintain reliable connections between components. The average trace thickness is 10 mil (0.254mm). The minimum size for most designs is about 6 mil (0.152mm).
Traces that are too wide have greater resistance but can carry higher currents. A trace that needs to carry more than a 0.3-A signal may need to be wider than the typical size.
Traces that are too thin cannot carry as high of a current, which can lead to overheating. Thinner traces are also more prone to damage and more difficult to reliably manufacture.
The routing should also provide the shortest, most direct path between components whenever possible. With a single-layer or two-layer PCB, you can visually inspect the trace routes. A multi-layer PCB with more than two layers requires a different inspection method to analyze the traces.
4. Decoupling Capacitors
Decoupling capacitors used for input and output signals should sit in series with the traces. Capacitors used for eliminating noise on a power signal, the capacitors should be placed parallel to the paths. The capacitors should also be placed parallel to a resistor when using them to reduce high-frequency EMI.
A decoupling capacitor is also often necessary when isolating digital and analog grounds. The capacitor can help reduce interference between the components. In all cases, the capacitors should be placed as close to the source that requires decoupling as possible.
5. Length of PCB Traces
The length of the PCB traces can impact signal quality, reliability, and transmission speed. A common best practice is to use trace length matching when multiple signals run between components.
For example, two pins from component A connect to two pins from component B. The two traces should be as close to the same length as possible. PCB trace length matching helps prevent a mismatch that can result in timing issues.
Proper spacing is also required to prevent unwanted interference between signal traces. The typical rule is to maintain a spacing of at least 3 mils (0.0762mm). However, optimal spacing depends on the width of parallel traces.
Many manufacturers use the 3W rule, which requires a spacing of three times the width of the trace to prevent coupling the signals. The distance is measured from the center of each trace.
For example, if the traces have a width of 6 mils, the distance between the center of the two traces should equal at least 18 mils.
6. Antenna Feedlines
Antenna feedlines need to match the impedance of the antenna. The impedance is often determined based on the width and thickness of the trace that carries the antenna signal between the rest of the PCB circuitry.
Increasing the width of the antenna feedline increases its impedance. If the antenna feedline appears too thin or too long, it may not produce optimal results. You may end up with a delay or error when transmitting or receiving data through the antenna. Depending on the design, the antenna feedline may need a width of 20 mils.
7. Grounds & Ground Planes
In the past, many PCB designs included separate ground planes for the analog and digital components, but this is not always an option with a two-layer PCB. Using a single ground plane on one layer of the PCB is sufficient.
With a two-layer board, a large ground plane can be used on one layer. The other layer contains the power traces and signal traces.
With a multi-layer PCB that contains four or more layers, it is possible to use multiple ground planes. Separate ground planes make it easier to accommodate different power supplies for components with varying power requirements.
When connecting through-hole components to the ground plane, the PCB may also benefit from one or more thermal reliefs.
Thermal reliefs are small reliefs cut into the board to help prevent heat loss during soldering. It allows for a better solder joint when connecting through-hole components, as it prevents heat from getting trapped near the surface.
Useful Tips to Ensure PCB Quality
Creating a quality circuit board starts with the design process. The design needs to follow many of the best practices discussed, such as maintaining proper trace width and length and using decoupling capacitors.
PCB quality must also be maintained throughout the entire fabrication and assembly process. This may involve several different types of quality control inspections, including manual inspections and automated optical inspections.
Automated inspections rely on equipment or software to inspect PCBs for potential issues, such as defects and routing issues.
Burn-in testing may also be performed to test the PCB’s ability to perform under specific stresses, such as functioning under continuous use.
These tests and inspections should be performed after the initial production run to ensure quality before fabricating additional PCBs. Catching any defects or errors early can help reduce waste.
Summary
PCB quality depends on a wide range of factors that you can visually inspect. Start by examining the PCB surface for any signs of defects, such as missing pads or scratches in the trace paths.
A quality PCB also requires the proper placement of components, antenna feedlines, ground planes, and decoupling capacitors. The trace width, length, and routing must also follow certain rules to prevent interference or unwanted resistance.
Along with manual inspections, additional testing, such as automated inspections and burn-in testing, may be necessary to verify the quality of the PCB before producing more units.
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