LCD Display Inverter

Display Inverter / VGA Board / LCD Controller

As a PCB engineer, you need to understand these design guidelines

When starting a new design, because most of the time is spent on circuit design and component selection, in the PCB layout stage, there is often insufficient experience and insufficient consideration.

When starting a new design, because most of the time is spent on circuit design and component selection, in the PCB layout stage, there is often insufficient experience and insufficient consideration.

If sufficient time and energy are not provided for the design of the PCB layout stage, it may cause problems in the manufacturing stage or defects in the function when the design is transformed from the digital domain to the physical reality.

So what is the key to designing a circuit board that is real and reliable on paper and in physical form? Let’s explore the top 6 PCB design guidelines you need to know when designing a manufacturable, reliable PCB.

01. Fine-tune your component layout

The component placement stage of the PCB layout process is both science and art, requiring strategic consideration of the main components available on the circuit board. Although this process can be challenging, the way you place your Electronic components will determine how easy your circuit board is to manufacture and how it meets your original design requirements.

Although there is a general order for component placement, such as placing connectors, printed circuit board mounting devices, power circuits, precision circuits, and critical circuits in order, there are some specific guidelines to keep in mind, including:

Orientation-Ensure that similar components are positioned in the same direction, which will help achieve an efficient and error-free welding process.

Arrangement-Avoid placing smaller components behind larger components, so that small components may be affected by the soldering of large components and cause mounting problems.

Organization-It is recommended to place all surface mount (SMT) components on the same side of the circuit board, and place all through-hole (TH) components on the top of the circuit board to minimize assembly steps.

One final PCB design guideline to note-when using mixed technology components (through-hole and surface mount components), the manufacturer may require additional processes to assemble the circuit board, which will increase your overall cost.

Good chip component orientation (left) and bad chip component orientation (right)

Good component placement (left) and poor component placement (right)

02. Properly place power, ground and signal wiring

After placing the components, you can place power, ground, and signal traces to ensure that your signal has a clean and trouble-free path. At this stage of the layout process, please keep the following guidelines in mind:

▶ 1) Locate the power and ground plane layers

It is always recommended to place the power and ground plane layers inside the circuit board while maintaining symmetry and centering. This helps prevent your circuit board from bending, which is also related to the correct positioning of your components.

For powering the IC, it is recommended to use a common channel for each power supply to ensure a sturdy and stable trace width, and avoid component-to-component daisy-chain power connections.

▶ 2) Signal wire routing connection

Next, connect the signal wires according to the design in the schematic. It is recommended to always take the shortest possible path and direct path between components.

If your components need to be fixed and placed in the horizontal direction without deviation, it is recommended to route the wires basically horizontally where the components of the circuit board exit, and then perform vertical wiring after the exit.

In this way, with the migration of the solder during soldering, the components will be fixed in the horizontal direction. As shown in the upper part of the figure below. The signal routing method in the lower half of the figure below may cause component deflection as the solder flows during soldering.

Recommended wiring method (arrows indicate the direction of solder flow)

Not recommended wiring method (arrows indicate the direction of solder flow)

▶ 3) Define the network width

Your design may require different networks, which will carry various currents, which will determine the required network width. Considering this basic requirement, it is recommended to provide 0.010″ (10mil) width for low-current analog and digital signals. When your line current exceeds 0.3 ampere, it should be widened. Here is a free line width calculator to make this conversion process easy.

03, effective isolation

You may have experienced how large voltage and current spikes in the power supply circuit can interfere with your low-voltage current control circuit. To minimize such interference issues, follow these guidelines:

Isolation-Ensure that each power supply is kept separate from the power ground and control ground. If you must connect them together in the PCB, make sure it is as close as possible to the end of the power path.

Layout-If you have placed a ground plane on the middle layer, make sure to place a small impedance path to reduce the risk of any power circuit interference and help protect your control signal. You can follow the same guidelines to keep your digital and analog separate.

Coupling-In order to reduce capacitive coupling due to the placement of a large ground plane and traces above and below it, please try to cross the analog ground only through analog signal lines.

Example of component isolation (digital and analog)

04. Solve the heat problem

Have you ever caused the degradation of circuit performance or even the circuit board damage due to heat problems? Since heat dissipation is not considered, many problems have plagued many designers. Here are some guidelines to keep in mind to help solve heat dissipation issues:

▶ 1) Identify troublesome components

The first step is to start thinking about which components will dissipate the most heat on the board. This can be achieved by first finding the “thermal resistance” rating in the component’s data sheet, and then transferring the generated heat in accordance with the recommended guidelines. Of course, radiators and cooling fans can be added to keep the component temperature down, and remember to keep key components away from any high heat sources.

▶ 2) Add hot air pad

The addition of hot air pads is very useful for producing manufacturable circuit boards. They are essential for wave soldering applications on high copper content components and multilayer circuit boards. Because it is difficult to maintain the process temperature, it is always recommended to use hot air pads on through-hole components to make the soldering process as simple as possible by slowing down the heat dissipation rate at the component pins.

As a general rule, always use hot air pads for any through holes or vias that connect to the ground plane or power plane. In addition to the hot air pad, you can also add teardrops to the pad connection line to provide additional copper foil/metal support. This will help reduce mechanical and thermal stress.

Typical hot air pad connection method

05. Popularization of hot air pads

Engineers responsible for Process or SMT technology in many factories often encounter solder empty, de-wetting, or cold solder on circuit board components. For non-wetting defects, no matter how the process conditions are changed or the temperature of the reflow soldering furnace is adjusted, there is a certain rate of non-wetting failure. what the hell is it?

Leaving aside the problem of component and circuit board oxidation, it is found that a large part of this type of poor soldering actually comes from the lack of layout design of the circuit board, and the most common one is the soldering of certain components. The pins are connected to a large area of ​​copper, causing poor soldering of the solder pins of these components after reflow soldering. Some hand-soldered components may also cause false soldering or oversoldering problems due to similar conditions, and some may even be heated for too long. The component is broken by soldering.

Generally, PCBs often need to lay a large area of ​​copper foil for power supply (Vcc, Vdd or Vss) and ground (GND, Ground) during circuit design. These large-area copper foils are generally directly connected to the pins of some control circuits (IC) and electronic components.

Unfortunately, if we want to heat these large-area copper foils to the temperature of melting tin, it usually takes more time than independent solder pads (that is, the heating will be slower), and the heat dissipation is faster. When one end of such a large area of ​​copper foil wiring is connected to small components such as small resistors and small capacitors, but the other end is not, soldering problems are likely to occur due to the inconsistent tin melting and solidification time; if the temperature profile of reflow soldering is changed If the adjustment is not good, and the preheating time is insufficient, the solder feet of these components connected to a large piece of copper foil are likely to cause the problem of virtual soldering because they cannot reach the melting temperature.

During Hand Soldering, the solder feet of these components connected to a large piece of copper foil will dissipate too quickly and cannot be soldered within the specified time. The most common undesirable phenomena are package soldering and virtual soldering. The solder is only soldered on the solder feet of the component and not connected to the pads of the circuit board. From the appearance, the entire solder joint will form a spherical shape; moreover, the operator constantly increases the temperature of the soldering iron in order to solder the solder foot to the circuit board, or heats it for too long, which causes the component to exceed the heat-resistant temperature. Damaged without knowing it. As shown below.

Package welding, cold welding or virtual welding

Now that the problem is known, there can be a solution. Generally, we will require the use of the so-called Thermal Relief pad (hot air solder pad) design to solve this type of soldering problem caused by the solder feet of the large copper foil connecting components. As shown in the figure below, the wiring on the left does not use hot air pads, while the wiring on the right has been connected with hot air pads. It can be seen that the contact area between the pad and the large copper foil is only a few small lines. In this way, the temperature loss on the soldering pad can be greatly restricted, and a better soldering effect can be achieved.

Use Thermal Relief pad (hot air solder pad) comparison

06. Check your work

When you hum and hum all the parts together for manufacturing, it is easy to find the problem at the end of the design project and become overwhelmed. Therefore, double and triple check of your design work at this stage may mean the success or failure of manufacturing.

To help complete the quality control process, we always recommend that you start with electrical rule checking (ERC) and design rule checking (DRC) to verify that your design fully meets all the rules and constraints. Using these two systems, you can easily check gap widths, line widths, common manufacturing settings, high-speed requirements and short circuits.

When your ERC and DRC produce error-free results, it is recommended that you check the wiring of each signal, from the schematic to the PCB, and check one signal line at a time to carefully confirm that you have not missed any information. In addition, use the detection and shielding functions of your design tool to ensure that your PCB layout materials match your schematics.

Carefully check your design, PCB and constraint rules

Concluding remarks

When you have this – the top 5 PCB design guidelines that our PCB designers all need to know. By following these suggestions, you will soon be able to design powerful and manufacturable circuit boards handily with real quality Printed circuit board.

Good PCB design practices are critical to success. These design rules lay the foundation for building and consolidating practical experience of continuous improvement in all design practices.

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