For a new design, because most of the time is spent on circuit design and component selection, it is often inexperienced and inconsiderate in the PCB .
Failure to provide sufficient time and effort for the design during the PCB layout can lead to problems in the manufacturing or functional deficiencies when the design is transformed from the digital realm to physical reality.
So what is the key to designing a board that's authentic and reliable on paper and in physical form? Let's explore the 6 PCB design guidelines when you design a manufacturable and functionally reliable PCB.
1. Slightly adjust your component layout
The component placement of the PCB layout process is both a science and an art, requiring strategic consideration of the main components available on the board. While this process can be challenging, the way you place your electronic components will determine how easy your board will be manufactured and how well it meets your original design requirements.
While there is a general order of component placement, such as the placement of connectors and devices for printed circuit boards in order. There are some specific guidelines to keep in mind, including:
Orientation - Ensuring that similar components are positioned in the same orientation will help to achieve an efficient and error-free soldering process.
Placement - Avoid placing smaller components behind larger components, because smaller components may be affected by soldering of larger components and assembly problems may appear.
Organization - It is recommended to place all Surface Mounted Technology (SMT) components on the same side of the board and all through-hole (TH) components on top of the board to minimize assembly steps.
One final PCB design guideline to note - when you use mix technology components (through-hole and surface-mount components), manufacturers may require additional processes to assemble the 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)
2. Appropriate placement of power supply, grounding and signal wiring
After the components are placed, power supply, ground wires and signal wires should be placed to ensure that your signal has a clean and trouble-free path. At this stage of the layout process, here are some guidelines to keep in mind:
1) the power supply and ground plane layer
It is always recommended to place the power supply and ground plane layers inside the board while remaining symmetrical and centered. This helps prevent your board from bending, and is also related to whether your components are positioned correctly.
For powering the IC, it is recommended to use a common path for each power supply, and to ensure a robust and stable path width. Daisy-chained power connections from component to component should be avoided.
2) Signal wiring connection
Next, connect the signal cables as designed in the schematic. It is recommended that you always take the shortest possible path and direct path between components.
If your components need to be placed horizontally without deviation, it is recommended to make the components wire outlet almost horizontal, and then vertical after the exit.
In this way, the component will be fixed in the horizontal direction as the solder moves during soldering, which is shown in the upper part of the figure below. In the lower part of the figure below, the signal wiring method may cause deflection of the component as the solder flows during soldering.
Recommended wiring (arrows indicate solder flow direction)
Bad wiring (arrows indicate solder flow direction)
3. Effective isolation
You may have experienced how large voltage and current spikes in power circuits can interfere with your control circuitry for low voltage currents. To minimize this type of interference, follow these guidelines:
Isolation - Ensures that each power supply is separated from control. If you have to connect power supply and control together in the PCB, make the control as close to the end of the power path as possible.
Placement - If you have placed a ground plane in 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 signals. The same guidelines can be followed to keep your digital and analog separate.
Coupling - To reduce coupling capacitance due to the placement of a large ground plane and the wiring above and below the plane, try to pass the simulated crossing with analog signal lines only.
Component Isolation Example (Digital & Analog)
4. Solve the heat problem
Have you ever experienced a thermal problem that has caused circuit performance decrease or even the board damage? Because heat dissipation is not taken into account, many designers have been troubled by many problems. Here are some guidelines to keep in mind to help resolve heat dissipation problems:
1) Identify troublesome components
The first step is to start thinking about which components produce the most heat on the board. First, you need to find the "Thermal Resistance" rating in the component's datasheet and then follow the recommended guidelines to transfer the heat generated. Of course, heat sinks and cooling fans can be added to keep the components temperature dropping. It is necessary to remember to keep critical components away from any high heat sources.
2) Add thermal relief
The addition of thermal relief is useful for producing the boards, and thermal relief is essential for high-copper components and wave soldering applications on multi-layer boards. Since it is difficult to maintain the process temperature, it is always recommended to use thermal relief on through-hole components to make the soldering process as simple as possible by slowing down the heat dissipation rates at the component pins.
As a general guideline, thermal relief should be always adopted to connect any through holes or vias that are connected to the ground plane or power plane. In addition to thermal relief, you can also add teardrops at the pad connection wires location to provide additional copper foil/metal support. This will help reduce mechanical and thermal stress.
Typical thermal relief connection
5. About thermal relief
Many engineers in charge of process or SMT technology in factories often encounter the problem of solder empty, de-wetting or cold soldering of circuit board components. No matter how the process conditions are changed or how the furnace temperature of reflow soldering is adjusted, there is a certain ratio of non-wetting. What's going on here?
Aside from the problem of oxidation of components and circuit boards, it is found that a large part of these soldering defects actually come from the lack of layout design of the circuit board, and the most common is that a large area of copper is connected to a few solder pins of the components, resulting in poor soldering after reflow soldering of these components. Some hand soldering components may also cause de-wetting or over-soldering problems due to similar situations, and some components are broken due to excessive heating.
Generally, a large area of copper foil for power supply (Vcc, Vdd or Vss) and ground (GND, Ground) is laid during PCB circuit designs.
These large-area copper foils are typically connected directly to the pins of some control circuits (ICs) and electronic components.
Unfortunately, if we want to heat these large areas of copper foil to the temperature of tin-melting, it usually takes more time than a separate solder pad (i.e., it heats up more slowly) and the heat dissipates faster. When one end of such a large-area copper foil wiring is connected to small components such as small resistance and small capacitor, and the other end is connected to big resistance and big capacitor, it is easy to cause soldering problems due to the inconsistency of tin melting and solidification time; If the temperature profile of reflow soldering is not adjusted well and the preheating time is insufficient, the solder pins of these components connected to a large piece of copper foil are prone to the problem of empty soldering due to the failure to reach the tin melting temperature.
In Hand Soldering, the solder pins of these components connected with a large piece of copper foil cannot be soldered within the required time due to the rapid heat dissipation. The most common undesirable phenomenon is over-soldering and empty soldering, where only component pins are soldered and the component is not soldered and connected to the pads of the circuit board. From the outside, the entire solder joint will form a ball; What's more, the operator keeps increasing the temperature of the soldering iron in order to solder the pins to the circuit board. Or the operator heats too long, so the components are damaged beyond the heat-resistant temperature, which the operator does not know. This is shown in the figure below.
There are solutions. We usually use a so-called Thermal Relief pad design to solve this kind of soldering problem caused by component soldering pins connected to large copper foil. As shown in the figure below, the wiring on the left does not use the thermal relief, while the wiring on the right has adopted the connection method of the thermal relief. With thermal relief, there are only a few small lines left in the contact area between the pad and the large piece of copper foil, which can greatly limit the temperature loss on the solder pad and achieve a better soldering effect.
6. Check your work
When you assemble all parts together, you are very likely to find problems after the design. So Double and triple checks for your design may decide whether the production is successful or not.
To help with the quality control process, we always recommend that you start with an Electrical Rule Check (ERC) and Design Rule Check (DRC) to verify that your design fully complies with all rules and constraints. With these two systems, you can easily check gap widths, line widths, common manufacturing settings, high-speed requirements, and short circuits among other things.
When your Electrical Rule Check (ERC) and Design Rule Check (DRC) show no error, it is suggested to check the wiring of each signal, from the schematic to PCB. You can check one signal line each time to make sure that you do not miss any information. And detection and shielding capabilities of your design tools should be used to ensure that your PCB matches with your schematic.
