parts of a circuit board

Commonly used components in PCB

Crystal oscillators
Switches and relays
Silicon controlled rectifiers (SCRs)


Wiring arrangements between printed circuit board components

(1) No crossover circuits are allowed in printed circuits.

For lines that may cross, the two methods of “drilling” and “winding” can be used to solve the problem. That is, let a lead “drill” through the gap under the feet of other resistors, capacitors, and transistors, or “wind” through one end of a lead that may cross. In special cases, if the circuit is very complicated, wire jumpers are allowed to simplify the design to solve the crossover circuit problem.

(2) There are two installation methods for components such as resistors, diodes, and tubular capacitors: “vertical” and “horizontal”.

Vertical means that the component body is installed and welded perpendicular to the circuit board. Its advantage is space saving. Horizontal means that the component body is parallel and close to the circuit board. The advantage is that the mechanical strength of the component installation is better. The component hole spacing on the printed circuit board is different for these two different installation components.

(3) The grounding points of the same level circuit should be as close as possible, and the power filter capacitor of the circuit should also be connected to the grounding point of the same level.

In particular, the grounding points of the base and emitter of the transistor at this level should not be too far apart, otherwise the long copper foil between the two grounding points will cause interference and self-excitation. The circuit using such a “one-point grounding method” is more stable and less prone to self-excitation.

(4) The total ground wire must be strictly arranged in the order of high frequency-medium frequency-low frequency from weak current to strong current.

It must not be connected randomly. It is better to connect longer wires between levels, but this rule must be followed. In particular, the grounding arrangement requirements for frequency conversion heads, regeneration heads, and frequency modulation heads are more stringent. If they are not arranged properly, self-excitation will occur and the circuit will not work.

Frequency modulation heads and other high-frequency circuits often use large-area enclosing ground wires to ensure good shielding effect.

(5) High-current leads (common ground wires, amplifier power leads, etc.) should be as wide as possible to reduce wiring resistance and voltage drop, which can reduce self-excitation caused by parasitic coupling.

(6) High-impedance traces should be as short as possible, while low-impedance traces can be longer, because high-impedance traces are prone to whistling and absorbing signals, causing circuit instability.

Power lines, ground lines, base traces without feedback components, emitter leads, etc. are all low-impedance traces. The base traces of the emitter follower and the ground lines of the two channels of the recorder must be separated and each of them forms a path until they are combined at the end of the function. If the two ground lines are connected back and forth, crosstalk is very likely to occur, which will reduce the separation.


Six Tips for Selecting PCB Components

1. Consider the selection of component packages

During the entire schematic drawing stage, component packages and pad pattern decisions that need to be made in the layout stage should be considered.

The following are some suggestions for considering when selecting components based on component packages.

Remember that the package includes the electrical pad connections and mechanical dimensions (X, Y, and Z) of the component, that is, the shape of the component body and the pins connected to the PCB. When selecting components, you need to consider any installation or packaging restrictions that may exist on the top and bottom layers of the final PCB.

Some components (such as polarized capacitors) may have height clearance restrictions that need to be considered during the component selection process. When you first start your design, you can draw a basic board outline shape and then place some of the large or location-critical components you plan to use (such as connectors).

This will allow you to quickly and intuitively see a virtual perspective of the board (without routing) and give a relatively accurate relative positioning of the board and components and component heights. This will help ensure that the components will fit properly into the outer packaging (plastic, chassis, frame, etc.) after the PCB is assembled. Invoke the 3D preview mode from the tool menu to browse the entire board.

Land patterns show the actual pads or via shapes of the components that are soldered to the PCB. These copper patterns on the PCB also contain some basic shape information. The land pattern needs to be sized correctly to ensure proper soldering and ensure the correct mechanical and thermal integrity of the connected components.

When designing the PCB layout, you need to consider how the board will be manufactured, or how the pads will be soldered if hand soldering. Reflow soldering (flux melting in a controlled high temperature oven) can handle a wide variety of surface mount devices (SMDs).

Wave soldering is generally used to solder the reverse side of the board to secure through-hole components, but it can also handle some surface-mount components placed on the back of the PCB. Usually when using this technique, the bottom surface-mount components must be arranged in a specific direction, and the pads may need to be modified to accommodate this soldering method.

The choice of components can be changed throughout the design process. Determining which components should use plated through holes (PTH) and which should use surface-mount technology (SMT) early in the design process will help with the overall planning of the PCB. Factors to consider include device cost, availability, device area density, and power consumption.

From a manufacturing perspective, surface-mount devices are generally cheaper than through-hole devices and generally have higher availability.

For small and medium-sized prototype projects, it is best to use larger surface-mount devices or through-hole devices, which are not only convenient for manual soldering, but also help to better connect pads and signals during error checking and debugging.

If there is no ready-made package in the database, it is generally created in the tool.

2. Use good grounding methods

Make sure the design has sufficient bypass capacitors and ground planes. When using integrated circuits, make sure to use appropriate decoupling capacitors close to the power supply terminal to the ground (preferably the ground plane).

The appropriate capacitance of the capacitor depends on the specific application, capacitor technology, and operating frequency.

When bypass capacitors are placed between power and ground pins and close to the correct IC pins, the electromagnetic compatibility and susceptibility of the circuit can be optimized.

3. Assign virtual component packages

Print a bill of materials (BOM) to check virtual components. Virtual components have no associated packages and are not transferred to the layout stage.

Create a bill of materials and then review all virtual components in the design. The only entries should be power and ground signals, because they are considered virtual components and are only handled specifically in the schematic environment and are not transferred to the layout design.

Unless used for simulation purposes, components shown in the virtual section should be replaced with components with packages.

4. Make sure you have complete bill of materials data

Check whether there is enough complete data in the bill of materials report.

After creating the bill of materials report, carefully check and complete the incomplete device, supplier or manufacturer information in all component entries.

5. Sort by component designator

To help sort and view the bill of materials, make sure the component designators are numbered consecutively.

6. Check for redundant gates

Generally, all redundant gates should have signal connections to their inputs to avoid floating inputs.

Make sure you check all redundant or missing gates and that all unconnected inputs are fully connected. In some cases, if the inputs are floating, the entire system will not work properly. Take the dual op amps that are often used in designs.

If only one of the op amps in a dual op amp IC component is used, it is recommended to either use the other op amp or ground the input of the unused op amp and lay out a suitable unity gain (or other gain) feedback network to ensure that the entire component works properly.

In some cases, an IC with floating pins may not work properly within the specification range. Usually, the IC can only work within the specification requirements when the IC device or other gates in the same device are not working in saturation, and the input or output is close to or at the power rail of the component.

Simulations usually cannot capture this situation because simulation models generally do not connect multiple parts of the IC together to model the floating connection effect.


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