Best PCB design methods: Six things to consider when selecting components based on component packaging.

1. Consider the choice of component packaging

Throughout the schematic drawing phase, component package and land pattern decisions that need to be made during the layout phase should be considered.

1.1 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 that connect the circuit board. When selecting components, consider any mounting or packaging restrictions that may exist on the top and bottom layers of the final board. Some components, such as polar capacitors, may have a high headroom limit and need to be considered in the component selection process.

When initially designing, you can first draw a basic circuit board frame shape, and then place some large or location-critical components (such as connectors) that you plan to use. In this way, you can quickly and intuitively see the virtual perspective view of the circuit board (without wiring), and give the relative positioning and component height of the circuit board and components. This will help ensure that after the circuit board is assembled, the components can be properly placed in outer packaging (plastic products, chassis, etc.). Invoke the 3D preview mode from the tools menu to browse the entire board.

1.2 The land pattern shows the actual land or via shape of the soldered device on the board. These copper patterns on the board also contain some basic shape information. The size of the land pattern needs to be correct to ensure proper soldering and to ensure the correct mechanical and thermal integrity of the connected components.

When designing a circuit board layout, you need to consider how the board will be manufactured, or how the pads will be soldered by hand soldering. Reflow (flux melting in a controlled high-temperature furnace) can handle a wide variety of surface-mount devices (SMD). Wave soldering is generally used to solder the reverse side of a circuit board to fix through-hole devices, but it can also handle some surface-mount components placed on the back of the circuit board. Usually when using this technology, the underlying surface-mount devices must be arranged in a specific direction, and in order to adapt to this soldering method, the pads may need to be modified.

1.3 Component selection can be changed throughout the design process. Determining which devices should use plated through holes (PTH) and which should use surface-mount technology (SMT) early in the design process will help the overall planning of the board.

Factors to consider are device cost, availability, device area density, and power consumption. From a manufacturing perspective, surface-mount devices are usually cheaper than through-hole devices and generally have higher availability. For small and medium-scale prototype projects, it is best to choose larger surface-mount devices or through-hole devices, which are not only convenient for manual soldering, but also better for connecting pads and signals during error checking and debugging.

1.4 If there are no ready-made packages in the database, custom packages are usually created in the tool.

2. Use a good grounding method

Make sure the design has adequate bypass capacitors and ground planes. When using integrated circuits, be sure to use appropriate decoupling capacitors near the power supply to ground (preferably the ground plane). The proper capacitance of a capacitor depends on the specific application, capacitor technology, and operating frequency. When the bypass capacitor is placed between the power and ground pins and placed close to the correct IC pin, the electromagnetic compatibility and susceptibility of the circuit can be optimized.

3. Assign Virtual Component Package

Print a bill of materials (BOM) to check the virtual components. Virtual components have no associated packaging and are not transferred to the layout stage. Create a bill of materials and view all the virtual components in your design. The only entries should be power and ground signals, as they are considered virtual components and are only processed specifically in the schematic environment and are not transferred to the layout design. Except for simulation purposes, the components shown in the virtual part should be replaced with components with a package.

4. Make sure you have complete bill of materials data

Check for sufficient complete data in the BOM report. After you create a bill of materials report, you need to double-check to complete incomplete device, vendor, or manufacturer information in all component entries.

5. Sort by component number

To facilitate sorting and viewing of the bill of materials, ensure that component numbers are numbered consecutively.

6. Check for excess gate circuits

In general, all redundant gate inputs should have signal connections to avoid floating the input. Make sure that you have checked for any excess or missing gates and that all unwired inputs are fully connected. In some cases, the entire system will not work properly if the input is floating. Take the dual op amps often used in design. If only one of the two op amp ICs is used, it is recommended to either use the other op amp, or ground the input of the unused op amp and arrange a suitable unity gain (or other gain) ) feedback network to ensure that the entire component can work properly.

In some cases, ICs with floating pins may not work properly within the specifications. Usually only when the IC device or other gates in the same device are not operating in a saturated state-the input or output is close to or is on the component power rail, can this IC work to meet the specification requirements. Simulation usually cannot capture this situation, because simulation models generally do not connect multiple parts of the IC together to model the floating connection effect.