1. Consider the component package choice
Throughout the schematic drawing phase, we should consider making the decisions of the component package and pad patternin the layout stage. Here are some recommendations when selecting components according to component package.
Package includes electrical pad connection of components and mechanical dimensions (X, Y and Z),and namely the shape of element body and pins of the connection PCB .When selecting components,we need to consider any installation or packaging restrictions which may exist on the top and bottom layers of final PCB.
Some elements (such as polar capacitor) may have a clearance height restrictions,which need to be considered in the component selection process. In the very beginning the design,you can draw a basic board outline shape,then place large-scale or key lacation components which you plan to use,such as connectors.
In this way, you can quickly and visually see the virtual perspective of no wiring board ,and give the relative accurate circuit board and the relative positioning of components and component height.This will help ensure after pcb is mounted,the components can be suitably placed in the packaging,such as plastic products. Call three-dimensional preview mode from the Tools menu to browse the entire board.
Land pattern shows the actual pad and via shape of welding device on the PCB.These copper pattern on the PCB also contains some basic shape information.The right size of pad pattern can ensure the proper welding,and can ensure correct mechanical and thermal integrity of the connected elements .
In the PCB design layout, we need to consider how to manufacture PCB, or when manual welding, how to weld the pad. Reflow (solder melted in a controlled high-temperature furnace) can handle a wide variety of surface-mount devices (SMD). Wave soldering is generally used to weld the opposite side of the circuit board, to fix the through-hole devices, but can also handle the table components which are placed in the back of PCB.
Typically when using this technique, the underlying surface mount components must be arranged in a specific direction, and in order to adapt to this welding, we may need to modify the pad.
Throughout the design process,we can change the selection of the element. Early in the design process,we should determine which devices should use plated through hole,and which should use the surface-mount technology (SMT),which is good for the overall planning of the PCB.We need consider the factors,such as device cost,availability, device area density and power consumption, and so on.
From a PCB manufacturing point of view, surface-mount devices are generally cheaper than the through-hole devices, and with higher availability. For small-scale prototype of the project,it is better to select larger surface-mount devices or through-hole devices .It is not only convenient for manual welding,but also facilitate troubleshooting ,and has better connection between pad and signals in debugging process.
If the database has no ready-made package, usually create a customized package in the tool.
2. Use a good grounding method
Ensure that the design has sufficient bypass capacitor and the ground plane. When using an integrated circuit,ensure that we use the appropriate decoupling capacitors at the position where is close to the supply terminal to ground. Suitable capacitance depends on the particular application, capacitive technology and operating frequency. When the bypass capacitors are placed between the power and ground pins,and close to the right IC pins, it can optimize electromagnetic compatibility and susceptibility of the circuit.
3. Assign the virtual device package
Print a bill of materials (BOM) ,which is used to check the virtual element. The virtual element has no relative package,and can not transfer to the layout phase. Create a list of materials, and then view all the virtual components.
The only entry should be the power and ground signals, because they are considered a virtual element. Only in the schematic environment,can they have specialized treatment,and will not be transferred to the layout.Except for the purpose of simulation, elements ,which are shown in the virtual part ,should be replaced with packaging components.
4. Make sure you have a complete bill of materials data
Check whether material bill report has enough data. After you create a bill of materials report, you should carefully examine,and complete the incomplete elements and vendor or manufacturer information.
5. Sort according to the component label
To help sort and view bill of materials, make sure the element label are consecutively numbered
6. Check the extra gate circuit
In general, input of all the extra gates should have signal connection, to avoid floating inputs,to make sure that you check out all excess or missing gates, and all inputs that are not connected are completely connected. In some cases, if the input is in a suspended state, the entire system does not work properly.
In some cases, IC,which have a suspension pin, may not work properly in the target range.Usually only when the IC device or other gates of the same device do not work in saturation-when the input or output is near or at element power rails,the IC can meet requirements when it works. Simulation usually can not capture this case,because simulation models generally can not make the portions of the IC connected together for modeling suspension connection effect.
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