Six techniques for selecting PCB components

- Jun 04, 2016-

1 consider the choice of components package

In the whole schematic drawing phase, it is necessary to take into account the need to make the layout phase of the component package and pad design decisions. Some suggestions which are needed to be considered in the selection of components based on components are given.

(1) remember, the package includes the components of the electrical bonding pad and mechanical dimensions (X, Y and Z), that is, the shape of the element body and connected to the PCB pin. In the selection of components, you need to consider the final PCB of the top and bottom of any installation or packaging restrictions may exist. Some elements (such as a capacitor polarity) may have high clearance limit, need to be considered in the process of component selection. At the initial starting design, can draw a basic circuit board frame shape first, and then placed on some plans to use the large position or key components (such as connectors). In this way, we can see (without wiring) the Virtual Perspective of the circuit board, and give the relative positioning of the circuit board and the component and the height of the component. This will help ensure that the PCB is properly placed in the outer package (plastic products, chassis, frame, etc.). From the tools menu, you can call the 3D preview mode to browse the whole circuit board.

(2) the pattern of the welding disk shows the actual welding disk or over hole shape of the PCB welding device. These copper patterns on the PCB also contain some basic shape information. The size of the pads is required to be correct in order to ensure proper welding and to ensure the proper mechanical and thermal integrity of the connected components. In the design of PCB layout, you need to consider how the circuit board will be made, or manual welding, then the welding plate will be how to weld. Reflow soldering (flux in a controlled high temperature furnace) can be treated with a wide range of surface mount devices (SMD). Wave soldering is generally used to weld the opposite side of the circuit board to fix through hole devices, but also can handle some of the surface mount components placed in the back of the PCB. Usually in the use of this technology, the bottom surface mount devices must be arranged in a specific direction, and in order to adapt to this kind of welding, you may need to modify the pad.

(3) the selection of components can be changed throughout the design process. Early in the design process to determine which devices should be used in electroplating through the hole (PTH), which should be used in the table technology (SMT) will contribute to the overall planning of PCB. Factors that need to be considered include device cost, availability, device area density, and power consumption. From a manufacturing perspective, surface mount devices usually by hole device, cheap, and generally high availability. For small and medium scale prototype project, it is best to use a larger surface mount devices or through hole devices, not only to facilitate manual welding, but also conducive to the investigation and debugging process better connection pads and signals.

(4) if there is no existing package in the database, it is generally in the tool to create a custom package.

2 use a good grounding method

Make sure the design has enough bypass capacitor and ground plane. When using an integrated circuit, to ensure that the position of the ground (preferably in the ground plane) is located close to the power source to the ground (preferably the ground plane). The proper capacity of the capacitor depends on the specific application, the capacitance technique and the working frequency. When the bypass capacitor is placed between the power supply and the ground pin, and placed close to the correct IC pin, the electromagnetic compatibility and susceptibility of the circuit can be optimized.

3 Distribution virtual components package

Print a list of materials (BOM) for checking virtual components. Virtual components are not related to the package, will not be transferred to the layout stage. Create a list of materials, and then look at all the virtual components in the design. The only items should be the power and ground signals, because they are considered virtual components, only in the principle of the environment for specialized processing, will not be transmitted to the layout design. Except for the purpose of simulation, the components in the virtual part of the display should be replaced with a package of components.

4 make sure you have a complete list of materials data

Check if there is enough integrity in the material list report. After creating the material list report, check it carefully and complete the incomplete parts, suppliers or manufacturers of all component items.

5 sort according to component label

To help sort and view the list of materials, make sure that the component numbers are numbered consecutively.

6 check the excess gate circuit

In general, the input of all the extra doors should be connected to the signal, so as to avoid the input end of the signal. Make sure that you check all the redundant or missing gate circuits, and that all of the input terminals that are not connected are completely connected. In some cases, if the input end is in a state of suspension, the entire system is not working properly. Take the dual design is often used in the. If dual op amp IC element is only one operational amplifier, it is recommended or to another op amps to use, or not the op amp input grounding, and laying a suitable unity gain feedback network (or gain), so as to ensure that the components can work normally.

In some cases, the presence of a floating pin in the IC may not work properly in the target range. Usually only when the IC device or the other gate in the same device is not working in the saturation state - input or output close to or in the components of the power supply rail, the IC work to meet the requirements of the index. Simulation is often not captured in this case, because the simulation model generally does not connect the multiple parts of the IC together for modeling the suspension connection effect.

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