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TIPS FOR EEs

TIPS FOR electronic engineers 

For almost all the electronic engineers (EEs), it's their daily activity to design printed circuit board. However, no matter they are full of experience or a green hand in this field, it's not an easy job to design circuit boards with high quality. A lot of elements have to be taken into consideration. First of all, requirements in terms of functions must be fully met, of course, which in itself contains many details such as the routing design, schematic design, component placement and more. Besides, other aspects must be considered as well, including the schedule and cost. This passage will provide some tips of how to design high-quality PCBs.

A good beginning is half done.

All PCB design begins with some needs. For EEs, the needs usually come from customers. The first job of EEs is to convert customers' needs into electronic language and logic based on which the circuit schematic can be generated. Schematic can be regarded as the blueprint manufacturers must strictly follow in the process of PCB manufacturing. It directly influences the operating effect of PCBs.


The main content of schematic contains the functions of PCBs, the design's characteristics, the interconnections between circuits and components, component placement and more. Besides, the "hardware" PCB requires has to be mentioned as well, such as the temperature requirement, the material of PCB and components because they play a significant role in the smooth operation of PCBs.

Fine feathers make fine birds.

In the case of PCB, fine BOM (Bill of Material) makes accurate PCBs. The generation of BOM is done at the same time with the schematic. The BOM represents the list of components used on the printed circuit board and should be compatible with schematic. Once the design of the circuit is completed, the BOM list is passed on to the PCB layout engineer as well as component engineer who will procure the components required for the design. The selection of components not only depends on the analysis in terms of the maximum operating voltage and current of the circuit with tolerance criteria considered, but on the availability, cost and size of components as well.


The BOM must contain five aspects of requirement on components:

  • 1. Quantity: the number of each type of components.
  • 2. Reference designators: the identification of a component in a circuit on a PCB.
  • 3. Value: numeric value of ohms, farads, etc.
  • 4. Footprint: the locations of each component.
  • 5. Manufacturer part number.

Up to now, all the documents a PCB design requires have been achieved, including hardware size graph, schematic, BOM, layout files, component placement files, assembly graphs and instruction, and Gerber files.

Where should they be?

Each component has its own place on a PCB. The determination of the components' places is based on thermal management, function and electrical noise considerations. The component placement goes the following order: connectors, power circuits, sensitive and precision circuits, critical circuit components and then the rest. After individual components are placed at the suitable places on the PCB, a test must be carried out to verify the smooth operation so that some adjustments can be made for ideal performance.


Then it's time to consider and adjust placement and package sizes based on size and budget. Basically, if components are capable of accepting power more than 10mW or conducting current over 10mA, they should be reconsidered in terms of thermal and electrical management.

Routing

All the components are interconnected on PCBs through traces realized by routing. Some elements must be taken into the considerations including power levels, noise sensitivity or generation and routing capability.


Basically, traces with a width of 10-20mils are capable of carrying the current of 10-20mA and 5-8mils of carrying current lower than 10mA. For high-frequency and rapidly changing signals, more attention should be paid when they are routed with high-frequency nodes.

Thermal management

In the process of operation, the heat will definitely be a problem. The perfect thermal design is capable of keeping the whole board at the same temperature. However, a lot of elements will lead to the component temperature changes including copper thickness, layer number, board size and thermal paths design.


There are some methods to reduce operating temperatures. First, solid ground or power planes with more layers can be directly connected to the heat sources. Second, the heat transfer can be optimized by the establishment of effective heat and high-current routes. Third, maximizing the area used for heat transfer can lower the temperature. Of course, it must be carried out within the demanding size.

Checks

Almost all the details of a PCB have to be involved in the final check. At this time, design rule checking (DRC) is picked up. DRC is capable of determining whether the physical layout of a particular chip layout satisfies a series of recommended parameters called Design Rules. It is a major step during Physical verification signoff on the design, which also involves LVS (Layout versus schematic) Check, XOR Checks, ERC (Electrical Rule Check) and Antenna Checks. For advanced processes some PCB manufacturers also insist upon the use of more restricted rules to improve yield.


PCB design may be a toughing job, but it's possible that everybody can be good at it as long as he or she holds some technique and insist on it. Hopefully, the tips in this passage will be beneficial to EEs' jobs.