Printed Circuit Board Design ( PCB) Guide
From the smallest smart watch that we wear in our daily lives to the largest electronic equipment installed in an industry, nuclear plant or automation factory, there is one thing common and that is Printed Circuit Boards (PCBs). These PCBs are solid and flexible types and commonly available in green color. These are the foundation of any electromechanical equipment that we see in common industries.
As the advancing technology, these PCBs are becoming more complex and dense and hence the requirements of user are increasing day by day. Thanks to the fast innovation in the field of electronic engineering and talented design engineers that cope with the needs and demands of customers for particular PCB design work.
Nowadays, more advance PCB design CAD tools are available with better features like Altium, KiCAD and ORCAD etc. With the help of these licensed and open source software the more advance and complex PCB design work is possible. These complicated PCBs are multilayer and having small form factor with very small copper tracks. These are manufactured on large scale by means of automated PCB fabrication system. It is interesting to note that even with these advanced tools, the design engineer at some circumstances, struggles to design a PCB layout by restricting to the best practice of industry. The best practices to layout PCB design is essential for the quality of final PCB product.
Hence in this article we will discuss some key points that a design engineer must focus in order to maintain quality and best standards / practice of design rules
A - Determining the Customer Need
The very first step in design a quality PCB layout, is to determine the customer requirements. Hence it is very important that a design engineer must be able to translate the requirements or needs to the electrical schematics form. The needs of the customer also dictate the size of PCB and shape of PCB. Also the application of PCB will determine the PCB base material. Like for wearable devices, the rigid-flex PCB combination is suited and for industrial automation rigid PCBs are recommended. An example of the customer need is discussed.
For example, “A customer wants a temperature control PCB module which can maintain a particular temperature and in this the module has a cooling Fan and heater to operate at 220VAC. This PCB will have 4 mounting holes and dimensions will be L x W x H = 5”x3”x1”. This requirement is the first of all translated by design engineer or “circuit designer”. In this case the circuit designer will recommend a parts list that meets on the mechanical and electrical aspects like Voltage, Current and Power consumption and also select components that are SMT or THT having max height of 1 inch. After this these parts are integrated on the schematics by means of wires and connection as determined by “circuit design”. This module also has relays for ON/OFF control of fan and heater. Hence the design engineer must be capable of understanding the requirements set by the customer with 100% accuracy and then generate a schematic diagram and then move to PCB layout. “
B - Knowing PCB Fabrication Shop Constraints
Another important aspect that a design engineer must understand is the constraints and limits of the PCB fabrication shop. The PCB where it is intended to be fabricated, the designer should be aware of the limits of equipment in PCB fabrication shop. Like maximum panel size, minimum hole size, maximum number of internal layers, trace to trace and trace to hole clearance etc. By considering these constraints like panel size one can save extra space of PCB and traces can be minimized to lower the material cost. This results in a perfect layout with high quality and low cost.
C - Schematics
After brainstorming on the requirements set by the customers and knowing the limits of PCB fabrication shop equipment and doing some research on the off the shelf readily available components for prototype purpose, the design engineer will start the “circuit designing” task.
The “Circuit Designing” is an art that an electronic engineer make use of his knowledge in combining various electronic components and integrating them with each other electronically by keeping the rules and laws of electronic engineering into consideration.
Today many design engineers do their design work and calculations by hand on rough paper and draw electronic circuits and then they copy this design on CAD software to draw a “Schematics”
A schematics is a CAD drawing of the circuit design that has symbolic representation of electronic components, designated by a number, connected with each other via wires, tracks, node and connectors/ ports. This systematic placement and connection of components determine the ultimate requirements to meet the goals.
The schematic is also known as “Blueprint” or “Circuit Diagram”. The schematics is required to generate a PCB layout that will function according to the needs. Without error free schematics, error free PCB is not possible. Hence it is recommended to run DRC “Design Rule Check” for schematics to verify electrical connections. It is recommended that DRC may be run on often during the circuit design process and not run at the end of design. This way the designer can avoid a large bulk of errors that may arise at the end and running DRC at various design stage can resolve issues on spot where they arise and speed up the process of circuit design.
It is important to set the grid settings, like snap grid spacing. For more detailed/zoomed component placement the fine grid is considered finer step size. For less detailed component movement/placement, the coarse grid is used.
The schematics is then go through the “simulation” stage where the circuit functionality is tested and so designer engineer can catch any possible errors in the schematics and rectify it. After this preliminary design, the schematics is then finalized into flawless design and converted to “netlist”. The netlist file contains all the information about the components and pin connection.
D - Bill of Materials
After completing the schematics, the design engineer then creates a BOM ( Bill of Material) . This is also known as parts list. There are many PCB fabrication shops that have their own custom BOM templates, they prefer that the design engineer fills that BOM pro-forma to make the process of component sourcing easier. On the other hand the design engineer can generate BOM by CAD software available feature of BOM file. Also the BOM can be made manually by keeping in view the electrical characteristics, size and cost.
There are some very important things that need to be mentioned in the BOM. These are
- 1- Component Footprint type must be mentioned like SMT or THT, 0402, 0805
- 2- Component part number provided from manufacturer.
- 3- Component designator on schematics
- 4- Value of component like 100uF capacitor, 1K ohm resistor
- 5- Quantity of parts / components
Please also remember to mention the alternate part number of any IC or component that is discontinued or obsolete. This will save time at the component sourcing end by reducing the search of old components hence purchasing the newer alternative.
Also try to not to remove the power supply decoupling capacitors from the schematics for the sake of reducing the BOM cost. This practice is not good and it can compromise the quality of end product.
E - PCB Component Placement
It is now time to place the components on PCB worksheet. The component placement on the board is a very technical work. There are some components that require special attention towards placement and assembly. Like thermal components that generate heat are recommended to be placed with proper heat sink assembly and be applied with high width copper tracks with thick solder mask. Similarly high speed components are separated and assembled in a separate block on PCB away from high frequency noise generating circuits like SMPS and DC-DC converter. Likewise high current carrying components like MOSFETs, IGBTs and TRIACs require special PCB assembly and design procedure in terms of trace clearance, trace width and hole size with proper thermal management.
The components that are needed to be tested or verified in the design stage, they must be placed near the test points (TP). Test points are point in the PCB which are created by the designer to test the board functions like 5V, 3.3V test points or some type of waveform and frequency test points like 1 KHz square wave output at test point.
Another important thing is the silkscreen. The design engineer should be free to place silkscreen printing that provides vital information about the component placement, important information about the test points and jumpers, two way, three way switch position information and any information regarding the PCB board different sections like power supply, ADC, memory and signal processing sections.
F - Component Routing Techniques
As soon as the components are placed on the board, it is time now to connect them together. The process of connecting the components through wires is called routing. The wires can be traces with variable thickness depending on the type of trace and type of components connected. For example a power device like TRIAC if connected to a THT resistor will require a thicker trace as compared to the trace width of micro-controller SDI/SDO pin connection to a SMD resistor.
Hence a general rule of thumb is followed by many designers that a trace between 10 and 20 mils will carry 10mA to 20mA current and traces less than 5 to 8 mil will carry less than 10mA current. The other practice that is followed is that the separate ground plane or power plane is used in multilayer PCBs, this helps to reduce the source impedance and avoid noise issues. The power tracks must be routed according to the power plane in the PCB.
It must also be considered that the traces length be kept as small as possible. The longer traces can result in signal loss in high speed digital circuits and can introduce unnecessary impedance and parasitic effects that can result in false readings at analog voltage levels. Hence the shorter the trace the better it is.
In today’s advance CAD tools available, the automatic routing option is available. These AutoRoute method is achieved by means of “Netlist File”. The netlist contains all the information regarding each and every component interconnection with each other. Importing netlist into the PCB document will generate an optimized/best routing keeping in view the number of layers of PCB, the size of PCB and the density of components on PCB. The AutoRoute method takes more processing time of the PC/Laptop especially when the PCB design is complex and dense.
However if the designer do not like the AutoRoute of PC computer, then he can go for manual routing as seems appropriate.
G - PCB Final Checks (DRC)
Your PCB board is now ready for checks and inspection. In this stage, the designer can look for any possible errors in the PCB design. A common problem that many designs encounter is the heat management issue. The thermal management problem can arise due to inappropriate amount of copper thickness of trace, the absence of thermal pad, large board size, number of layers of PCB and absence of heat sink or smaller surface area of heat sinking pad.
It is therefore recommended that the PCB Design Rule Checker ( PCB DRC) may be run to identify the possible errors in the design. Some of the basic tips to mitigate thermal issues in PCB are vital to mention here. For example
- 1- Use separate Power or Ground plane to directly connect to the heat generating source like microprocessor or Graphic Processor Unit (GPU). These planes have high amount of copper and can dissipate heat efficiently.
- 2- Use high current routes to direct the heat dissipation for effective heat transfer.
- 3- Maximize the surface area of thermal pad for effective heat dissipation
There are more types of checks for PCB layout other than DRC, like LVS, ERC and XOR checks. These basic and advance checks are performed by design engineers to avoid any mishap in PCB fabrication / manufacturing stage
H - Searching for Assembly Service of Your PCB Design
It is very important to work on the research of PCB fabrication / manufacturing partner for your long term business relationship. A perfect PCB fabrication shop that completely suits your requirement is not easy to find. You have to do a thorough research on their capability and services.
At Rayming PCB we have rapidly growing PCB fabrication facility where we serve our precious client 24/7 with our dedicated and hardworking staff striving to deliver the best services in very affordable prices. We offer multilayer, single and double sided PCB fabrication. We also deal in Flex and Rigid-Flex PCB fabrication. Our capability in the domain of Rigid PCB can be checked here and SMT capacity can be checked here
Our Customer support staff is available at your service. Please feel free to get in touch with us and contact us anytime. A free quote for your design requirements please email us at firstname.lastname@example.org