Remembering your childhood, you understand the meaning of tolerance. At the baby stage, tolerance reigns supreme. We love watching year-old-babies move, explore, and generally tear up the house. When babies grow into toddler-hood, we still have an overall willingness to tolerate their behavior. But…every behavior that caused us to react with a, “Look at that—so cute!” now drives everyone to DEFCON 5 status. Bad behaviors erode our tolerance. Post toddler-hood age to teenage kids believe that tolerance is a myth. From the top-of-the-mountain perspective, they believe that parents see, question, and seek to alter every behavior.
In our magnificent world of design, prototyping, and manufacturing, tolerance is a big deal and has a slightly different meaning. Think of tolerance in this context. If you won a contest to become the first person to travel to Mars, you might feel better knowing that all manufacturers rigorously checked all components and assemblies for your spaceship. If the importance of tolerance in manufacturing components becomes an afterthought, the size of key parts like a control surface might vary and some assemblies that should contact other assemblies might not. Bon Voyage!
Tolerance? Precisely, My Dear Watson
Good PCB designs depend on tolerance. As a design team, your priority quickly shifts to achieving precise aspect ratios, board thicknesses, and impedances. Tolerance becomes a priority when selecting board materials and when establishing component clearances within an enclosure. Whether you have designed a double-sided PCB for a consumer device, a multiboard PCB for an automotive system, or rigid-flex an aerospace application, you need to carefully specify those tolerances when developing the concept, establishing your processes, validating the design, and producing the output documentation that flows to the fabrication house.
PCB design software allows you to establish, maintain, and test precise tolerances at each stage of your design. Tools within your software allow you to establish design rules and perform electrical rule checks. Component libraries provide the accurate information that you need for matching components to your application and the specifications of your customer. Testing routines demonstrate that your routing remains free of electromagnetic interference (EMI) and other the tiny transmission lines that make up your PCB paths do not become noise-emitting antennas. 3D design tools address electrical and mechanical tolerances so that your components operate correctly while fitting within a particular enclosure.
When working with the initial design of your PCB, establish the following benchmarks.
Laminate material type
Application requirements define the type of laminate selected for the PCB. Laminates differ in terms of dielectric constant, arc resistance, electrical strength, material strength, flammability, degradation temperature, and other properties. When your design team begins to select the laminate, each of those qualities has significance. For example, a laminate that offers superior material strength may not have the appropriate dielectric constant for the application. Other materials may not withstand temperatures or chemical processes.
Selecting PCB materials can mean a lot depending on the application of your board
PCB Thickness and How it Changes Tolerance
PCB thickness measures include the total thickness, the maximum finished thickness, copper thickness, standard thickness, and minimum thickness. Let’s take a quick look at each of the definitions as we examine tolerances.
Although standard PCBs have an approximate thickness of 1.6 millimeters and a thickness tolerance of +/- 0.15 millimeters, the definition of “thickness” sometimes changes based on the application, suppliers and fabricators.
The total thickness of a PCB equals the sum of the thicknesses of the dielectric cores, the epoxy prepreg, the copper, solder mask, silk screen, and plating seen within the stack. Because of the different values of each material, the value for total thickness ranges from 0.5 mm to 6.85 millimeters. Because the total thickness of a PCB includes the conductive layers, fabricators can produce these boards by using thinner cores.
A maximum thickness rating measures from copper to copper—before plating, solder mask, and silk screening occur. Fabricators use the maximum thickness measure to determine stack heights, for setting the permanent limits of processing equipment, and for calculating the aspect ratio.
The very thin PCBs used for sensors, cell phones, and other small devices have a minimum thickness of 0.2 mm. Along with weight and space savings, the minimum thickness measure becomes important because of the possible need for special handling and fabrication processes.
Copper thickness measures in ounces. Pressing one ounce of copper completely flat and spreading the copper over one square foot produces a copper layer that measures 1.37 mils (0.00137 inch) thick.
Multilayer PCB Manufacturing Specifications
Every PCB uses vias to connect traces on one layer to the next layer. Multilayer PCBs often use blind vias—or vias that only connect from the outer layer to a specific inner layer and buried vias that only connect internal layers together. Through-hole vias penetrate through the entire board and connect the outer layers and inner layers together. Dividing the maximum thickness by the smallest drill diameter gives the aspect ratio.
Each drilled portion of a via has an annular or copper ring that establishes the connective surface for components. In terms of tolerance, the aspect ratio affects the ability of plating solutions to flow through a via. If a design team specifies a thick PCB with small holes, the copper plating may not flow through the hole and create an unacceptable electrical connection. A large aspect ratio also increases the risk of laminate material cracks because of Z-axis expansion.
Get the Right Clearance
Good PCB designs avoid stray capacitance. Having the correct anti-pad tolerance reduces any stray capacitance that a via may have and eliminates insertion loss. When working with high-speed circuits, ensure that the copper plane has the clearance needed to allow a drilled hole to pass without electrically connecting. The copper clearance must exist on all routing and plane layers. Follow best practices and allow at least 0.25 mm annular ring clearance around the via.
Along with having the right anti-pad tolerance, you should also determine the right pad shapes for your design. Your PCB design software provides component libraries that recommend the pad shapes for the components that you select. Choosing the right pad size depends on the application, the components, and the PCB manufacturing process.
What is the Importance of Tolerance in Manufacturing?
In high-frequency circuits, mismatched impedances between the source and load cause reflections and standing waves. As the distance from source to load increases, standing waves also cause impedance to ebb and flow. In addition, impedance values can change because of poor trace design or because of the type of materials used for the trace. Because digital circuits depend on short transition times and high clock rates, problems with impedance decrease any signal gain, and generate noise. Those problems and any distortion in amplitude and phase cause errors.
When we work with digital circuits that have high clock rates and short transition times, we need to control impedance. Controlling circuit impedance allows faster, sequential signal transfer. Digital circuits deliver desired performance because of short transition times and high clock rates.
Make sure to understand your manufacturing specifications
Within PCB design, the need for the strict tolerances that allow controlled impedance begins with the traces. Your PCB design software assists with matching component impedances and establishing precise trace widths, spacing, and thicknesses. The software also assists with setting the correct signal and plane layer order, selecting the appropriate copper and dielectric thickness, and selecting the correct dielectric constant for your circuit.
Observing the importance of tolerance also allows you to adhere to Design for Manufacturability rules. Consulting with manufacturers about their capabilities can eliminate many communication problems, prototype issues, and errors. Eliminating errors and costs speeds the time from design to manufacturing.
With all this in mind, it’s a wonder how PCB designers don’t have even less tolerant memories. Thankfully, with design software like Cadence Allegro’s PCB Editor you can ensure any of your design and analysis needs are met.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.