PCB design seven basic and key techniques for beginners and experienced professionals(2)

- Nov 03, 2018-

Old-timers often use a hybrid approach to cabling - do key wiring manually, fix their positions, and then process non-critical wiring with an automatic cabling machine.The automatic routing region in the design helps to manage the "runaway state" in the routing algorithm, which sometimes achieves a good compromise between the controllability of manual routing and the speed of automatic routing.4. Considering circuit board size and current most people who are engaged in electronic design know that, like a river that runs along a river, the flow of electrons may also encounter choke points and bottlenecks.This point is directly applied to the design of automotive fuse.By controlling the thickness and shape of the routing (u-bend, v-bend, s-shape, etc.), the fuse can be calibrated to fuse at the throat when the current is overloaded.The problem is that design engineers occasionally encounter similar electrical throats in their PCB designs;For example, where it is possible to use two steep 45 degrees, use a 90 degree Angle;When the bending degree is greater than 90 degrees, the word shape is adopted.Those wires will only slow the signal down.The worst case scenario is that they will fuse at a resistance point like a car fuse.

5. Sliver avoidance is a manufacturing error that can be best managed through proper circuit board design (figure 1).To understand the cracking problem, we need to review the chemical etching process.Chemical etching is to break down unwanted copper, but if the part to be etched is particularly long, thin, and flaky, the shapes sometimes peel off in whole before they are completely broken down.The lobes float in a chemical solution and may randomly land on another circuit board.

There is also a risk that the lobes will remain on the original circuit board;If the lobes are narrow enough, the acid pool may corrode enough copper below to separate the lobes.The lobes cling like flags to the circuit board, and eventually fall on the board and cause other lines to short.So where to look for potential lobes?How do you avoid lobes?It is best to avoid leaving a very narrow copper wire area (as shown in figure 2) during PCB wiring.This area is usually caused by the intersecting of the wire and pad gap and the filling of the plane (figure 3).Set the minimum width of the copper wire to be greater than the manufacturer allows, and your design should not have this problem.The standard minimum etching width is 0.006 inches.

6. Care about the setting of DRC automatic wire-laying device is usually for Design functions, while Design Rule Checker (DRC) is usually used to capture the manufacturer's Design constraints.Although the setup process is equally tedious, it is much better than the automatic wiring device.Most design teams end up with a set of design rules designed to standardize the cost of making a bare board, maximize yield, and make assembly, inspection, and testing as consistent as possible.In addition to benefiting the design, these design rules - by maintaining the design within the pre-defined manufacturing limits - also contribute to establishing consistency in the purchasing department;If the price of PCB manufacturing is consistent, usually procurement can reduce the number of specific PCB manufacturing agreements that need to be maintained.To solve all these problems, DRC is built in - some tools call them "constraint managers" - and when you edit, DRC marks design violations in an interactive manner.And once you have set DRC rules for your chosen manufacturer, be ready to take the mistake seriously.DRC tools are generally conservative in design, and they make mistakes when reporting potential errors, and you have to decide.Sifting through hundreds of "possible" questions can be tedious, but it has to be done anyway.This list of questions may hide the reasons why your first stream was doomed.In addition, if your design triggers a large number of possible errors, represent the way your routing needs to be improved."Take the time to understand and properly set the constraints provided by wiring tools and review all levels of constraints," says Dave Baker, a Sunstone Circuits circuit design engineer with more than two decades of experience.Constraint tools can be powerful and flexible, but can also be confusing and dangerous.Wrong constraints can easily lead to defective or unmanufactured circuit boards, and errors in constraint Settings are likely to limit DRC or render it inoperable."What could happen is that DRC is passed each time, but the circuit board is still unable to make or function," he says.I have seen this before. The design team was happy because the circuit board passed DRC.To trace the cause of the failure, the team went back to the constraints manager of the CAD tools;The constraint manager has no design awareness, and it will let you do anything, no matter how bad."For example, Sunstone Circuits receives daily quotations for circuit board designs that are easy to make, except in key areas where design tolerances and gaps are substantially compressed.This has led to bad news for PCB foundries such as Sunstone: because tolerance is beyond our reach, we cannot make circuit boards;Or we can make a circuit board, but it's expensive and has yield limits.That type of customer would benefit if it were designed with the capabilities of a particular manufacturer in mind.

7. Get to know existing OEM partners after discussing DRC setup, the final PCB design tip is almost -- but not entirely -- redundant;In addition to helping you set DRC rules correctly, know your circuit board design is about to be sent to the manufacturer and get some additional assistance before entering the factory.A good OEM will provide some pre-order assistance and advice, including how to deal with your design to reduce design iterations, reduce the problem of error removal on the test bench, and improve PCB yield.Hugo, a doctoral student at carnegie-mellon University, commented in a blog post about the manufacturer's issues: "each manufacturer has its own specifications, such as minimum wire-width, spacing, number of layers, etc.Before you start designing, you should consider your requirements and find a manufacturer that will satisfy you.Your requirements also include grade PCB materials, from fr-1 (a paper-phenolic mixture) to fr-5 (glass and epoxy);Most PCB prototype manufacturers use fr-4, but fr-2 is also commonly used in a large number of consumer applications.The type of material will affect the board's strength, durability, hygroscopic and flame retardant (FR).Knowing the PCB process and what processes and methods your manufacturer will use will help you make better design decisions.It may come as a surprise to visit your preferred manufacturing service provider and see the process for yourself.Also, use manufacturability design (DFM) tools well before sending design drawings to production.