Determining the prerequisites for successful circuit board design

To help solve all of these problems, many PCB design tools have built-in DRC checkers (some tools call them "constraint managers") that interactively flag design rule violations as you edit. Once you have set up the DRC rules for your chosen manufacturer, take errors seriously. DRC tools are generally conservative. They will intentionally report possible errors and let you make the decision. Sifting through hundreds of "possible" questions can be tedious, but do it anyway. Buried deep within this list of problems may be why the first production run was doomed to fail. In addition, if your design generates a large number of possible errors, you should be alert that your routing methods may need improvement.

Tip 1: Focus on researching manufacturing methods and foundry chemistries In this era of fabless IC companies, it’s not surprising that many engineers really don’t know the steps and chemistries involved in generating a PCB from their design files. . This lack of practical knowledge often leads novice designers to make unnecessarily complex design choices. For example, a common mistake that novices make is to design the circuit board layout with extremely precise dimensions, that is, using orthogonal wires associated with a tight grid, only to find that not every circuit board fabrication shop can produce it. A design that maintains adequate reliability during its service life in the field.

Factories with these capabilities may not be able to offer the most economical PCB prices. Does the design really need to be that complicated? Can the board layout be designed on a larger grid, thereby reducing board cost and improving reliability? Other misunderstandings encountered by novice designers include via sizes that are too small and blind vias and buried vias. These advanced via structures are the product of powerful tools in the PCB designer's toolbox, but their effectiveness is highly context-specific. Just because they're in the toolbox doesn't mean you should use them.

Bert Simonovich's "Design Notes" blog has this to say about via cross-sectional aspect ratio: "A via with an aspect ratio of 6:1 is a good guarantee that your board can be fabricated anywhere." For large For most designs, with a little thought and planning, these HDI features can be completely avoided, again saving costs and improving the manufacturability of the design. The physics and fluid mechanics required to copper plate these ultra-small or single vias are not something all PCB shops are good at. Remember, one bad via can ruin the entire board; if you have 20,000 vias in your design, you have 20,000 chances of failure. Including unnecessary HDI via technology, the probability of failure will only increase.

Tip 2: Trust the flying line

Sometimes drawing a schematic when designing a simple circuit board seems like a waste of time, especially after you've done a design or two. But for beginning designers, drawing schematics can also be a daunting task. Skipping the schematic is a tactic often adopted by novices and those with intermediate proficiency. But you must resist this strong desire. Starting to develop your layout with a complete schematic that you can use as a reference will help ensure that your layout connections are all made. Some explanation below.

First, a schematic is a visual depiction of an electrical circuit that communicates information on many levels. Subsections of a circuit can be drawn in detail across several pages, and components can be arranged close to their functional blocks regardless of their final physical layout. Because every pin on every component is shown in the schematic symbol, it's easy to check for unwired pins. In other words, whether or not the formal rules for describing a circuit are followed, a schematic helps you quickly and visually determine this fact.

In a group discussion on Stack Overflow, one poster commented: "If a schematic is likely to mislead the person looking at it, then it is definitely a bad schematic, regardless of what it turns out to be... Correct schematic. The problem is clear. A technically correct but confusing schematic is still a bad schematic." While it's easy to agree with this point, in a CAD program, an unreadable schematic can still be a bad schematic. Expressing connection information that describes a circuit is still useful in layout design.

Bottom line: When designing a PCB layout, having a schematic as a golden reference makes the job much easier. Complete connections with symbols; you don't have to think about connections at the same time when meeting routing challenges. Finally, discovering a wire connection you forgot to make in the first version of the design can save you a lot of rework.

Tip 3: Use an autorouter, but don’t rely solely on it

Most professional-grade PCB CAD tools have autorouters. But unless you design the PCB very professionally, the automatic router will complete the wiring in one go; for PCB wiring, the automatic router is not a one-click solution. You should still know how to do hand wiring.

The autorouter is a highly configurable tool. In order to give full play to their role, router parameters must be carefully and thoughtfully set for each task, and even each module in a single PCB design must be set individually. There is no basic universal default setting suitable for any occasion.

When you ask an experienced designer "What is the best autorouter?" their usual answer is "What's between your ears (eyes)." This is no joke, they mean it. Wiring, as a craft, is as much an art as the algorithm; wiring itself is heuristic and therefore very similar to traditional backtracking algorithms. For constrained path selection applications (such as mazes and puzzles), backtracking algorithms are suitable for finding answers, but in open, unconstrained situations, such as printed circuit boards with pre-placed components, Backtracking algorithms are not good at finding optimal solutions. Unless the constraints of the autorouter are highly fine-tuned by the designer, the autorouter results will still require manual inspection of the backtracking algorithm results for weaknesses.

Wire size is another difficulty. The autorouter cannot reliably determine how much current will flow on a wire, so it cannot help you determine how wide a wire to use. The result is that most automatic routers do not route the required wire widths. Many autorouters allow you to specify reference wire constraints. In a forum post on the stackexchange.com website, author Martin Thompson wrote, "I use an autorouter (sorry, a very high-end router...) on every board I make. If Your constraints are something like this: only on this layer, these two signals form a differential pair, and these nets must match the lengths, then you have to tell the autorouter these conditions." When you want to use the autorouter , you have to ask yourself: "When I set up the autorouter constraints for the board, and maybe even set up constraints on each wire in the schematic, how much manual routing will be done in that time?"

Experienced designers put a lot of energy into the initial component layout, and almost half of the entire design time is spent optimizing the component layout:

Simplified wiring - minimize the intersection of flying lines, etc.;

Devices are close together – shorter routes mean better routing;

Signal timing considerations.

On the Sunstone Circuits user forum, one post reads, "Pay more attention to component placement. Place components in a way that makes them easier to route. Component placement accounts for 70% of the entire job. Before you start routing the first wire. To place all the components...use flying leads (these lines indicate connections that have not yet been routed) as a rough guide to routing complexity."

Old-timers often used a hybrid approach to routing—routing important key lines by hand and then locking those lines after routing. An autorouter is then used to handle non-critical wires and help manage "escape states" in routing algorithms. This approach is sometimes a good compromise between controlled manual routing and fast automated routing.

Tip 4: Circuit board geometry and current

Most people who work in electronics design know that, like a river along its course, electronics can hit choke points and bottlenecks. This has direct application in the design of automotive fuses. By controlling the thickness and shape of the wire (U-bend, V-bend, S-shape, etc.), the fuse is calibrated to blow at the choke point in the event of an overload. The problem is, PCB designers accidentally create similar electrical choke points in their PCB designs. For example: use a 90-degree bend where you can use two quick 45s to create an angle; bend greater than 90 degrees to create a zigzag shape. In the best case, these wires slow down the speed of signal propagation; in the worst case, they act like car fuses and blow at the point of resistance.

Tip 5: Oh, the pieces!

Debris is a manufacturing issue that is best managed with proper circuit board design (Figure 1). In order to understand the chipping problem, we first need to review the chemical etching process. The purpose of the chemical etching process is to dissolve away unwanted copper. But if there are particularly long, thin, strip-shaped pieces that need etching, these pieces sometimes break off in one piece before completely dissolving. The sliver then floats in the chemical solution, possibly landing randomly on another circuit board.

An equally risky situation is when debris remains on the circuit board. If the fragment is narrow enough, the acid pool can eat away enough of the copper underneath to partially detach the fragment. Now the debris is traveling everywhere, clinging to the circuit board like a flag. Eventually it will end up on your own circuit board, causing shorts to other wires.

So where do you look for potential debris and how do you avoid it? When designing a PCB layout, it's best to avoid leaving very narrow areas of copper (Figure 2). This area is usually created when copper is applied at the intersection of a wire and a pad gap (Figure 3). Set the minimum copper width beyond the minimum allowed by the manufacturer and your design should have no problems with this. The standard minimum width for etching is 0.006 inches.

Figure 1 In this example, the narrow shielding pattern between the conductors appears solid on the circuit board substrate.

Figure 2: A very narrow flake area, such as this example in the original design document, can peel uncontrollably during manufacturing, creating short circuits and yield issues.

Tip 6: Pay attention to DRC

While setting up the autorouter is often done specifically for specific design functions, design rule checking (DRC) is generally used to enter the manufacturer's design constraints. While this setup is tedious, it's not as bad as the autorouter. Most design teams end up establishing a set of design rules aimed at: standardizing bare board build costs and maximizing yield; making assembly, inspection, and testing as consistent as possible. In addition to design benefits, these design rules—which keep designs within predefined manufacturing limits—also help create better consistency within the procurement department. If the price of circuit board manufacturing is consistent, purchasing can often reduce the number of professional PCB manufacturing agreements that need to be maintained.

Figure 3: In this example, chemical etching changes the shape/size of a narrow strip of fill. Unexpected scales and flaps can occur when the fragments are peeled off.

Here's what Dave Baker, a PCB designer at Sunstone Circuits with more than 20 years of design experience, advises. "Take the time to understand and correctly set up the constraint system provided by the layout tools. Take the time to review all levels of constraints. Constraint tools can be powerful and flexible, but they can also be confusing and dangerous. The wrong constraints can be very dangerous. Can easily lead to defective or unmanufacturable boards. Errors in constraint settings are likely to limit the DRC check or make it unusable. It is possible to have a situation where the DRC passes every time, but the board still cannot be manufactured or Not working properly. I've seen this happen before. The design team was happy because the board passed the DRC inspection, but the test bench was smoking on the first product. Tracking this failure would take the team back to the CAD tool A constraint manager. A constraint manager has no design awareness; it makes you do anything, no matter how bad it is."

For example, at Sunstone Circuits, we receive quote requests almost every day for circuit board designs that are easy to manufacture, but sometimes the design tolerances and clearances in critical areas are too tight. This situation leaves PCB foundries (such as Sunstone) with bad news: either we can't make the board at all because the tolerances are beyond our capabilities, or we can make the board but at a higher price and with higher yields Probably lower. These customers would be nice if they were designed with the capabilities of a specific manufacturer in mind.

Baker added: "If your layout software allows you to shelve DRC violations, be careful when using this feature. It is easy to shelve DRC violations and leave it to be dealt with later, and it is often easy to forget about it. Remember to always check for any pending DRC errors before sending your design out for manufacturing.”

Bob TIse, an experienced PCB designer currently working for Sunstone Circuits, believes: "You must resist the temptation to completely shelve DRC errors and follow the rules you set at the beginning."

Tip 7: Know the foundry you are using

After discussing DRC settings, this tip is almost - but not quite - redundant. In addition to helping you set up DRC rules correctly, knowing which foundry your boards will be shipped to can provide some additional pre-manufacturing assistance. A good foundry will provide you with some helpful help and advice before you place an order, including how to improve your design to reduce design iterations, reduce final problems encountered when debugging on the test bench, and improve the yield of the circuit board. .