This article discusses seven basic (and important) techniques and strategies that can be used by both novice and veteran alike. By paying more attention to these techniques during the design process, the number of design returns, design time and overall diagnostic difficulties can be reduced.
Tip 1: focus on manufacturing methods and OEM chemical processes
In this era of factory-free IC companies, it’s not surprising that many engineers really don’t know the steps and chemical processes involved in generating PCB’s from their design documents. This lack of practical knowledge often leads novice designers to make unnecessarily complex design choices. For example, a common mistake beginners make is to design a circuit board layout with extremely precise dimensions, that is, to use orthogonal wires connected to tight grids. It turns out that not every circuit board factory can produce designs that are reliable enough to last the lifetime of the site.
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 circuit board layouts be designed on a larger grid to reduce circuit board costs and improve reliability? Other pitfalls encountered by novice designers include too small overhole sizes and blind and buried holes. These advanced through-hole structures are the product of powerful tools in the PCB designer’s toolbox, but their effectiveness is highly dependent on the situation. Just because they’re in the toolbox doesn’t mean you should use them.
Bert Simonovich’s design notes blog talks about the cross-sectional aspect ratio: “a 6-to-1 aspect ratio is a great way to make sure your circuit board can be built anywhere.” For most designs, with a little thought and planning, these HDI features can be completely avoided, again saving money and improving the design’s manufacturability. The physical and hydrodynamic capabilities required for copper plating on these ultra-small or single through holes are not unique to all PCB factories. Remember, one bad hole can destroy the circuit board; If you have 20,000 holes in your design, you have 20,000 chances of failure. If unnecessary HDI through hole technology is included, the failure probability will only increase.
Tip 2: trust the fly line
Sometimes drawing a schematic diagram while designing a simple circuit board seems like a waste of time, especially if you’ve done one or two designs. But drawing schematic diagrams can also be a daunting task for budding designers. Skipping schematic diagrams is a tactic often used by beginners and intermediate level of proficiency. But resist the urge. Developing your layout from a complete schematic that you can use as a reference helps ensure that your layout connections are complete. Here are some explanations.
First, a schematic is a visual representation of a circuit that communicates information at many levels. Subsections of a circuit can be drawn in detail over several pages, and components can be arranged close to their functional blocks, regardless of their final physical layout. Since the schematic symbol shows each pin on each component, it is easy to check for unconnected pins. In other words, schematic diagrams help you quickly and visually determine this fact, whether or not the formal rules describing the circuit are followed.
In one set of discussions on stack overflows, one poster commented: “if a schematic can mislead viewers, it must be a bad schematic, regardless of the final indication that… It’s actually the right schematic. The question is clear. A technically correct but confusing schematic is still a bad one.” Although this view is easy to agree with, in CAD programs, an unreadable schematic diagram can still express the connection information describing the circuit, which is still useful in layout design.
The conclusion is that when designing PCB layout, having a schematic diagram as a golden reference can make the work easier. Complete the connection with symbols; You don’t have to think about connectivity at the same time you’re dealing with the wiring challenge. Finally, it turns out that wire connections you forgot to make in the first version of the design can save you rework times.
Tip 3: use automatic cabling, but don’t rely entirely on it
Most professional PCB CAD tools have automatic cabling. But unless you design the PCB professionally, the automatic cabling will only complete the wiring once. Automatic cabling is not a one-click solution for PCB wiring. You should still know how to do manual wiring.
Automatic cabling is a highly configurable tool. In order to give full play to their role, cabling parameters should be carefully and thoughtfully set for each task, and even each module in a single PCB design should be set separately. There is no basic general default setting suitable for any occasion.
When you ask an experienced designer “what’s the best automatic cabler?” the usual response is “the thing between your ears.” it’s not a joke. As a technology, wiring is as artistic as algorithm. Cabling itself is heuristic and therefore very similar to traditional backtracking algorithms. For constrained path selection applications (such as mazes and puzzles), backtracking is good for finding answers, but in open, unconstrained situations, such as prearranged printed circuit boards, backtracking is not very good at finding optimal solutions. Unless the cabling constraints are carefully adjusted by the designer, the cabling results still need to be manually checked for weaknesses in the backtracking algorithm results.
Wire size is another difficulty. Automatic cabling can’t reliably determine how much current will flow through a wire, so it can’t help you determine how wide a wire to use. As a result, most automatic cabling devices do not produce wires of the required width. Many automatic cabling devices allow you to specify reference wire constraints. In a BBS post on stackexchange.com, author Martin Thompson wrote, “every board I built used an autocabler (excuse me, a very high end cabler… ). If your constraints are similar to this: only at this level, the two signals form a differential pair, and the networks have to match the length, then you have to tell the automatic cabler about these conditions.” When you want to use an autocabling, ask yourself, “how much manual wiring will I have to do when I set the autocabling constraints on the circuit board, and perhaps even on each wire in the schematic diagram?”
Experienced designers put a lot of effort into the initial component layout, and almost half of the design time is spent optimizing the layout of components:
Simplify wiring – minimize crossing of flying wires, etc.
Proximity of devices – shorter routes mean better wiring;
Signal timing considerations.
“Pay more attention to component layout,” reads a post on Sunstone Circuits’ user BBS. Layout components in a way that makes wiring easier. The component layout accounts for 70% of the total workload. Place all the components before starting the first thread… Use flying wires, which indicate connections that have not yet been wired, as a rough guide to the complexity of wiring.
Old-timers often used a hybrid approach to wiring – hand-woven key wires that were then locked down. Automatic cabling is then used to process non-critical wires and help manage the “escape state” in the routing algorithm. This approach is sometimes a good compromise between controlled manual cabling and fast automatic cabling.
Tip 4: circuit board geometry and current
Most people who work in electronics design know that, like a river along a river, electronics can run into choke points and bottlenecks. This point is applied directly in the design of automobile fuse. By controlling the thickness and shape of the wire (u-bend, v-bend, s-bend, etc.), the calibrated fuse will fuse at the throat point in case of overload. The problem is that PCB designers occasionally produce similar electrical choke points in their PCB designs. For example, use a 90-degree corner where you can use two quick 45s to form an Angle. The curvature is greater than 90 degrees, forming the shape of the word. At best, these wires slow down the signal; In the worst case, they fuse like car fuses at resistance points.
Tip 5: oh, shards!
Fragmentation is a manufacturing problem that can be best managed by correct circuit board design (figure 1). To understand the fragmentation problem, it is first necessary to review the chemical etching process. The purpose of the chemical etching process is to dissolve unwanted copper. But if there are particularly long, thin, sliver fragments that need to be corroded, these fragments will sometimes detach as a whole before fully dissolving. The sliver then floats in a chemical solution, possibly landing at random on another circuit board.
Also risky is when debris remains on the circuit board. If the debris is narrow enough, the acid pool can corrode enough copper underneath to partially peel off the debris. Now the pieces are floating around, attached to the circuit board like flags. Eventually it will land on your own circuit board, short-circuiting other wires.
So where do you look for potential debris and how do you avoid it? When designing PCB layouts, it is best to avoid leaving very narrow areas of copper (figure 2). This area is usually caused when copper is applied at the intersection of the wire and pad gap (FIG. 3). Set the minimum width of the copper sheet to exceed the minimum allowed by the manufacturer. Your design should not have this problem. The standard minimum width for etchings is 0.006 inches.