As a PCB engineer, you must master how to quickly design a successful and excellent PCB with layout, which means that you are on the road to achieve a fast, reliable and professional PCB.
This article will discuss some concise (and critical) skills and strategies that are applicable to both novice and veteran PCB designers. As long as you pay more attention to these skills in the design process, you and your team can reduce the number of redesigns, shorten the design time and reduce the task of diagnosing the overall design results. Let’s find out one by one.
1. Familiar with factory manufacturing process
In this era of fabless IC industry, many engineers don’t know the production steps and chemical treatment process of PCB manufactured according to their design files. This is not surprising. However, this lack of practical knowledge often leads novice engineers to make unnecessary and complicated design decisions.
Does the design really need to be that complicated? Can’t a larger grid be used for wiring, thus reducing the cost of the circuit board and improving the reliability? Other mistakes that novice designers are prone to make include unnecessarily small through hole size, blind via and buried via. Those advanced via structures are the sharp tools of PCBlayout designers, but their effectiveness is highly contextual. Although they are available tools, they do not mean that they must be used.
A blog post by Bert Simonovich, a PCB design expert, talked about the size ratio of through holes: “A through hole with an aspect ratio of 6:1 can ensure that your circuit board can be produced anywhere.” For most designs, with a little thinking and planning, those high density (HDI) features can be avoided, and the cost can be saved again, and the manufacturability of the design can be improved.
All PCB foundries are not good at the physical and hydrodynamic capabilities required for copper plating with ultra-small size or dead-ended vias. Remember, as long as there is a bad through hole, the whole circuit board can be destroyed; If there are 20,000 through holes in your design, then you have 20,000 chances of failure. Unnecessary use of HDI through holes, the failure rate immediately soared.
2.layout design uses automatic router, but do not rely on it.
Most professional PCB CAD tools have automatic router, but unless you design PCB professionally, automatic router can only be used to make layout design pass the preliminary test at best; For PCB circuit connection, automatic router is not a one-click solution. You should still know how to route by hand.
Automatic router is a highly configurable tool. In order to give full play to their functions, the router parameters should be carefully and comprehensively set in each task, and even each module on a single PCB should be considered individually. In short, there is no proper basic general preset value.
When you ask an experienced design engineer, “Which automatic router is the best one to use?” They will answer, “The thing between the ears on both sides (eyes);” And they are serious. As an algorithm, this routing process is more like an art, which is heuristic in itself, so it is very similar to the traditional backtracking algorithm.
The backtracking algorithm is very suitable for finding solutions, especially in situations where path selection is limited, such as mazes or puzzles. However, in an open and unrestricted situation, such as PCB with pre-placed components, backtracking algorithm can’t give full play to its strengths in finding optimal solutions. Unless the constraints of the automatic router are carefully fine-tuned by engineers, the finished wiring products still need to be manually checked for weaknesses in the results of the backtracking algorithm.
Route size is another problem. The automatic router can’t be 100% sure how much current you intend to pass on a route, so it can’t help you determine how wide the route to use. As a result, the trace width generated by most automatic routers does not meet the specification.
When you consider using an automatic router, first ask yourself, “After I have set the constraints of the automatic router for the board, and even for every line on the circuit diagram, how much time do I have to manually route?” Veterans of design engineers will devote most of their energy to the initial part layout, and almost half of the whole design time will be devoted to optimizing the component layout from the following three aspects:
Simplify the wiring—minimize the crossing of the rat’s nest, or rat-line, rat-trace network, etc. Proximity of components-the shorter the winding, the better. Signal timing considerations.
Older generations often use hybrid routing methods-key routing by hand, fixing their positions, and then processing non-critical routing by automatic router; The designed automatic routing area helps to manage the “runaway state” in the routing algorithm. This method can sometimes make a good compromise between the controllability of manual routing and the speed of automatic routing.
3. The circuit diagram can simplify the design task.
Sometimes it seems to be a waste of time to design a simple circuit board by layout and draw a schematic circuit diagram; Especially if you have already had the experience of completing one or two designs. But for those who design PCB for the first time, drawing circuit diagram will also be a difficult task. Skipping the circuit diagram is a strategy often adopted by novices and design engineers with moderate experience. However, please start with a complete circuit diagram that can be used as a reference to develop your wiring, which will help to ensure that all your wiring connections can be completed. Here are the reasons.
When designing PCB, if a circuit diagram can be used as a basic model, it can simplify the wiring task. Use circuit diagram symbols to complete the connection, and at the same time, you can overcome the routing challenge without thinking about those connections repeatedly; In the end, you will save design rework by catching the missing wiring links in the first revision.
First of all, the circuit diagram is the visual presentation of PCB circuit, which can convey multiple levels of information; The sub-area of the circuit is drawn in detail in several pages, and the components with corresponding functions can be arranged in adjacent positions, regardless of the final physical layout. Secondly, because the circuit diagram symbol will mark each pin of each component, it is easy to check out the unconnected pins; In other words, whether the formal rules for describing the circuit are followed or not, the circuit diagram helps you to make a quick visual judgment and ensure the integrity of the circuit.
4.PCB circuit board avoids the risk of cracking
Sliver is a kind of manufacturing error, which can be best managed by proper circuit board design. In order to understand the splitting problem, we need to review the chemical etching process. Chemical etching is used to decompose unwanted copper, but if the part to be etched is particularly long, thin and flaky, those shapes will sometimes peel off in whole before being completely decomposed; This kind of splinter will float in the chemical solution and may randomly land on another circuit board.
There is also a possible risk that the lobes remain on the original circuit board; If the lobes are narrow enough, the acid pool may corrode enough copper underneath, so that the lobes are partially peeled off. So the splinters stuck to the circuit board like flags and floated around, and finally they inevitably landed on that board, causing other traces to be short-circuited.
5. Consider circuit board size and current.
Most people engaged in electronic design know that, like a river along a river course, flowing electrons may also encounter throat points and bottlenecks; This is directly applied to the design of automotive fuse. By controlling the thickness and shape of the wiring (U-bend, V-bend, S-shape, etc.), the fuse can be calibrated and blown at the throat when the current is overloaded.
The problem is that layout design engineers occasionally encounter similar electrical choke points in their PCB design; For example: where two steep 45-degree angles are acceptable, use a 90-degree bend angle; When the curvature is greater than 90 degrees, it is zigzag. Filling those wires will only slow down the speed of signal transmission; In the worst case, they will blow at the resistance point like a car fuse.
