Routing is one of the steps that is used during electronic and manufacture of Printed circuit boards (PCBs) and also integrated circuits. It adds on to the placement step which involves the location of every active element or component of a PCB from where it adds the wires required to connect the components that have been placed. In simpler terms routing involved creating the wires needed to connect placed components in a PCB. While doing this, one has to consider all the design rules for the PCB’s. As experts in the PCB world, we know PCB routing might be hectic especially for beginners. Please join us as we discuss more about the requirements of PCB routing.
The basic task of a router is come up geometries in such a way that all the terminals assigned to the same net are joined and that no terminals are joined to non-matching nets. The simplest routing problem is that of obtaining the shortest route for a particular net in a particular layer there being no obstacles. This routing problem is referred to as the Steiner tree problem.
In addition to that, to perfectly connect the nets, there is an expectation that the router ensures the design meets timing, meets the metal density requirements, it doesn’t have crosstalk problems, and that it does not suffer from any antenna effects. Routers hardly attempt to get an optimum result. However, majority of the routing is based on heuristics which try to come up with a solution that is “good enough”.
- Ensure that traces are not routed parallel to one another for a significant length
During routing it is important to ensure that signal lines are not routed parallel to each other, not unless there is a desire of the micro-strip effects. Unintentional electro-magnetic coupling that comes about between traces that lie close to one another on a PCB is referred to as crosstalk. Crosstalk occurs on a 3 dimensional axis when traces on adjacent layers are coupled to one another (broadside coupling) and also when traces on the same layer couple (trace-to-trace coupling). Inductive coupling reduces when long parallel traces running in proximity to one another are minimized. It is also advisable to maintain a gap which is at least three time the signal trace width.
It is also best to ensure that signal traces that require high isolation be routed on separate layers, if they cannot be distanced well enough it is better to run them orthogonally to one another there being a grounding plane between them.
- Ensure that digital traces do not pass through analogue sections of the board, or vice versa
The circuit of an Opamp is of an analog form, which is quite different from a digital circuit. Therefore by use of special layout techniques, it is important to partition it in its own section of the PCB board. This is because the PCB effects come out clearly in analogue circuits operating at high speed. It is important to minimize effects arising out of the PCB itself in such a way that during the operation of the actual analogue circuit its performance will be as that of the design and its prototype.
The main reason behind the partitioning of the analogue circuit from the digital circuits is because of the primary limitation of analogue circuits which is noise. The noise sources arise from conducted emissions, internal Opamp noise, radiated emissions, conducted susceptibility, radiated susceptibility, etc. Also if during design the ground of the analogue circuit is connected to the same ground as the digital circuit the auto-router responds thus creating a disaster, hence it is important to ensure that the grounds of the analogue circuitry at connected differently from those of the digital circuit. Emphasizing further on the reason for separation of the digital circuit from the analogue circuit, if the digital and analogue circuits were mixed on the same PCB circuit board it would be almost impossible to separate the grounds as it would require the designers to lift every ground in the analogue circuit from the board so as to connect them together therefore creating a mess of floating IC leads and passive components. It is therefore essential to ensure that designers do not compromise on the performance as they go along PCB routing process by ignoring simple basics rules that involve relationships between different electronic circuits.
- Ensure that vias are avoided during PCB routing
It is important to ensure that routing constraints do not force a via during PCB routing. A via is basically a connection between the layers of a PCB. This connection of the layers leads to formation of a parasitic inductance. This is because a resonant circuit is formed when inductive bias combines with parasitic capacitance. Another challenge that comes about due to the presence of vias is the creation of ground loops since holes are put into the ground planes by the vias. The best practice to ensure that the vias are eliminated or minimized is come up with an analogue layout that routes the signal traces on the upper layer of the PCB.
- Ensuring that right angle traces are not made when turning by using curves instead
When a PCB trace turns a corner at, a reflection can occur. A reflection comes about due to the change in trace width. The trace width is multiplied by a factor of 1.41 as its width, at the apex. The transmission line characteristics are upset more so the distributed self-inductance and capacitance of the trace hence resulting into a reflection.
When performing PCB routing one has consider the above requirements so as to come up with high quality PCB’s. Designers can choose to either depend on their knowledge when doing the PCB routing, use the knowledge and then use an auto-router to perform a sanity check and others do all the routing on an auto-router software. However, with recent advances in technology designers have auto-router software at their disposal to assist them in being more productive. Auto-routers utilize the available board space and are very useful in making digital designs by eliminating crosstalk problems that come about due to proximity of the traces.
Working with auto-routers helps designers see is required to be done without necessarily going through the conditional design rules and keying in of complex tradeoffs. However is needs to understand that auto-routers do not work like a push-button that one just presses and a routed PCB is produced instantly. Humans need to tell it what it has to do after comprehensively deciding what needs to be done. After deciding on the routing one then command the router on how to proceed.
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