Overview:
This post is primarily made for absolute beginners who are interested in PCB designing. In this post we will NOT learn about PCB designing but some important knowledge to be learned about PCBs even before you learn PCB designing, the knowledge gained from this post can be applied while designing a PCB to get best results for the final product.
It is very much possible for a novice who can be trained within a week to design a PCB without knowing much electrically and physically about PCBs and this knowledge gap can lead to several issues while designing a complicated production PCBs especially that involves in high frequency, high voltage and high current applications and PCB designing is not just only about routing traces.
We will see about: Quick history about PCBs, the deconstruction of a PCB and the electrical and physical aspects to be understood.
Quick & short history of PCBs:
The modern world that we know today won’t exist without printed circuit boards because it is the backbone of the electronics which holds all the components mechanically and conducts electrically. A PCB can hold hundreds or even thousands of components and makes sure the components and its connections stay unaltered even when we apply mechanical shocks and this is the primary function of a PCB.
The idea of PCBs or something very close to that can be tracked back around 1900s and several famous inventors were also got involved in it including Edison and several patents has been granted to several inventors for their method of fabricating PCBs.
Despite the idea of PCB existed as early as 1900s, till 1950s PCBs did not conquer the consumer market, but still people had electrical appliances like televisions and radios at their home. The home appliances were manufactured without PCBs looked like this:
The above image is a vintage television set manufactured by Motorola around 1948, the method to assemble the components like this is called point to point connection. We can only imagine how difficult it was to assemble and how fragile the television sets were back then, this was how the electronics looked like without PCBs and just imagine if our computers and smartphones were constructed like this.
After 1950s things started to catch up, more and more consumer electronics goods were manufactured with PCBs. As early as 1947 double layered PCBs were fabricated and in 1960 4+ layered PCBs were fabricated and as the time progresses several advanced fabrication methods were invented and several software design tools were introduced using which designing and fabricating PCBs are not hard as it used to be.
Now enough of history lesson, let’s about learn important aspects of a PCB.
Deconstruction of the PCB:
The physical components present in a PCB:
· Copper traces
· Layers
· Via
· Pads
· Substrate
Copper Traces:
The copper tracks or the traces are the one conducts electricity which is etched on a hard / soft substrate which is non-conductive; traces are the equivalent of conductive wires in a circuit. The primary task of a PCB designer is routing the copper traces according to a given schematic.
The traces can be made long or short in length depending on the components and their orientation and can make left or right turn or can shoot at an angle, but one has to route the traces with in a given space and not every time a PCB designer get the luxury of space and sometimes one has to cramp the traces in a small foot print according to the product specifications.
On top of the copper trace a green shiny material is coated is called solder mask or solder resist. Solder mask is used for insulating the copper traces to prevent unintentional short circuit between traces when the traces comes in contact with metal particles such as glob of solder and other foreign metal particles. The solder mask also prevents the copper traces from getting oxidized.
Layers:
As more and more electronic components were getting packed at one side of the PCB, manufactures decided to put components on both the sides, now the traces are also made on the other side too, this type of PCB is called double layer PCB.
As the electronic component density and pin count per components increased like computer CPUs with 1000s of pins, single and double layer PCBs were simply not feasible. To tackle the issue manufacturers decided to add more layers between the two sides of the PCB.
Though the layers between PCB cannot hold any components and you cannot see the traces with naked eye, but it can route the copper traces efficiently to accommodate more number of pins and components on both the sides of a PCB. Example: A modern smartphone has 4+ layers of PCB to accommodate 100s of components in a small footprint.
Via:
Now we know that PCB traces can be laid in multiple layers but how do you interconnect the traces from a layer above or below? The answer is “via”.
Via is the name given to the connector that connects number of traces from different layers. The above image illustrates a cutout view of a multilayer PCB.
The above image illustrates via on a PCB which looks like holes.
Via has three parts:
1. Barrel, the conductive tube.
2. Pad, the ends of the barrel which are connected to the traces.
3. Anti-pad, it is an insulating material around the “via” to prevent short between unintended traces.
Pads:
Pads are the exposed copper traces where a component’s terminal to be placed and soldered. The pads are gold plated or solder tinned to prevent the pads from getting oxidized and other atmospheric affects acting on the pads.
There are two types of pads, one is “through hole” pad and the other is “surface mound” pad.
Through hole pad as the name suggests it has a hole to insert a component’s terminal, the component is inserted on one side and the soldering is done on the other side of the PCB.
Surface mount pad has no holes; the components are placed on the pads on the same side where the soldering is done.
Substrate:
The substrate is the one which holds the traces and the components and it is non-conductive and there are two types of substrates, hard and soft.
Hard substrate is sold, does not bend and made up of materials like: Alumina, Aluminum nitride, Beryllium oxide etc. each material is selected depending on the application.
Soft substrates are flexible and used in applications where space is limited or where the components will be in motion (example: popup camera in smartphones).
Here is an illustration of flexible PCB:
Image courtesy: Wikipedia (CC BY 2.0)
This is an SLR with soft substrate and SLRs are complicated mechanical equipment with electronic brain and implementing hard substrate will take a lot of space and makes the camera bulkier in hand.
Till now we learned all about physical aspect of a PCB, in the next section we will about the electrical aspects in the form of questions.
Electrical aspects of a PCB:
Why we should avoid PCB Traces taking a 90 degree turn (right angle)?
There are several reasons why you should not take a 90 turn and here are few reasons:
1) Right angled traces has sharp corner and in high voltage circuits sharp edges can leak charges (the effect is called corona discharge) and may cause shorts and unwanted Electromagnetic interferences to nearby equipment and components.
2) Right angled traces act like an antenna and may emit electromagnetic interferences if the traces are operated at high frequencies, not just emitting but also can absorb them effectively when compare to rounded corners or with two 45 degree turn.
3) During the fabrication process the 90 degree corner may get etched narrower than standard width of the trace.
Why there must be sufficient gap between high voltage traces and low voltage traces?
The high voltage and low voltage sides must have sufficient gap between them to prevent arcing. Arcing is not only bad for the product also very dangerous for the users using the product. The above PCB is from a DVD player’s switch mode power supply.
There are several standards which govern about high voltages on PCBs and gaps between traces and I recommend the readers to learn about them on online or offline.
Why some PCB traces take snake / wave shape?
The PCB traces are laid like snake / wave shape which can be found in high speed or high frequency communication lines between master and slave devices.
The snake like PCB trace is to make the trace longer, so that the length of this trace can match the length of the other PCB traces of the same bus and a bus may have 2 or more traces.
Traces with different length pass signal with different timings and a microprocessor can misinterpret the signal.
Conclusion:
I hope I could give some insight about the physical and electrical aspects of a PCB and this don’t end here, there are several things to be understood before you learn PCB designing and PCB designing does not mean simply routing traces on a piece of software.
