Printed Circuit Board Design and routing Tips
A printed circuit board is also known as a PCB. A PCB is a printed circuit board. It’s used in every aspect of our daily lives, including computers, cell phones, calculators, wristwatches, and other electrical components.This article is for professionals with a background in PCB design and Hardware design.
Shaping the PCB
The rectangle is the most popular shape for PCB. People also prefer the corners to be rounded as it reduces the risk of edge-cracking. The shape of a PCB greatly depends on the location you intend to place and your mechanical requirements (the final box in which the product is placed).
There are usually four large holes in the board. Each hole is located in one corner. These holes can be used to secure the board using a patch or a PCB holder. It is plated if the diameter exceeds 2 millimeters.
What number of layers should I use?
We now move on to the next step. How many layers should you use? This will depend on how often you use the design, the number of components, whether you have Ball Grid-Array components, and, most importantly, how dense the design is.
Systems running at 80 MHz are usually allowed to use two layers if it is possible to route the board using them. Consider C.E. Take FCC regulations and certification into consideration. The FCC regulations require that public radio bands (FM 80-108MHz) emit a maximum of -130dBm. This is possible if your clock operates between 40 and 80 MHz The second harmonic would be between 80 and 160 MHz These rules can easily be broken.It is important to use more layers for systems with a higher speed than 80 MHz
There are two tactics within four layers.
- The top and bottom layers can either be Ground or Power planes. The middle layers are used for routing.
- Signal layers are top and bottom, and middle layers are for planes.
The signal quality of the first method is very high, as signals are sandwiched between power planes. This results in minimal emission.
This second method is easier to route because you don’t need a via (vertical interconnect access) for each pin. The pins are on the same signaling layers. You can also have multiple islands on the internal planes to meet all your power requirements, further reducing the via count. This method is not for everyone.
It is important not to break power planes when high-speed signals are being sent. This could lead to a return path loop. Unwanted emission is more likely to occur.
While more layers will result in a better product, it will be costlier to develop, particularly during the prototyping stage. The difference between a prototype with two layers and 4-6 layers can reach several hundred dollars.
The six-layer+ method almost works. The use of top and bottom layers as power-planes, and internal layers as routing layers, can reduce emission and noise and significantly reduce design efforts. This section will be covered for high-speed signals.
Layer arrangement for Impedance-matching
Assume you are dealing with a high-speed system with SSTL/HSTL, LVDS/RSDS, GTL+ and other high-speed interconnections such as USB HS, 2.5Gbps PCI Express, etc. In market, impedance control PCB are preferred for various reasons. These routings need special considerations. These lines need impedance-matching. It can be very confusing for beginners. It is important to understand the difference between resistance and impedance. You can use a resistor to match resistance.
On the other hand, Impedance matching has nothing to do with resistors. It all depends on the track’s Width, the underside power-plane and whether it is Strip-Line (situated between two power-planes) or uStrip. This means a power plane under it but that the other side is free as in Top Layer (or bottom layer).
These parameters are important to achieve the desired impedance for a track. You can use an impedance calculator to achieve the desired impedance (usually 50 to 75 ohms).
A mismatched impedance connection can cause the board to fail, especially on RF, SATA, or High-Speed USB. You will need to start looking for the next prototype without knowing what caused it to fail.
It is not possible to capture everything about PCBs but, we hope this article will help you knowing more tips and tactics about Printed Circuit Boards. You few minutes of reading will result beneficial for you. DO more and more research for best PCB designs.
Passionate Writer, Blogger and Amazon Affiliate Expert since 2014.