Article Technical Rating: 9 out of 10 (very technical)
There are a few mistakes that I see over and over when it comes to hardware design.
More specifically, I see errors with the design of the Printed Circuit Board (PCB) that connects and holds all of the electronic components together.
Okay, lets now look at 5 of the most common PCB mistakes that I see when reviewing other designs.
#1 – Incorrect landing patterns
I’ll start with the mistake that I’ve been known to make myself. Shocking I know.
All PCB design software tools include libraries of commonly used electronic components. These libraries include both the schematic symbol, as well as the PCB landing pattern. All is good as long as you stick with using the components in these libraries.
Problems begin when you use components not in the included libraries. This means the engineer has to manually draw the schematic symbol and the PCB landing pattern.
It’s very easy to make mistakes when drawing a landing pattern. For example, if you get the pin-pin spacing off by a fraction of a millimeter it will make it impossible to solder the part on the board.
#2 – Wireless antenna layout is not optimal
If the product has any wireless functionality the PCB layout of the antenna is super critical. Unfortunately, it’s done wrong more often than right, so watch this one very closely.
For maximum power transfer between the transceiver and the antenna their impedance must be matched. This means two things are required.
First is a proper microstrip connecting the antenna and the transceiver.
A microstrip is a type of transmission line fabricated on a PCB for carrying microwaves (high frequency radio waves). It’s a conducting strip separated from a ground plane by a dielectric layer.
In most cases the microstrip needs to be designed with a 50 ohm impedance for maximum power transfer with the antenna.
This is done by setting the microstrip’s width based on the dielectric specifications for the PCB. I suggest you calculate this width by using a microstrip calculator.
In addition to using a 50-ohm microstrip transmission line, it’s also necessary to usually add some type of LC matching circuit like a pi-network. This allows fine tuning of the antenna impedance for optimum matching and maximum power transfer.
There are many other common mistakes in wireless design and I’ll be writing an article just on this topic very soon.
#3 – Decoupling capacitors incorrectly located
Critical components need a clean, stable voltage source. Decoupling capacitors are placed on the power supply rail to help in this regard.
However, for decoupling capacitors to work their best they must be as close as possible to the pin requiring the stable voltage.
The power line coming from the power source needs to be routed so it goes to the decoupling capacitor before going to the pin needing a stable voltage.
Also it’s critical to place the output capacitor for the power supply regulator as close as possible to the output pin of the regulator.
This is necessary for optimizing stability (all regulators use a feedback loop that can oscillate if not properly stabilized). It also improves transient response.
#4 – Insufficient width for power traces
If a PCB trace will have more than roughly about 500mA flowing through it then the minimum width allowed for a trace probably won’t be sufficient.
The width of the trace required depends on several things including whether the trace is on internal or external layer, and the thickness of the trace (or copper weight).
For the same thickness, an external layer can carry more current for the same width than an internal trace, because the external traces have better air flow allowing better heat dissipation.
The thickness depends on how much copper is being used for that layer. Most PCB manufacturers allow you to choose from various copper weights from 0.5 oz/sq.ft to about 2.5 oz/sq.ft. If preferred you can convert the copper weight to a thickness measurement such as mils.
When calculating the current carrying capability of a PCB trace you must specify the permissible temperature rise for that trace.
Generally a 10C rise is a safe choice, but if you need to squeeze down the trace width more you can use a 20C or higher allowed temperature rise.
Although the calculations for trace width are pretty simple I usually recommend using a trace width calculator.
#5 – Blind/buried vias not manufacturable
A typical through via goes through all layers of the board. This means even if you only want to connect a trace from layer one to layer two, that all of the other layers will also have this via.
This can act to increase the size of a board since the vias reduce the routing space on layers not even using the via.
A blind via connects an external layer to an internal layer, whereas a buried via connects two internal layers. However, they have strict limitations on which layers they can be used to connect.
It’s all too easy to use blind/buried vias that can’t actually be manufactured. I’ve seen PCB designs with a whole menagerie of blind/buried vias with most not being manufacturable.
To understand their limitations you must understand how the layers are stacked to make the board. For all of the technical details see my blog Making Your Printed Circuit Board (PCB) as Small as Possible.
These are just 5 mistakes that I frequently see in PCB designs.
Whether you are the engineer doing the design, or just the one paying for the design, I highly encourage you to obtain a second opinion before prototyping. Doing so will pay for itself many times over.
If you are looking for PCB design software, I personally recommend a software package called DipTrace. Diptrace is really affordable, powerful, and easy to use. I’ve been happily using it for years with great results.