Have an idea you’d like to bring to market? In this series, John Teel walks through the process of scaling up from prototype to production. Follow each installment for a closer look at how to incorporate individual components.
Designing electronics is complex, and many of the most common mistakes often occur on the Printed Circuit Board (PCB). The PCB is what connects and holds all the electronic components together.
Whether you choose to do the design yourself, or hire an engineer, I strongly encourage you to get a design review from an independent engineer before you produce any prototypes. I always have other engineers review my designs before prototyping. Independent design reviews are one of the best ways to prevent costly errors such as the ones listed below.
1. High current traces not wide enough
If a PCB trace must handle current greater than a few hundred milliamps, the minimum width probably won’t be sufficient.
A trace on an external layer can carry more than an internal trace for the same thickness. This is because the external trace has superior air flow and heat dissipation.
Copper weight measures the thickness of the copper used on each trace. Most PCB manufacturers allow you to choose from copper weights of 0.5 oz./sq. ft. to about 2.5 oz./sq. ft.
I recommend a trace width calculator for proper calculations. You must specify the permissible temperature rise for a trace to calculate its current carrying capacity. A safe choice is a 10C rise, but if you need a narrower trace width, you can use a 20C or higher temperature rise.
2. Errors in landing patterns
PCB design software packages provide libraries of electronic components. These libraries include the schematic symbol and PCB landing pattern for each component. You won’t usually have problems if you stick to these components in your design.
However, if you use components outside of these libraries then you must manually draw the schematic symbol and the PCB landing pattern. It’s easy to make mistakes especially on the landing pattern. For instance, if the pad to pad spacing is off by as little as a fraction of a millimeter, the pins won’t align properly making soldering difficult or impossible.
3. Incorrect antenna layout on wireless designs
If your product uses wireless technology, the layout of the antenna on the PCB is extremely critical. However, it is commonly layed out incorrectly, even by electrical engineers.
For maximum power transfer between the transceiver and the antenna, you must match their impedance. This means their complex impedance and not their simple resistance. You’ll need a proper transmission line to connect the antenna and the transceiver.
Most of the time, the transmission line should have 50 ohm impedance for maximum power transfer with the antenna.
You can determine the PCB transmission line dimensions for achieving proper impedance by using a special calculator such as the free tool AppCad from Avago. I recommend this particular AppCAD because it can handle lots of different types of transmission lines (microstrips, coplanar waveguides, etc.). It’s also the tool that I use myself on a regular basis. Other online calculators are more limited in their options.
A matching circuit, like an LC pi-network, is also usually required between the antenna and the transceiver. This allows you to fine tune the antenna impedance.
4. Decoupling capacitors not used or improperly placed
Most components in a design need a clean, stable voltage. Decoupling capacitors on the power supply rail help serve this function.
However, decoupling capacitors need to be as close as possible to the pin that needs the stable voltage to be effective. The power trace from the supply rail also needs to go through the decoupling capacitor before it reaches the pin requiring the stable voltage.
For highly sensitive components, such as analog-to-digital converters, a series inductor should also be added. This creates a low-pass LC filter to remove any supply noise.
5. Layout of switching regulators not optimized
Two types of voltage regulators exist in electronic designs. The first type is a linear regulator. They can waste a lot power but they’re inexpensive and usually simple to layout correctly. Although for especially high power or ultra low-noise applications the layout for a linear regulator becomes more critical.
The second type is a switching regulator. Switching regulators are more complex, but much more efficient (less power wasted = longer battery life). However, they require more careful layout on the PCB, so the datasheet guidelines should be followed closely.
6. Blind/buried vias used or non-manufacturable
A through via passes through all layers of the board. Even if you only want to connect a trace from layer one to layer two, all other layers have the via too. The problem with this is it can increase the size of the PCB, because the via reduces routing space on every layer.
A blind via connects an external layer to an internal layer and a buried via connects two internal layers. However, it’s not that simple. Blind and buried vias have severe limitations based on how the layers stack up on the board. They also drastically increase prototyping costs so I don’t recommend them for most applications.
7. High-speed traces are too long
High-speed signals should follow the shortest, straightest path possible. For the majority of designs this means at least the routing of any high-frequency crystals.
Most microcontroller based designs require few super high-speed signals. However, if your product uses a high-speed microprocessor with an external data and address bus, then the trace path becomes much more important.
These are just a few errors that can occur in a printed circuit board layout. If you obtain a second opinion (i.e. an independent design review), it greatly improves your chances of a first pass success. Otherwise, you might end up with a prototype that doesn’t work, costs you more money to correct, and takes longer to reach the market.