Over the years of working with hardware entrepreneurs, we have seen too many cases of poorly designed printed circuit boards (PCBs) that never meet the quality requirements of commercial production.
In some cases, these committees are designed by the entrepreneurs themselves, and sometimes by freelance engineers.
In particular, if you’re hiring a hypothetical expert to design a PCB, it’s important to be able to judge the quality of their work.
As I think many electrical engineers would agree, most engineers have never been taught PCB design in school, so make sure you avoid hiring engineers who lack actual PCB design experience.
While it does take a PCB design expert to perform a proper complete design review, there are ways to quickly determine the quality of a PCB design.
A schematic can show you how the components are connected together to provide a given function. However, on its own, it provides very limited information about how the components are actually placed and connected together to provide a functional product.
For example, schematic lines are converted to alignments on a printed circuit board (PCB). However, schematic diagrams provide little information about the kinds of signals these lines carry unless they are explicitly documented in the schematic diagram.
This schematic document is especially important if the PCB layout is designed by a different engineer than the one who designed the schematic.
The signals in these lines may be low-level, low-noise signals that must be routed away from the noisier board alignments to avoid noise.
Or, they may be fast data or clock signals that fan out to many pins on multiple chips. In this case, the alignments should be matched in length and kept short to avoid latency mismatches.
If these alignments are not designed correctly, some circuit boards may work and some may not, depending on the characteristics and tolerances of the components used to populate each board.
In other words, even if the board faithfully interconnects all the components of a complete working schematic, the final product may not work as expected.
This article describes 7 ways to quickly determine the quality of a PCB design.
The focus here is on layout and component placement, not on the actual quality of the board structure itself (which depends entirely on the board manufacturer).
Finally, this article is not intended to be highly technical and certainly does not cover all possibilities, especially for highly complex designs or designs with unique requirements.
The purpose of this article is to show you how to quickly determine if you have a bad PCB design, because there are specific areas in PCB design where new designers are most likely to do these things wrong.
#1 – Circuit board alignment
Take an overall look at the visible traces on the board. These will be covered with solder resist, which is a thin lacquer-like layer of polymer that covers the copper traces to prevent oxidation and shorting.
The layer is usually green, but other colors can be used. Note that the white solder resist layer tends to make the traces the hardest to see. In most cases, just use the standard green color.
Also, only the top and bottom layers are actually visible, and if the board has more than two layers, you will not see the inner layers. Nevertheless, reviewing only the outer layers should still provide some clues about the quality of the design.
First, check to see that all traces extend in straight sections without sharp bends. Sharp corners can be troublesome for some high power and high frequency alignments.
Rather than trying to determine which traces can be bent 90 degrees, you can just avoid them. In any case, most CAD PCB layout packages can be set up to avoid this problem.
Please note that there are some exceptions. Some printed inductors are square with concentric spirals, and some printed antennas have sharp curves. However, both are easy to identify.
#2 – Decoupling Capacitor
All chips need power to function, but what happens if the power supply is some distance away from the chip that needs it? In these cases, power must be supplied to the chip through the board alignment (though usually through the PCB power board on the inner layer).
The decoupling capacitors are positioned very close to the power pins of the chip to filter out the negative effects of high frequency noise on the chip.
Usually, if a chip has multiple VDD pins, each such pin requires at least one decoupling capacitor, and sometimes more.
The physical location of these decoupling capacitors should be very close to the pins they are supposed to decouple. If this does not happen, then their effectiveness will be greatly reduced.
If your PCB design does not place decoupling capacitors next to the power pins of most microchips, your design is not done correctly.
If you hired someone to design a PCB and they didn’t handle the decoupling capacitors correctly, then you should get a new designer.
#3 – PCB alignment length equalization
In designs that require precise timing relationships between multiple signals, the length of the PCB traces must be matched. This is critical, for example, when routing high-speed clock signals to multiple chips or data and address buses running between the microprocessor and RAM memory.
This ensures that all signals arrive at their destination with the same delay, thus preserving the relationship between signal edges. This requires access to the schematic and knowing which set of signal lines need the exact timing relationship.
Then, the alignment is traced to see if some sort of alignment length equalization (called delay lines) has been achieved. These delay lines usually look like curved lines, as shown in Figure 1 below.
Please note that vias in the signal path can cause additional delays. If you cannot avoid these problems, check all alignments that require precise timing relationships and make sure they have the same number of through-holes. Alternatively, you can use delay lines to compensate for the delays caused by through-holes.
#4 – Antenna feed line
If your design includes a radio transmitter, receiver, or transceiver (transmitter and receiver combined), it must have an antenna.
For best performance, the feedline between the radio frequency (RF) pins on the RF chip should match the impedance of the feedline to which it is connected. In turn, that feedline must match the impedance of the antenna.
This impedance matching is necessary to maximize the power transfer between the antenna and the radio chip.
Any mismatch will result in a reduction in actual transmitted power and thus a reduction in operating range. The feedline is simply a PCB alignment with a controlled impedance that matches the antenna impedance (typically 50Ω).
If the output impedance of the transmitter does not match the impedance of the feeder, a matching network consisting of inductors and capacitors is usually used.
To achieve controlled impedance, the feeder is a PCB alignment with a calculated width extending over the grounding layer. The width of this alignment depends on the thickness of the copper alignment, the thickness of the PCB substrate, and the dielectric constant.
There are many online tools available to calculate the exact width needed for a given copper thickness and substrate material, and it is best to confirm this in an actual PCB. My favorite is the free software called AppCad that can be downloaded from Broadcom.
If the antenna is a PCB antenna, it should be located on the side of the PCB without any ground plane. It should be clear of any other traces and away from any large components.
Screen printed markings around the antenna are usually fine, but copper markings (such as PCB numbers or company names) can detune the antenna.
#5 – Component placement
In addition to placing decoupling capacitors, there are a number of other considerations for placing components on the board.
Here are some notes:
If the circuit contains inductors, they should not be placed too close together. Inductors create magnetic fields. Placing them close together, especially end-to-end, may cause unnecessary coupling between them.
In addition, inductors should not be placed close to large metal objects. The magnetic field induces currents in these objects, which can change the value of the inductor.
Ring or toroidal inductors are usually less susceptible to stray magnetic fields, so they have less impact. If placing the inductors close together is unavoidable, they should be placed perpendicular to each other to reduce unwanted mutual coupling.
If the board contains power resistors or any component that generates a lot of heat, the effect of the heat on other nearby components needs to be considered.
For example, if a circuit contains a thermistor to compensate for ambient temperature effects, it should not be placed close to any power resistors. The same is true for temperature compensating capacitors.
If the circuit contains on-board switching regulators, all components associated with them should be physically positioned on a portion of the PCB and as far away as possible from the section that handles small signals. These tend to generate a lot of switching noise that can negatively impact sensitive circuit sections.
If the PCB normally has AC power applied directly to it in the power section, the AC side should be located on a portion of the board.
In addition, the PCB itself should have a physical barrier to separate the AC from the rest of the board. Typically, this is achieved by having a slot in the PCB that separates the two sections.
#6 – Trace width and routing
Alignments carrying high currents should be sized appropriately. Figure 2 below shows the recommended alignment (sometimes called alignment) widths for IPCs (Institute of Printed Circuits) with different current ratings:
Due to noise pickup issues, alignments carrying small analog signals should not be parallel to alignments carrying digital or rapidly changing signals.
Likewise, as a rule, the alignments connecting inductors should not exceed the necessary width. They may work like antennas and produce harmful RF emissions.
#7 – Ground and floor
For any moderately complex PCB, it is best to use at least four layers of board, with two inner layers for power and ground.
If the design contains both analog and digital sections, the grounding layers should be separated and connected only at the common point (usually the negative side of the power supply). This prevents large ground current spikes from the digital section from adversely affecting the analog section.
If only two layers are used, the ground loop alignment for each subcircuit should be separated and then they should all be connected to the negative terminal of the power supply.
It is bad design to have the ground loop of any subpart or IC connected to a common ground loop path and back to the negative side of the power supply, as shown in Figure 3 below.
The problem here is that PCB copper alignments do have a certain amount of resistance. Therefore, the current through the alignment will cause the voltage to drop. In the example above, the chip on the far right of the alignment will see its ground reference voltage higher than the actual ground reference voltage.
And, its ground will bounce based on the return current of all the chips on the left side of the graph.
Conclusion
Whether you are learning to design your own PCBs or plan to outsource them to electrical engineers, you need to be able to judge the quality of your PCB designs.
If you have no design experience and outsource your PCB design, take note of the seven areas highlighted in this article to determine if your engineer is worth the cost.
In fact, if they don’t meet any of these seven criteria, then I suggest you consider finding a new designer. On the other hand, if you are going to design your own PCB, make sure you avoid these common mistakes.
In any case, it is always a good idea to have an independent engineer perform a complete design review before proceeding with PCB prototyping development.
