6 Horribly Common PCB Design Mistakes

in my years of working on new electronic products I've seen too many cases of poorly designed printed circuit boards that would never be of sufficient quality for commercial production and although it takes an expert in PC be designed to do a proper full design review there are ways to quickly judge the quality of a PCB design whether you designed the board yourself or you had the design outsourced in this video you're going to discover six ways to quickly judge the quality of your PCB design obviously it's best if you can catch these mistakes before you actually prototype or manufacture any boards but all of these mistakes can also be easily identified on a finished board and speaking of mistakes I make them all the time and the first time I recorded this entire video I realized I had the mute on so mistakes are something I I do know quite a bit about okay let's go ahead and get started the first mistake is incorrect PCB traces the first thing you want to do is take an overall look at the traces on the PCB first look to see if all the traces are running in straight line segments with no sharp bins or turns because sharp angles can be Troublesome for high power and high frequency traces instead of trying to determine which traces can acceptably have a 90 degree Bend it's best to Simply avoid them all together in any case most PCB design layout packages can be set up to avoid this problem note that there are exceptions to this rule some printed inductors are square concentric Spirals and some printed antennas have sharp bends but both of these are generally easy to recognize traces carrying High currents should be sized appropriately and traces carrying critical analog signals should not run parallel to traces carrying digital or fast changing signals due to noise pickup issues also in general Trace is connecting inductors should not be any wider than necessary these can act like antennas and produce unwanted radio frequency emissions mistake number two is incorrectly placed decoupling capacitors all microchips need power to function but what happens when the power source is a long distance away from the chip that's actually needing the power well in these cases power has to be brought to the chip via a PCB Trace although more typically via a PCB power plane on an internal layer decoupling capacitors are placed very close to the chips power pins for two reasons one to provide a stable Supply voltage during any transient spikes and two to filter out high frequency noise that may negatively affect the chip commonly you will see two decoupling capacitors being used the larger one is typically around 10 microfarads and is called the bulk capacitor and this capacitor helps keep the voltage stable during a load transient the second smaller capacitor is usually around 0.1 microfarads and mainly helps to filter out high frequency noise in general if a chip has more than one Power Pin then each Power Pin requires at least one decoupling capacitor these decoupling capacitors should be physically placed very close to the pins they're supposed to decouple if this doesn't happen their effect is greatly reduced if your PCV design doesn't have decoupling capacitors placed right next to the power pins on most of the microchips then that is a big indicator that your design with is not properly done and if you hired someone to design your PCB and they don't deal with decoupling capacitors correctly then you should probably find a new designer mistake number three is no length Equalization of high-speed PCB traces the length of the PCB traces must be matched in designs that require a precisely timed relationship between multiple signals for example this is critical when routing a high-speed clock signal to multiple chips or for the data and address bus lines running between a microprocessor in RAM memory this ensures that all of the signals arrive at their destinations with the exact same delays thus preserving the relationship between the signal edges this requires a knowledge of which set of signal lines require precise timing relationships these traces should incorporate some type of Trace length Equalization or just commonly called delay lines these delay lines will often look like squiggly lines as you can see here on this board also be aware that V is in the signal path can cause additional delays if these Vias can't be avoided then check that all sets of traces that require a precise time in a relationship have the same number of Vias or you can use delay lines to compensate for the delays caused by the additional Vias mistake number four is improperly designed antenna feed lines if your design includes a radio transmitter receiver or transceiver then it has to have an antenna to achieve the best performance the feed line between the RF pin on the radio chip should be impedance matched to the feed line connected to it this antenna feed line in turn must match the impedance of the antenna itself this impedance matching is necessary in order to maximize the power transfer between the antenna and the radio chip any impedance mismatches will cause a decrease in the actual transfer power and hence A reduced operating range the feed line is simply a PCB Trace with controlled impedance that matches the antenna impedance which is usually 50 ohms if the transmitter output impedance does not match the impedance of the feed line then a matching Network consisting of inductors and capacitors is usually used when designing this feed line don't confuse Trace with resistance with Trace impedance the 50 ohms we're talking about is not the resistance of the trace itself so please don't add a resistor to your antenna feed line or make the feed line longer than necessary trying to add extra resistance the 50 ohms requirement that we're talking about is actually the impedance from the antenna feed line Trace to the underlying ground plane in order to achieve a controlled impedance the feed line needs a specific width running over this ground complain the width of this Trace depends on the thickness of the copper Trace itself the thickness and the dielectric constant of the PCB substrate there are many online tools that you can use to calculate the exact dimensions required for a given copper thickness and substrate material my favorite is a free software tool that you can download from broadcom called appcad if the antenna is a PCB Trace antenna it should be on one edge of the PCB free and clear of any ground plane it should be clear of any other traces and away from any large components silk screen markings around the antenna are usually okay the copper markings such as a PCB number or company name can actually detune the antenna mistake number five is non-optimized component placement in addition to the placement of decoupling capacitors that we already discussed there are some other considerations for placing components on the circuit board if the circuit contains inductors they should not be placed too closely together because inductors create magnetic fields placing them close together and specifically end to end can cause unwanted coupling between them furthermore inductors should not be placed close to large metallic objects the magnetic fields can actually induce currents in these metallic objects and this can change the value of the inductors if you can't avoid placing inductors close together then they should be placed perpendicular to each other to reduce unwanted Mutual coupling if the board contains power resistors or any component with significant heat generation you need to consider the effects of this heat on any nearby components for example if the circuit contains thermistors to compensate for ambient temperature effects then these thermistors should not be placed close to any power trans resistors the same applies to temperature compensation capacitors if the circuit contains an onboard switching regulator then all components is associated with the switching regulator should be physically localized to a section of the PCB and as far away as possible from sections handling small analog signals these switching Regulators tend to generate significant switching noise that can negatively affect sensitive circuit sections if the PCB has an AC Mains applied directly to it usually in the power supply section then the AC side should be localized to one section of the board in addition the PCB itself should have a physical barrier separating the AC from the rest of the board typically this is accomplished by having a slot in the PCB separating the two sections mistake number six is incorrect design of grounds and ground planes for any moderately complex PCB it's best to use at least a four layer board with the two inner layers being the supply and ground planes if the design contains both analog and digital sex sections the ground plane should be split and only joined at a common Point usually the power supply negative this avoids large ground current spikes from the digital section adversely affecting the analog section if using only a two layer board then each sub-circuit ground return Trace should be separate and all of them should then join at the power negative terminal it's bad design practice to have the ground returned at any subsection or IC join into the Common Ground return path back to this power supply negative the issue here is that the PCB copper traces do have some resistance they're not superconductors after all thus current through these traces will cause voltage drops along that Trace in the example that you can see here the chip at the far right end of the trace will see its ground reference actually at a higher voltage than the true ground reference what's more its ground will bounce around depending on the return currents of all the chips before it if you found this video helpful then definitely check out this video here where I review 10 of the most common product development mistakes

Channel: Predictable Designs

Views: 177,442

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Id: Z9nycymUd-I

Length: 10min 39sec (639 seconds)

Published: Wed Aug 16 2023