How to approach EMI is a really big question and as has already been pointed out, one for which there are multiple answers, often working as trade-offs one against another. There has already been a good number of answers given, and quality suggestions along with it. Obviously there is no single answer and because there are many different kinds of products, there will be many cases to consider. A few will require extreme measures to be taken to make emissions or susceptibility low enough for the design as a whole to function as it needs to.
When we implement our designs on a PCB suggestions from manufacturers often result in adding common mode chokes, ferrites, shielding cans and many other things and this adds to the cost. Naturally we want to keep the cost down but this is hard to do if we choose to follow all the manufacturer’s advice. But often the advice is given as a precaution, though we sometimes interpret it as a mandatory.
It is tempting to say that the PCB itself is often one of if not the most important component that determines the EMI the product will unintentionally emit or capture. In many designs, it is fair to say that unless there are very special needs, most EMI issues can be solved by appropriate PCB design. Of course, you might say that your product can’t stand the cost of a 4-layer PCB, but it may be that this solution is cheaper than a 2-layer PCB with additional ferrites, filters and whatever else. Making a 2-layer PCB work from an EMI perspective can be tricky at best, but a correctly implemented 4-layer board without these extra parts can work out quite a lot easier to design and be the more cost effective solution.
Excepting trivial circuits, 1-layer PCBs are very difficult for controling EMI, but if the solution is to put it in a can, would it have been easier to skip the can and use a 2-layer, or better a simple 4-layer stack-up? This may also facilitate a smaller PCB too which also saves cost.
This is a rather long preamble to answering the first question, but it sets the scene. When I think about the EMI concerns on a new design, my first thought is “does it make sense to design this as a 4-layer (S-G-G-S) board?” If it does, then I’m happy because most of the varied things that I get asked to design can be implemented in this way with a very low EMI risk. If the answer is “no” but it is because 4 layers is not enough, then adding more is not a big deal, and again I expect an easy “pass” at the EMI test lab. I get a lot more concerned if it has to come down to a 2-layer board, as whatever parts have to be added to make this pass, it is much more of a struggle. Single layer designs I won’t do unless they are really simple things like just power relays and very low-speed things only, and it can be made very small.
It is perhaps useful to add a bit of context. About half of the boards I design are multiprotocol routers of one sort or another. They have 10Gbps USB, 1Gbps Ethernet, HDMI, various kinds of serial interfaces, Cellular, WiFi/Bluetooth, LORA, and multiple switching PSUs. For this a 4-layer PCB is a consistent easy win at the EMI labs. It doesn’t require extra protection on any of the interfaces to the outside world. It is about as good whether the unit is in a metal case, a plastic case or no case at all. With a 4-layer board, the only time I have to think further about EMC is if something special is needed, like ability to withstand a higher voltage while surge testing etc.
It is quite justifiable to say that the best investment to make EMI a much lower risk is quality training. The most recent example that I recall from Sierra Circuits was the Rick Hartley training course back in February. Learn how to design PCBs right and a great deal of the EMI issues vanish. The best courses don’t just tell you what to do, they explain why. Once you know the principles, the application of the rules makes a lot more sense, and may give you a good steer when things can’t be done perfectly. Look out for the training courses Sierra Circuits host and invest in your future.