Much like everything in the electronics world, there is a standard for how closely you should (or should not) place components together. For those that are familiar with such standards, PCB:NG assumes that components will be spaced as per IPC-7351B, using Nominal courtyard values. Additionally, very tall components should have a much larger courtyard. Boards that do not adhere to this standard may be delayed, and in extreme cases, the order cancelled.
Not everyone has heard of IPC-7531B, or the term courtyards, or thought much about component spacing. So, a bit more detail:
Not surprisingly, many PCB designers have never heard of the IPC-7351B standard. For those that are not acquainted, imagine that there's an imaginary barrier around a given component. It might look like this for an 0402 resistor:
The blue areas are suggested pad size and location, and that magenta enclosure? That's what IPC-7361B considers Nominal spacing to the next component courtyard (i.e., courtyards shouldn't overlap - don't cross the streams). This is for an 0402 - other parts will have different values. There's also Most (way more space) and Least (a bit less space), but Nominal is reasonable for all concerned. Every component has a suggested courtyard value, and you may actually find that info in your EDA package.
For those of you that are not using such a design package, here's a few things that might be helpful:
- If you have more than 8-12 unique components per average square inch on a given side, your board may be too crowded. The higher that value, the more likely it is that you have an overly crowded board. This is a very blunt rule of thumb - it's easily possible to have less unique components, and still have a crowded design. But if you have more than this, you probably should consider moving parts to the other side of your board, or increasing the size of your board.
- You'll note that the courtyard of the 0402 passive, above, is just about the same size as the next size up - an 0603. This is a very easy way to keep things spaced, at least for passives - select a component, one size up, and use it as a spacing template (and then discard it from your design - don't want to put down spurious parts!)
- Or - create the courtyards in your silk layer, and lay those out first.
- The IPC makes a free "calculator" available. It may or may not be useful, as it's a Windows only package, and it has a very limited selection of parts. But hey, free.
Despite being chill about the IPC standards, we will look for proper courtyards for tall components. For example, consider this lovely tact switch:
My, he's a strapping fellow, and so tall, too - about 5.5mm. Now, consider this real-life example - someone wanted to place an 0402 resistor right next to one of these bad boys, kind of like so:
Unfortunately, Pick and Place machines use a pick-up device that's shaped kind of like this:
And this is the result:
Now, normally, taller components go down last, but sometimes parts run out, or there are mispicks, or other issues that flip that order. Either way, spacing issues such as these cause parts to ricochet around, and while we'll do everything possible to rectify, it's going to relegate your design to best-effort status.
So, how much space is required for tall components? Again, whatever the IPC defines as nominal - your component manufacturer can answer that question. But for tall components, when in doubt - add space.