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I'm learning PCB CAD. I want to know why components seem to be placed at right angles and orthogonal to the PCB (assuming that it's rectangular). Since the traces are secondary to the placement of the components, I'm not particularly concerned with traces. Why can't my TO-220 transistor be at 18.31 degrees to its decoupling capacitor? Why can't a ring of 13 LEDs be placed with each rotated by 27.69230769230769230769 degrees?

The thing that really confuses me, is the connector. It's pretty much irrelevant which way a resistor is placed alongside a capacitor on a PCB. They only relate to each other. But connectors are different. They relate to the outside world which is not entirely orthogonal. If I have a triangular bedside thingy, I might want connectors /controls on all three sides. So at 180 degrees to each other. This might also have an effect on the traces if close to the connector pads.

I don't personally think that there is any issue with components at any angle. You just never see them, even at the school level. Why?

Three pointers:

  • I'm only concerned with DIY projects. So no component auto placing by cyborgs operating to standard programs/industry norms.

  • There is a similar question from 2010. That is broader in scope than mine as it does not restrict the manufacture to DIY-only projects. And six years in CAD / software development is a lifetime and rarely relevant. Software that is easier to design in 45 degrees steps is not a necessarily still relevant. If I use Illustrator, Blender, AutoCAD or Inkscape there are no restrictions on angles.

  • I'm making a distinction between the orientation of traces and the components themselves.

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    \$\begingroup\$ I don't know what software you are using, but the PCB design package I use allows me to place components at any angle. There is no reason for orthogonal placement other than it looks good and it's easy to route traces around. \$\endgroup\$ – Majenko Sep 4 '16 at 21:31
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    \$\begingroup\$ Also PCB packages work on grids. If you rotate to strange values the footprints no longer line up with the grid. You can still use them, but it's not as easy. \$\endgroup\$ – Majenko Sep 4 '16 at 21:36
  • \$\begingroup\$ Totally agree Majenko (+1) but I doubt if any software or manufacturer can place a component to the OP's stated desired rotational accuracy of +/- 0.000000000000000000005 degrees \$\endgroup\$ – JIm Dearden Sep 4 '16 at 21:36
  • \$\begingroup\$ Go right ahead and place as best suits you. I believe it is historical all the way from manual to fully automated design and assembly where a grid is easier. There are many older full analogue designs that were free form. There are many space constrained modern designs that also have arbitrary rotation for the reasons you specify. Basically if you cannot save money fiddling with angles they are kept orthogonal. \$\endgroup\$ – KalleMP Sep 4 '16 at 21:36
  • \$\begingroup\$ The Pi has some capacitors and resistors at 45° - cdn-shop.adafruit.com/1200x900/3055-05.jpg \$\endgroup\$ – Majenko Sep 4 '16 at 21:37
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Having designed a few PCBs in my time, including some with components at weird angles, I'll give you a quick summary of my experience.

As I see it, there are several reasons to stick to 90 degree angles.

  1. Grids - If you are trying to route everything nicely on a grid, then having off-angles is a pain in the neck as stuff no longer lines up with the grid. Each time you then try to move the component around, you have to redraw the traces to get the trace back on a grid so you can route the rest of it. This is a real pain in Eagle, though not so much in other software where the traces are more fluid (e.g. Mentor Xpedition).

  2. Escape Routing - This is especially true for SMD components with many pins. As you try to bring the trace out, if you are sticking to a 45 degree routing pattern for traces, you end up having to zig-zag all over the place to try and get the traces out which is just a mess.

  3. Machine Assembly - As @PlasmaHH points out in the comments, some pick and place machines can't handle arbitrary angles. Many are limited to just orthogonal placement, while others to 45 degrees. This may more may not be applicable to DIY boards depending on whether you are planning on hand or machine soldering.

  4. Soldering - Laying components down that are at all sorts of angles can be a pain. I find it much easier to have everything orthogonal as you can tape the PCB down or put it in a holder, and then just solder each in turn. Though this one is somewhat subjective.

  5. Aesthetics - Someone once said to me that if a PCB looks neat, it will work good. That's not entirely true, but it is a good thing to keep in mind. It requires some care and attention to lay out a PCB neatly, with traces kept nicely at 45 degree angles, neat routing of data buses, and so forth. This attention to detail and extra time spent increases the likelihood that you will spot any mistakes.

    Why do I mention this? Personally I find PCBs with traces going off at all sorts of odd angles, and components placed off-grid at odd angles, just look messy or rushed.

Now in some cases, especially TQFP or QFN packages, it is far easier to place them on a 45 degree angle. This actually makes escape routing much easier and more compact as you can go out easily in any of the 4 orthogonal directions after a short 45 degree trace. Notice how in the below image at the top right of the TQFP package, you can actually route the pins onto a 90 degree routing grid within the border of the IC.

Furthermore, some times space restrictions necessitate odd angles. Normally I would stick all passive resistors and caps orthogonally. In some cases, if for example a data bus with terminations is running on a 45 degree grid, it becomes necessary to rotate the passives to match to avoid increasing the overall size massively. Notice also in the image below that I placed the crystal at 45 degrees to match the TQFP package.

TQFP at 45 Degrees

There are cases where I have put components on odd angles. However I only tend to do this if there is a very specific reason. One example is if you need to place LEDs in a pattern - for example edge mount on a circle. In this case it is necessary to put them on angles. I show this as an example below.

Example PCB

In fact you may notice that many of the components are rotated as well. This is indeed the case, and it is also another specific case. In this design, there are 11 copies of the exact same subsystem, each containing 32 degrees of LEDs in a circle. The individual sections were designed with all components but the LEDs placed orthogonally. 11 rotated copies were then placed to build up the circle. In this case components are not orthogonal over the whole design, but they are orthogonal in their sub-block.

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    \$\begingroup\$ A point that you might want to add to the list: not all fabs have pick n place machines that are comfortable placing passives at odd angles \$\endgroup\$ – PlasmaHH Sep 5 '16 at 8:07
  • \$\begingroup\$ @PlasmaHH I agree, though the question specifically mentions DIY assembly and not industrial. \$\endgroup\$ – Tom Carpenter Sep 5 '16 at 12:16
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    \$\begingroup\$ Borders are not really sharp here. I have once gotten a good deal on some boards and had them fully assembled for a DIY project. Also some DIYers are so passionate they have a small pick n place machine. Also a lot of things you see in DIY mimic professional setups and/or are a phase of learning what todo (or not to do) in professional environments. \$\endgroup\$ – PlasmaHH Sep 5 '16 at 12:24
  • \$\begingroup\$ @PlasmaHH fair point. I've added it to the list :). \$\endgroup\$ – Tom Carpenter Sep 5 '16 at 20:56
  • \$\begingroup\$ I wonder if anyone considered the possibility of making a part's pin spacing be a multiple of sqrt(2) times a common grid multiple, so that when rotated 45 degrees everything would line up nicely on the grid? \$\endgroup\$ – supercat Sep 12 '16 at 23:28
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Some components are mechanically significant, as the O.P. have already gleaned. But majority of the PCB component (resistors, capacitors, small signal transistors, OpAmps, logic ICs) aren't mechanically significant. With these non-mechanically-significant components dense and convenient packing is often the goal. Most PCB components can be approximated with a rectangle. This is why PCB layout software defaults to right angle horizontal-vertical rotation.

Having said that, all PCB layout packages1 which I've used had a way of assigning an arbitrary rotation angle to any component. This is important for mechanically significant components. Automated pick&place assembly machines can place components at arbitrary angles as well.

enter image description here

On this board, most of the bottom (blue) layer components are at oblique angles. They are mechanically significant. All of the top (red) layer components are placed at right angles, because they aren't mechanically significant.

1OrCAD, Eagle, Altium. The layout in the illustration was done with Eagle.

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A compelling reason to place two lead surface mounted components parallel with respect to one another and perpendicular to the printed circuit board travel into the oven is too prevent the tombstone effect. Tombstoning is when uneven heating melts solder at the leading lead pulling the surface mounted component into a standing position.

enter image description here

The question was specifically for do it your self printed circuit boards. So Tombstoning may not be an immediate concern. However it would be good to create manufacturable boards just in case.

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