I'm designing a double layer PCB to make an Arduino UNO shield.
When I tried to route the components I realize I can't route all components with only one layer.
I attach schematic and board design (with Eagle autorute with two layers).
The components are through-hole and I don't know how I could weld the components (top layer or bottom layer).
If I place a component on top layer and I do the welding in the bottom layer, How do I connect the component with the track.
Commercially made PCBs have [u]plated-through holes[/u]. If you are having your boards professionally made, you can get them that way (as long as your files call for them), and Eagle should take care of that if you design the board correctly.
Or if you're using through-hole parts, you can simply make a pad on both sides and solder both sides. For vias with no component installed you can make a pad on both sides and solder a wire through.
Apparently there are was of making a homemade plated-through via, but I'm not sure if it's worth it. (To me, it's not worth it making homemade PCBs at all.... I've made single-sided boards a long time ago, but I've never made a double-sided board and I don't plan on making my own boards again.)
Alternate solution: 0 ohm resistors, also known as "jumpers".
Put a "resistor" into the schematic on a trace that is blocking you. This will let you have a break in it on the PCB that you can route traces through. Just solder a wire into the position instead of a resistor to complete the trace.
Home made, then make a single sided PCB and use jumpers.
You only need four or five jumpers.
I often use zero ohm resistors, PTH or SMD.
It is NEVER worth the time and effort to do a DIY double sided PCB.
Recommend you widen the traces where possible to 50mil.
Edit.
Place two vias, run traces to be jumped in between, run a trace on other side between vias so DRC checks out.
1/2 inch is a good jumper width if you are not using SMDs as jumpers.
Generally, you try to avoid needing to do this - I used a lot of 0 ohm resistors to jump over traces when I was making my own boards. I used SMD resistors, since drilling holes in the board is so godawful unpleasant.
(also - maybe you've got better technique than I did, but I used to not be able to reliably put traces between pins of through-hole header when making boards myself . I switched to SMD pin header for home-made boards)
Unless you're either a masochist, or you have need of simple PCBs with super fast turnaround times, I'd really suggest you consider sending the design out to a board house instead of making your own boards. The boards you get back are just so much nicer - soldermask, silkscreen, plated through holes, much finer resolution than you'll do at home, and you don't have to spend productive time on it or stain your workspace with etchant - and they're insanely cheap, all things considered.
dirtyPCBs will give you 10 copies of a 4" x 4" board for like $25 shipped by oxcart, $40 shipped DHL - I find it really hard to cost justify making boards at home with that kind of deal available. PCBWay hits a similar price point.
DrAzzy:
Generally, you try to avoid needing to do this - I used a lot of 0 ohm resistors to jump over traces when I was making my own boards. I used SMD resistors, since drilling holes in the board is so godawful unpleasant.
(also - maybe you've got better technique than I did, but I used to not be able to reliably put traces between pins of through-hole header when making boards myself . I switched to SMD pin header for home-made boards)
Unless you're either a masochist, or you have need of simple PCBs with super fast turnaround times, I'd really suggest you consider sending the design out to a board house instead of making your own boards. The boards you get back are just so much nicer - soldermask, silkscreen, plated through holes, much finer resolution than you'll do at home, and you don't have to spend productive time on it or stain your workspace with etchant - and they're insanely cheap, all things considered.
dirtyPCBs will give you 10 copies of a 4" x 4" board for like $25 shipped by oxcart, $40 shipped DHL - I find it really hard to cost justify making boards at home with that kind of deal available. PCBWay hits a similar price point.
I agree with these.
It is nice to be able to whip off a board in an hour though.
It looks like your board is pretty simple, and with a little re-routing traces and adjusting some resistors, you can make it a 1 sided board. Traces on the back, components on the top. The tricky part would be putting the pin headers in to engage the arduino headers. For that, I would use the extended length pins and put the spacer on the top of the pcb to hold them in place while soldering, then clip the excess off, pry the spacer off and add the spacer on the bottom. Make sure and have pads up top (and solder)to keep from ripping the copper traces off the bottom when you separate the shield from the arduino.
