Electronic – PCB design few questions

Congratulations. Welcome to the club.

You MUST have through wires in vias.

• Solder "glops" will not work. If they work sometimes they will not work always, and one which do happen to work will die during rework/touchup/alterations.

• Threading some single strand wire through the holes and then soldering it and then cutting it off is advised. Do not use multi-strand wire - tends to make larger ends, harder to deal with overall - no better result.

• Wire can be left vertical in hole and soldered and clipped as above. this "tends" to stay in place when resoldered for many values of tend, but an enthusiastic hot irom may draw it out. For extra safety bend wire at right angle on each side before soldering. Clipping before soldering leaves a smaller solder blob than otherwise.

• You can buy special pins made for this purpose but they tend to be ugly, cost money and work no better.

• You can use dressmaking pins for this but make sure they they solder OK. Nickel plated brass ones solder well (or, the one that I used long ago did). Solid steel pins are often very very very hard to solder). Small head sizes are desirable - large head sizes take up more room than needed.

• I've seen people suggest running a wire through the same hole as an IC pin and using that to gain access to the connection on the top side. I've not tried that and it sounds messy but I mention it as a possibility.

With tracks that go to the top of IC leads you must either

• Solder to track on top of PCB or

• Provide an alternative link from top to bottom as well (via a "via" or other.)

Some devices are easy to top solder to, some aren't. If wires come to IC pads from under the IC, having enough pad on the outside to solder to is wise (at least).

If using sockets (and they are often a good idea as long as not rubbish) then choose a type that allows iron top access to the top of the pin when inserted. "Machine screw" round pin sockets are generally well spoken of and work well enough for this purpose. I've had good results from them over the years.

Whatever socket you use, choose an acceptable quality one. this need not be the dearest but will probably also not be the cheapest. Here brand name often counts. Cheap and no-name is often no-quality.

1) Crystals should not be routed this way. Traces should be shorter and as symmetrical as possible. You should connect capacitors to GND in a single point, so that you are not picking any noise from the ground plate. This is especially important for RTC crystal. With current routing you might get problems with generation start/failure if you are unlucky.

2) Checkout my single-layer board for ARM : http://hackaday.com/2011/08/03/an-arm-dev-board-you-can-make-at-home/ - even this nightmare works (only 1 decoupling cap). Defenitely what you have here will work. You may add some extra caps (like some 25uF electrolytic + 2.2uF ceramic) on the backside of the board, you have plenty of space there, and both VCC & GND together. The only thing I don't like is thin traces to your caps. They should be as wide as possible. In my design, the only capacitor was connected by like 2mm-wide traces.

Also, look at C5: You can move it to the right a little, move via closer to the cap and connect it with short wide track. When you via is under the chip, you cannot have wide tracks. Same for C6 and C7.

Also, if you are going to manufacture this at home,you'll have problems making vias under QFP chips.

3) Ground plate is more than enough. There is no much need to have solid ground plane except a square under chip where all decoupling caps are connected, it won't help with ground noise much. Ground plate is needed for controlled impedance, which is not important in your case. But your GND connection to contacts should be as wide as possible. This is general rule: VCC & GND nets should have wide tracks.

4) Yes, this is perfectly ok for low-speed ARMs.

In my case I even had no back side, and it was still working ;-) The only thing to improve if you are manufacturing on a factory is to have a small VCC square on the bottom layer in the middle of the chip, and connect it to the top using some 4-9 vias instead of 1. For VCC & GND planes you always need to have as low as possible resistances and inductance so that caps can easier filter noise => you need wider and shorter tracks and more parallel vias. But in this specific design it is not a requirement.

So, it will work even now without modifications. After mentioned changes it will be perfect.