Electronic – ESD potection in the PCB

ESD is difficult to deal with, and solutions are more black magic than science. That being said, what you want is for the impedance to ground to be smaller than the impedance to the chip you're protecting. There are several ways to do this, and the most practical solution will probably involve several of these things at once.

1. Placement and routing of traces is a good start. As you noted, MCU <-> Diodes <-> DB25 is probably the best, although MCU <-> DB25 <-> Diodes can work. To make it work, the traces to the diodes should be thick and short. The traces to the MCU should be long-ish and thin. But, IMHO, just doing this is not enough for a commercial product.

2. Put some sort of of resistor or ferrite bead between the DB25/Diodes and the MCU. I prefer resistors for this because their impedance is more predictable at high frequencies, but a bead could work too. A resistor of around 10 to 50 ohms is good, depending on the nature of the signals you're running. This resistor/bead will increase the impedance to the MCU, guiding the ESD to ground through a different way.

3. Put a capacitor in parallel with the diodes. A value of 3 nF is ideal for ESD protection. But depending on your signal you might have to use a smaller or larger one., or none at all. The largest you can get away with will also reduce your EMI problems. The basic function of the cap is to quickly absorb the ESD shock and re-emit it more slowly and with a smaller voltage. If the cap is large enough then the diode is not required. This cap also forms an RC filter with #2 above and prevents EMI from going in or out of the box.

4. Connect the shield of the DB25 to chassis ground, and make sure your chassis make a good shield.

Recently I had an issue with a USB device that would crash whenever an ESD zap happened within 8 feet of the box. In the end I had to connect the USB shell to Chassis, add 33 ohm resistors to the USB data lines, add caps, and diodes. Until I did all that I still experienced failures. If I left off one of those, any one, it would fail. Now it runs solid, even with 1 inch long sparks right to the chassis.

Here is the general approach you should take.

Any connectors and contact points to the outside world should have protection. The outer perimeter of the board should have a guard ring running around it with protection on it. This is because people tend to handle the board by the edges. This guard ring should be the ground for the rest of the system with a another ring concentric just inside.

All other internal nodes probably can't be protected, in fact bare boards are never considered to be ESD proof for all possible scenarios.

You can encapsulate the board to provide a little more protection, depending upon the encapsulation it can be quite good. 32 um of parylene would really help but is expensive (this is a mil standard).

But ultimately, the whole board should be inside of an enclosure to prevent contact and discharge.