Electronic – PCB Design Considerations – Copper Pouring

In general, I would say keep the top-side pour; it certainly does no harm, and it has some secondary benefits, such as less etching required and less thermal stress on the board during reflow.

You do still need to pay attention to current loops and place the vias appropriately, not just scattering them about randomly. Since the FT232R is the only active chip on the board, focus on its outputs. There are two LEDs that are powered by V_USB, and a few outputs associated with the serial port that are powered by V_CC. Where do the currents flow when any of these outputs change state? Try to keep the paths as short and direct as possible.

Note in particular, the ground path for the USB connector in your non-pour example. It has to go down, cross below the chip, then come up on the right before it gets to the ground pins on the top of the chip. The top-side pour shortens this considerably. In either case, it would help if you adjusted the vias near pin 1 of the chip so that the bottom pour is continuous there.

One side point about your design: Try to avoid having three etches come together at an acute angle, like you have on your Vcc trace. Make that a right-angle tee connection.

first allow me to clearly state my answer. A four layer board will give better performance. The difference is not stark for all boards, but the four layer board will allow you to route signal, power, and ground, directly over each other in a larger variety of ways. Keeping ground directly under the power plane will reduce crosstalk for close proximity lines and reduce noise by allowing overall better routing choices. With four layers, single point ground is an achievable feat without also cutting the ground plane in two. Essentially, if the two layer solution requires cutting across the ground plane, then you are creating two ground planes connected by two points. This allows a current loop, causing noise, and this can be exacerbated by temperature differentials, nearby electronics, etc. I am not sure as to the use case of this device, but the temperature, nearby EMI, and operating frequencies will all play in. If this is a semi-high speed design, then a four layer will definitely save the integrity of those signal lines. A lot of this comes down to how much resolution you need from those ADCs. If you truly need 16 bits, then I would say a 5% performance increase from a four layer board is worth it.

A great reference which I pull my design guidelines from is... http://www.ti.com/lit/an/szza009/szza009.pdf

Cheers