Designing a Phono Preamp - Part VII: PCB Layout

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Audiovista

Designing a Phono Preamp - Part VII: PCB Layout
« on: 6 Jun 2009, 05:58 pm »
It's been a while since the last installment. A lot was done in the past few weeks and I'll try to catch up with my writing....

Now that the Schematics is ready, all parts have been selected and footprints created, a Netlist can be generated, The Netlist links connections between components ("nets") with footprints from Layout libraries assigning each component pin to a particular net.

There are several custom footprints created for this project. Operational amplifiers and precision resistors in the passive RIAA have Surface Mount (SMT) and Thru-Hole (TH) footprints merged. That significantly increases number of parts that can be used. Some of the newest high precision, low-noise amplifiers come only in SMT packages, we definitely want to be able to try them.

New file for the Printed Circuit Board (PCB) layout is created, then we outline the board and import the netlist. All the components are then sorted on the screen without regard for their actual relations. Picture below shows imported netlist, where I placed connectors, LED and mounting hole in positions predetermined by mechanical package. Those components are locked in place to prevent moving them accidentally during layout.



Yellow lines represent nets - they show which pins are mutually connected and they move as the component is moved. They can also be optimized to minimize their length, which is all a great help. Old fashioned method would be laying out PCB without netlist and checking schematics to determine which parts and pins are connected. It goes without mentioning that netlist saves a lot of time and eliminates errors. Then, when all components have their initial placement, we just click on the yellow line and start moving it around as needed to lay down traces.

This particular software offers options of autoplacement and autorouting - that is when computer decides where the parts should lay and how they should be connected. I typically do not use the features and I prefer to manually place every component to its optimum location and select the routing and trace thickness as appropriate for each individual connection.

Next step is finding a spot for Through-Hole components.



Now we need to place Surface Mount components. These parts will go on the other side of the board, right underneath TH components. This not only minimizes required space, but also minimizes trace length, parasitics and makes it possible to further optimize placement (for example - bypass capacitors are placed just 0.03" from the  pins that they are bypassing). This is just about impossible to do with standard, single sided component placement using through-hole technology. Component leads alone are much longer than that.

As we work, we can pull all kinds of different statistics that give us a snapshot of where we are and how long it takes. The screen shot below is taken towards the end of the layout.



We can see that we worked on the design for 8:15 hours, placed 100% of components and routed all of them.

Top side, thru-hole components:



Bottom side (SMT components):



At this point the software offers an option of using an automated Design Rule Check (DRC). We can set the rules and computer checks for spacing between pads, traces or footprints and makes sure that all electrical connections are done as per netlist.

Once we get a clean bill of health a little cleanup is in order to make sure that component designators are moved outside of the footprint so they can be read when the component is installed and other minor cosmetic stuff that gives a board nice, professional look.

Finally, top side of the finished board is ready for picture taking:



as is the bottom layer:



The bottom layer is shown as seen "through the board" so all components are shown in mirror image.

Big green and red mass is a ground plane, which comes great to shield sensitive components and traces and provide low impedance ground connection throughout the circuit. The ground plane is vertically split in two about 1/5 on the left, to isolate and contain power supply ripple and noise, to make sure right side has only clean, quiet ground. Ground plane is only 0.02" far from components and traces that are shielded - now, this just does not happen with point-to-point wired boards. Wire insulation is thicker than that.

One of the last design steps is creation of Gerber files - the files are used by PCB manufacturers to automate fabrication of the boards and hold the information about board size, traces (length, width, routing), holes (locations, dimensions), component locations, pads, masking, etc. This is another step in automating the process and minimizing errors. The screen shot below illustrates Gerber setup and preview.



In the end, when we are completely happy we need to create the so called "Detail and Spec" drawing which is a main tool for communicating with PCB manufacturers.



The drawing specifies board size and thickness, thickness of the popper, plating, tolerance for all the parameters and illustrates placement of the layers.

Most of the time layering is obvious and many designers do not create such a detailed drawing, but I just feel more comfortable having all in one place and minimizing interpretations by PCB manufacturers. That ensures that I receive the board exactly as I like it.

Gerber files along with the Detail and Spec drawing are now ready to be sent out for quotation and boards can be placed on order.
« Last Edit: 6 Jun 2009, 07:30 pm by Audiovista »

mgalusha

Re: Designing a Phono Preamp - Part VII: PCB Layout
« Reply #1 on: 6 Jun 2009, 06:33 pm »
Great thread Boris, I wish I'd seen something like this before I tried my first board layout. This deserves to be collected into a tutorial somewhere.  :thumb:

mike

Audiovista

Re: Designing a Phono Preamp - Part VII: PCB Layout
« Reply #2 on: 7 Jun 2009, 01:08 am »
Thanks for the kind words Mike.  :thumb:

Well, I am always available should advice be needed. Even though I am not a master PCB designer, this is probably one of the simplest boards I designed. I have other people do most of the PCB design these days and I do the final check. From time to time, however, I do my own boards, just to keep in shape.... This board was fun to work on - low noise and high gain requirements in presence of the AC voltage, all on a really small outline.

Audiovista

Re: Designing a Phono Preamp - Part VII: PCB Layout
« Reply #3 on: 7 Jun 2009, 07:13 pm »
Whenever possible I like to make a mockup board for the final check of the mechanical fit. Ordering PCBs involves tooling charges and minimizing number of iterations is always a good idea. I print the PCB layout on a thick paper, add several layers to emulate original thickness and create a mockup board. When doing this, one should be reasonably certain that the printer is capable of printing in 1:1 scale - that is not always the case, but it's easy to check.



Now we can attach components critical for good mechanical fit to verify their location.



The board was carefully designed, electrically and mechanically checked.... NOW we can move ahead with the order....
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