Feedback on my first pcb layout

[Matador]

I always flooded my layouts with ground, and it's 100% effective until it isn't. . Adding a ground flood adds a capacitor to ground of a few pF's to every trace on the board. Most of the time, it doesn't matter, but it should be a deliberate decision, especially if you have high impedance nets that you are using to form filters.

As an example: you have a 220k feedback resistor bypassed with a 100pF cap, used in a LPF tuned to 7.2kHz. 10pF parasitic capacitance on a long trace over a ground plane could shift the frequency down almost 800Hz!

At a high level, ground should follow signal, and you should consider the layout as a differential input of your source/signal voltage and ground. As I said, in this design, it may not make much difference, but doing it in this way minimizes problems.

 

[Matador]

Here is my stab:

I start with an overall board outline, guestimating 2x3in, and roughly landscape shaped. Import the parts, and place them in roughly the shape of the schematic. I find this is a good first step, as we go to great pains to draw the schematics so that wires and parts don't overlap, which is a pretty good starting point for the PCB (because it must do the same).

I place the input on the left, and output on the right, coincident with the schematic. Visually, the power rails connect on the top and the bottom, so the power input can either go at the top, or on either side, and feed upwards and downwards. NPN and PNP transistors are using the TO-92 package in a standard EBC configuration, and the output pair is TO-220 in ECB.

Once the initial placement is done, and before routing, the PCB can be compacted down easily to 2x2.5", and with additional work could easily be made smaller than a 2" square. However this is with about 30 minutes invested.

You can see that the power and ground nets route easily along the top and bottom as predicted by the schematic, and ground follows signal through the layout. Ground and power are routed with 50 mil traces, which are good for about 3 amps with standard 1oz/ft^2 copper thickness (in other words, severely overkill here), and all other traces are 25 mils. Design rules are set to 10x10 (10 mil minimum trace, 10 mil clearance), which is roughly double what most fabrication houses can do.

Power and ground are routed first, and since we placed carefully those components that connect with rails, those connections reside along the top and bottom (matching the schematic). It's easy to connect. Then, using the bottom layer, we make every 'trivial' connection that runs 'top' to 'bottom' (or vertically). Here, 'trivially' means a short run between pads that is in a (mostly) vertical direction. Doing this knocks out roughly 30% of the connections.

Once I exhaust all trivial vertical traces, I switch to the top layer, and do all connections that are trivially left or right, or horizontal looking. This completes another 35% of the traces. This leaves only 5 or 6 nets to be connected to finish the board.

This is of course but one way to do it, but I find with 2 layer audio projects I can get a layout out and working very quickly. You can see if we moved to SMD, you could get this down to less than a square inch easily, which would make this compatible with existing discrete opamp layout formats.

 

[JMan]

Wow @Matador , thank you for taking the time to do this and explain it so thoroughly! This does indeed look much different (and better) than mine. The rationale makes sense to me, so I will try to implement this thought process in future attempts at board design.

With your permission, I’d like to try to recreate what you’ve done here for my current project. I think that it would be a good practice opportunity for me, and also will get me a better end result!

 

[Matador]

Of course, you can use whatever you'd like. Keep in mind I didn't carefully select the footprints: the resistors are all standard 1/4W dimensions, and I guessed at the cap sizes rather than researching.

Looking at the schematic you linked to: I would also add a bypass cap to the +24V rail next to the output transistor pair.

 

[wlinart]

Here is my stab:

I start with an overall board outline, guestimating 2x3in, and roughly landscape shaped. Import the parts, and place them in roughly the shape of the schematic. I find this is a good first step, as we go to great pains to draw the schematics so that wires and parts don't overlap, which is a pretty good starting point for the PCB (because it must do the same).

I place the input on the left, and output on the right, coincident with the schematic. Visually, the power rails connect on the top and the bottom, so the power input can either go at the top, or on either side, and feed upwards and downwards. NPN and PNP transistors are using the TO-92 package in a standard EBC configuration, and the output pair is TO-220 in ECB.

Once the initial placement is done, and before routing, the PCB can be compacted down easily to 2x2.5", and with additional work could easily be made smaller than a 2" square. However this is with about 30 minutes invested.

You can see that the power and ground nets route easily along the top and bottom as predicted by the schematic, and ground follows signal through the layout. Ground and power are routed with 50 mil traces, which are good for about 3 amps with standard 1oz/ft^2 copper thickness (in other words, severely overkill here), and all other traces are 25 mils. Design rules are set to 10x10 (10 mil minimum trace, 10 mil clearance), which is roughly double what most fabrication houses can do.

Power and ground are routed first, and since we placed carefully those components that connect with rails, those connections reside along the top and bottom (matching the schematic). It's easy to connect. Then, using the bottom layer, we make every 'trivial' connection that runs 'top' to 'bottom' (or vertically). Here, 'trivially' means a short run between pads that is in a (mostly) vertical direction. Doing this knocks out roughly 30% of the connections.

Once I exhaust all trivial vertical traces, I switch to the top layer, and do all connections that are trivially left or right, or horizontal looking. This completes another 35% of the traces. This leaves only 5 or 6 nets to be connected to finish the board.

This is of course but one way to do it, but I find with 2 layer audio projects I can get a layout out and working very quickly. You can see if we moved to SMD, you could get this down to less than a square inch easily, which would make this compatible with existing discrete opamp layout formats.

What a nice masterclass in pcb design! Thank you for that!

 

[JMan]

Of course, you can use whatever you'd like. Keep in mind I didn't carefully select the footprints: the resistors are all standard 1/4W dimensions, and I guessed at the cap sizes rather than researching.

Looking at the schematic you linked to: I would also add a bypass cap to the +24V rail next to the output transistor pair.

Thanks! I'll be mindful of the footprints, particularly of the caps.

Okay, so a bypass capacitor...I also know very little about this, but I read this article about how they work and I think I get the concept. So as far as implementing it, would it just be an electrolytic cap added directly between the power rail and ground, like in the attached drawing?

I also tried to use this calculator to determine what value I might want to use, but where I'm getting stuck is that I can't figure out how that parallel resistance is determined. Maybe I've misunderstood where to place the capacitor, or I am supposed to glean that resistance from other information in the circuit?

 

[MidnightArrakis]

Thanks! I'll be mindful of the footprints, particularly of the caps.

Okay, so a bypass capacitor...I also know very little about this, but I read this article about how they work and I think I get the concept. So as far as implementing it, would it just be an electrolytic cap added directly between the power rail and ground, like in the attached drawing?

I also tried to use this calculator to determine what value I might want to use, but where I'm getting stuck is that I can't figure out how that parallel resistance is determined. Maybe I've misunderstood where to place the capacitor, or I am supposed to glean that resistance from other information in the circuit?

EDIT: I've continued reading other articles and now I think I've got this wrong. The problem is that I keep seeing the examples in the articles and going, "ah, yes, I understand," then returning to this schematic to apply it and going, "wait, nope, I don't have it yet."

View attachment 89220

Perhaps the attached PDF files may help you in your PCB design endeavors!!!

/

 

[Matador]

So as far as implementing it, would it just be an electrolytic cap added directly between the power rail and ground, like in the attached drawing?

Yes, exactly. I'm lazy, and I hate managing BOM's, so if it were me, I'd find a 100uF/35V electrolytic cap, and use the same one for all of the electrolytic caps. Then you have one footprint, and only one value to buy three times. Looking at Mouser, a standard footprint for a 100uF/35V cap is 6.3mm with 2.5mm lead spacing, so I would use that one (which is smaller than what I used, which was 10mm with 5mm lead spacing).

The key is to place it close to the output transistor pair, since that is where most of the current is circulating.

 

[JMan]

Perhaps the attached PDF files may help you in your PCB design endeavors!!!

/

Wow! Thanks so much for those resources!

 

[JMan]

Yes, exactly. I'm lazy, and I hate managing BOM's, so if it were me, I'd find a 100uF/35V electrolytic cap, and use the same one for all of the electrolytic caps. Then you have one footprint, and only one value to buy three times. Looking at Mouser, a standard footprint for a 100uF/35V cap is 6.3mm with 2.5mm lead spacing, so I would use that one (which is smaller than what I used, which was 10mm with 5mm lead spacing).

The key is to place it close to the output transistor pair, since that is where most of the current is circulating.

Got it! Thanks for your guidance here, I sincerely appreciate all of the time you've spent with my questions. It's a humbling experience to be asking questions of folks who are so knowledgeable and receiving such thorough and patient answers.

 

[MidnightArrakis]

Here is a complete "tome" on everything about "How To Design PCB's" in one single place. The original PDF file is rather huge, so I had to break it down into much smaller parts in order to upload them onto your thread. If you do a search on "PDF file merger", you will come across several programs which will allow you to input all of these individual files and put them all back together as its original single PDF file. I use a program called "IceCream PDF Split & Merge" and ADOBE even has its own PDF "Split & Merge" utility program online.

Essentially.....most anything that you need and/or want to know about the world of "PCB Design" is contained within this series of files. Once you put all of these files back together as a single file, you will then have an excellent "PCB Design Reference Manual" that will serve you well for many years to come!!!

GOOD LUCK!!!

/

 

[MidnightArrakis]

Wow! Thanks so much for those resources!

Attached is my "IceCream PDF Split & Merge" program. Because this forum does not allow you to upload -- .exe -- program files, I have merely renamed this file as a -- .pdf -- file. All ya gotta do is download the attached file here into a folder (i.e., IceCream PDF Utility or whatever) and then simply rename this attached file back to being -- pdf_split_and_merge_setup.exe -- and you will be "Good To Go"!!! Just double-click on the -- .exe -- file and then it will install itself. Once you get that done, then launch the program, add all of my previous PDF files into the program (in numerical sequence 1 thru 12), then click on MERGE and you will end up with a single file just as I had started out with originally!!! Then.....you can spend many enjoyable hours and hours reading and learning all about the in's and out's of professional PCB Design!!! COOL, HUH???

GOOD LUCK!!!

/

 

[JMan]

Thank you again for such a wealth of resources! I've downloaded everything and look forward to diving in.

 

[JMan]

Based on Matador's wonderful example above, along with all the other feedback in this thread, I've tried reconfiguring this board completely. I've totally overhauled my layout to more closely resemble the schematic layout a la Matador's. I've also completely changed the board size -- I was previously trying to keep it super compact because the project that I intend to use it in will have limited space, but I've now changed it to match the dimensions of a turret board that will house a separate part of the project so that they can be stacked. If anything, the board seems almost too spread out now.

I've added the suggested bypass capacitor. I've also added a footprint for two "optional" resistors to the bases of the output transistors. These were suggested by someone in another thread, but I have yet to clarify whether or not they will actually be needed -- if not, these will be populated with jumpers, which actually solves some minor routing problems anyway.

Before I send off for a couple of boards, any last changes you'd make? Am I still way off the mark?

 

[Matador]

Nice! You notice that traces top to bottom really only cross at 90 degree angles, and the loop area for the output current is away from the feedback traces.

The only other recommendation I can make is to size R14/R15 to 1/2W layout, as I'm not sure what impedances you'll be driving. If you short the output the 47 ohm resistors take the brunt of the abuse.

 

[JMan]

Nice! You notice that traces top to bottom really only cross at 90 degree angles, and the loop area for the output current is away from the feedback traces.

The only other recommendation I can make is to size R14/R15 to 1/2W layout, as I'm not sure what impedances you'll be driving. If you short the output the 47 ohm resistors take the brunt of the abuse.

Thanks! I feel like I really learned a lot from your example and it gave me some confidence with what I was doing. I may still have a long way to go before I can call myself an expert lol, but this helped tremendously. Obvious as it seems, the simple notion that ground wants to follow signal was a big "a-ha" moment for me.

I'll make R14/R15 half watt-ers, that's easy enough. I think I also may still have a couple traces crossing at 45 degree angles, but those should be simple to adjust.

 

[moamps]

The power connector should be placed on the right side of the PCB, as close as possible to the output (high current) part.

 

[JMan]

The power connector should be placed on the right side of the PCB, as close as possible to the output (high current) part.

Ah, yes, okay I've made that adjustment as well. Thank you for the reminder!

 

[MidnightArrakis]

Wow! Thanks so much for those resources!

HEY!!! I had forgotten all about that I had this "other" large file "Splitter/Joiner" utility program and I just split the large PCB Design Handbook for you. The neat thing about this other program is that it also generates its own automatic "joiner" file, so it becomes much, much easier for you to put everything back together again. OK, so.....here's what you need to do:

1. Make a folder on your hard-drive and call it whatever you want to call it.
2. Download all of the files attached to this message into the new folder.
3. One of the files has a -- .pdf -- file extension so you need to rename it to -- .exe -- so it reads as -- The Hitchhikers Guide To PCB Design.pdf.1.exe --
4. Rename all of the other -- .txt -- files to have an extension of -- .gfs -- so they all read similar to -- The Hitchhikers Guide To PCB Design.pdf.1.gfs --
5. After you have renamed all of the files as detailed above, double-click on the -- .exe -- file and it will then automatically join all of the other files together as a single 39MB PDF file just like my original PDF file.
6. Take the time to carefully read through all of the contents of this PDF file and become an "Expert PCB Designer"!!!
7. Enjoy life while having fun designing excellent PCB's for all who needs them.

/

 

[wlinart]

HEY!!! I had forgotten all about that I had this "other" large file "Splitter/Joiner" utility program and I just split the large PCB Design Handbook for you. The neat thing about this other program is that it also generates its own automatic "joiner" file, so it becomes much, much easier for you to put everything back together again. OK, so.....here's what you need to do:

1. Make a folder on your hard-drive and call it whatever you want to call it.
2. Download all of the files attached to this message into the new folder.
3. One of the files has a -- .pdf -- file extension so you need to rename it to -- .exe -- so it reads as -- The Hitchhikers Guide To PCB Design.pdf.1.exe --
4. Rename all of the other -- .txt -- files to have an extension of -- .gfs -- so they all read similar to -- The Hitchhikers Guide To PCB Design.pdf.1.gfs --
5. After you have renamed all of the files as detailed above, double-click on the -- .exe -- file and it will then automatically join all of the other files together as a single 39MB PDF file just like my original PDF file.
6. Take the time to carefully read through all of the contents of this PDF file and become an "Expert PCB Designer"!!!
7. Enjoy life while having fun designing excellent PCB's for all who needs them.

/

Maybe it's easier to link to where everyone can request a full pdf:
The Hitchhikers Guide to PCB Design (Free eBook)