Complete newcomer to PCB layout - where do I start?

[tautech]

Test points.
Once you've done a few repairs the one thing you like to add to your own designs are a few test points which is simple enough to do for TH and often just raising a component lead is enough to make a connection but you can't do that with SMD.

Instead I've found Keystone SMD test points to be very useful as you can place them most anywhere on a Net and assign their pads to it so to not have Net errors.
Your call as to how small you want to get these but I find 0805 a good size to work with as they are just large enough to get a probe hook onto without the need for grabbers if something smaller is used.

Once a design is proven these test points can be removed and used again if there is no need for them to remain on the PCB. Most of the bigger parts suppliers will stock them in cut tape quantities.
https://www.keyelco.com/category.cfm/Test-Points-Surface-Mount/Micro-Miniature/p/523/id/741

[eti]

Thank you SO much, everyone who helped. I now have to work out what "power flags" are, and how to attach them to things; this is all a new language to me, replete with a whole new set of words and systems.

[tautech]

I now have to work out what "power flags" are, and how to attach them to things; this is all a new language to me, replete with a whole new set of words and systems.

Power flags, power Nets 
Your CAD program should have an assortment of them and if it's any good you should be able to assign custom names to them. Typically you might want a couple of power rails where VDD normally means a digital supply rail and you should be able to edit the name and add a voltage. Likewise with VCC that you might have positive and negative rails of the same voltage as is common with Opamps.
The trick to having the CAD program do all the work for you is to specify the correct power Net for IC supply pins and although some CAD programs hide IC supply pins when schematics are transferred to the PCB all the Net connections will be correct it you've set it all up correctly.

[eti]

I now have to work out what "power flags" are, and how to attach them to things; this is all a new language to me, replete with a whole new set of words and systems.

Power flags, power Nets 
Your CAD program should have an assortment of them and if it's any good you should be able to assign custom names to them. Typically you might want a couple of power rails where VDD normally means a digital supply rail and you should be able to edit the name and add a voltage. Likewise with VCC that you might have positive and negative rails of the same voltage as is common with Opamps.
The trick to having the CAD program do all the work for you is to specify the correct power Net for IC supply pins and although some CAD programs hide IC supply pins when schematics are transferred to the PCB all the Net connections will be correct it you've set it all up correctly.

Ah, well that’s good but I’m still at a loss as to the specifics of what these “power flags” do, and when it’s apt to use them. Any help would be useful, as YouTube is hit and miss as regards finding a specific guide. Thanks

[nctnico]

The trick to having the CAD program do all the work for you is to specify the correct power Net for IC supply pins and although some CAD programs hide IC supply pins when schematics are transferred to the PCB all the Net connections will be correct it you've set it all up correctly.

Back in the days when logic was powered from 5V, -5V and -2V power nets and hidden power pins where a good thing because you didn't need to draw all the power pins. Nowadays hidden power pins are horrible. Even a simple digital circuit with 3 chips (each 48 pins or less) can have a 4 or 5 power nets.

The nicest things about the power net markers / symbols is that they connect across sheets. A net typically is local to the schematic sheet it is on (like a variable declared static in C). You'll need to use off-sheet ports to transport a net to other sheets. Power nets which use a power net marker / symbol don't need that.

[tautech]

The trick to having the CAD program do all the work for you is to specify the correct power Net for IC supply pins and although some CAD programs hide IC supply pins when schematics are transferred to the PCB all the Net connections will be correct it you've set it all up correctly.

Back in the days when logic was powered from 5V, -5V and -2V power nets and hidden power pins where a good thing because you didn't need to draw all the power pins. Nowadays hidden power pins are horrible. Even a simple digital circuit with 3 chips (each 48 pins or less) can have a 4 or 5 power nets.

No worries if you assign them all to the correct IC pins. Who on earth needs to see power rail connectivity in a schematic ? 
Assign rail nets/flags to the appropriate pins and supply rails and anyone can follow it.

[bson]

but I’m still at a loss as to the specifics of what these “power flags” do, and when it’s apt to use them. Any help would be useful, as YouTube is hit and miss as regards finding a specific guide. Thanks

If you open the symbol editor (ctrl-e or cmd-e while hovering over the symbol) and then look at the pins (with the E key) you'll see pins say things like "input", "output", "power input", "bidirectional", "no connection" etc.  Most ICs have power inputs.  You'll connect these to say the GND and VCC nets, which then connect to a header someplace where you intend to hook up power.  But the header pins are probably all defined as "passive" meaning they have no function associated with them.  So you end up with a lot "power inputs" on a power net, with no "power output" to drive it.  The power flag silences this particular rule violation (a bunch of power inputs with no output anywhere on the net).  A fancier solution is to clone the header symbol and create your own, change pin functions on the header to reflect your actual use, and then use this symbol instead of the stock header.  This also has the added advantage that you won't accidentally say connect the GND header pin to your VCC net or vice versa, which is easily done if you just drop down power flags willy nilly.

[PlainName]

Who on earth needs to see power rail connectivity in a schematic ?

How else do you know which is the power pin without either seeing it there or looking it up in the datasheet? When debugging some hardware, an unambiguous connection to a specific signal/rail is what I need. Further, with battery and low-power stuff the power pin may not actually be on a power rail, but if it's hidden it's not simple to see that.

One solution is to have a table of pins vs rail off to one side, but that suffers from the same thing as comments in code: you gotta keep remembering to do maintenance on it. And you still have to actually say "This pin connects to that rail" when you really meant 'the other rail' but since no-once can see it after that you won't find out until the hardware doesn't work.

[tautech]

Who on earth needs to see power rail connectivity in a schematic ?

How else do you know which is the power pin without either seeing it there or looking it up in the datasheet? When debugging some hardware, an unambiguous connection to a specific signal/rail is what I need. Further, with battery and low-power stuff the power pin may not actually be on a power rail, but if it's hidden it's not simple to see that.

One solution is to have a table of pins vs rail off to one side, but that suffers from the same thing as comments in code: you gotta keep remembering to do maintenance on it. And you still have to actually say "This pin connects to that rail" when you really meant 'the other rail' but since no-once can see it after that you won't find out until the hardware doesn't work.

Again I ask, please take the trouble to incorporate the poster into your Copy/Paste quotes.

Isn't the PSU section also part of the set of schematics ? Well it certainly should be and in doing so every power rail will be labelled and if the designer intended the device to be repairable power rails will be labelled too.
There are choices on how to do this if the designer wants to take the trouble to help with a properly serviceable design and a overlay is the most common used in good designs but marked test points or even symbols in the copper are used if there is sufficient space on the layout.
All these features are in CAD tools if we take the trouble to use them......

Once you've done a few designs you know what things you need to include into both the schematic and layout to qualify as a good serviceable and easy to follow design. However as PCB designs get more compact for whatever reason much of what makes a good design is omitted to fit miniaturization requirements or lower the call on the worlds resources.
However for commercially made products too much info on the PCB gives away some of the manufacturers IP........

[nctnico]

There are two things that need to be seperated here:

1) Hidden power pins: in modern designs these are a nuisance. Don't do this because it makes a circuit harder to debug due to the many different supply voltages and package variations in existence nowadays.

2) Draw all lines for a power net: don't do this, use power net symbols for this purpose or net labels if you want to save even more space.

[PlainName]

There are two things that need to be seperated here:

1) Hidden power pins: in modern designs these are a nuisance. Don't do this because it makes a circuit harder to debug due to the many different supply voltages and package variations in existence nowadays.

2) Draw all lines for a power net: don't do this, use power net symbols for this purpose or net labels if you want to save even more space.

Don't normally quote in full, but this can't be said too much

[eti]

So am I to take it that labelling the 0V GND & the +5V rail as "POWER_FLAG", one for each of them, it tells the software "derive the supply from here"?

[tautech]

So am I to take it that labelling the 0V GND & the +5V rail as "POWER_FLAG", one for each of them, it tells the software "derive the supply from here"?

Simply, yes.
When you place/add a Power flag whatever its name it becomes a power Net and any device connected to that Net will show one of its pads properties as being so.
For an IC that's connected to that power Net you can edit the devices properties to hide power Nets and in some programs you can do this globally and still the connectivity will show when you transfer the design to the PCB.

In a lot of cases to keep the schematic tidy you'll use power flags rather than run power traces everywhere.

 

[eti]

Could someone give me a clear but definitive explanation of the following, please:

What is a "net", exactly?
What is "mil" and "thou", and how do they differ from millimetres etc?

So grateful, this noob.

[tautech]

Could someone give me a clear but definitive explanation of the following, please:

What is a "net", exactly?
What is "mil" and "thou", and how do they differ from millimetres etc?

So grateful, this noob.

Technically a Net is each and every connection between components. However a power Net can of course connect to many components. Some such as GNG and power Nets need be named and for IC's their power pins should be assigned to the appropriate Net.

Mil = thou = 1/1000th inch
~40 mil = 1mm.

Mil is a US term as in Milli inch I believe whereas thou is of European origin.

[PlainName]

Technically a Net is each and every connection between components.

Just to be a bit clearer on that,  think of a net as a signal. There should be at least two connections to components, possibly more.

[nctnico]

Technically a Net is each and every connection between components.

Just to be a bit clearer on that,  think of a net as a signal. There should be at least two connections to components, possibly more.

A better analogy to a net is a wire that connects components. In the end a PCB is just that: wires between components.

[PlainName]

Maybe. A wire implies point-to-point whereas a signal may have many connections. Indeed, a connected signal may comprise many wires.

[nigelwright7557]

For beginners a CAD package with lots of design rule checking is good.
Maybe even a Gerber viewer too, to be sure the output is what you expect.

There are a wealth of packages out there now from crap free to good free to paid a little and to pay a lot.

[Bassman59]

Thank you SO much, everyone who helped. I now have to work out what "power flags" are, and how to attach them to things; this is all a new language to me, replete with a whole new set of words and systems.

First: a "net" is the term for a connection between two or more pins.

A "Power Flag" is Kicad's mechanism to fix an issue that comes up in Electrical Rules Checks. (ERC is in schematics, Design Rule Checks or DRC are in PCB.)

The pins on schematic symbols can have electrical types, like Input, Output, Open Collector, and so forth. For power connections there are Power In pins and Power Out pins. Most of your ICs have power pins, and those get a Power In type.

Any pin that's a Power In must be driven by one (and only one ...) Power Out pin. After all, the goal is to provide an admittedly primitive way of checking connections: all power pins have to have a power source, right?

When you place a Power Symbol (like +3V3 or +5VA or the like) from the Power library, you add a global net (same net on all sheets in the design) with that net name (+3V3, +5VA, whatever) to the schematic, and you connect that net to the pins on which you place that symbol.

Here's the rub, though, and it's somewhat non-obvious. The Power symbols are a one-pin symbol, and that pin is declared as a Power In type. It is not a power source. This makes sense, when you think about it: all the power symbol does is to declare a global net. It does not "drive."

The output pin of an LM317 regulator is a Power Out pin. When you connect that pin to a Power symbol, you have satisfied the ERC requirement that all Power In pins on that net must have a driver.

But what if your board doesn't have a regulator, and instead you have a connector through which you get your supply voltage. Say you get your supply from a board-to-board connector, or a Phoenix connector with pins, or whatever. Connectors are usually designed with their pins designated as Passive, because you don't know how they'll be used from one design to the next. So you place your connector on your board, connect a Power symbol (+3V3, etc) to the pins you designate for supply voltage, and you run ERC. And it fails because there is still no Power Out pin driving the power net.

This is where the Power Flag comes in. A Power Flag is a one-pin symbol and that one pin has a type Power Out. (Like a Power symbol, it has no footprint and doesn't appear on the BOM.) When you place a Power Flag on the undriven supply net, the Power Flag provides that driver. Run ERC and it will be happy because now your +3V3 net has a Power Out pin driving it.

Other uses for Power Flags include:

1. Say you have an AC mains plug connected to a transformer, and the transformer drives a diode bridge, and the diode bridge drives your LM317's input pin. That pin is a Power Input. A diode's pins are of type Passive. So while you "know" the LM317 has a power input, the ERC doesn't know that. Place a Power Flag on the net that connects the bridge output to the 317 input, and now the 317 has a proper Power Out driving it. ERC is happy.

2. Say you have a switching regulator with post filtering, and you connect that filter output to your +3V3 rail by placing a Power symbol on it. Now you have a global net with name +3V3 and that's connected to all of your logic chips' voltage supply pins. The filter components have Passive pins, so ERC thinks that all of your logic devices' Power In pins have no driver. Place a Power Flag on the +3V3V rail somewhere and now ERC is satisfied.

Don't forget: ground pins on your ICs and such are designated as Power In also, so your GND net must also connect to a Power Out pin. In the cases described above, you will need to put a Power Flag on that net to satisfy ERC.

Is this a hack workaround? I don't know. I don't care. It is a clever solution to a problem. Of course you could always just not use the ERC feature and Kicad won't care, but why? It's there, use it.

[nigelwright7557]

I think I have worked it out now, and it's pretty obvious when I consider it. Connecting pads between components which are not connected on the schematic, ergo the nets are not connected, won't work, else that would be changing the circuit, wouldn't it...

A DRC check should throw up any wrong connections.
If you use the rats nest to start with wrong connections shouldnt happen.

[EPAIII]

I have made PCBs for over 40 years now and I am "starting" for the 25th time once again with KiCAD and YouTube:

https://www.youtube.com/results?search_query=pcb+design+tutorial+for+beginners

https://www.youtube.com/results?search_query=kicad+pcb+design