Full transcript
0:00all right we're gonna introduce you guys
0:01to some basic concepts in PCB design so
0:04for a lot of you this will be the first
0:06time that you've ever designed the PCB I
0:08suspect for most of you that's the case
0:10and so there's a couple of important
0:13concepts that you'll want to kind of
0:15understand and some sort of basic
0:17terminology that you'll need to know to
0:19be able to understand what it is that
0:21you're doing now the other video that
0:23Chandan has produced will describe how
0:26to go through and create your first
0:29design so I wanted to give a little bit
0:30of an introduction to some of the
0:31concepts and some of the things that he
0:33discusses in there so first of all the
0:37process that he's going to take you
0:39through is to go through some of the key
0:43steps in the design process which
0:45essentially begin with schematic capture
0:47so at this point it's sort of assumed
0:50that you've already kind of gone through
0:51your basic design you know what your
0:53circuit is and you place your schematic
0:55into the the layout tool alright so
1:00you'll do that in OrCAD what you then do
1:03is you get that schematic ready for
1:05layout so John Don will walk you through
1:07that process basically how you attach
1:09package symbols specifically what we
1:11call footprints in other words how we
1:13actually go about putting in the
1:16specific ways in which the the chip
1:19itself or the component itself is laid
1:22out so there's some basic things sort of
1:24mechanically that we need to get you
1:25introduced to to understand this piece
1:27and then creating a netlist which is an
1:29important element of the manufacturing
1:33process this netlist basically creates
1:36for us a list of all the nodes in the
1:38circuit and tells us where all the nodes
1:41are connected then it basically goes
1:44through setting up the PCB design
1:46environment essentially creating some
1:50basic standards that he's going to use
1:52so things like what basic line widths
1:56should be what the vias are going to be
1:58we're going to talk a lot more about
1:59this later on in the semester we're just
2:01getting introduced to it right now
2:03but we're going to talk a little bit
2:04about why we we have some of those
2:07constraints then you're going to go
2:09through the process of actually laying
2:10out the circuit and placing the
2:12components
2:14and so I guess there's really two things
2:15one is you're placing the components in
2:17to here then routing which means that
2:20what you're doing is you're you're
2:22routing the nets you're basically
2:23connecting the individual components
2:26with various traces we call them then
2:29you're kind of going through your last
2:31steps to kind of prepare for
2:32manufacturing so getting your what's
2:34called your silkscreen ready so we'll
2:36talk about what that is then basically
2:38you're going to generate the files that
2:39are used to actually create this then
2:43the initial video Chandan doesn't really
2:45talk about these elements we'll talk
2:47about some of those later but basically
2:49what we're talking about is what we call
2:50our Gerber files or artwork these are
2:53the files along with our drill files
2:55that tell the manufacturer how to
2:58manufacture the board we then generate
3:02some basic documentation things like a
3:05bill of materials for our for our
3:07components so that we know what
3:10components go on there and what
3:12components we need to purchase in order
3:13to complete the actual build of the
3:15board and then what we do is we submit
3:17our files for a fabrication check so for
3:21that we actually send them to the
3:22manufacturer to see if there's any
3:24issues that might prevent the board from
3:28working these issues are kind of
3:30mechanical things as we'll see there
3:33still might be other issues that arise
3:36electrically that you know really can't
3:40get picked up in the by by the
3:43manufacturer things that you know may
3:45make the circuit not work but this is
3:48really meant to see if you know things
3:50get too close to each other here's the
3:53these are the basic steps I want to talk
3:56a little bit about some of the things
3:57that some of the terms I guess that I
3:59used in there first of all one of the
4:02things I described was a netlist we're
4:05not going to talk much about the
4:07specifics of the netlist but it's one of
4:10the critical elements and it's one of
4:11the steps that John Tom will take you
4:13through what he calls net listing so in
4:15this particular case what I see a very
4:17simple circuit with a you know battery
4:19resistor and an LED if I look carefully
4:22this this circuit has three nodes right
4:25it's got a ground
4:26it's got a node here between that
4:28battery and the resistor and it's got a
4:30node between the resistor and the LED so
4:33what I see is this guy defines three
4:36nets and you'll see in your designs
4:39these net names will appear on the wires
4:42that run around through the circuit okay
4:45so you'll see these you'll see names
4:47like this what you have then is a net
4:50connections table down here again I'm
4:52not going to go through the structure of
4:53this but basically what this does is it
4:55describes how individual components in
4:58the circuit are connected via these
5:00nodes alright so this is one of the last
5:03critical steps as we do as we go from
5:05the schematic capture into the actual
5:08PCB design layout all right so when we
5:13get to the PCB itself the printed
5:15circuit board just introduced some basic
5:18concepts right so the board itself is
5:22made up of a couple of different layers
5:24what I'm showing here is a basic two
5:26layer board the critical layers you have
5:30are the copper layers these are where
5:32the the traces are that actually are the
5:35wires within the circuit right that are
5:37connecting the different components in
5:39the circuit in this two layer board then
5:42I have another one here on the bottom
5:44layer so we'll refer to this as the top
5:46copper layer very often we just say the
5:48top layer and then the bottom copper
5:51layer or just the bottom layer between
5:53those is a dielectric material which is
5:56essentially fiberglass sometimes called
5:59the substrate that material is referred
6:02to that fiberglass material is referred
6:04to as fr4 and fr4 is as I said a
6:08particular type of fiberglass has a
6:10particular dielectric set of dielectric
6:13properties and such that can be looked
6:16up so typically if you look at a basic
6:18board what you have is copper on the top
6:22and on the bottom of one of those pieces
6:24of fr4 substrate now on in addition what
6:31most boards also have is this sort of
6:35layer on top of that copper
6:37it's on top both on the bottom and on
6:40the top called solder mask we'll take a
6:42look at that in a second but with solder
6:43mask is is basically a layer of
6:45insulation that sits on top of the
6:47copper so that things won't touch each
6:49other and then very often what you do is
6:51you have a letter of silkscreen on top
6:53the silkscreen is basically text that
6:56allows us to identify what the
6:57components are so just as a basic
7:01example of what what we might have let's
7:04say if we had a multi-layer board which
7:06is eventually what you guys will get to
7:08is you basically have a sandwich of
7:10these individual boards like this so
7:14what I have here is I have copper and
7:17then some fr4 and then I have a layer a
7:19copper and then I have an another one
7:22down here with copper fr4 and copper and
7:25in between those I have a material
7:27called prepreg which is a cheaper
7:29material which which helps the
7:32individual elements to adhere alright so
7:35this in this particular case I have a
7:36four layer board and we'll see you know
7:39that's pretty common and most likely
7:42what most of you will ultimately develop
7:44when you get into lab 3 where you need
7:46to use more than two layers just so you
7:49have an idea if you're looking at a
7:51motherboard in a computer or something
7:53of that level of complexity you might
7:54have something upwards of say 16 layers
7:57and it might be a pretty complicated
7:59design most designs you know are
8:03somewhere between two and six layers
8:06alright now I used a couple of terms
8:08there I wanted to just kind of show
8:10these things some of you or hopefully
8:12all of you have seen a PCB at some point
8:15the solder mask here is that green layer
8:18of material and if I look carefully at
8:21the board what I can see these things
8:24here are are called pads alright
8:27whenever I have exposed copper like this
8:29as that silvery look that's a pad and
8:33that's something to which I can solder
8:35and so in this particular case I've got
8:37two types of pads I have these sort of
8:39annular rings which are around what I
8:41call a plated through-hole so a hole
8:44that's been drilled through the board
8:45and is plated with copper all the way
8:48through the board those are the sorts of
8:50things that we typically solder to now
8:53there's another type of through-hole
8:57that we don't solder to that's called a
9:00via alright that's one of that this is a
9:02via right here what that via is doing is
9:05it's connecting one layer to another so
9:09it's connecting a wire on one layer to
9:10another layer if I look carefully at
9:13this board what I can see is I have
9:15these sort of kind of lighter green
9:18areas I guess and then these kind of
9:20darker green areas where I have these
9:23lighter areas that's basically where
9:25there's copper sitting on top of that
9:28fr4 substrate so here I can see I've got
9:31a big copper area right this is what is
9:34sometimes called a copper poor where
9:37copper is effectively in a large plane
9:39alright so we'll use that terminology a
9:42copper poor these other sort of skinnier
9:45areas right here these are copper traces
9:48so essentially what's been done is the
9:50silkscreen has been sat on top of the
9:54the top layer and so wherever it crosses
9:57over copper you get this sort of lighter
9:59color these darker areas basically
10:02that's just open board
10:04all right that's basically just this the
10:06solder mask sitting on top of just blank
10:11substrate that's our four substrate so a
10:17couple of things here so I said these
10:18are plated through holes these plated
10:21through holes right here with these big
10:23annular rings around them you can solder
10:24to those are for what we call
10:28through-hole components I also have pads
10:30that are sitting right here these pads
10:33are for what we call surface mount
10:34components right and we would solder the
10:36pins directly to these pads so we'll
10:39talk more about those in the first lab
10:42you're gonna deal mostly with surface
10:44mount components in your design although
10:47later on we're gonna switch over more to
10:49the boards that you're actually going to
10:50manufacture in those particular cases
10:53we'll deal with through-hole components
10:57I did mention here quickly this this via
11:00that's something we typically don't
11:02solder too
11:04we'll talk a little bit more about that
11:05here in a bit the other element I should
11:08talk about is this silkscreen so all of
11:10this white see this white component
11:13outline here or this white text where it
11:16says rx with the plus and the minus
11:18those are what we call silkscreen and
11:20that's often a critical thing to allow
11:22us to know what components were actually
11:24looking at when we're looking at the
11:26board now just to understand so this is
11:30a this is a six layer board so this is a
11:33pretty complicated one but just to see
11:35it and stack up what I have is basically
11:39so one two three of those fr4 layer
11:43layers right where I've got copper on
11:46both sides
11:47so here's layers 1 & 2 3 & 4 5 & 6 and
11:52what I have is holes that run through
11:56the board ok so I've got two holes that
11:59are running through the board and then I
12:01can see some some copper traces up on
12:03top and it looks like in here I've got a
12:06pretty big layer of copper that's pretty
12:09common on our inner planes so in this
12:11particular case in this example what's
12:13calling that a ground plane and then
12:16down here where it's also pretty big we
12:18see that it's called a VCC plane so
12:21we'll talk later in more detail about
12:23power distribution but that's a pretty
12:26common thing to do to have a lot of
12:27ground plane a lot of power plane and
12:29then in addition to that I sort of have
12:31my signal traces on the other ones I
12:35mentioned vias vias basically are when
12:39Nets have to cross so whenever I have to
12:42cross from one node over another one
12:45what I have to do is I have to take one
12:48of those nets and I have to change
12:51copper layers when we do that we make
12:54use of a via so here's kind of a simple
12:56example what I have going on is I might
12:58have a trace on top a wire or trace that
13:03then goes through a plated through hole
13:05and then to the wire that it's going to
13:09go to on the layer beneath it all right
13:12that is a pretty common thing to do we
13:14try to avoid it as much as possible
13:16through careful layout but if we need to
13:19do it we need to do it all right all
13:21boards have these things we just do our
13:23best to try to avoid them as much as
13:25possible through careful layout and
13:27routing okay so one thing about these
13:31guys is they although they do have a
13:33ring around them that ring is usually
13:35much smaller since we don't need to be
13:37soldering to those particular components
13:40now when you're done the final process
13:44and that's not talked about in the in
13:46the initial video what you ultimately
13:48generate is a set of artwork those
13:52artwork files are called Gerber files
13:55these dot art files are come Gerber
13:58files what they what they what they do
14:00is they basically provide to the board
14:02manufacturer some indication as to you
14:05know how they should be laying out the
14:07copper and how they should be drilling
14:10the fie the the board's so if you look
14:13at this if what you have is the so it
14:16says top and bottom so these would be
14:18the top and bottom copper layers and
14:21then you've got a silkscreen on top the
14:24solder mask the bottom the bottom piece
14:30here and then some other elements that
14:33we'll talk about later in addition what
14:35you have is a drill file which basically
14:37tells the manufacturer where to drill
14:40holes in the XY plane and also tells
14:44them how big those holes need to be okay
14:47so we'll talk more about how we can
14:49identify those sorts of things later
14:51these files ultimately are the ones are
14:54part of what gets submitted to the
14:57manufacturer and are reviewed when we go
15:00through something called the DRC or
15:02design rule check so we'll talk about
15:04that in more detail later on now when
15:08you're actually going through the design
15:10process we'll talk some more about this
15:11later there are certain standards that
15:14one uses and a common standard is IPC -
15:182 - 2 1 the generic standard for printed
15:21circuit board design this as you can see
15:25comes from this Institute for inter
15:26connecting and packaging of electronic
15:28circuits IPC
15:30there's there's other flavors of this
15:32but the critical elements that it
15:34outlines for us are some things that we
15:36need to know when we're designing boards
15:38so just as an example of what I'm kind
15:43of referring to you you know
15:48from taking fields right you know that
15:51okay if there's a voltage on this line
15:53let's let's say for sake of argument
15:55that this wire right here is it a
15:56hundred volts and well I guess in this
15:59particular case this this guy is
16:00actually grounded I see that this is
16:03connected to the ground layer all right
16:06so because this is ground layer and it's
16:08connected physically to this one to this
16:11plated through hole and then to this
16:13line this is a grounded line let's say
16:16this guy right here is at a hundred
16:18volts all right so if there's a hundred
16:20volts here and there's ground here
16:22there's a electric field between those
16:25two things there is a possibility when I
16:28have an electric field over some
16:31distance like this then I can having a
16:33phenomenon called dielectric breakdown
16:34right where I can actually get some some
16:37jump some electrons to jump from one
16:40side to the other right we want to avoid
16:42that and so one of the critical things
16:44that we have is standards about how what
16:47the distance between lines should be and
16:49that distance is largely generated by
16:52controlled by voltage because we're
16:54trying to have to prevent that
16:55dielectric breakdown that you talked
16:57about in fields so one of the things
17:00that's in that that that design standard
17:04is it gives us things like okay well if
17:06the if the voltage between conductors is
17:11between 0 and 15 volts and depending on
17:14where it is whether it's an internal
17:16conductors or whether it's on the
17:18external sides alerts internal layers or
17:20external layers there's different widths
17:23that I need to maintain notice that as
17:25the voltages go up the distances go up
17:28now notice in this particular case these
17:32units are in millimeters you'll see a
17:35mix and match of of units when you do
17:38PCB design very often you see
17:40millimeters but the other unit that you
17:42often see is something called
17:44Mills which is thousandths of an inch
17:46all right we'll look at that in a little
17:48bit so you know this as I said this
17:52standard lays out you know some basic
17:54guidelines one of them being how do i
17:56space conductors to prevent that
17:58dielectric breakdown the other really
18:00critical thing that we often worry about
18:02is what size are the traces so if I look
18:06back to you know these traces right here
18:09well how does one decide what what with
18:11those traces need to be well the width
18:15of traces is basically set by the
18:17current so if I have a lot of current
18:19going through these things then my
18:22concern would be how much voltage is
18:26being lost as that current goes through
18:28that resistance of this trace and then
18:32ultimately how much power is lost is
18:34that current traverses through this
18:35trace so what I want to do is I want to
18:38minimize the resistance but the question
18:40is how do I you know ultimately how
18:42small do I make that resistance
18:43well resistance is is kind of a tricky
18:47thing what we tend to think about is
18:49temperature rise I know that if I have a
18:51lot of current running through these
18:53traces that I'm going to be dissipating
18:54power and if I'm dissipating power that
18:57means the temperature is going to rise
18:58so one thing that is included in here is
19:02a set of grass and ultimately a set of
19:04equations that allow us to determine how
19:06to size our traces on the basis of what
19:10temperature rise we won't want to have
19:12for a given current all right so as an
19:15example of that one simple way to do
19:17this to a company that will make use of
19:20eventually advanced circuits basically
19:23has this thing called a trace width
19:25calculator which is taken from this IPCC
19:28- 2 to 1 IP C - to 2 to 1 right so what
19:34I see here is I can input a particular
19:37current so I put in in this case what
19:39say I point 1 amps going through a
19:41particular trace and I in this
19:44particular case I'm saying let's say the
19:45ambient temperature is 25 degrees C the
19:49trace is 1 inch long and I only want a 1
19:52degree temperature rise so that means I
19:54only want the temperature to rise 1
19:56degree above
19:57the ambient well what that tells me in
20:00this particular case if I'm on an
20:02internal layer I need something that's
20:04two point six mils I said mils were a
20:07thousandth of an inch so I need a trace
20:09it's 2.6 million I have associated with
20:15that and if I have an external trace I
20:20see that it has a width 0.99 now notice
20:24that the internal layers basically give
20:29us wider traces for the same temperature
20:33rise notice as well if I look at that
20:36that the resistance of this guy is lower
20:38we would sort of expect that something a
20:40wider wire right a wider trace would
20:44give us a lower resistance now if you
20:47notice on the top it's a it's a skinny
20:49guy and he has a higher resistance well
20:52notice the critical thing that we set
20:55here the critical specs that we give as
20:57inputs is how much of a temperature rise
21:00do I want to have so on the external
21:03pieces of the board I have air that's
21:06able to move over the board and so that
21:09that resistance and ultimately that
21:11power drop doesn't matter as much
21:13because the air is able to move over the
21:15top of the board and carry the heat away
21:17whereas I have a slightly different
21:19problem in the internal layers of the
21:20board right on the internal layers
21:23effectively that heat is very hard to
21:25get out of there and so I need a thicker
21:28trace a wider trace to be able to have
21:33the same amount of temperature rise
21:35alright so notice the critical things
21:37that you tell this are how much current
21:40and how much temperature rise you want
21:42to have the other piece is this thing
21:45called thickness of copper and notice
21:48what it actually says here it says 2
21:50ounce per square foot typically in the
21:53industry we refer to this as 2 ounce
21:55copper copper weights we talk about
21:59these terms so basically how thick is
22:01the copper in other words how thick is
22:03it in the in the z-direction
22:05so how thick of it thick is it off the
22:08board surface
22:11what we see here is it you know so that
22:14that ounce idea you can you can take a
22:16look at this but basically it's if I
22:19have two ounce copper that's basically
22:22what it says is in one square foot I've
22:25poured as much copper that it weighs two
22:27ounces if I do that then that copper is
22:30about 2.8 millicent 2 ounce copper sort
22:36of the standard values that we utilize
22:38when we're doing a lot of power work we
22:40often refer to the 2 ounce copper where
22:42it's going to be thicker the thicker the
22:44copper gets obviously the lower the
22:46resistance is gonna be all right so just
22:49introducing some basic concepts now
22:53Chandan in the video introduces concepts
22:56around line widths and via sizes and he
22:59basically goes in and he sets minimum
23:01values for those things the minimum
23:04value that he sets is 20 mils for the
23:06line widths which basically means
23:08minimum of 20 mil traces so what that
23:12what that means when I have 20 mil
23:14traces is those traces are 20 mils wide
23:18so here I had one that was 2.6 so
23:21effectively Chandan is setting it to be
23:23about 10 times that his circuit is a
23:27power circuit and typically if it's a
23:30power circuit then we'll out we know the
23:32currents are going to be higher and
23:33typically they're gonna be thicker
23:35traces so how we set these things in
23:38here ultimately gets decided by going
23:41through calculations like this which are
23:43laid out in industry standards ok so
23:47there's there's a lot for you guys to
23:48learn and to go through as we go through
23:50the semester but those are those are
23:53some key basic concepts the other thing
23:56that you'll see Chandan talk a little
23:57bit about is how we ultimately choose
24:00our packages so you guys lay out a
24:04circuit you capture a schematic and say
24:06okay well here's the here's the circuit
24:08schematic and for instance you may
24:10utilize an op-amp that's that's an LM
24:12324 and I see that that LM 324 has it
24:17would appear for different package types
24:20has these two dip chips
24:24dip means dual inline packages right and
24:27that's two basic surface mount
24:29components one of which this TS SOP
24:32package is smaller these things here see
24:34dippy dippy SOI cts SOP these are
24:38industry standard footprints for four
24:41packages so typically in the datasheet
24:43what I find if I go later into the
24:47datasheet here I have mechanical
24:51information that tells me about the
24:53footprint of the of the device right
24:56we'll need this information we'll be
24:58looking at this information to make sure
25:00that we we align and have our holes so
25:04in this case this would be a
25:05through-hole component where I want to
25:07have you know these pins going through
25:10the board I need to make sure that holes
25:13are placed in the right spots and that
25:16the pads that are placed around those
25:19holes are of the right width right so I
25:22need to have this information about the
25:24footprint of the component to make sure
25:26that the board itself is set up properly
25:28alright so there's a lot a lot to learn
25:31but some key terminology is kind of
25:34introduced here and when you go through
25:36that video that Shawn Don has you should
25:39get a pretty good sense of of how to use
25:42the specific tool that we'll be using or
25:44CAD to be able to go through this this
25:47process