... nihil est tam difficile et arduum quod non humana mens vincat

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Eagle Cad: track size


The track size of any pcb is really important particularly when high currents are involved. Such a materia is strictly ruled by a lot of directives, almost always of military origins, not really important for a daily use.

All these directives (for people intereted in) are listed, public and published: here the link 

In an hobbystic environment such directives are not really important, what is really necessary is to follow some simple rules. Pcb are sold or realized normally into two different thickness: 35(micro)m and 70(micro)m. According to such a condition we can set the size according to the following table:


Current (amp)

width for 35um (mils)

width for 70 um (mils)

1 10 5
2 30 15
3 50 25
4 80 40
5 110 55
6 150 75
7 180 90
8 220 110
9 260 130
10 300 150


But what happens if, for instance, a big track must be connected from the bottom to the top layer? the secret consists in create simple pads to join the two tracks (top and bottom) in multiple points





Eagle Cad - Net classes


Using the autorouter of Eagle for the realization of a pcb it is really important to establish the size of the tracks. Using the DRC function (on the right side of the menu) we can change a lot of parameters regarding the layers, the path, the distance between tracks, the shapes and so on, but unfortunately these parameters regard the whole pcb.

Very often, almost always we can say, in a single pcb tracks must have a different size. While could be normal, for a signal track, a size of 0.3-0.4 mm for instance, in many cases the size of the tracks must be increased a lot, particularly for power supply lines or for any other track that needs a wider shape for instance.

The problem can be really easily solved using the "net classes" function of  Eagle: here follows a sample, using a simple schematics:

 netclasses We are going to realize a pcb according to the following schematic: a simple voltage regulator, just to give an idea.
 netclasses  ... and this is the simple component layout, the pcb we are going to realize.
 net classes eagle  Pressing the autorouter button all tracks are automaticcaly generated according to the size established on the DRC menu (track size, drill size, distances etc.). The DRC menu allows to change the size of the track but in this case the new rule applies to the whole pcb. Very often we need to create only few tracks with a size different from the others: normally a 0.3-0.4 mm size could be perfect for a signal track but could be considered too small to provide all current necessary to the circuitery. To change the track size individually we have to select on the menu: edit - net classes
 eagle net classes

 We will create a new class for the track simply giving the new attributes: name and size (if '0' value is left the value will be accordingly the DRC menu). In this case we create a new class: name - Ground (any name is ok) size - 24 mils.

Cliking now with the right button of the mouse on the schematic on the wire we wish to change the size ...

 eagle net classes  ... we will assign to the net GND the net class nr1: Ground
 eagle net classes  Clicking on the autorouter now all the track relative to the net 'GND' will have a size of 24 mils.

This is a really important matter: if tracks are drawn using the autorouter, it is really important to establish the right size of the tracks, particularly for the supply nets.


Gerber format RS274x


While many PCB makers or board houses accept an eagle .brd format, many others request a Gerber file set to produce a prototype or a small series of pcbs. The whole process, quite simple, has been probably not enough cleared in other sections. Here follows the whole procedure. ...

eagle rs274x  1) Open a board create with Eagle. In the example we have used one of tutorial files include with the package 
 eagle rs274x  2) Run the ULP drillcfg. It is part of the configuration process to create the excellon drill file 
eagle rs274x   3) Select INCH as unit measure (please check this parameter accordingly with the board house. Many of them pretend millimeters as unit measure) and press OK
 drill cfg eagle  4) Press OK to confirm the size of the bit necessary to create the holes of your PCB
 eagle rs274x  5) Save the file regarding the drill configuration
 eagle rs274x  6) Start the cam job by pressing the small icon on the top menu as indicated
 eagle rs274x  7) Press on the top menu OPEN - JOB
 eagle rs274x  8) ..  and select Confirm pressing Open
 eagle rs274x  9) Select EXCELLON-RACK, choose the previously saved file (demo.drl), verify that 44-Drills and 45-Holes are greyed and then press PROCESS JOB to create demo.drd excellon file.

 AT THIS POINT, USUALLY, THE PCB MAKERS PROVIDE THEIR OWN GERBER.CAM (downloadable fom their respective web sites).

If they accept a generic eagle Gerber274x format follow this procedure otherwise follow their procedure

 eagle rs274x  10) As before FILE - OPEN - JOB and select
 eagle rs274x

 11) Press the button PROCESS JOB to create all the files necessary to producethe yearned PCB

 12) The job is done. Put all the files in a zip format and send the file to the PCB maker.

 I say again. Usually any pcb maker prefers all files named using their preferred extensions and for this reason they provide a modified gerber cam file.

Ask to the manifacturer before send them the job done  

Mill-outlines.ulp (HPGL format)


Here follows a simple example, just to understand the procedure, how to produce a printed circuit board in an hpgl format (directly millable using kcam4, for instance).   The picture shows a simple schematics: a 5v power supply:

Step 1: draw the schematics

eagle hpgl


Step 2: Study the pcb according to the rules of the auto-routing and save it

hpgl pcb 

Step 3: Run the script mill-outlines.


 Be careful to the parameters:


bottom: (bottom side) must be mirrorized;
rubout: avoid rub out (it's a long process just to remove all the copper not used;
mill file: chose the file.

Then generate the hpgl file pressing OK.

Step 4 - END

At this point the hpgl file power.plt, will be ready and generated. Be careful because in this case the PCB is expressed in inches. Using KCAM4, or any other program able to import and/or convert an hpgl file, proceed directly to the milling of the printed circuit to realize considering that many (probably all) cnc machine pilot programs provide a direct conversion of the measures (inch-mm) and of the file (hpgl->gcode).




Smashing a PCB

Eagle cad is a great software. 
Many of his functions are automatized processes but something must be done by hand. Names and values of the components , for istance, usually are not optimized (for direction and position) but this can be changed by hand just to make a pcb more readable.

smashing pcb

 As you can see the names and the values of the resistors are very badly positioned making them almost unreadable. We desire, to make the pcb look more professional, to change this position.

This process can be done in a snap using the function SMASH (menu -  edit - smash) and click on the components where we ant to modify these attributes.


smash pcb


 Just clicking on any components a cross compares under the value and the name of the components itself.

Immediately after will be possibile to change the position of these attributes making the whole job more professional and readable avoiding confusion and over-impressions of the shapes of the components and their names-values.


smash pcb


Probably it is more difficult to explay the process than to do the job. Have a look to the picture and you can realize by yourself the difference.

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