ELEC 332

Basic PCB Milling

Getting Ready to Mill

1. Set up directory structure
These instructions assume that you have set up a directory structure as described in the PCB Layout Tutorial. A skeleton of this structure, along with some template files is contained in the pcb.zip file. If you have not set up this structure previously, you should download and unpack this file before proceeding.

At this point you should have the following structure. 

pcb
|-- blanks              ; outlines of the standard sized blank boards
|   |-- blank3x6.gbr
|   `-- blank9x6.gbr
|-- build_panel.exe     ; a program for combining multiple boards into a single panel
|-- example_board       ; the artwork and drill files used in these instructions
`-- your_board          ; your board would go here in the directory sturcture
There are other files and directories included in the zip file, but these are the ones we will be using here. It is assumed that the board you will be milling is in the example_board directory.
2. Collect Artwork Files.
If you have not already done so generate the artwork and drill tape files for your board as described in the PCB Layout tutorial.

The output of the PCB layout software is a set of files describing the features (outline, traces, holes, etc.) which comprise the board. Traditionally the files describing the copper patterns or traces are called artwork or Gerber files. The files containing the hole sizes and locations are called drill files.

For a basic two layer board, you will need three files: one artwork file for each of the top and bottom layers and a drill file. Each pcb layout program has a different set of conventions for naming these files. These instructions will assume they are named top.gbr, bottom.gbr, and holes.drl.

3. Verify Design Rules
Boards suitable for basic milling have the following restrictions:
1. Traces and spaces must be at least 12 mils wide.
2. Large holes (>40 mils) may be of any size, but small holes (<= 40 mils) must be one of the following sizes (in mils): 12, 16, 20, 25, 30, 36, 40.
These restrictions may be relaxed in the advanced milling processes described elsewhere.
4. Verify File Formats
If you followed the instructions in the PCB Layout Tutorial, then your files will have the correct format, but the artwork files will have the extension .art rather than .gbr. If you have used a different layout process, you should verify that your artwork files have the following format:
Units: English
Digits: Integer 5, Decimal 3
Mode: Absolute
Leading zeros suppressed
Your drill files should have the same parameters, except that the number of integer digits should be 2 instead of 5.
5. Assemble Boards into a Panel
If your board is a module for the ELEC 332 breadboard system, or if you wish to fabricate several boards (or several copies of the same board) in a single run, then you should create a set of combined artwork files as described in Combining Multiple Boards. If not, proceed to the next step.

Process Layout Files

1. Start IsoPro
You should see a display like this. The rectangle in the middle of the screen represents the working area of the milling machine.

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2. Import Gerber files
Select File->Import->Gerber File(s)... from the menu bar. Browse to the directory where you have placed your artwork files (e.g. example_board in the above directory sturcture). Select top.gbr and bottom.gbr, and click Open. Accept the defaults and click Import.

Hint: If you select top.gbr first, then shift- or control-click to select bottom.gbr, the top layer will be displayed on top of the bottom layer. If you have a ground plane on the bottom layer, this will make it much easier to see the top layer traces.


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3. Import drill file
Select File->Import->Drill File(s)... from the menu bar. Browse to your board directory, select holes.drl, and click Open. Again, accept the defaults and click Import.

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At this point your screen should look something like this.

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4. Examine your data
Carefully examine the top and bottom layers, the drill holes, and their relationship to each other. Verify that everything is present, the right size, and in the right place. Unlike the layers in Cadence Allegro, the layers in IsoPro are not transparent. To view the hidden layers, click on the entry in the Layer Palette for the occluding layers until their status changes from view to hide.

Note: After changing the status of a layer, no change takes place in the display until the cursor is moved from the layer palette back in to the main window.


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5. Set up the layer table
It is necessary to tell IsoPro which is the bottom layer so that it will know to mirror the tool motions. Click on the Layer Table button () to bring up the layer table. Click on the Type field for bottom.gbr and select Solder from the drop-down list that appears. Do not check the mirror box. It will be checked automatically in the next step.

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6. Create bottom drill layer
Drilling completely through the board can create burrs on the back side of the board and the surface of the backing material. These burrs can prevent the board from seating firmly against the backing material and cause the depth of the isolation cut to be uneven. To prevent this, the drills are set to cut only halfway through the board and are drilled from both sides. Since the back side is mirrored, it is necessary to create a mirrored drill layer.

The layer table should still be displayed from the previous step, if not, bring it back up. Select the drill layer by clicking on the > symbol at the left end of the drill layer row. Click the Copy button on the right hand side of the dialog box. This will create a new layer named "Copy of holes.drl". If you like, change this to something more meaningful such as "bottom holes". Check the Mirror box for this new layer. Verify that both the bottom Gerber layer and the bottom holes layer have their mirror boxes checked. Close the layer table by clicking OK.


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7. Run the isolator
Select Edit->Select...->All Data from the menu bar. Then select Tools->Isolate from the menu bar or press the Isolate button (). In the Isolate Layers dialog box, enter "11" into the first line of the tool table. Select top.gbr and bottom.gbr in the layers table. Check the Isolate selected only box. In the Tool Type field, select Pointed Tool. Finally, click the Isolate button. When the isolation is complete, select Edit->De Select...->All Data from the menu bar.
8. Examine the isolation
Carefully examine both isolation layers, making sure there are no gaps in the isolation. If the spacing between two pads or traces is too small for the tool to pass through, they will be left connected, creating a short. If this happens, you will have to redo your layout to increase the spacing.

Position the Board on the Blank

If you have used the build_panel program your board should already be properly positioned. In this case you can skip to the next step: Starting the T-Tech Machine. If not, it will be necessary to position your board so that it properly fits the blank material.
1. Choose the appropriate sized blank
Blank board material is available in the following sizes:
Blank Size      Maximum Board Size
6 x 3 4 x 1.5
6 x 4.5 4 x 3
6 x 6 4 x 4.5
6 x 9 4 x 7.5
9 x 12 7 x 10.5
You should choose the smallest blank which will accomodate your board.
2. Load board outline
Select File->Import->Gerber File(s)... from the menu bar. Browse to the blanks directory and select the file corresponding to the size of blank board you will be using. E.g. if you are using a 3" x 6" blank, select blank3x6.gbr, and click Open. Accept the defaults and click Import.

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3. Immobilize the board outline
If the layer palette is not visible (in the left-hand border of the main window), click on View in the menu bar, then click on the Layer Palette entry at the bottom of the menu to enable it.

In the layer palette, click on the entry for the board outline to change the status from Edit to View.

4. Move the board into the usable area of the blank.
Select Edit->Select...->All Data from the menu bar. Click on the interior of the board and drag until everything is inside the interior rectangle of the board outline, which represents the usable area of the blank. Select Edit->De Select...->All Data from the menu bar.

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5. Save your work
Select File->Save as ... from the menu bar. Choose a meaningful name for your project, or simply accept the default which will be something like IsoPro1.iso.

Starting the T-Tech Machine

1. Turn on
The power switch is on the lower right front panel of the machine.

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2. The major parts
Before proceeding, be sure you are familiar with the parts of the machine as described in this step and the next.

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3. The machine head

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4. Set Pressure
Verify that the air compressor is plugged in. Using the up and down buttons, set the air pressure to 15.0 psi in each of the three displays on the left hand side of the control panel.
5. Initialize
Select Initialize from the Mill menu in IsoPro. The head will move slowly toward the left edge, then toward the front of the work area. Be patient, as it can take several seconds for the T-Tech to recognize the Initialize command. Once the coordinate reference has been established, the head will move to the home position (0,0) at the front center. At this point, the machine is ready to use.

Warning
During operation, the T-Tech machine will move to where it thinks it should go. If this is not where you want it to go, press the red Stop button located in the lower left corner of the front panel.

Milling a Board

1. Pin the board to the table
Your blank board should have two 1/8 inch tooling holes, one in the center of the front edge, and one in the back. The backing material has two holes in the same locations, each of which should have a dowel pin in it. If either of the pins is missing, get one from the black plastic box next to the machine and place it in the hole. Place the holes in your board over the pins and press down until the board is seated firmly against the backing material. There should be no gap between the bottom of the board and the top of the backing material. If there is, check to see that the board is not warped and that there are no burrs on the surface of the backing material. If the board is warped, replace it with another. If there are burrs, remove them. The tops of both dowel pins should be below the surface of the PCB material. If either is above the surface of the board, DO NOT PROCEED, or you will damage the pressure foot.

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Warning
Never attach a board directly to the milling surface. There must always be a piece of backing material between the board and the table surface.
2. Set the contact target area
In order to control the depth of cut, the QCJ5 must know the z-axis location of the tool tip. This is determined by moving the tool over an unused area of the board and slowly moving it down until electrical contact is sensed between the tool and the copper surface of the board. To define the area where this test will be made use the Set target area tool (Tools->ContactByTouch->Set target area). Click and drag the tool across a small area (about 200 mils on a side) in a region of your board which contains no traces or holes. If your board is too dense to provide such an area, place it immediately adjacent to the active area of the board in either the +y or -y direction (left or right when facing the machine).
3. Activate the vacuum holddown
The board is held flat by vacuum, supplied through the holes in the surface of the table and backing material. Cover all exposed holes with pieces of paper. Start the vacuum by pressing the foot switch on the floor next to the vacuum pump.

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4. Run the drill layer
Select Mill-Run Layer from the menu bar. Select holes.drl from the list and click OK. The Layer tool list will appear. Click OK without changing any of the fields.

The machine head will move to the tool storage area and select the first drill. The spindle and vacuum will start and the machine head will move to the position of the first hole. After a brief wait for everything to come up to speed, the head will lower, pause briefly, raise, move to the next hole position, and repeat until all of the smallest sized holes are drilled. The spindle will stop, return to the tool storage area, select the next drill, and repeat the process until all the small (<40 mil) holes have been drilled.

To avoid having to stock a large number of drills, all holes larger than 40 mils are cut with a single tool: a 1 mm (39 mil) contour mill. If any large holes are required, the machine head will return to the tool storage area, select this tool, and use it to cut all of the remaining holes.

When all of the holes are finished, the last tool will be returned to the storage area and the machine head will return to the home position.

5. Run the top layer
Select Mill->Run Layer from the menu bar. Select the layer named "top.gbr (11.0 mils)" of type "Isolation" from the table. Click the RUN button.

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Warning
There are two layers named top.gbr. Be sure to select the one with the type of "Isolation" not the one with type "Component." If you choose the component layer, the mill will cut down the middle of the trace, not around its border, and your board will be ruined.
Monitor the run to insure that it continues to cut to the proper depth. If the board is warped, or there is a burr between the board and the backing material, the depth of cut will be uneven, leading to incomplete isolation. A consistent track of yellow powder on the sides of the cut is an indication that things are going well. Its absence is an indication that the mill is no longer penetrating the copper layer. If this happens, stop the run, correct the problem, and restart.
6. Turn the board over
When the top layer isolation is complete, the spindle motor will turn off and the machine head will return to the home position. Select Mill->Material Change from the menu bar. Turn off the vacuum holddown by depressing the foot switch. Remove the board and rotate it 180° about the axis formed by the two tooling holes. This will put what was the bottom of the board on top, with what was the front of the board still at the front. Replace the board, and click Return to previous position on the Material Change dialog. Restart the vacuum pump with the foot switch.
7. Set the contact target area for the bottom layer
Choose Tools->ContactByTouch->Set target area(mirrored) from the menu. Click and drag the tool across a small area chosen as for the top layer.
8. Run the bottom layer
If there is any doubt in your mind that you remembered to set the type of the bottom layer to Solder, now is the time to resolve it. If the back of the board is not mirrored, the cuts will be in the wrong place.
Select Mill->Run Layer from the menu bar. Select the layer named "bottom.gbr (11.0 mils)" of type "isolation" from the table. Click OK. Watch the first few cuts to insure that they are in the right place and still of the correct depth.
9. Drill the bottom holes
Select Mill-Run Layer from the menu bar. Select "bottom holes" from the list and click OK. The Layer tool list will appear. Click OK. without changing any of the fields.
When the run is complete, select Mill->Material Change from the menu bar. Turn off the vacuum pump.
10. Inspect board
Carefully examine your board to insure that all isolations have cut completely through the copper and into the underlying substrate. Make sure that all holes that are supposed to be in your board have actually been drilled. Check all corners for copper slivers, as described in the instructions on soldering milled boards.
Because the tooling holes precisely define the position of the board on the milling table, you can remove and replace it and the milling tool or drill will be able to return to exactly the same place. This means that if you find a mistake before you cut out your board, you can fix it by putting the board back in the machine and recutting or redrilling the problem area.
11. Turn things off
Turn off the T-Tech machine and close the cover. Close IsoPro and logoff. If you are using a flash drive to store your board files, remember to remove it.
12. Separate the boards
Separate your boards (or board) by cutting along the lines of 16 mil holes with the the shear. If you are very careful, you can line the board up by eye, but a much more accurate way is to place two pieces of 28 or 30 ga wire in the furthest separated holes of a row and use them as guides to align the row with the edge of the lower blade of the shear.