Making our first circuit board.

Prep for this week’s class

Watch the corresponding Fab Academy lecture. We’ll be covering part of this content.

If you like, watch this short YouTube playlist showing the wole milling process.

Baseline

Who has:

  • Made something with electronics?
  • Used a CNC machine of any kind?

You don’t need to know any electronics this week.

We’ll start by producing a board, before we do design - you may not understand what the parts are, or why they are connected this way, we’re just learning how to put it together.


How to make a circuit

Just need a way to support and connect all the electrical components:

  • Jumper wires and crocodile clips (not useful for any level of complexity)
  • Conductive thread sewn into garments (good for components, but not for microcontrollers)
  • Copper tape cut with the vinyl cutter, stuck to anything (giant programmer / picture)
  • Breadboards (picture) (unreliable, not scalable, a first step only - even on milled boards, you can edit afterwards)
  • Stripboard (picture) (like the inside of a breadboard, needs soldering)
  • PCBs (printed circuit boards)

Making PCBs

Most are made from copper laminated onto a non-conductive substrate, and then copper selectively removed to form traces, pads, vias, etc.

Etching

Commercial process, hobbyist/prototyping level

  • Uses nasty chemicals, safe disposal is non-trivial
  • You must manage the lifecycle

Milling

How we do it.
Great for prototyping - every board can be different.
Not so good for batch production - slower than etching.
Move a tool bit around removing copper to leave circuit behind.
Video series of whole process

The Milling Machine

Roland MODELA MDX-50 Benchtop CNC Mill

Accuracy:

  • positioning (< 1/1000”)
  • tolerances (approx. 1/1000)
  • e.g. bit size for traces: 0.3mm diameter

Tools

  • Cutting outline of board (1mm diameter)
  • Cutting traces (0.3 or 0.4mm diameter)

Tools are a consumable - maybe make 20 boards/tool if used properly.
Hard but brittle - don’t drop or knock.

Materials

FR1 - paper-reinforced (not FR4, which is glass-based)

Scaling up

The desktop mill is good for 1-10 boards.
What if you want to make loads of boards for an art project or small product run?

  • Board houses can mill boards, but also stuff, or even source components, and fabricate.
  • Or go to Shenzhen (or go through an online intermediary).

Process

See our Fab Academy docs

1. Starting point: a PNG file of the circuit you want to mill

<#alt_text#>

2. Generate a tool path (CAD to CAM)

CAM software:

Fab Modules settings

Input format

Choose your png traces file.

pcb-mill-workflow-1.png

Traces are white, material to be removed is black - may need to invert your png.
Check resolution is the same as your png file.

pcb-mill-workflow-2.png

Output format
Roland mill (.rml)

Process
PCB traces (1/64”) or PCB outline (1/32”)

Output

Machine: MDX-40   

speed (mm/s):   4  

x0 (mm):  0  
y0 (mm):   0  
z0 (mm): 0  

zjog (mm):  2  (a safe height above board)

xhome (mm):  0  
yhome (mm):  0  
zhome (mm):  0  

cut depth (mm): 0.1mm (can adjust this)   

tool diameter (mm): 0.4 or 0.3 (Make sure it’s the same as the tool!)

number of offsets (-1 to fill):  
1 to verify.  
2 is enough clear space to solder.  
4 makes life a bit easier.  
-1 removes all non-circuit material. Looks nice, but takes ages.

Calculate
Hit calculate to see a preview of tool path.

pcb-mill-workflow-3.png

You will probably need to go back several steps in the workflow here to address problems.

Typical problems:

  • Traces and pads merging
  • Traces running underneath components

Fixes:

  • Move components in the board layout
  • Reduce pad sizes (don’t change pitch!)
  • Use a different milling bit
  • Lie to the machine about which milling bit you’re using

Save
Once it’s working, hit save to generate RML file. (A similar process to generate RML file for cutting outline.) Save the rml file somewhere you can find it. Rml looks like this; very simple instructions:

PA;PA;VS4;!VZ4;!PZ0,200;!MC1;
PU350,4075;
Z350,4075,-10;
Z304,4077,-10;
Z276,4097,-10;
Z262,4129,-10;
Z264,4167,-10;
Z282,4193,-10;
Z306,4207,-10;
Z336,4211,-10;
...

3. Set up the milling machine (docs)

Prepare bed

  • Sacrifical material
  • Must be flat and clean

Attach board to bed

  • Double-sided tape
  • Got to get board flat - secure taping, and remove warps

Select tool

  • Know what tool is in what cartridge (1-6).

Set x, y, z zero

  • Set x and y home (allow some space, but make your your design fits on your board)
  • Lower tool to surface, set z0. Always spin the tool when setting z

4. Back to the computer: Send your rml file to the mill.

Open your rml file in VPanel.

vpanel.png

When you hit output, the machine will start milling. Stand by in case something goes wrong.

5. Mill it

Keep an eye on things!

6. Cut outline

  • Similar process to cut outline. See docs for more detail.

7. Clean and check board

  • Deburr board with abrasive sponge
  • Clean skin oils off copper
  • Verify your traces look correct (clean, not shorting, right size)

Design rules

Consider the machine, tools, materials and process when designing your circuit (next week).

  • Machine has a level of stability ( = accuracy)
  • Tools are round (how to cut a right angle?)
  • Tools have a thickness (how close can traces be?)
    • Neil: 15mil (mil = 1/1000 of an inch, not millimetre - see Eagle)
  • Some tools are conical, so diameter varies (what if your board or bed isn’t flat?)

Now we have the traces on a board. What next?

Components

Surface-mount (SMD) - picture

Through-hole - picture
Often used by hobbyists - not us. Becoming obsolete.

Chip scale - picture

Soldering

Guide: Adafruit Guide To Excellent Soldering

Videos:
SMD Soldering - the easy way - YouTube (Arthur Guy)
EEVblog #186 - Soldering Tutorial Part 3 - Surface Mount (enthusiastic Aussie, unavoidable)

Remember:

  • You want shiny, smooth joints
  • Keep soldering iron tip shiny and clean
  • Melting solder wets the joints
  • Heat both parts of the joint with the iron
  • Feed in small amount of solder - helps conduct heat
  • Wait
  • Once parts are at the right temperature, more solder will flow up and down parts
  • Wait, then remove iron

Recognising a bad joint

Common Soldering Problems - Adafruit Guide To Excellent Soldering

Picture

Techniques

Holding the board

  • Tape
  • Vice

Holding the part

  • Tacking
  • Note orientation of chips, capacitors, diodes, etc.
  • Order of stuffing - inside out

Desoldering

  • Braid - tin the braid first to help heat flow
  • Pump
  • Hot air gun - use directional tip (plus tweezers and gravity). Clean up with braid afterwards.

Safety

Fumes - an issue if you’re soldering for several hours
Leaded solder - Wash your hands!


Assignment

Mill and stuff an ISP programmer. Follow this design and instructions: Building the FabTinyISP
To really verify that is works, load the board with the programming software.
(What is an ISP programmer, and why do I need one?)

What do I need to do to pass?

Mill and stuff a board that passes a visual inspection (good traces, viable solder joints, correct components in the right places).

(If there isn’t enough time for everyone to mill a board, we may do this in batches.)

Document all your work on your student blog, with photos and videos to show what you did, what went wrong, and how you fixed it. Cite external sources where you have used someone else’s work.

Extra credit

  1. Mill a board and teach someone else how to operate the machine
  2. Test the board you make with oscilloscope or the multimeter and document what you find
  3. Test that the board really works by verifying it does what it’s supposed to do (i.e. accept a program loaded onto it)

For next week

You will need access to a functioning programmer for next week.
Ideally, it should be one that you have made.


Materials we need this week

  • Cutting files for programmer: http://fab.cba.mit.edu/classes/863.16/doc/projects/ftsmin/index.html
  • FR1 for milling
  • Some pre-milled boards WIP: make these
  • USB extension cables (for USB key-style programmer boards)
  • Components:
    • 1x ATtiny45 or ATtiny85
    • 2x 1kΩ resistors
    • 2x 499Ω resistors
    • 2x 49Ω resistors
    • 2x 3.3v zener diodes
    • 1x red LED
    • 1x green LED
    • 1x 100nF capacitor
    • 1x 2x3 pin header

Failsafe for next week

Make some working programmers so students can use them to program their boards