Students route copper traces cell by cell, find that a trace can't touch another net (a short circuit), route the shield on one layer with a detour, then on two layers with a trace on the bottom, and compare the copper.
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Students will be able to
I can route copper traces that join every net without a short circuit.
I can explain when a trace goes round and when it goes to the other layer.
I can explain what a via is for.
Timing
Predict and Try 10 min · Worked example 8 min · Vary 12 min · Practise and Make 15 min
Before class
The board model: one trace per grid cell per layer, through-hole pads on both layers, steps of one cell (no diagonals). Real tools route at angles of 45°, check gaps (clearance) between copper, and route at finer grids; KiCad and EasyEDA both have an interactive router that pushes other traces aside.
Keyboard: the board can be focused (Tab); arrow keys move a cursor and Enter acts on the cell.
Curriculum hooks: Design and Technologies, engineering principles and systems; Digital Technologies, digital systems (hardware).
Running the lesson
Predict: hold up two wires and ask what happens if two bare copper strips touch.
Try: when GND is blocked, stop and ask for both answers (round, or under).
Vary: the one-layer route takes a while; let early finishers try a different GND path.
Make: compare copper lengths across the class.
The hard part (on purpose)
Routing takes patience: one cell at a time, and some nets block others. Suggest routing the short nets first and using Undo freely.
When students get stuck
What you will see
A question to ask
The fix
A trace won't start.
Did you click a pad (or a trace) of a net?
Start on a pad.
A step is refused.
Is that cell next to the last one, and free of other nets on this layer?
Go round, or switch layer.
A net still has a thin line after a trace finished.
How many pads does the net have?
Start a new trace from a joined pad to the next one.
Answers to the checks
crossing: GND must get from J1 to R2 and D1, but the OUT trace is in the way. On a board with copper on one side only, what can you do? Go round. Copper traces are bare metal, so touching joins them. With two layers there is a second answer: go under, on the bottom.
VCC: Route VCC: click J1's top pad, then step to S1's left pad.
One net done: its thin ratsnest line has gone.
OUT, then GND: Route OUT (J1's third pad to R1's left pad), then try to route GND (J1's bottom pad) towards R2 on the top layer.
OUT blocks the direct way: GND must go round, or change layer.
What happened?: What happened when GND's trace reached OUT's copper? The board refused: it would be a short circuit Touching copper joins nets. Here that would short GND to OUT.
Two ways past OUT: A detour adds 9 cells of copper. How many extra mm is that, to 1 decimal place? 22.9 9 × 2.54 = 22.86 ≈ 22.9 mm.
When you need a via: A trace on the bottom must end at a surface-mount pad, which is only on the top. What does it need? A via near the pad, to come up to the top Through-hole pads go through both layers. Surface-mount pads sit on one side, so a trace on the other side needs a via.
Vary: one side only: Route every net on the top layer only.
Possible, with a long way round for GND: about 119 mm of copper.
Short: Two traces of different nets touch. What is it called? A short circuit Touching copper joins nets: a short circuit.
Via: What is a via? A plated hole that takes a trace from one layer to the other A via joins copper on the top to copper on the bottom.
Copper: A board's traces are 52 cells long in all. How much copper is that, in mm, to the nearest mm? 132 52 × 2.54 = 132.08 ≈ 132 mm.
Short copper: Route every net, using both layers, with 100 mm of copper or less.
GND under OUT on the bottom saves the long way round.
Solutions
One possible solution for each task. Other correct programs are fine.
Vary: one side only
Open task: any program that does what the step asks.
route it short › Short copper
Open task: any program that does what the step asks.
route it short › Evaluate
Open task: any program that does what the step asks.
Stretch and support
Stretch: Route the shield so GND is a wide area of copper on the bottom (a ground plane, many cells). Why do real boards do this?
Support: Give the VCC and OUT paths as cell lists to follow, then let students route GND.
Exit ticket
Two nets must cross on a board. Give two ways to do it, and one cost of each.
Go round on the same layer (longer copper, more space) or go to the other layer (a two-layer board; vias if surface-mount pads are involved).
What each step is for
crossing: Students know wires can cross; copper can't. Committing to a plan for a crossing sets up the two answers routing gives: go round, or go under.
Try: route on the top: Routing a simple net shows the mechanics; routing GND on the top only shows the cost of a crossing: a long detour.
Worked example: round or under?: The worked example compares the two answers by copper length and by layers. The tasks turn cells into millimetres and say when a via is needed.
Vary: one side only: Routing everything on one layer forces the detours, and makes the value of a second layer obvious.
routing: What a short is, what a via does, and measuring copper.
route it short: With both layers allowed, students aim for the shortest copper: the two answers to a crossing, chosen net by net.
Where it leads
Next: Design rules and ordering.
Real tools route at 45°, check clearance between copper, and pour ground planes; the Capstone project Design a PCB uses one.