Students set trace widths against current from a table, find a trace too wide to squeeze between header pins and route it around, then work out the cost of a board order (setup plus per board).
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Students will be able to
I can choose a trace width from the current it carries.
I can explain why two design rules can force a longer route.
I can work out what an order of boards costs, per board.
Timing
Predict and Try 8 min · Worked example 10 min · Vary 8 min · Practise and Make 14 min
Before class
The width table is IPC-2221 for 1 oz copper on an outer layer with a 10 °C rise, rounded down. Real design tools have calculators for this; the numbers change with copper thickness, inner layers and the rise you accept. The 0.4 mm squeeze assumes 2.54 mm pin spacing, 1.7 mm pads and 0.2 mm clearance each side.
Board makers' prices change; the lesson uses round numbers. Check a current price before ordering for a class.
Curriculum hooks: Design and Technologies, materials and technologies (properties of copper), engineering principles and systems; Science, electric circuits (Year 9); Maths, rates and costs.
Running the lesson
Predict: show a photo of a burnt trace, if you have one.
Try: ask why SIG can stay thin.
Worked example: point at both rules on the board: the width table and the gap between pins.
Make: discuss what else a design rule check catches (hole sizes, gaps between copper, unrouted nets).
The hard part (on purpose)
Students want every trace wide 'to be safe'. Ask what it costs in space, and where the trace has to go.
When students get stuck
What you will see
A question to ask
The fix
A trace still fails after widening.
Does it squeeze between pins?
Route it around, then widen it.
A student picks the first width that looks big.
What does the table say that width carries?
Find the narrowest width whose current is at least the net's.
A student divides the setup cost wrongly.
What does the whole order cost?
Setup + boards × price, then divide by the number of boards.
Answers to the checks
which trace burns?: Every trace is 0.25 mm wide. The motor draws 1.5 A. Which trace is most likely to overheat? VM (and GND, MA, MB): they carry 1.5 A. SIG carries a few thousandths of an amp, so a thin trace is fine for it.
Widths: Set each net's width so every trace passes. Use the table under the board.
1.5 A needs 1 mm (safe to 2.4 A); 0.5 mm only carries 1.4 A. SIG stays at 0.25 mm.
Why not wide?: Why not just make every trace 2 mm wide? Wide traces take room: they can't pass between close pins, and the board grows Width costs space. Use what the current needs, with some margin, and keep signals thin.
Choose the width: A heater trace carries 2.8 A. Which is the narrowest width that passes? 1.5 mm 1.5 mm is safe to 3.2 A, which is at least 2.8 A.
Two rules meet: MA must carry 1.5 A, and its shortest path squeezes between two header pins (0.4 mm of room). What should the designer do? Route MA around the header, at 1 mm wide Two rules meet: width for current, and room between pins. The longer route around satisfies both.
Vary: a squeeze: In this version, MA's path squeezes between header pins. Make every trace pass: widths, and a path for MA.
MA at 1 mm, routed around the header; SIG stays thin and can squeeze through.
Width: A trace carries 0.9 A. Which is the narrowest width that passes? (0.25 mm 0.8 A · 0.5 mm 1.4 A · 1 mm 2.4 A) 0.5 mm 0.25 mm is safe only to 0.8 A; 0.5 mm to 1.4 A.
An order: A board maker charges $20 to set up an order and $1.50 a board. What do 10 boards cost, in dollars? 35 20 + 10 × 1.50 = $35.
Per board: Same prices: what does each board cost when you order 40, in dollars? 2 20 + 40 × 1.50 = $80, and 80 ÷ 40 = $2 a board. The setup cost is shared by more boards.
Pass the rules: Make every trace pass the design rules.
Ready for the design rule check in a real tool, then the files the maker needs (Gerbers).
Order: Your class needs 24 boards. The maker charges $20 to set up and $1.50 a board. What is the cost per board, in dollars? 2.33 20 + 24 × 1.50 = $56, and 56 ÷ 24 ≈ $2.33 a board.
Solutions
One possible solution for each task. Other correct programs are fine.
Vary: a squeeze
Open task: any program that does what the step asks.
ready to order › Pass the rules
Open task: any program that does what the step asks.
ready to order › Order
Open task: any program that does what the step asks.
ready to order › Evaluate
Open task: any program that does what the step asks.
Stretch and support
Stretch: Some makers charge less per board for more boards (a price break). How would you decide how many spares to order?
Support: Give the width table on paper with a column to write each net's current.
Exit ticket
A trace carries 2 A. Which width (0.5 mm 1.4 A, 1 mm 2.4 A, 1.5 mm 3.2 A) do you choose, and why not the others?
1 mm: the narrowest safe for 2 A; 0.5 mm would overheat, and 1.5 mm takes more room than needed.
What each step is for
which trace burns?: Students think of all wires as the same. Committing to which trace overheats ties width to current before the table appears.
Try: set the widths: Students use the table (in the board's note) to choose each width, and see SIG stay thin.
Worked example: width from current: The worked example reads the table with a margin, then checks the second rule: a trace between two header pins has only 0.4 mm of room.
Vary: a squeeze: Two rules in one board: students set widths and choose a path for the squeezed net.
rules and orders: Width from the table, and the cost of an order: a fixed setup cost plus a cost per board.
ready to order: Students fix every rule on the board, then reason about the order: the full move from design to manufacture.
Where it leads
Next: the Capstone projects' Design a PCB, where students design, check and (optionally) order a real board.
Real tools: KiCad's and EasyEDA's design rule check; board makers' instant quotes.