A trace that carries a motor's current must be wider than one that carries a signal, or it heats up. Students set trace widths against current, meet a trace too wide to pass between pins, then work out what an order of boards costs.
The motor board works on the bench, then a trace turns brown and lifts off. Why that trace, and not the others?
A trace is a strip of copper, so it has resistance: the more current through it, and the thinner it is, the hotter it gets. Design rules are the limits a board must meet before it is made: trace width for current, gaps between copper, hole sizes. The design tool checks them all (a design rule check, DRC).
This board drives a motor. VM brings the motor's power in from J1, U1 switches it, and MA and MB go out to the motor on J2. SIG is the control signal from J3.
Predict: Read the problem and decide what you think before you try anything. A wrong guess is fine: it shows you what to look for.
Try: Try it on the board and compare the result with your prediction.
Investigate: Work out why it happens, one step at a time.
Modify: Change the problem on purpose and see what happens.
Practise: A few on your own, to check you have it.
Make: Make your own problem using what you have learned. Expect a few tries; that is normal.
Make your prediction first. Help opens once you've locked it in.
PredictRead the problem and decide what you think before you try anything. A wrong guess is fine: it shows you what to look for.
Which trace burns?
TryTry it on the board and compare the result with your prediction.
Try: set the widths
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Set each net's width so every trace passes. Use the table under the board.
Use the board. This ticks when you get there.
InvestigateWork out why it happens, one step at a time.
Worked example: width from current
Worked exampleBefore each step, say what you think comes next.
VM carries 1.5 AFind the current each net carries (the motor's datasheet gives it).
0.5 mm→1.4 A<1.5×Too thin: it would run hot.
1 mm→2.4 A≥1.5✓The narrowest width that passes, with a margin.
between header pins: ≤0.4 mmA second rule: the gap between pins leaves room for 0.4 mm only.
1 mm>0.4 mm⇒route aroundA trace that needs 1 mm can't squeeze between pins, so it takes the long way.
ModifyChange the problem on purpose and see what happens.
Vary: a squeeze
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In this version, MA's path squeezes between header pins. Make every trace pass: widths, and a path for MA.
Use the board. This ticks when you get there.
PractiseA few on your own, to check you have it.
Rules and orders
A board maker charges $20 to set up an order and $1.50 a board. What do 10 boards cost, in dollars?
Same prices: what does each board cost when you order 40, in dollars?
MakeMake your own problem using what you have learned. Expect a few tries; that is normal.
Ready to order
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Make every trace pass the design rules.
Use the board. This ticks when you get there.
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?
For teachers
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). About 40 minutes.
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.
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
A trace still fails after widening. Ask: Does it squeeze between pins? Fix: Route it around, then widen it.
A student picks the first width that looks big. Ask: What does the table say that width carries? Fix: Find the narrowest width whose current is at least the net's.
A student divides the setup cost wrongly. Ask: What does the whole order cost? Fix: Setup + boards × price, then divide by the number of boards.
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.
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.
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.
Now you can
Choose a trace width from the current it carries.
Explain why two design rules can force a longer route.
Work out what an order of boards costs, per board.