Place the parts

Classroom resources

Load these into Waypoints Prepare: choose Load URL and paste the copied address, or download the file and import it. In Prepare, the worksheet's Teacher edition shows the answers; the student copy hides them.

• Slides Download Copy address for Prepare Present from the browser, or export to PowerPoint or PDF.
• Worksheet Download Copy address for Prepare Print the student copy; the Teacher edition has the answers.

Students will be able to

• I can explain what a net and a ratsnest line are.
• I can place parts so the joins are short and the connector is at the edge.
• I can measure a board in millimetres from its grid.

Timing

Predict and Try 10 min · Worked example 8 min · Vary 10 min · Practise and Make 12 min

Before class

Do Arduino › From simulator to breadboard (or any breadboard circuit) first: this course turns a breadboard circuit into a board.

The board is a teaching model of a real PCB tool (KiCad, EasyEDA): a 2.54 mm grid, through-hole parts, nets, a ratsnest. Real tools add footprints for every part, a schematic that makes the nets, and surface-mount parts on finer grids. The Capstone project Design a PCB uses a real tool.

Curriculum hooks: Design and Technologies, engineering principles and systems, and materials and technologies; Digital Technologies, digital systems (hardware).

Running the lesson

1. Predict: show a photo of a real PCB and ask where the copper goes.
2. Try: have students say out loud which pads light up for GND.
3. Worked example: model the order on the board: connector, signal path, the many-net part, gaps, numbers.
4. Make: compare layouts; whose was shortest, and what did they place first?

The hard part (on purpose)

Students move parts at random. Ask them to fix the connector first, then place one part at a time, watching the ratsnest total.

When students get stuck

Answers to the checks

1. where to put the parts: Which placement will be easiest to join up with copper?
   Close together, in signal order. Every ratsnest line becomes copper, and crossing lines become traces that must find a way round each other.
2. A net: Click any pad on the GND net (J1's bottom pin is one). Which pads light up?
   
   GND joins J1.4, R2 and D1: three pads that must end up joined.
3. Count the pads: How many pads are on the GND net?
   3
   J1.4, R2.2 and D1.2.
4. Move a part: Click D1 to select it, then click an empty cell close to R1 to move it there. Watch the ratsnest total.
   
   D1 shares a net with R1 (LEDA): closer together, a shorter line.
5. Placement, step by step: A ratsnest line runs 10 cells straight across. How long is it, in mm?
   25.4
   10 × 2.54 = 25.4 mm.
6. Why the edge?: Why must J1 sit at the edge of the board?
   A cable or a micro:bit plugs into it, so it must be reachable
   Connectors, switches and LEDs go where people need them: at the edge, or facing out. Then the rest is placed around them.
7. Vary: meet the brief: Get the total ratsnest to 100 mm or less, with every part on the board, none touching, and J1 at the edge.
   
   Short joins, a gap between parts, J1 at the edge: the board is ready to route.
8. Millimetres: Two pads are 4 cells apart. How far apart are they, in mm?
   10.16
   4 × 2.54 = 10.16 mm.
9. Joins: Three pads are on the same net. How many joins does the ratsnest need?
   2
   Joining three things in a chain takes two links: the ratsnest uses the shortest two.
10. Gaps: Why leave at least one empty cell between parts?
    So traces can pass between them and the parts can be soldered
    Copper runs between parts, and a soldering iron needs room.
11. A small board: On a board 12 × 8 cells (about 30 × 20 mm), get the ratsnest to 70 mm or less, with every part on the board, none touching, and J1 at the edge.
    
    A small, tidy layout: cheaper to make, and easy to route.

Solutions

One possible solution for each task. Other correct programs are fine.

Vary: meet the brief

Open task: any program that does what the step asks.

a smaller board › A small board

Open task: any program that does what the step asks.

a smaller board › Evaluate

Open task: any program that does what the step asks.

Stretch and support

Stretch: Rotate J1 so it points out of a different edge. How does the best layout change?

Support: Give the placement order as a checklist, and the target as a number to beat by steps of 20 mm.

Exit ticket

Name two rules of thumb for placing parts on a board, and say why each helps.

Connectors at the edge, so cables reach them; parts that share nets close together, in signal order, so the copper is short and doesn't cross; leave gaps between parts for copper and soldering.

What each step is for

• where to put the parts: Placement decides most of how hard routing will be. Committing first makes the point land: short, uncrossed ratsnest lines now mean short, easy copper later.
• Try: nets and moves: Clicking pads shows what a net is; moving one part and watching the total ratsnest drop shows what placement is for.
• Worked example: read the board: The facts line is the designer's checklist: ratsnest length, parts on the board, a gap between parts, connectors at the edge. Converting cells to millimetres keeps the grid honest.
• Vary: meet the brief: A target turns placement into a design problem with a measure: students move and turn parts until the board meets every criterion.
• placement: Millimetres from cells, why parts need a gap, and what a net is.
• a smaller board: A smaller board forces real trade-offs: every part must still fit, with gaps, and the ratsnest must be shorter still.

Where it leads

Next: Route the traces.

Real tools: KiCad (free, open source), EasyEDA (in the browser, linked to a board maker). The Capstone projects' Design a PCB uses one.

Sources

• KiCad documentation: PCB editor (benchmark for the model; wording our own) (checked 12 October 2026)