Lesson · Problem 17
Carry 3 Amps Safely
Current, heat, and copper area
You should be able to
- Relate trace resistance to length, width, and copper thickness.
- Keep heatsink copper on ground and off the supply pad.
Power · Routing
What it is
Resistance of a trace grows with length and shrinks with cross-section. Cross-section is width times copper thickness. Thermal vias and extra copper area give heat a path into the board. This challenge grades the resistance, the copper area, and the via count it states, using the course geometry rather than a lab temperature rise.
Why this matters
Three amperes in a thin trace drops voltage and heats the copper. Width, length, and extra copper are different knobs.
What you are building
A 12 V trace sized for the resistance limit, and ground copper with vias on the exposed pad.
Prerequisites
This lesson uses trace width, copper cross-section, via, net, pad. It introduces current density, trace resistance, voltage drop, thermal via, thermal spreading.
Component guide
What the parts are. The requirements panel is still the list that is graded.
Wide trace
- What it does
- Copper whose cross-section is width times thickness. Resistance falls as that area grows.
- Why it is here
- Three amperes in a thin trace drops voltage. Width and length are both knobs.
- Symbol
- A trace on the 12 V net, separate from the ground pad.
- Constraints
- The resistance limit uses the course formula, not a lab temperature.
- Beginner mistake
- Pouring 12 V onto the exposed ground pad and calling it a heatsink.
- What an engineer checks
- The current is context. The graded numbers are width, length, area, and via count.
Thermal via
- What it does
- A via used to move heat into another layer.
- Why it is here
- On this board those vias are on the exposed ground pad, not on the 12 V trace.
- Symbol
- Vias inside the ground pad.
- Beginner mistake
- Putting the vias on the supply copper.
- What an engineer checks
- The via count and which net the pad is.
Interview lens
Resistance is ρL / (width × thickness). Say which net the heatsink is. Here it is ground.
What PCBGrade measures
That resistance, the copper area, and the via count. Not a temperature rise and not CFD.
Where this shows up
A connector that runs hot is often a thin trace or a heatsink pad tied to the wrong net.
Terms
- Thermal via
- A via used to move heat into another layer, not to complete an unrelated signal.
- Trace resistance
- The electrical resistance of that copper path. It grows with length and shrinks with cross-section.
- Voltage drop
- Current times that resistance. Width is one knob. Length is the other.
What is happening electrically
Current in a trace crowds into the copper’s cross-section. A narrower trace has higher current density, higher resistance, and more heat for the same current. Resistance is ρL / (width × thickness). The voltage drop is that resistance times the current. Heat does not stay in the trace: it spreads into the board, into nearby copper, and through vias into other layers. A via array under a hot pad is a thermal path, not a signal trick.
Why the geometry matters
Widen the supply path, or shorten it, until the resistance meets the limit. Width alone is not the whole problem: a wide trace that is very long can still drop too much, and a wide pour on the wrong net is a short. The heatsink copper in this challenge is ground, on the exposed ground pad, with ground vias. Pouring the supply onto that pad ties the supply to ground.
How an engineer reasons
Compute the path you drew, not the path you meant. Length, width, and the stated thickness give the resistance. Then look at the thermal pad and ask which net it is. Extra copper only helps the heat if it is the net the pad already is. Doubling width halves resistance only when the length and the thickness stay the same. A via array under the pad spreads heat into the plane. It does not widen the 12 V trace.
Worked example
At 35 µm copper, resistance is ρL / (width × thickness). A 50 mm trace that is 0.50 mm wide is about 50 mΩ. Doubling the width cuts that resistance in half. The challenge publishes its own length and its own resistance limit. The thermal region is ground copper and ground vias on the exposed pad, which is a different net from the 12 V trace.
Good and bad
A short, wide 12 V trace under the resistance limit, plus ground copper and ground vias on the exposed pad, separates the electrical path from the thermal path. A long 0.25 mm trace fails the resistance even if it fits the gap. A 12 V pour that covers the ground pad can look like “more copper for heat” and is a supply-to-ground short.
Common mistake
Pouring 12 V over the exposed ground pad, or using a long 0.25 mm trace because it fits the gap.
Where this rule stops
The resistance uses the course copper model. It is not a lab temperature rise and not an IPC chart. Real heating also depends on copper weight, nearby planes, and airflow. The principle that survives is: cross-section and length set the drop, and thermal copper has to be on the net that is hot. Current density is the current divided by that cross-section. A wide short trace and a narrow long trace are different problems even when both of them connect the pads.
Before the challenge
The challenge limits resistance of the supply path and asks for ground copper and vias on the exposed pad. Those are different nets.
Ready for the challenge
You can widen the 12 V path without pouring 12 V onto the ground pad.
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