Once you have narrowed a short to a branch of a rail, the last step is finding the exact part. The principle is the same for every method: push a controlled current into the short and look for the part that turns that current into heat. What differs is how you see the heat. A thermal camera, freeze spray and rosin are the three classic tools, and each has situations where it is best.
The common setup
All three methods depend on voltage injection:
- Disconnect the battery and all other power.
- Set a bench supply to a voltage at or below the rail's normal voltage — start around 1 V for most rails.
- Set the current limit low, a few hundred milliamps, and connect positive to the rail and negative to ground.
- Raise the current gradually until something warms.
The faulty part dissipates power roughly in proportion to the current squared times its resistance, so a small increase in current makes a large difference in heat. Increase slowly. The injection itself is covered in Bench Power Supply for Laptop Repair: How to Inject Voltage.
Method 1: thermal camera
A thermal camera shows the board's temperature as an image, so the warm part simply appears as a bright spot.
Strengths
- Fast — you see the whole area at once.
- Works at low currents if the camera is sensitive enough.
- Shows parts on both sides if you flip the board.
- No residue to clean.
Limitations
- Shiny metal surfaces reflect heat and read incorrectly; the camera sees the plastic body of a part better than a metal shield.
- Low-resolution cameras struggle with 0201 and 0402 parts packed closely together.
- Cost.
Technique
- Start with a wide view, then move closer once you see the warm area.
- Watch for the first spot to warm up — after a while heat spreads into nearby copper and parts.
- Use the boardview to identify the part at the warm spot.
Whether a camera is worth buying is discussed in Thermal Cameras for Short Detection: Are They Worth It.
Method 2: freeze spray
Freeze spray cools the area to well below room temperature, coating it with a thin layer of frost. When you inject current, the shorted part warms first, and the frost melts there first.
Strengths
- Cheap and effective at locating small parts precisely.
- Works well under a microscope.
- Shows the melting point clearly even in crowded areas.
Limitations
- Consumable and messy; condensation leaves moisture on the board.
- Thermal shock can, in rare cases, crack fragile parts.
- Needs re-spraying for each attempt.
Technique
- Spray the suspect area briefly to form a thin frost layer.
- Inject current and watch under the microscope.
- The first spot where the frost melts is your suspect.
- Let the board dry completely before powering it normally; use isopropyl alcohol and warm air to help.
Method 3: rosin
Rosin (colophony) is solid flux. Heated with a soldering iron, it produces vapour that condenses on the cool board as a thin white layer. When injected current warms the shorted part, the coating melts or evaporates there first, leaving a clear spot.
Strengths
- Very cheap and very sensitive.
- Precise on small parts.
- No special equipment beyond an iron and a block of rosin.
Limitations
- Fumes — work with extraction or ventilation.
- Leaves residue that must be cleaned afterwards.
- Slower to set up than a thermal camera.
Technique
- Touch a hot iron to the rosin block and hold it a short distance above the area so the vapour settles evenly on the board.
- Inject current and watch for the coating to clear.
- Clean the board afterwards with isopropyl alcohol.
Choosing a method
| Situation | Best method |
|---|---|
| Large area, unknown location | Thermal camera |
| Small parts packed tightly, location roughly known | Rosin or freeze spray under the microscope |
| Very low-resistance short (little heat) | Thermal camera with higher current, or rosin |
| Parts under metal shields | Remove the shield first; any method |
| Budget setup | Rosin |
Many technicians combine them: a thermal camera to find the area quickly, then rosin or freeze spray to pinpoint the exact part.
Example: two capacitors side by side
A short has been narrowed to a branch near the CPU with a dozen tiny capacitors in two rows. The thermal camera shows a warm area but cannot separate two neighbouring capacitors clearly — the spot spans both.
The technician coats the area with rosin vapour, injects the same current and watches under the microscope. The coating clears first on the left capacitor and only later spreads to its neighbour. Removing the left one restores a normal reading on the rail. One minute with rosin settled what the camera's resolution could not.
When the warm spot is on the other side
A common surprise: the top side warms in one place, you remove the capacitor there, and the short is still present. Heat travels through the board, and a shorted part on the bottom side directly under a chip or a cluster of vias can make the top side warm first.
- Check both sides early. After the first warm spot appears, flip the board and look at the same area from below before removing anything.
- Use the boardview. Highlight the shorted net and look at what sits on the bottom side under the warm area. Capacitors directly beneath a processor or chipset are frequent offenders.
- Lower the current and watch closely. The true source warms first and hottest; heat that has travelled through copper spreads out and lags behind.
- Mind the injection leads. The clip and the pad you injected into also warm slightly. Keep them away from the area you are watching.
Mistakes to avoid
- Too much current too fast. Heat spreads, the wrong part looks warm, and healthy parts can be stressed.
- Injecting above the rail voltage. It can damage chips on the rail.
- Forgetting the other side. The shorted part may be on the bottom, directly under the spot that warms on top.
- Not confirming. Always measure the rail again after removing a part.
Safety
- Ventilate when using rosin; avoid breathing the fumes.
- Use freeze spray in short bursts and keep it away from your skin and eyes.
- Keep injection current limited, and never inject with the battery connected.
- Let the board dry fully after freeze spray before applying normal power.
FAQ
Do I need a thermal camera to find shorts?
No. Rosin and freeze spray find shorts reliably and cheaply. A camera makes the process faster, especially across large areas.
Why does nothing warm up even at higher current?
Either the short has extremely low resistance, so little heat is produced in it, or it is somewhere you are not looking — another side, under a shield, or inside a large chip. Split the rail further or increase current carefully.
Can I use my finger to feel for heat?
Some technicians do on low-voltage injection, but it is imprecise and parts can become hot enough to burn. The methods above are more reliable.
Is the rosin residue harmful?
Rosin itself is not corrosive in normal conditions, but clean it off so it does not trap dirt or hide future problems.
Which side of the board should I inject from?
Anywhere on the rail works electrically. Choose a large pad that is easy to clip to and keeps the leads away from the area you want to watch.
Bottom line
Inject a limited current at a safe voltage, then let heat reveal the part — on a thermal camera for speed, with freeze spray or rosin for precision. Raise current slowly, watch for the first spot to change, check both sides of the board and always measure again after removing a part. The overall method is in How to Find a Short Circuit on a Laptop Motherboard.