A preheater warms the whole board, or a large area of it, from below while you work from above. It sounds like a luxury, and for small resistors and capacitors it is. But on large parts, on ground planes and in BGA work it changes everything: parts release faster, pads lift less, boards warp less and nearby components suffer less. This guide explains why preheating works, what kinds of preheater exist, and when you really need one.
Why preheating helps
When you heat a part from above only, the heat flows away into the board's copper and the board's cold underside. To melt the joints you must push more heat, for longer, which:
- overheats the part's top and its neighbours;
- creates a big temperature difference across the board, which can warp it;
- risks lifting pads and damaging the board's layers.
If the board is already warm from below, the top heat only has to cover the last part of the temperature rise. The part releases sooner, at a lower nozzle temperature, with less stress everywhere. The tool is described under preheater.
Types of preheater
| Type | How it heats | Strengths | Weaknesses |
|---|---|---|---|
| Infrared (IR) | Ceramic or quartz emitters radiate heat | Even heat over large areas, no air flow | Dark and light surfaces absorb differently |
| Hot plate | A flat heated surface | Simple, very even for flat boards | Poor for boards with parts on the underside |
| Hot-air bottom heater | A large, gentle air nozzle under the board | Works with parts on both sides | Can move small bottom-side parts if airflow is high |
For laptop boards, which carry parts on both sides, IR and hot-air bottom heaters are the most practical. Hot plates suit flat, single-sided areas.
Temperatures
The aim is to warm the board, not to melt anything. Preheating temperatures are kept well below the melting point of the solder, high enough to reduce the work the top heater must do, and low enough that parts on the underside stay attached and plastic parts survive. Measure, do not guess: a thermocouple taped to the board near the work area shows the real board temperature, which can differ a lot from the preheater's display.
Raise the temperature gradually rather than switching the preheater on at full power with the board on it.
Supporting the board
- Hold the board in a frame or holder that supports its edges and keeps it flat.
- Leave space underneath for heat to reach it evenly.
- Do not let the board sag in the middle; a warm board flexes more easily.
- Keep the area under the board clear of parts that could be knocked off.
Protecting heat-sensitive parts
Before preheating, remove or protect:
- batteries — never preheat a board with a battery attached;
- plastic connectors and cables;
- electrolytic capacitors and other temperature-sensitive parts close to the hot area;
- labels and stickers, which may scorch.
Shielding methods are covered in Heat Shielding: Kapton Tape, Aluminium Tape and Other Tricks.
When you need a preheater
| Job | Preheater? |
|---|---|
| 0402 and 0603 resistors and capacitors | Rarely needed |
| Power MOSFETs and inductors on ground planes | Very helpful |
| Connectors with large anchor tabs | Very helpful |
| Chips with large thermal pads | Very helpful |
| BGA chips — removal, reballing, refitting | Essential |
| Boards with heavy copper and many layers | Very helpful |
For BGA work, controlled preheating is part of the process, as described under bga rework.
Using a preheater with hot air
A typical combined routine:
- Prepare: flux, shielding, board in its holder.
- Preheat until the board near the work area reaches the planned temperature.
- Apply hot air from above, as described in Hot Air Station Basics for Laptop Repair.
- Lift the part when all joints are molten.
- Let the board cool slowly, not by blowing cold air on it.
Fresh flux remains important: preheating dries flux over time, so apply it shortly before the top heating, or add more. Flux choice is covered in Flux, Solder and Wick: Consumables for Board Repair.
Preheating for soldering, not only for removal
Preheating helps when fitting parts as well. A new power stage or connector on a heavy ground plane solders more evenly when the board is already warm: the solder wets the whole pad at once, the part settles flat, and there is less risk of cold joints on the ground connections. The same applies to tinning large pads before fitting a chip.
Cooling down
Cooling too fast stresses joints and the board. Turn the preheater down gradually or let the board cool in its holder. Do not move a hot board around; it is softer and easier to damage while hot.
Measuring the board temperature
A thermocouple is cheap and turns preheating from guesswork into a process. Tape the probe to the board close to the work area with a small piece of polyimide tape, on the top side, away from the nozzle's direct airflow. Watch it rise during preheating and note the temperature at which you start top heating. Record that for the type of board; next time, the same numbers give the same result. Many multimeters accept a thermocouple, so no special instrument is needed.
Preheating and moisture
Boards and some components absorb a little moisture from the air over time. Rapid heating can turn that moisture into steam inside the board or a chip package, causing small bubbles or internal damage. A gentle, gradual preheat drives moisture out slowly and reduces the risk. Boards that have been through liquid damage and cleaning should be thoroughly dried before any rework for the same reason.
Safety around a preheater
A preheater stays hot long after it is switched off. Keep it in a fixed place with clear space around it, never leave cables or flux bottles resting on it, and let it cool before covering or moving it. Boards come off hot, so use a heat-resistant mat to set them down.
Budget options
A dedicated IR or hot-air preheater is the best option. On a tight budget, some technicians use a hot plate with a support frame for flat areas, or a second hot air station on a stand pointing upwards at low airflow. These improvised methods work, but need extra care to avoid uneven heating and moving bottom-side parts.
Example: an inductor that would not move
A shorted power stage sits under a large inductor on a multilayer board with heavy ground planes. With hot air alone, the inductor does not release even after a long heat, and the board around it begins to discolour. With an IR preheater warming the board from below and a thermocouple confirming the board temperature, the inductor lifts after a short heat from above at a lower nozzle temperature. The power stage comes out cleanly underneath, and the board shows no discoloration.
FAQ
Do I need a preheater for laptop repair?
Not for small passive parts. For large parts, ground planes and BGA work, it makes a big difference.
What temperature should I preheat to?
Well below the solder's melting point. Measure the board with a thermocouple rather than trusting the display.
Can I preheat a board with the battery attached?
No. Always remove the battery first.
Which type is best for laptop boards?
IR or hot-air bottom heaters, because laptop boards have parts on both sides.
Why did parts fall off the underside?
The board got too hot from below, or the bottom airflow was too strong.
Bottom line
A preheater warms the board from below so the top heat has less to do. Use it for large parts, ground planes, connectors and BGA work; measure the board's real temperature; support the board flat; protect batteries, plastics and sensitive parts; and let the board cool slowly.