Hot air rework is surgery: a focused stream reflows one component while sparing its neighbours. With the right settings and patience, QFNs and connectors come off clean and go back on invisibly.
At a glance: 6 minute guide · part 9 of 10 in the tools and equipment complete guide track · includes a worked example and a quick-reference table.
Settings before surgery
Here is the working theory in one pass. Match the nozzle to the part (cover all joints without roasting the block), 340–380 °C typical for lead-free, moderate flow. Preheat the board gently from below or with wide passes a cold board warps while a local spot screams.
| Step | Setting/Action | Watch for |
|---|---|---|
| Preheat | ~150 °C board | No sudden steam/pop |
| Flow | Moderate, not hurricane | Blowing small parts away |
| Nozzle | Part-sized | Neighbour protection |
| Remove | Nudge test, lift clean | Never pry |
| Replace | Flux + settle | Self-alignment, bridges |
Removal technique
Circle the part with steady airflow for 20–60 s, nudge with tweezers at intervals when the solder lets go, the part lifts with zero force. Never pry: pads and traces come with the stubborn ones. Wick the pads clean afterwards.
Replacement
Flux the footprint, align the new part (surface tension does the fine alignment during reflow). Reheat with the same circular pattern until it settles. Inspect under magnification for tombstones and bridges before declaring victory.
How to apply this in your build
Work through the sequence below each step assumes the previous one passed. For numbers that need calculating, the linked tools at the end of this guide do the arithmetic instantly.
- Preheat the board before local reflow
- Use the smallest nozzle that covers all joints
- Test readiness by nudging, never prying
- Reflow replacement with generous flux
Worked example
A USB connector replaced at 360 °C with a preheated board lifted off in forty seconds with every pad intact. The same job attempted cold peeled four pads first try. Run the numbers yourself with the related calculator and the result should agree to within rounding.
Practical note from the bench. Rework patience rule: when in doubt, add five seconds of airflow never force. Pads are forever, parts are stock.
Pitfalls that cost real hardware
- Full-blast airflow launching 0402 parts into orbit
- Skipping preheat on thick boards
- Prying “almost ready” parts and donating pads
Key takeaways
- Settings before surgery the foundation of this guide; revisit it if any measurement here surprises you.
- Removal technique the foundation of this guide; revisit it if any measurement here surprises you.
- Replacement the foundation of this guide; revisit it if any measurement here surprises you.
Who this guide is for
Beginners get a single focused topic instead of a whole textbook chapter. It assumes the track’s earlier pages in the tools and equipment complete guide path. Intermediate readers use it as a reference the table, the worked example and the mistake list answer the questions that come up mid-build. If you teach, the structure (theory, application, example, failure modes) maps cleanly onto a lab session.
What you need before starting
Nothing exotic: the parts or tools named in the guide, a multimeter. A notebook for the numbers. Preheat the board before local reflow before you begin the guide assumes it and keep the quick-reference table above within sight while you work through the steps.
Quick reference card
| Aspect | Where to find it in this guide |
|---|---|
| Core theory | Settings before surgery |
| Application steps | How to apply this in your build |
| Worked numbers | Worked example |
| Failure modes | Pitfalls that cost real hardware |
How this fits the tools and equipment complete guide track
This guide is one stop in the structured learning path. Start from the tools and equipment complete guide complete guide for the full map, or continue with SMD iron work and defect guide.
Frequently asked questions
Do I really need preheat? On multilayer boards, yes the ground planes drink heat; local air alone struggles and pads pay.
Can hot air replace a reflow oven? For singles and repairs, thoroughly; for whole boards, stencil + oven wins by hours.
Where do I go next? Back to the tools and equipment complete guide complete guide it indexes every guide in this track and updates as new ones are published.
Continue this track
- Building a foundation? The tools & equipment complete guide maps every step in order.
- Next: Cathode Ray Oscilloscope: Working Principle, Block Diagram
- Next: Logic Analyzers: Watching Digital Buses Talk
- Next: Oscilloscope Triggering: The Skill That Makes Waveforms Stand Still
- Work the numbers: frequency converter · trace width tool
Keep going with this track
- The complete tools & equipment guide: Tools & Equipment complete guide
- Read next: how to use a multimeter: every mode that matters
- Also in this track: oscilloscope triggering: the skill that makes waveforms stand still
- Continue with: probe compensation: the one-minute scope calibration
- Calculate as you go: frequency and wavelength tool · trace width calculator · resistor decoder
- Bookmark this page against the day a measurement surprises you. Most readers return to the table and the mistake list first, and that is the correct order.
Working method notes
When a result here disagrees with your expectation, write down both numbers before changing anything. The gap between predicted and measured is where the real engineering lives. It is usually a tolerance, a parasitic or an assumption that was never checked.
Keep a lab notebook entry for every build in this track. The measured values, the deviations from the guide and the reason for each. Six months from now, those notes are worth more than any tutorial. They describe your bench and your components rather than a general case.
Formulas and checks from this guide
Verification checklist for this track: compensate probes before trusting amplitudes, verify meter fuses before current work. Keep one known-good reference to sanity-check instruments. Calibration you can demonstrate beats calibration you assume.
Bookmark this page against your next build in the track. The checklist above is the same one used across 12 guides in this series.
What the bench taught us
After every move between scopes, and monthly as ritual. It costs one minute.
Warm-up drift is real in both. Swap in a known-good reference to decide which side drifts.
Last updated 23 August 2026