Component Placement with Hot Air

Section Overview

Removal takes a part off; placement is the reverse — and the reason you cleaned those pads so carefully. Hot air placement is setting a new part onto its prepared pads, aligning it, and reflowing all of its joints at once with hot air so the whole part solders into place together (8.4). The sequence mirrors removal in reverse: prepare, place, reflow, and inspect. You start from the clean, flat pads left after removal (8.4; 6.6) and get solder onto the joints — either by tinning the pads and adding flux, or by applying solder paste (a sticky mix of tiny solder particles and flux) to the pads; for a fine-pitch part, a stencil — a thin metal foil with an aperture cut for each pad — lays down the paste evenly and in the right amount. You pick the nozzle and settings for the part (8.2; 8.3) and confirm its orientation and pin-1 mark from what you noted at removal (8.4; 6.1). Then you place the part on the pasted or tinned pads, square and correctly oriented to the silkscreen outline and pin-1 mark (the alignment ideas from 7.5 apply, though here the part rests on tacky paste). Now the reflow: you heat evenly until the solder flashes shiny and wet, and as it melts something wonderful happens — surface tension pulls the part into alignment on its pads, the self-alignment you met with chip passives (6.3). A slightly-off part is drawn square by the molten solder, so you hold the board still and let it settle. But self-alignment only corrects small offsets: a grossly misplaced or wrongly-oriented part will bridge (7.3) or tombstone (7.4) instead, so you place it close to right and never rely on reflow to fix a bad placement. When it is reflowed, you let the joints cool undisturbed — moving them while they freeze makes a disturbed joint (5.4) — and then inspect: every joint wetted, no bridges, no tombstones, pin-1 correct, the part flat (6.7). For a reballing job on a BGA — replacing the balls under the package before placement — and other advanced prep, the chip-replacement chapter goes deeper (Chapter 9). Clean pads, paste and flux, place aligned, reflow and let it self-align, cool undisturbed, inspect — that is hot air placement.

Why This Matters

Placement is the half of rework that puts the new part on — a removal is only useful if you can seat a replacement cleanly, and hot air placement is how you do it for the parts an iron can't. This matters because it completes the repair: removing a failed chip is pointless unless you can solder a good one in its place, so placement is what actually fixes the board. It matters because reflow gives you self-alignment, a real advantage: the same surface tension that stands a chip up when you get it wrong (7.4) pulls a part square when you get it close (6.3), so reflow placement is forgiving of small errors in a way hand-soldering every lead is not — if you understand and use it. It matters because knowing the limit of self-alignment prevents disasters: believing reflow will fix any placement leads to bridged and tombstoned parts, so the "place it close to right" discipline is what makes self-alignment help instead of hurt. It matters because the solder source matters: whether you tin the pads or use paste, and whether a stencil is worth it, shapes how evenly the part reflows — choices that separate a clean placement from a bridged mess. It matters because the cooling rule protects the joints: a placement can be perfect at reflow and ruined by a bump while it freezes (5.4), so the don't-disturb habit is as important here as anywhere. And it matters because inspection closes the loop: a placed part is not done until you have confirmed every joint is sound and the orientation is right (6.7), because a hidden bridge or a backwards chip fails just as surely as no repair at all. Learn to place and reflow a part cleanly, and you can complete a board-level repair from removal to a working replacement.

Required Prerequisites

• Component Removal with Hot Air — Section 8.4 took a part off and cleaned the pads; this section puts a new one on. You should be able to clean pads flat (8.4; 6.6), set the nozzle and settings and heat evenly to reflow (8.2; 8.3), align a part to the silkscreen and pin-1 (7.5; 6.1), and you should know self-alignment (6.3), bridging (7.3), and tombstoning (7.4).

Recommended Consumables

• Solder paste, or fresh solder and flux (Chapter 3) — the solder source for the new joints; paste for fine-pitch, tinned pads plus flux for simpler parts
• A stencil for the footprint, if you have one — to lay down paste evenly on a fine-pitch site
• Flux (Chapter 3) — for even wetting and clean self-alignment
• A scrap board with cleaned sites and sacrificial parts to place — to practice without risking a real board
• Isopropyl alcohol and a brush — to clean flux residue and inspect (6.7)
• Eye protection and ventilation; nitrile gloves for handling paste — paste holds leaded particles and active flux (Chapter 2; Chapter 3)

Recommended Practice Hardware

• A hot air rework station with a nozzle suited to the part (8.1; 8.3)
• Fine tweezers and, ideally, a vacuum pen — to place the part precisely without disturbing the paste (8.4)
• A preheater if you have one — to warm the board and reduce thermal shock (8.2)
• A magnifier and good light (Volume 2, Chapter 9) — to place accurately, watch the reflow, and inspect
• A board holder and an ESD-safe, heat-tolerant surface

Real-World Applications

Placing a part with hot air is the second half of every board-level component swap, done right after the old part comes off. A technician fitting a replacement QFP pastes the pads, sets the chip square to the silkscreen with pin-1 to its mark, reflows evenly, and watches surface tension pull the leads perfectly onto their pads (6.3). A repairer installing a new QFN applies paste, places the part, and reflows — letting self-alignment seat the hidden pad and leads at once. Someone replacing a BGA works from a reballed package or a pasted site, places it to the pin-1 corner, and reflows, relying on the balls self-centering on the pads (Chapter 9). A builder who placed a chip slightly crooked sees it snap square as the solder melts — self-alignment doing its job — and leaves it alone to cool. And someone who placed a chip a full pad off learns the limit the hard way when it bridges (7.3), and after that places every part close to right. The failures placement prevents or causes: a completed, working repair when done right; a bridged, tombstoned, or backwards part when the placement is careless — so the difference is in the preparation, the alignment before reflow, and respecting what self-alignment can and cannot fix.

Common Challenges

• The part bridges after reflow. Too much solder or paste, or the part placed too far off for self-alignment — use less paste (a stencil helps), place it closer to square, and re-wick any bridge (7.3).
• The part tombstones or shifts. Uneven heating or a gross misplacement — heat evenly, place it square, and check the pads are balanced (7.4).
• A joint looks dull or grainy. You disturbed it while it cooled — hold everything still until the solder fully solidifies (5.4).

Safety Notes

Risk Level: Medium. The hot-air hazards all apply, and placement adds solder paste — a material with tiny solder particles and active flux you must handle with care.

Professional Tips Before Starting

• Place it close to right — don't count on self-alignment. Surface tension pulls a slightly-off part square (6.3), but only slightly — place it aligned to the silkscreen and pin-1 first, and let self-alignment finish the job, not do it (7.5).
• Use the right amount of solder. Too much paste bridges, too little starves the joint — a stencil meters it evenly for fine-pitch; for simpler parts, a thin tinning plus flux is plenty.
• Hold still while it cools. A placement perfect at reflow is ruined by a bump as it freezes — keep everything still until the solder is fully solid (5.4).

Setting a New Part with Hot Air

What Hot Air Placement Is

Hot air placement is the technique of setting a new component onto its prepared pads and reflowing all of its joints at once so it solders into place together. It is removal run backward (8.4): where removal melts every joint and lifts the part off, placement sets the part down and melts every joint to fix it in place. The heat is delivered the same way — a controlled, non-contact stream that brings the whole part and all its joints to reflow at once (8.1) — which is exactly why hot air is the tool for placing the same multi-lead and no-lead parts it removes: a quad flat package, a QFN, a ball-grid array, a big IC (6.1). The logic is "set it, then melt every joint together", and because all the joints reflow at the same moment, the part solders uniformly and — crucially — can self-align as the solder melts (6.3). Placement is the payoff of a clean removal: the flat, even pads you cleaned (8.4) are what let a new part seat correctly, so the two halves of a swap are joined at the pads. Understand placement as reflow-in-place: the part goes down cold on a solder source, and hot air melts every joint at once to make it a permanent, self-aligned connection — the completion of the repair a removal only started.

Preparing the Site and Part — Paste, Flux, Settings

A clean placement starts with getting the right amount of solder onto clean pads and setting up for the part. Begin with the pads: they should be clean, flat, and even from the removal cleanup (8.4; 6.6) — a new part cannot seat on lumps. Then get solder onto the joints, by one of two routes. For simpler parts you can tin the pads with a thin, even coat of solder and add flux — the part sits on the tinned pads and reflows into them. For fine-pitch and area-array parts, solder paste is better: a sticky mix of tiny solder particles and flux, applied to the pads, that holds the part in place and reflows into joints. To lay paste down evenly on a fine-pitch footprint, you use a stencil: a thin metal foil with an aperture cut over each pad — you spread paste across it with a squeegee and it deposits a precise, even dab on every pad at once, far more consistently than by hand. Add flux for even wetting (Chapter 3). Then set up the tool: choose a nozzle for the part (8.3) and set temperature and airflow for its thermal mass (8.2) — but keep airflow gentle, because a strong stream blows wet paste and its tiny solder balls around. Finally, confirm the orientation: check the pin-1 mark and how the part goes, from what you noted at removal (8.4; 6.1). Good preparation — flat pads, the right solder source, flux, gentle settings, and a confirmed orientation — sets up a clean reflow.

Placing and Aligning the Part

With the site prepared, you set the part down square and correctly oriented before any heat is applied. Pick up the part with fine tweezers or a vacuum pen (8.4) and lower it onto the pasted or tinned pads. Align it as you place it: the body inside its silkscreen outline, and pin-1 to the board's pin-1 mark (6.1) — the same alignment discipline you learned for tacking (7.5), except here the part rests on tacky paste rather than being held by a tack. The paste helps: it is sticky enough to hold a placed part roughly in position while you check it. Get the placement square and the orientation right now, while the part is cold and easy to move — nudge it with tweezers until the body sits within its outline and the leads or balls line up over their pads. This pre-reflow alignment is the placement that matters most: self-alignment will refine it, but only if you start close. A multi-lead part especially must go down with pin-1 correct and the leads roughly over their pads, because reflow cannot rotate a part that is a whole pad off or turned the wrong way — it will only bridge it (7.3). Place the part as accurately as you can by eye and magnification, check it against the silkscreen and pin-1 one more time, and only then reach for the hot air: a good placement is square and correctly oriented before the first heat.

Reflowing It and Letting It Self-Align

Now you reflow the part, and watch reflow's best trick — self-alignment — do its work. Bring the hot air over the part and heat evenly — soak the whole area with a moving nozzle or a box nozzle so all the joints come up together (8.2; 8.3) — until the solder or paste flashes shiny and wet, the sign it has reflowed (reflow soldering). As the solder melts, surface tension goes to work: the molten solder on each pad pulls itself into a tight shape, and because it wets both the pad and the part, it draws the part into alignment centered on its pads — this is self-alignment (6.3), the same physics that stands a chip up when things go wrong (7.4) but working for you when the part is placed close to right. You will often see a slightly-crooked part visibly snap square as the joints melt — a satisfying confirmation that the reflow is good. Your job during this is simply to hold the board still and let it happen: deliver even heat, watch the whole area go wet, and let surface tension center the part. Do not poke or nudge a part that is self-aligning — it is finding its own position, and interfering only disturbs it. Once every joint is molten and the part has pulled square, you have a reflowed, self-aligned placement: stop the heat and let it cool — the melting and the self-alignment are done in the same even, patient reflow.

When Self-Alignment Isn't Enough

Self-alignment is powerful but limited, and knowing its limit is what keeps it an asset instead of a trap. self-alignment corrects small offsets — a part a fraction of a pad off, a slight rotation — because surface tension can only pull the part as far as the molten solder's reach (6.3). It cannot fix a gross error. A part placed a whole pad off, badly rotated, or with pin-1 in the wrong corner will not be rescued by reflow: instead the misplacement becomes a defect. Leads that span the wrong pads bridge (7.3); a chip pulled toward one end tombstones (7.4); an area-array part settles its balls onto the wrong pads. So the rule is to place the part close to right and treat self-alignment as a finisher, not a fixer. If a part comes out grossly misaligned after reflow, do not try to shove it while it is solid: reheat the joints and nudge it into place (7.5), or if it is badly wrong, remove it and start again (8.4). And prevent it next time by placing more carefully. The mental model is simple: self-alignment is a gentle helper that squares a nearly-right placement, not a magic hand that fixes a careless one — place it close, and let self-alignment finish; place it far off, and it will bridge or tombstone instead.

Cooling and Inspecting

The placement is not finished at reflow — it is finished after it has cooled undisturbed and you have inspected it. The moment the part has reflowed and self-aligned, your task is to keep everything perfectly still while the solder solidifies: moving the board or the part while the joints freeze makes a weak, dull disturbed joint (5.4), undoing a good reflow. Take the heat away, hold steady, and let the joints cool and set fully before you touch anything. Then inspect (6.7). Look at every joint you can see: is it shiny and smoothly wetted to the pad and lead? Are there any bridges between leads (7.3)? Is any chip standing up or lifted (7.4)? Is the part flat, square, and centered on its pads, with pin-1 in the right corner (6.1)? For a part with visible leads, magnification shows you the fillets directly. For a ball-grid array you cannot see the joints at all — they are hidden under the body — so you inspect what you can see (the part sitting flat and square) and verify the connections electrically, with continuity or a functional test (6.7) — though continuity only confirms a connection exists, not that every hidden joint is sound, so the definitive inspection for a ball-grid array is X-ray, covered later (Chapter 9). Clean off the flux residue and look once more. Only when every joint is sound, the orientation is right, and the part sits flat is the placement done: reflow, cool undisturbed, and inspect — a placement you have verified, not just made.

Common Mistakes

• Placing the part too far off. Self-alignment fixes small offsets only (6.3) — a gross misplacement bridges or tombstones (7.3; 7.4); place it close to square.
• Too much paste or solder. It bridges when it reflows — use a stencil to meter paste, or a thin tinning for simpler parts (7.3).
• Nudging a part while it self-aligns. You disturb the position it is finding — hold still and let surface tension center it.
• Disturbing the joints while they cool. It makes dull, weak disturbed joints (5.4) — keep everything still until the solder is fully solid.
• Skipping inspection. A hidden bridge or a backwards part fails (6.7) — check wetting, bridges, tombstones, pin-1, and flatness before you call it done.

Troubleshooting Guidance

Placement problems trace to the solder amount, the pre-reflow alignment, or the cooling. If the part bridges: too much paste or placed too far off — use less paste (a stencil helps) and place it closer to square, then re-wick the bridge (7.3). If it tombstones or shifts: uneven heat or a gross misplacement — heat evenly and place it square (7.4). If a joint is dull or grainy: you disturbed it while it cooled — hold still until it is fully solid (5.4). If the part didn't self-align: it was placed too far off, or the joints didn't all reflow — place closer and heat evenly to full reflow (6.3). If pin-1 is wrong: it was placed backwards — reflow can't fix orientation — remove and replace it correctly (8.4). If a BGA doesn't work after placement: hidden joints may be open or bridged — verify electrically and reflow or redo if needed (6.7; Chapter 9). If paste balls scattered: airflow was too high for wet paste — lower the airflow. The throughline: right amount of solder, place close and square, reflow evenly and let it self-align, cool undisturbed, and inspect.

Verification & Testing Methods

Use this as a placement check:

• [ ] I can explain that hot air placement sets a new part on prepared pads and reflows all its joints at once so it solders into place together.
• [ ] I prepare the site with clean flat pads and the right solder source — tinned pads and flux, or solder paste (a stencil for fine-pitch) — and confirm the orientation (8.4; 6.1).
• [ ] I place the part square and aligned to the silkscreen and pin-1 mark before any heat (7.5).
• [ ] I reflow evenly and let surface tension self-align the part, understanding it corrects only small offsets (6.3) — a gross misplacement bridges or tombstones (7.3; 7.4).
• [ ] I hold everything still while the joints cool, so I don't make a disturbed joint (5.4).
• [ ] I inspect every joint for wetting, bridges, tombstones, pin-1, and flatness — verifying a BGA electrically (6.7).

Then try the practice exercises below — placement practice; scenarios differ from the quiz.

Practice Exercises

1. Paste and place a part (10 minutes, applied). On a cleaned scrap site, apply paste (or tin the pads), place a multi-lead part square to the silkscreen and pin-1, reflow evenly, and watch it seat. Inspect for wetting and bridges.
2. Watch self-alignment (7 minutes, applied). Place a chip deliberately a little off-center, then reflow and watch surface tension pull it square — then try one placed a whole pad off and see it bridge or tombstone instead. Note the difference (6.3; 7.3).
3. Cool and inspect (8 minutes, applied). Reflow a placed part, hold everything still until the solder is fully solid, then clean the flux and inspect every joint for wetting, bridges, and pin-1 (6.7).
4. Reason about a bad placement (5 minutes, reasoning). A part came out bridged after reflow. Explain whether self-alignment could have fixed it, what you'd do now, and how you'd place it next time (7.3; 8.4).

These core ideas — what hot air placement is, preparing the site with paste or solder, placing aligned to pin-1, reflow and self-alignment and its limits, cooling undisturbed, and inspecting — are tested in the Chapter Quiz at the end of this chapter, where a score of 80% is required to continue.

Key Takeaways

• Hot air placement sets a new part on prepared pads and reflows all its joints at once so it solders into place together (8.4) — the completion of a repair and the payoff of a clean removal.
• Prepare the site: clean, flat pads (8.4; 6.6), the right solder source (tin the pads and flux, or solder paste — a stencil deposits paste evenly for fine-pitch), flux, a nozzle and settings for the part (8.2; 8.3), and a confirmed orientation and pin-1 (6.1).
• Place the part square and aligned to the silkscreen and pin-1 mark before any heat (7.5) — because reflow refines a good placement but cannot rescue a bad one.
• Reflow evenly and let surface tension self-align the part onto its pads (6.3): a slightly-off part snaps square, but self-alignment corrects only small offsets — a gross misplacement or wrong orientation bridges (7.3) or tombstones (7.4) instead.
• Hold everything still while the joints cool (a bump makes a disturbed joint, 5.4), then inspect every joint for wetting, bridges, tombstones, pin-1, and flatness (6.7) — verifying a BGA electrically; advanced prep like reballing a BGA is covered later (Chapter 9).

Skills Learned

• You can now explain how hot air placement reflows a new part into place.
• You can now prepare a site with solder or paste, using a stencil for fine-pitch.
• You can now place a part square and aligned to the silkscreen and pin-1 mark.
• You can now use self-alignment and recognize when a placement is too far off.
• You can now cool the joints undisturbed and inspect the finished placement.

Glossary Additions

• stencil — a thin metal (or sometimes polymer) foil with an aperture cut over each pad of a footprint, laid on the board so that solder paste spread across it with a squeegee is deposited evenly and in a controlled amount on every pad at once; a stencil gives far more consistent paste volume than applying it by hand and is especially valuable for fine-pitch and area-array parts, where too much paste bridges and too little starves the joints. Stencils range from small part-specific foils to full-board framed stencils used in assembly.
• reballing — the process of attaching fresh solder balls to the underside of a ball-grid array package after its original balls have been removed (for example when reclaiming or reusing a chip), so that it can be placed and reflowed onto a board again; reballing uses a small stencil or preform to position the balls and hot air or a reflow step to attach them. It is an advanced BGA-rework task, treated in depth in the chip-replacement material rather than here.

Suggested Next Sections

Must read next:

• Protecting Adjacent Components — you can now remove and place parts with hot air; the next section addresses the constant hazard of both — keeping the heat from reflowing, shifting, or cooking the components around your work, with shielding, nozzle choice, and technique.

Recommended:

• Component Removal with Hot Air — the reverse operation, and the source of the clean pads a placement starts from.
• Tack Soldering for Alignment — the alignment discipline you apply when placing a part square to the silkscreen and pin-1 before reflow.