2.3 Rework Flow Control
BGA/QFN hot air, bottom heaters, thermocouples, and safe removal/placement—done the same way every time.
Reworking BGAs
(ball grid arrays) and QFNs (quadis flatone no-leads)of the most delicate operations in electronics manufacturing, where the true joints are hidden from sight and the margin for error is lessslim. aboutSuccess brutecomes heatfrom andprecise morethermal about controlled, repeatable process. These components hide their solder joints under the package, so success depends oncontrol: balancing bottom preheat andwith targetedcarefully topdirected airhot whileair, trackingguided temperatures withby thermocouples (TCs)that in real time. The tools—hot-air heads, adjustable nozzles, vision alignment, mini-stencils, shields, and vacuum lifts—are chosen to apply heat precisely, protect nearby parts, and handle components onlyverify when the solder is fullytruly molten.liquid. ProfilesWith arethe tunedright soprofiles, boardstools, and padsdocumentation, surviverework multipleshifts cyclesfrom withoutrisky damage, with each step loggedimprovisation to keepa repairsstructured traceable.process Sitethat preparation,protects solderpads, deposition,preserves boards, and carefuldelivers placementreliable ensuresecond thatchances reflowfor happensexpensive smoothly and joints form reliably. When the process is steady and documented, rework becomes not a gamble, but a repeatable craft.
2.3.1 The rework mindset (before you grab the nozzle)
- Plan → Heat → Prove. Decide the method, set the profile, then prove with TCs and photos.
- Local, not global. Bottom preheat carries most of the load; top air just finishes.
- One attempt is a choice. Two is a risk. Track attempt counts and thermal cycles per board/part (ticket + MES).
- Same alloy, same cleanliness. Rework with the product alloy; don’t “save” a board with low-temp mixes unless an approved deviation exists.
2.3.2 Core kit (and why each matters)
2.3.3 Profiles that don’t hurt boards (numbers you can start with)
Targets (tune to product alloy & component limits):
Thermocouples: one on copper 2–5 mm from site, one on package top. Lift/place only when the site TC and package TC say you’re truly at liquidus.
2.3.4 Safe removal (don’t take pads with you)
BGA
- Mask & preheat. Shields up; bottom heater to ~120 °C surface.
- Top air on centered nozzle; ramp to TAL.
- Nudge test. Tiny lateral tap with a pick—if it “floats,” it’s molten.
- Vertical lift with vacuum; no prying. If it resists, re-heat—never force.
- Site redress: gel flux + light wick to flat pads; preserve solder mask dams; clean gently.
QFN/LGA
- Flux the perimeter; preheat.
- Top air; slice the fillet with a fine knife tip while molten; lift vertical.
- Center pad: wick with gel flux using a broad chisel; aim for thin, even tinning, not bare copper.
Do not scour mask or over-wick into VIPPO/microvia-in-pad—you’ll create sinks for future opens.
2.3.5 Site prep & deposition (how much solder to bring back)
- BGA: prefer flux-only on the site, place a pre-balled new component. Paste on pads can trap volatiles and raise voids.
- QFN/DFN: use a mini-stencil (100–130 µm) for edge pads; window the thermal pad to 50–65% total area (small tiles + chimney).
- Through-hole touch-ups: liquid flux pen; preheat; larger chisel tip.
2.3.6 Placement & reflow (line up once, don’t chase)
- Vision & Z: align with split-vision; lower until light contact. Don’t drag to “find” pads—heat and surface tension do that.
- Hold-down: minimal pressure; let collapse happen freely.
- Reflow: run the same style profile as removal (Section 14.3.3). Watch TCs; do not exceed part/board limits to chase a quick collapse.
- Cool-down: passive air; avoid fans directly on hot joints (cracking risk).
2.3.7 Aftercare & proof (what “done” means)
- BGA: AXI—look for uniform collapse and voids within limit; no “cap-and-ball” seams (HIP).
- QFN/DFN: AOI side/3D—continuous perimeter fillet; thermal pad spread reasonable (void pattern OK).
- Electrical sanity: smoke test or ICT spot on the reworked area.
- Cleanliness: remove sticky flux where accessible; don’t flood solvents under BGAs.
2.3.8 Common pains → smallest reliable fix
2.3.9 Controls that make rework traceable (and auditable)
- Ticket fields: SN, component refdes, reason, attempt #, profile ID, nozzle/airflow, flux lot, operator.
- MSL discipline: parts out of bag? Bake per label before rework.
- Limits: post a max reflow cycle count (e.g., BGA ≤ 2, QFN ≤ 3).
- Attach proof: AXI/AOI images tied to the ticket; “before/after” photos of the site.
2.3.10 First-article script (15 minutes, saves hours later)
- Mount TCs (copper near site + package top).
- Run a dry profile on a scrap board; record ramp/TAL/peak.
- Shield neighbors; verify airflow clears plastics.
- Remove the device by the book; redress pads; deposit (flux or paste).
- Place & reflow; log profile.
- Inspect (AXI/AOI). If OK, freeze settings as the rework recipe; if not, change one variable and repeat.
2.3.11 Pocket checklists
Setup
- Bottom heater on; board surface 100–140 °C at site
- TCs fixed: copper near site + package top
- Nozzle sized; airflow modest; shields in place
- Flux type chosen (BGA flux-only; QFN paste via mini-stencil)
Removal
- TAL reached (by TC); nudge float confirmed
- Vertical lift—no pry; site wicking light & flat
- Clean without mask damage
Placement
- Align; light Z contact; reflow to same TAL/peak
- Cool naturally; no fan blast
Verify & log
- AXI (BGA) / AOI side/3D (QFN) passed
- Ticket updated (attempt #, profile ID, images)
- MSL/bake recorded if used