2.3 Rework Flow Control

Reworking complex surface mount devices (SMD), such as BGAs and QFNsQFNs, is one of the most delicate operations in electronics manufacturing,manufacturing. whereSuccess thehinges trueon jointsachieving areprecise, hiddencontrolled fromthermal sightprofiles that minimize component stress and theprotect marginsurrounding forpads. errorThis isprocess slim. Success comes from precise thermal control:requires balancing bottombottom-side preheat with carefully directedtargeted hot air, guided by thermocouples that verify when solder is truly liquid. Withensuring the right profiles, tools, and documentation, rework shifts from risky improvisation to a structuredstructured, processauditable that protects pads, preserves boards, and delivers reliable second chances for expensive assemblies.procedure.

2.3.1 The reworkRework mindsetMandate: (beforeControlled youHeat graband Traceability

Rework is an expensive, high-risk process where the nozzle)primary goal is to protect the Printed Circuit Board (PCB) laminate from excessive heat damage and to maintain pad integrity. Every step must be documented, as the reliability of the finished product depends on the quality of this "second chance" reflow.

• PlanThermal → Heat → Prove.Control: DecideThe process relies heavily on the method,bottom set the profile, then prove with TCs and photos.
• Local, not global.preheater Bottomto preheat carriescarry most of the load;thermal topload, airreducing justthe finishes.required intensity of the top-side hot air.
• One attempt is a choice. Two is a risk.Traceability: TrackThe attemptnumber counts andof thermal cycles per(rework board/partattempts) (ticket + MES).
• Same alloy, same cleanliness. Rework with the product alloy; don’t “save”on a board withor low-tempcomponent mixesmust unlessbe anstrictly approvedtracked deviation(e.g., exists.BGA
  maximum 2 reflows) via the repair ticket system (Chapter 5.5).
• Proof: Inspection (AXI for BGAs, 3D AOI for QFNs) must prove the joint quality is equal to or better than the original assembly run.

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2.3.2 Rework Core kitKit (and whyProfile eachTargets

matters)

The hot-air rework station requires specific tooling and a strictly validated thermal profile.

A) Core Kit for Complex Rework

Tool	Why you need itFunction	NotesMandate
Hot-air rework head with changeable nozzles & vision	Localize heat; align/place	Nozzle Ø ≈ package + 1–2 mm; adjustable airflow
Bottom heater / preheaterPreheater	ShrinksProvides ΔT;bulk savesheat padsto the board; shrinks the thermal differential (Delta T).	PlateBoard orsurface IR;temperature aimmust 100–be established at 100 – 140 °C boardbefore surfacehot pre-liftair is applied.
Thermocouples (TCs)	Measures +actual loggertemperature on the board and package top.	MakeTCs heatmust visible	Tape/epoxybe taped or epoxied to the copper near the site and the package top to verify liquidus and peak temperature.
Hot-Air Head & Nozzle	Localizes top-side heat for reflow.	Nozzle diameter must be sized approximately 1 – 2 mm wider than the package body.
Rework mini-stencilsMini-Stencils	ControlledApplies paste volumeto QFN pads.	Mandatory for QFN/DFN; BGA sites oftentypically use flux-only; QFNsto useminimize paste via mini-stencil
Flux (no-clean gel + pen)	Wetting without floods	Gel for perimeter/drag; pen for hole/pad wetvoids.
Shields/tapeTape	Protects (aluminum/Kapton)nearby plastic connectors and electrolytic capacitors.	Aluminum or Kapton tapeProtect neighbors	Buildis used to build quick fencesthermal around plastics
Vacuum pick & tweezers	Vertical lift; gentle align	Never pry; lift only when molten
Cleaning (solvent, swabs, wick)	Site redress	Wick lightly; don’t scrub mask
Inspection (AOI/AXI)	Prove the joint	AXI for BGAs; AOI + height/side view for QFNsfences.

B)

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Standard Profile Mandates (SAC Alloy)

The

2.3.3profile Profilesmust thatprotect don’t hurt boards (numbers you can start with)

Targets (tune to product alloy &the component limits):while ensuring sufficient Time Above Liquidus (TAL).

Step	BGABGA/QFN (SAC)Target	QFNRationale
Ramp (SAC)Rate	Why1 – 3 °C/second maximum.	Protects the board laminate and components from thermal stress.
Preheat (bottom)Bottom)	Board coppersurface 100–100 – 140 °C	100–140 °C.	Reduces topthe heat;risk preventsof pad craters
Ramp	1–3lifting °C/s	1–3(cratering) °C/s	Gentleand onminimizes laminatetop &heat partsrequirement.
TAL @(Time ≥217Above °CLiquidus)	50–50 – 80 sseconds	40–70 s(above 217 °C).	WettingEnsures time,full avoidball over-cookcollapse and good wetting; crucial for mitigating Head-in-Pillow (HIP).
Peak (package top)Temperature	235–235 – 245 °C	235–245 °C(on package top).	EnoughMust reach the minimum to reflow;reflow staywithout withinexceeding devicethe speccomponent's maximum thermal rating.

2.3.3 The Rework Process Flow

Thermocouples:Rework oneis ona copperfour-stage 2–5process: mmremoval, from site, one on package top. Lift/place only when the site TCpreparation, deposition, and package TC say you’re truly at liquidus.

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2.3.4 Safe removal (don’t take pads with you)

BGAplacement/reflow.

1. MaskSafe &Removal preheat.(Protecting Pads):
   • Preheat the board to the required bottom temperature.
   • Ramp top air until the site TC Shieldsconfirms up;liquidus.
   • Perform bottoma heaternudge test (a tiny lateral tap) to ~120confirm °Cthe surface.part is floating.
   • Lift the component Top airvertically onusing centereda nozzle;vacuum ramppick. Never pry or force a cold part.
2. Site Preparation (Redress):
   • Apply no-clean flux gel to TAL.the site.
   • Use Nudgesolder test.wick Tiny lateral tap(flux-coated) with a pick—ifbroad, itclean “floats,”chisel it’stip molten.to gently flatten the pads.
   • Vertical liftMandate: The site must be flat with vacuum;minimal nosolder prying. If it resists,remaining; re-heat—never force.
   • Site redress: gel flux + light wick to flat pads; preserve solder mask dams; clean gently.
     

QFN/LGA

1. Flux the perimeter; preheat.
2. Top air; slice the fillet with a fine knife tip while molten; lift vertical.
3. Center pad: wick with gel flux using a broad chisel; aim for thin, even tinning, not bare copper.
   

Dodo not scourscrub mask or over-wick into VIPPO/microvia-in-pad—you’ll create sinks for future opens.

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2.3.5 Site prep & deposition (how muchthe solder mask, as this leads to bringpad back)lifting during the next reflow.

Solder Deposition:

• BGA: preferUse the new, pre-balled component (flux-only deposition on the site, place a pre-balled new component. Paste on pads can trap volatiles and raise voids.site).
• QFN/DFN: useUse a mini-stencil (100–130 µm) forto edgeprint pads;fresh paste onto the pads and thermal slug. Mandate a windowwindow-pane pattern for the thermal pad to vent gases.50–65% total area (small tiles + chimney).

Through-holeReflow touch-ups:and Inspection: liquid flux pen; preheat; larger chisel tip.

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2.3.6 Placement & reflow (line up once, don’t chase)

• VisionPlace &the Z:component align withusing split-vision;vision lowerfor precise alignment. Lower the part until light contact is made.
• Run the validated reflow profile (Section 2.3.2). 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).
  

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2.3.7 Aftercare & proof (what “done” means)

• BGA:Inspection: AXI—look is mandatory for uniformBGAs to check for voids and proper collapse; 3D AOI is used for QFN fillet formation.

2.3.4 Traceability and Auditing Controls

Rework data must be recorded to ensure accountability and feed back into the overall quality system (Chapter 5.5).

• Audit Trail: The repair ticket must log the collapseprofile ID used, the operator's badge, and the attempt number.
• MSL Discipline: andAny voidscomponent withinremoved limit;from noits “cap-and-ball”Moisture seamsBarrier Bag (HIP)MBB) must be tracked. If the part exceeds its floor life, baking is mandatory before reflow to prevent popcorning (internal component damage).
• QFN/DFN:Damage Control: AOIIf side/3Dlifted pads—continuous perimeteror fillet;solder thermalmask paddamage spreadoccurs reasonableduring rework, the board must be immediately sent to Material Review Board (voidMRB) patternfor OK)disposition (e.g., scrap, or repair via approved procedure).
• Electrical sanity: smoke test or ICT spot on the reworked area.
• Cleanliness: remove sticky flux where accessible; don’t flood solvents under BGAs.
  

Final

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Checklist:

Rework

2.3.8Flow CommonMandates pains → smallest reliable fix📝

SymptomStep	Likely causeMandate	First fixRationale
BGA head-in-pillow (HIP)Prep/Shielding	UnevenBoard heatpreheated /to short100 TAL;– oxidized140 balls°C; sensitive neighbors shielded (tape/shields).	LengthenProtects TALsurrounding 10–15components s;and improvePCB bottomlaminate heat;from freshthermal flux; check package MSL/bakedamage.
BGAProfile voids highVerification	Two thermocouplesPaste use(site /and trappedpackage volatilestop) used to verify TAL and Peak Temperature.	SwitchEnsures tothe flux-onlyjoint site;reaches slowliquidus ramp;without ensureexceeding boardthe iscomponent dryrating.
QFN open cornersDeposition	BGA: Flux-onlyPaste starvation;on poorsite; releaseQFN: Mini-stencil paste with ventilation pattern.	SlightlyMinimizes largervoiding edgeand apertures;controls nano-coatedsolder mini-stencil;volume +5–10 s TALprecisely.
LiftedRemoval pad (site damage)Technique	Vertical liftPried cold;initiated over-wickingonly after confirming molten float (nudge test).	MorePrevents bottompad heat;lifting liftand onlydamage whento floating;the wickcopper with broad tip + gel, minimal pressurepads.
Neighbor parts skewedTraceability	Attempt numberAirflow tooand high;profile noID shieldingare logged against the board's serial number.	LowerEnforces airflow;the addmaximum shields;thermal preheatcycle more,limit top(e.g., airBGA less
Dull$\mathbf{\le joints2}$ / de-wet	Cold peak; cooked flux	+5 °C peak or +5–10 s TAL; refresh flux; verify finish agereflows).

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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.
  

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2.3.10 First-article script (15 minutes, saves hours later)

1. Mount TCs (copper near site + package top).
2. Run a dry profile on a scrap board; record ramp/TAL/peak.
3. Shield neighbors; verify airflow clears plastics.
4. Remove the device by the book; redress pads; deposit (flux or paste).
5. Place & reflow; log profile.
6. Inspect (AXI/AOI). If OK, freeze settings as the rework recipe; if not, change one variable and repeat.
   

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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
  

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Conclusion: Standardizing rework through controlled profiles, verified thermocouple data, and strict logging ensures that hidden joints are reflowed safely and consistently. This approach minimizes damage, maintains traceability, and turns complex BGA and QFN repairs into dependable, repeatable operations.