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2.3 Rework Flow Control

Reworking BGAs and QFNs is one of the most delicate operations in electronics manufacturing, where the true joints are hidden from sight and the margin for error is slim. Success comes from precise thermal control: balancing bottom preheat with carefully directed hot air, guided by thermocouples that verify when solder is truly liquid. With the right profiles, tools, and documentation, rework shifts from risky improvisation to a structured process that protects pads, preserves boards, and delivers reliable second chances for expensive assemblies.

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)

Tool

Why you need it

Notes

Hot-air rework head with changeable nozzles & vision

Localize heat; align/place

Nozzle Ø ≈ package + 1–2 mm; adjustable airflow

Bottom heater / preheater

Shrinks ΔT; saves pads

Plate or IR; aim 100–140 °C board surface pre-lift

Thermocouples (TCs) + logger

Make heat visible

Tape/epoxy to copper near site and package top

Rework mini-stencils

Controlled paste volume

BGA sites often flux-only; QFNs use paste via mini-stencil

Flux (no-clean gel + pen)

Wetting without floods

Gel for perimeter/drag; pen for hole/pad wet

Shields/tape (aluminum/Kapton)

Protect neighbors

Build quick fences 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 QFNs



2.3.3 Profiles that don’t hurt boards (numbers you can start with)

Targets (tune to product alloy & component limits):

Step

BGA (SAC)

QFN (SAC)

Why

Preheat (bottom)

Board copper 100–140 °C

100–140 °C

Reduces top heat; prevents pad craters

Ramp

1–3 °C/s

1–3 °C/s

Gentle on laminate & parts

TAL @ ≥217 °C

50–80 s

40–70 s

Wetting time, avoid over-cook

Peak (package top)

235–245 °C

235–245 °C

Enough to reflow; stay within device spec

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

  1. Mask & preheat. Shields up; bottom heater to ~120 °C surface.
  2. Top air on centered nozzle; ramp to TAL.
  3. Nudge test. Tiny lateral tap with a pick—if it “floats,” it’s molten.
  4. Vertical lift with vacuum; no prying. If it resists, re-heat—never force.
  5. 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.

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

Symptom

Likely cause

First fix

BGA head-in-pillow (HIP)

Uneven heat / short TAL; oxidized balls

Lengthen TAL 10–15 s; improve bottom heat; fresh flux; check package MSL/bake

BGA voids high

Paste use / trapped volatiles

Switch to flux-only site; slow ramp; ensure board is dry

QFN open corners

Paste starvation; poor release

Slightly larger edge apertures; nano-coated mini-stencil; +5–10 s TAL

Lifted pad (site damage)

Pried cold; over-wicking

More bottom heat; lift only when floating; wick with broad tip + gel, minimal pressure

Neighbor parts skewed

Airflow too high; no shielding

Lower airflow; add shields; preheat more, top air less

Dull joints / de-wet

Cold peak; cooked flux

+5 °C peak or +5–10 s TAL; refresh flux; verify finish age



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)

  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.



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


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.


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