2.3 Rework Flow Control

Reworking complex surface mount devices (SMD), such as BGAs and QFNs, is one of the most delicate operations in electronics manufacturing. Success hinges on achieving precise, controlled thermal profiles that minimize component stress and protect surrounding pads. This process requires balancing bottom-side preheat with targeted hot air, ensuring the rework shifts from risky improvisation to a structured, auditable procedure.

2.3.1 The Rework Mandate: Controlled Heat and Traceability

Rework is an expensive, high-risk process where the 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.

Thermal Control: The process relies heavily on the bottom preheater to carry most of the thermal load, reducing the required intensity of the top-side hot air.
Traceability: The number of thermal cycles (rework attempts) on a board or component must be strictly tracked (e.g., 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.

2.3.2 Rework Core Kit and Profile Targets

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

A) Core Kit for Complex Rework

Tool	Function	Mandate
Bottom Preheater	Provides bulk heat to the board; shrinks the thermal differential (Delta T).	Board surface temperature must be established at 100 – 140 °C before hot air is applied.
Thermocouples (TCs)	Measures actual temperature on the board and package top.	TCs must be 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-Stencils	Applies paste to QFN pads.	Mandatory for QFN/DFN; BGA sites typically use flux-only to minimize paste voids.
Shields/Tape	Protects nearby plastic connectors and electrolytic capacitors.	Aluminum or Kapton tape is used to build quick thermal fences.

B) Standard Profile Mandates (SAC Alloy)

The profile must protect the component while ensuring sufficient Time Above Liquidus (TAL).

Step	BGA/QFN Target	Rationale
Ramp Rate	1 – 3 °C/second maximum.	Protects the board laminate and components from thermal stress.
Preheat (Bottom)	Board surface 100 – 140 °C.	Reduces the risk of pad lifting (cratering) and minimizes top heat requirement.
TAL (Time Above Liquidus)	50 – 80 seconds (above 217 °C).	Ensures full ball collapse and good wetting; crucial for mitigating Head-in-Pillow (HIP).
Peak Temperature	235 – 245 °C (on package top).	Must reach the minimum to reflow without exceeding the component's maximum thermal rating.

2.3.3 The Rework Process Flow

Rework is a four-stage process: removal, site preparation, deposition, and placement/reflow.

Safe Removal (Protecting Pads):
- Preheat the board to the required bottom temperature.
- Ramp top air until the site TC confirms liquidus.
- Perform a nudge test (a tiny lateral tap) to confirm the part is floating.
- Lift the component vertically using a vacuum pick. Never pry or force a cold part.
Site Preparation (Redress):
- Apply no-clean flux gel to the site.
- Use solder wick (flux-coated) with a broad, clean chisel tip to gently flatten the pads. Mandate: The site must be flat with minimal solder remaining; do not scrub the solder mask, as this leads to pad lifting during the next reflow.
Solder Deposition:
- BGA: Use the new, pre-balled component (flux-only deposition on the site).
- QFN/DFN: Use a mini-stencil (100–130 µm) to print fresh paste onto the pads and thermal slug. Mandate a window-pane pattern for the thermal pad to vent gases.
Reflow and Inspection:
- Place the component using split-vision for precise alignment. Lower the part until light contact is made.
- Run the validated reflow profile (Section 2.3.2).
- Inspection: AXI is mandatory for BGAs 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 profile ID used, the operator's badge, and the attempt number.
MSL Discipline: Any component removed from its Moisture Barrier Bag (MBB) must be tracked. If the part exceeds its floor life, baking is mandatory before reflow to prevent popcorning (internal component damage).
Damage Control: If lifted pads or solder mask damage occurs during rework, the board must be immediately sent to Material Review Board (MRB) for disposition (e.g., scrap, or repair via approved procedure).

Final Checklist: Rework Flow Mandates 📝

Step	Mandate	Rationale
Prep/Shielding	Board preheated to 100 – 140 °C; sensitive neighbors shielded (tape/shields).	Protects surrounding components and PCB laminate from thermal damage.
Profile Verification	Two thermocouples (site and package top) used to verify TAL and Peak Temperature.	Ensures the joint reaches liquidus without exceeding the component rating.
Deposition	BGA: Flux-only on site; QFN: Mini-stencil paste with ventilation pattern.	Minimizes voiding and controls solder volume precisely.
Removal Technique	Vertical lift initiated only after confirming molten float (nudge test).	Prevents pad lifting and damage to the copper pads.
Traceability	Attempt number and profile ID are logged against the board's serial number.	Enforces the maximum thermal cycle limit (e.g., BGA $\mathbf{\le 2}$ reflows).