Sub-3nm logic board architectures pack billions of transistors into a chip smaller than a postage stamp. When a phone experiences heavy drop damage or severe thermal cycling, BGA (Ball Grid Array) solder joints underneath the CPU or PMIC can fracture, resulting in continuous bootloops or complete no-power states.
In this guide, we outline the exact 5-step process used in the AGNISIO cleanroom lab to reball high-density BGA packages safely.
Thermal Expansion & PCB Flexure
Modern smartphone PCBs feature sandwich dual-layer designs joined by perimeter interposer pads. Drops exert localized shear stresses:
Lead-free SAC305 solder joints have high tensile strength but lower ductility than legacy leaded solder. Repeated mechanical shocks lead to micro-fractures along the boundary layer.
Step 1: Pre-Heating & Thermal Stabilization
Placing a cold board directly under a hot air station causes catastrophic PCB warping. Always utilize a bottom-side infrared pre-heater:
- **Pre-Heat Temperature:** 150°C (Held for 3 minutes)
- **Target Board Temp:** 130°C uniform surface heat
- **Hot Air Station:** Quick 861DW @ 360°C, Airflow 50
Step 2: Pad Site Cleaning & Surface Preparation
Once the target IC is removed: 1. Flood the pad array with premium rosin flux. 2. Use a broad chisel soldering iron tip (350°C) with copper braid to wick away remaining solder. 3. Clean with 99.9% anhydrous isopropyl alcohol under ultrasonic vibration. 4. Inspect pad surfaces under 40x magnification to confirm zero torn copper traces.
Step 3: Stencil Placement & Solder Paste Application
Solder Paste Specification: Sn63/Pb37 or SAC305 Particle Size: Type 5 (15-25 microns) Stencil Material: 0.12mm Laser-Cut Japanese Steel
- Align the high-precision laser stencil over the BGA pad layout.
- Apply Type 5 solder paste evenly using a flexible rubber blade.
- Wipe away excess paste cleanly across the stencil face.
- Apply uniform low airflow hot air (280°C, airflow 20) until solder spheres pool cleanly into shiny spheres.
Final Inspection & Reseating
Inspect the newly formed BGA array under a 3D stereo microscope. Every sphere must match uniform height (±5 microns). Lower the IC onto the logic board, heat until self-alignment surface tension occurs, and allow to cool naturally.