AMAOE S20 G988U Middle-Layer BGA Reballing Stencil for Samsung Galaxy S20 Ultra 5G (SM-G988U) – Dual/Single-Stackable, 0.12mm Strong Magnetic
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AMAOE S20 G988U MIDDLE-LAYER BGA REBALLING STENCIL TECHNICAL REWORK MANUAL
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1. ARCHITECTURAL PURPOSE & DUAL-LAYER PCB DESIGN
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The Samsung Galaxy S20 Ultra 5G (SM-G988U Snapdragon variant) uses a compact, vertically stacked
dual-layer (sandwich) logic board to accommodate its large 108MP periscope camera module, 5000mAh
battery, and 5G mmWave/Sub-6 RF hardware:
* Primary Lower Board (AP / Logic Layer): Houses the Qualcomm Snapdragon 865 SoC, LPDDR5 RAM, UFS 3.0
NAND storage, primary PMICs, and display/charging interfaces.
* Secondary Upper Board (RF / Modem Layer): Houses the Snapdragon X55 5G modem, RF transceivers,
power amplifiers, and antenna front-end modules.
* Middle Interposer Frame (中层): A dual-section peripheral PCB ring populated with fine-pitch copper
pads that bridge power rails, MIPI lines, PCIe/RF control signals, and ground planes between the
two stacked halves.
This AMAOE stencil features a scored vertical fold line separating the two interposer footprints
(left L-shaped primary ring and right elongated secondary ring), enabling technicians to plant solder
balls cleanly on either one board half or both halves depending on the rework requirement (可单堆可双堆).
2. KEY STENCIL ENGINEERING SPECIFICATIONS
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* 0.12mm Calibrated Thickness: Delivers precise solder paste volume per aperture so re-stacked board
halves settle to exact factory Z-height without cold joints or inter-pad bridging.
* Strong Magnetic Alloy (强磁性): Locks securely onto magnetic S20 G988U reballing platforms and
base jigs, preventing stencil shift during squeegee passes.
* Dual-Zone Scored Layout: Can be used flat as a single sheet or split/folded along the center score
line for independent clamping on smaller magnetic fixtures.
* High-Temp Japanese Stainless Steel: Resists thermal warping during direct hot-air reflow.
3. CRITICAL SIGNALS ROUTED THROUGH THE MIDDLE LAYER
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Cracked, oxidized, or bridged solder joints along the SM-G988U middle interposer typically trigger:
* No Service / Baseband Unknown / 5G Dropout: Open joints on RFFE clock/data, transceiver IQ lines,
or modem power rails bridging the AP board to the upper RF board.
* No Boot / High Standby Current: Squeezed solder balls shorting VPH_PWR / VDD_MAIN rails to adjacent
ground pads during improper board re-stacking.
* Wi-Fi / Bluetooth Failure: Interrupted PCIe/UART control lines or missing 1.8V/3.3V RF supply rails
across the interposer perimeter.
* Camera / Display / Charging Faults: Broken MIPI or I2C/SPI control lines routed between the main
logic board and upper-board connectors.
4. STEP-BY-STEP MIDDLE-LAYER REBALLING & RE-STACKING WORKFLOW
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A. BOARD SEPARATION & PAD PREPARATION:
1. Place the SM-G988U motherboard onto a dedicated pre-heating platform set to 175°C–190°C.
2. Once the factory middle-layer solder reaches liquidus, lift the upper RF board vertically using
curved ESD tweezers without applying sideways shear force.
3. Sweep both upper and lower interposer rings with a wide knife-edge soldering iron tip (320°C),
low-melt solder (138°C Bi/Sn), and tacky flux to lower the melting point of residual lead-free solder.
4. Clean all interposer pads thoroughly with 0.8mm/1.0mm desoldering wick and 99.9% Isopropyl
Alcohol (IPA) until every pad is flat and mirror-bright.
B. STENCIL ALIGNMENT & SOLDER PASTE APPLICATION:
1. Seat the lower (or upper) PCB half onto a magnetic SM-G988U reballing jig or flat silicone pad.
2. Align the corresponding side of the AMAOE S20 G988U stencil over the interposer perimeter under
a stereo microscope.
3. Apply 183°C Sn63/Pb37 (or 158°C/138°C medium-temp) solder paste across the apertures using a
stiff stainless scraper at a 45-degree angle in one smooth pass.
4. Hold the stencil flat and heat evenly with a hot air gun at 260°C–280°C (low airflow, nozzle
~2cm away) until all paste forms uniform silver solder spheres, OR lift the stencil vertically
first and reflow the printed paste directly on the board.
5. Inspect every perimeter solder ball under the microscope for uniform height and zero bridges.
C. BOARD RE-STACKING (REJOINING):
1. Apply a thin film of no-clean tacky flux along the opposing board's clean interposer pads.
2. Align the upper and lower halves on the pre-heating platform.
3. Heat at 185°C–195°C (matching the chosen solder alloy) until the upper board settles naturally
by surface tension (apply a gentle 1mm nudge with tweezers to verify self-centering).
4. Allow the board to cool naturally to room temperature before testing with a DC bench power
supply for shorts on VPH_PWR / VDD_MAIN.
5. POST-REWORK VERIFICATION & BENCH DIAGNOSTICS
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* Diode Mode Check: Probe key connector lines (Display, Camera, Charging, RF/SIM) in diode mode
before powering on to verify zero open lines (OL) or dead shorts to ground.
* DC Power Supply Boot Test: Connect to a regulated 4.2V DC bench supply; verify zero pre-boot
leakage current and normal boot current progression.
* Full RF & Connectivity Audit: Confirm 5G/LTE signal registration, Wi-Fi/Bluetooth MAC address
presence, NFC, and 120Hz Dynamic AMOLED 2X display output.
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