WYLIE Repair Spot Welding Piece

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WYLIE BGA PAD RESTORATION & MICRO-SOLDERING REWORK TECHNICAL GUIDE

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1. SUBSTRATE DAMAGE MECHANICS & RESTORATION PRINCIPLES:

- Root Cause of Ripped Pads: During IC desoldering (CPU, NAND, PMIC), excessive mechanical prying before solder reaches liquidus (SAC305 @ 217°C), improper underfill clearing, or board drop impact fractures the adhesive bonding layer between the copper foil pad and the underlying BT-resin / polyimide PCB core.

- The Traditional Jumper Wire Failure Mode:

* Hand-winding a 0.02mm enameled wire into a circular loop creates a non-planar, curved mound.

* When hot air is applied to install the BGA chip, surface tension pulls the jumper loop sideways, creating an internal bridge to neighboring balls or lifting off the via, resulting in open-circuit faults under thermal stress.

- The WYLIE Solder Lug Solution:

* Chemically etched planar copper lugs maintain uniform 0.025mm thickness across the entire footprint.

* Flat mating surface ensures 100% coplanar contact with the incoming reballed solder sphere, replicating OEM factory PCB lands and maintaining controlled impedance.

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2. SOLDERING ALLOY SELECTION & UV CURING CHEMISTRY:

- Solder Wire / Paste Alloy for Lug Attachment:

* High-Reliability Alloy: Sn63/Pb37 (Melting point: 183°C) or Sn62/Pb36/Ag2 (179°C).

* Trace Splicing Alloy: Use low-melting Sn42/Bi58 (138°C) ONLY for anchoring the micro-tail to the delicate via if nearby components cannot tolerate heat; otherwise, Sn63/Pb37 is strictly preferred to prevent secondary reflow during the final chip mounting stage (330°C hot air).

- Flux Formulation:

* Highly active synthetic no-clean paste flux (Amtech NC-559-ASM or Kingbo RMA-218). Minimizes copper oxidation while providing sufficient surface tension to hold the micro-lug in place prior to iron contact.

- Dielectric Insulation & Structural Bonding:

* High-viscosity UV-curable solder mask epoxy (Green/Black UV curable ink).

* Curing Light: Optical 365nm UV curing lamp (minimum 10W optical output). Curing duration: 10 - 20 seconds for localized spot cure; 60 seconds for complete cross-linked polymerization.

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3. STEP-BY-STEP MICROSURGICAL WORKFLOW FOR TORN PAD RECONSTRUCTION:

STEP 1: Microscopic Inspection & Trace Excavation

- Position motherboard under stereomicroscope at 20x - 45x magnification.

- Clean the damaged pad cavity thoroughly using 99.9% anhydrous IPA and lint-free micro-swabs.

- Identify whether the trace enters via a surface microstrip or a buried blind via (micro-via).

- Use an ultra-fine scalpel (No. 11 surgical blade or QianLi 007 carving blade) held at a 15-degree angle to gently scrape the solder mask overlying the incoming copper trace for approximately 0.3mm to 0.5mm, exposing bright, pristine copper. Never cut downward into the dielectric fiberglass to prevent severing underlying layer-2 copper planes.

STEP 2: Trace Tinning

- Apply a pinhead-sized drop of tacky rosin flux over the exposed trace/via.

- Using a micro-pencil soldering tip (JBC C115-101 / C210-018 ultra-fine bent cone) set to 320°C, apply a minute amount of Sn63/Pb37 solder to the exposed copper trace until a flat, shiny solder mound is formed.

- Clean residual flux with IPA.

STEP 3: Solder Lug Selection & Extraction from WYLIE Sheet

- Compare the damaged pad diameter against the WYLIE sheet under the microscope.

- Select a lug matching the exact pad diameter (e.g., 0.20mm for CPU signals; 0.35mm for PMIC power; 0.50mm for ground).

- Using precision titanium anti-magnetic tweezers (e.g., 2UUL or VETUS 0.1mm tips) and a sharp scalpel, cut the micro-tail of the chosen lug to the exact length required to reach the tinned trace.

- Gently peel the lug off the backing sheet. Avoid bending or folding the pad to preserve planarity.

STEP 4: Lap-Joint Micro-Soldering (Spot Attachment)

- Apply a micro-layer of flux over the repair site.

- Align the WYLIE lug so that the round contact pad sits precisely inside the original pad coordinates, with its tail overlapping the tinned copper trace.

- Hold the pad firmly flat against the PCB substrate using the tip of fine tweezers in one hand.

- Lightly touch the overlapping tail with the micro-soldering tip (300°C) for 0.5 seconds. The solder will instantly wet across the lap joint. Remove the iron tip while maintaining tweezer pressure for 2 seconds until the alloy solidifies.

STEP 5: Dielectric Passivation & Mechanical Anchoring

- The bonded tail requires mechanical isolation to prevent it from lifting when the BGA chip is reflowed.

- Using a tungsten acupuncture needle (0.1mm tip), apply a microscopic droplet of UV solder mask green oil strictly over the soldered tail joint and the outer perimeter of the lug.

- CRITICAL: DO NOT allow UV oil to cover the central circular contact pad. If oil flows onto the pad surface, immediately wick it away with a dry micro-applicator.

- Expose the site to a 365nm UV curing lamp for 30 - 45 seconds until the epoxy solidifies into a rock-hard mechanical anchor.

STEP 6: Surface Planarization & Final Pre-Reflow Conditioning

- Under the microscope, gently scrape the circular contact surface of the restored pad using the flat edge of a scalpel blade to remove any residual cured film or oxidation, exposing bare, shiny metal.

- Apply a drop of liquid flux and touch the pad lightly with the iron tip to form a micro-dome of leaded solder, or leave it flat for direct BGA ball seating.

- The reconstructed pad is now 100% thermally and mechanically stable, capable of withstanding standard 350°C hot-air BGA installation without shifting.

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4. ELECTRICAL INTEGRITY & DIAGNOSTIC VERIFICATION MATRIX:

A. Cold Diode-Mode Ground Reference Validation (Red Probe on Ground, Black on Repaired Pad):

┌──────────────────────┬───────────────────────────────┬────────────────────────────────────────────────────────┐

│ Circuit Net Class │ Expected Diode Reading │ Fault Indicator & Corrective Action │

├──────────────────────┼───────────────────────────────┼────────────────────────────────────────────────────────┤

│ High-Speed Data Line │ 0.450V - 0.650V │ "OL" (Open Loop) = Micro-tail solder joint failed; │

│ (UFS / MIPI / PCIe) │ (Must match symmetrical line) │ 0.000V = Solder bridged to adjacent PCB ground plane. │

│ Clock / Reset Lines │ 0.400V - 0.550V │ "OL" = Via copper fractured internally under pad; │

│ (CLK / RESET_N) │ │ Requires deeper excavation down to layer-2 trace. │

│ Power Buck Rails │ 0.120V - 0.350V │ Readings lower than 0.050V indicate accidental bridge │

│ (VDD_CORE / VPROC) │ (Low resistance domain) │ to inner ground plane during trace excavation. │

│ Battery / VPH_PWR │ 0.380V - 0.480V │ High-current lines require reinforced solder lugs with │

│ (Switched Bus) │ │ dual-trace soldering to handle >3.0A current draw. │

│ Ground Net (GND) │ 0.000V - 0.002V │ Ensure lug is firmly soldered to ground plane; loose │

│ │ (Continuous beep) │ ground pads cause floating references and RF noise. │

└──────────────────────┴───────────────────────────────┴────────────────────────────────────────────────────────┘

B. High-Frequency & Impedance Considerations:

- For high-speed differential signal pairs (e.g., PCIe Gen3, UFS 2.1/3.1, USB 3.1 SS, MIPI D-PHY):

* Keep the lug tail as short as possible (< 0.4 mm) to prevent stub inductance and signal reflection.

* Ensure the restored pad diameter does not exceed adjacent OEM pads, preserving 50-ohm single-ended / 100-ohm differential characteristic impedance.

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WYLIE Repair Spot Welding Piece

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