MY-CK01 (CPU Degumming Knife
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YCS MR. YANG MY-CK01 CPU DEGUMMING & UNDERFILL REMOVAL TECHNICAL GUIDE
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1. MATERIAL METALLURGY & MECHANICAL DESIGN PRINCIPLES:
- Metallurgy & Elastic Deflection:
* Constructed from vacuum heat-treated SK5/65Mn high-carbon spring steel, quenched and tempered to 58 – 62 HRC.
* High tensile elasticity allows the blade to flex up to 15 degrees when pushed along the motherboard plane without taking permanent plastic deformation or snapping.
- Ground Bevel Geometry:
* Features a precision flat-chisel spade profile with a single micro-beveled cutting wedge.
* The flat underside of the spade glides flush against the FR4/polyimide solder mask layer. The angled top wedge directs upward shearing force strictly into the brittle epoxy underfill, preventing the knife tip from digging downward into the copper ground plane or slicing off surface-mount components.
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2. THERMAL WINDOWS & EPOXY UNDERFILL CHEMISTRY:
Modern smartphone motherboards deploy three distinct underfill chemistries around CPUs and PMICs:
A. Black Rigid Epoxy Resin (Qualcomm, MediaTek, Samsung Exynos):
- Highly cross-linked silica-filled aromatic epoxy.
- Glass Transition Temperature (Tg): ~130°C – 150°C.
- Softening Rework Window: 230°C – 250°C at 35% hot-air airflow.
- Characteristics: Becomes rubbery and crumbly under localized heat, allowing the MY-CK01 single-bevel blade to slice cleanly through perimeter borders.
B. Clear / Amber Capillary Epoxy Resin (Apple A-Series BGA Underfill):
- Low-viscosity anhydride-cured epoxy with high adhesion to solder mask.
- Softening Rework Window: 220°C – 240°C at 30% airflow.
- Characteristics: Softens into a pliable gum; requires continuous flat skimming with the MY-CK01 blade to peel resin ribbons away from the substrate.
C. White / Gray Silicone-Modified Underfill (Baseband / RF ICs):
- Lower bond strength; softens rapidly at 180°C – 210°C. Slices effortlessly with minimal tool pressure.
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3. STEP-BY-STEP WORKFLOW FOR UNDERFILL REMOVAL & MOTHERBOARD RESTORATION:
STEP 1: Perimeter Underfill Clearing (Pre-Desoldering)
- Secure the motherboard into an alloy CNC preheating fixture.
- Adjust the stereomicroscope to 15x – 30x magnification.
- Set hot air rework station to 240°C (Airflow: 30% – 35%) using a 5 mm angled nozzle.
- Direct the hot-air nozzle along the perimeter gap between the target CPU/NAND and adjacent decoupling capacitors.
- Insert the MY-CK01 blade into a balanced chuck handle. Hold the blade at a 15° to 20° angle relative to the PCB.
- Advance the spade tip smoothly through the softened epoxy along all four edges of the chip.
- Keep the flat underside of the blade pressed against the chip flank to guide the stroke, ensuring the cutting edge stays clear of neighboring 0201 resistors and inductors.
STEP 2: Component Desoldering & Under-Die Release
- Protect surrounding plastic connectors and sensitive cameras with thermal copper foil and Kapton tape.
- Apply a small bead of NC-559-ASM tacky flux around the chip boundary.
- Increase hot air temperature to 350°C – 365°C (Airflow: 50% – 60%).
- Heat the top of the IC in continuous rectangular sweeps for 35 – 45 seconds.
- When the solder spheres transition to molten liquidus (SAC305 @ 217°C / Leaded @ 183°C), insert the ultra-thin 0.15 mm tip of the MY-CK01 blade into the corner seam beneath the chip.
- Gently slide the flat spade horizontally underneath the die to sever the internal underfill web.
- Lift the chip vertically. Never twist or pry with upward leverage while solder is semi-solid to avoid ripping copper pads from the PCB.
STEP 3: PCB Pad Scraping & Degumming (Hot-Air Aided)
- With the IC removed, hardened underfill remains bonded across the PCB ball grid.
- Set hot air station to 230°C – 250°C (Airflow: 25%).
- Direct the hot air stream 2 cm ahead of the MY-CK01 blade tip.
- Lay the blade completely flat (0° to 5° tilt) against the PCB land array.
- Push the spade forward in parallel strokes across rows of pads. The flat chisel edge shears softened epoxy lumps off the gold pads while skimming harmlessly over the smooth solder mask.
- Periodically wipe the blade edge on a lint-free wipe soaked in 99.9% anhydrous IPA to clear accumulated resin residue.
STEP 4: Soldering Iron + Copper Braid Final Leveling
- Apply fresh rosin flux across the scraped motherboard footprint.
- Run a wide chisel soldering iron tip (JBC C245-784 or T12-K) at 340°C combined with 2.5 mm desoldering wick (Goot Wick) across the pads to absorb residual lead-free alloy.
- If microscopic resin flakes remain trapped between pads, reheat to 220°C and use the corner tip of the MY-CK01 blade to flick the debris loose.
- Clean the area thoroughly with 99.9% IPA and inspect under the microscope to verify 100% planar, mirror-bright gold pads with zero solder mask scratches.
STEP 5: IC Underside Degumming (BGA Die Cleaning)
- Mount the removed IC (CPU, RAM, or NAND) upside down into a dedicated universal chip reballing fixture.
- Apply flux and set hot air to 240°C.
- Push the MY-CK01 spade flat across the bottom BGA ball array to scrape away residual epoxy and level the factory solder bumps.
- The chip is now fully prepared for stencil alignment and 183°C leaded solder paste reballing.
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4. TECHNICIAN RISK MITIGATION & FAULT PREVENTION MATRIX:
┌────────────────────────────────┬────────────────────────────────┬────────────────────────────────────────────────────┐
│ Operator Error / Hazard │ Resulting Board Failure │ Prevention & Corrective Technique │
├────────────────────────────────┼────────────────────────────────┼────────────────────────────────────────────────────┤
│ Blade angle too steep (> 30°) │ Slices solder mask; exposes │ Keep blade angle < 15°; flat underside must glide │
│ during perimeter cleaning │ internal ground planes/traces │ parallel to PCB surface; seal cuts with UV mask. │
├────────────────────────────────┼────────────────────────────────┼────────────────────────────────────────────────────┤
│ Cold scraping without adequate │ Rips copper BGA pads from the │ Never push blade cold; always apply 230°C–250°C │
│ hot-air preheating (< 200°C) │ FR4 motherboard substrate │ hot air to soften resin before advancing blade. │
├────────────────────────────────┼────────────────────────────────┼────────────────────────────────────────────────────┤
│ Lateral prying during chip │ Fractures silicon die; cracks │ Slide blade strictly horizontally underneath die; │
│ extraction under hot air │ internal solder ball lands │ confirm solder liquidus before lifting vertically. │
├────────────────────────────────┼────────────────────────────────┼────────────────────────────────────────────────────┤
│ Dull or chipped blade edge used│ Tears epoxy into clumps; chips │ Replace blade immediately if edge develops burrs; │
│ on high-density 01005 passives │ off adjacent micro-capacitors │ MY-CK01 10-pack ensures fresh razor-sharp edges. │
├────────────────────────────────┼────────────────────────────────┼────────────────────────────────────────────────────┤
│ Excessive downward force on │ Bends interposer frame; tears │ Use feather-light forward pushing force; let the │
│ iPhone sandwich motherboards │ delicate perimeter ring pads │ micro-wedge do the shearing work, not hand torque. │
└────────────────────────────────┴────────────────────────────────┴────────────────────────────────────────────────────┘
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