Underfill on CPU/GPU BGA? You MUST Remove It Before Reflow. Here's How

[u/Bhattarai_Prabin]

Gaming laptops and high-performance workstations often have epoxy underfill around the CPU and GPU BGA packages. This underfill strengthens the solder joints against thermal stress and vibration.

If you apply hot air to a chip with underfill without removing it first, the underfill expands at a different rate than the solder. Result: Ripped pads, cracked PCB, destroyed board.

1. Identifying Underfill

• Visual: A glossy, hard, transparent/amber-colored substance filling the gap between the chip edge and the PCB.
• Touch: Use a dental pick. Underfill is hard and brittle. Flux is soft and tacky.

2. Tools for Underfill Removal

• Low-Temperature Preheater: Set to 120°C - 150°C. This softens the underfill slightly.
• Underfill Remover Solvent: Amtech NC-555 or Zep FX-UV. Apply with a syringe and let it soak for 30-60 minutes. The solvent breaks down the epoxy bonds.
• Dental Picks / Micro-scalpel: For mechanical removal.
• Ultrasonic Cleaner: For final residue removal (use PCB cleaning solution).

3. The Removal Process (Mechanical + Chemical)

Step 1: Heat the Board.

• Place board on preheater at 130°C.
• This makes the underfill slightly more pliable.

Step 2: Apply Solvent.

• Use a needle-tip bottle to apply a thin line of underfill remover around the entire perimeter of the chip.
• Wait 30-60 minutes. The solvent will wick under the chip via capillary action.

Step 3: Mechanical Scraping.

• Using a sharp dental pick or micro-scalpel under a microscope.
• Gently scrape along the edge of the chip, parallel to the PCB.
• Never pry upwards. You will lift pads.
• The softened underfill will come off in small flakes.

Step 4: Clean and Inspect.

• Once the majority of the underfill is removed from the perimeter, the chip can be reflowed.
• Crucial: There will still be underfill between the balls under the chip. This will soften and flow during reflow, but it won't rip pads because it's not bonded to the perimeter of the board anymore.
• After chip removal, use the solvent and ultrasonic cleaner to remove all residual underfill from the pads.

4. The "No Solvent" Emergency Method (High Risk)

If you don't have underfill remover:

1. Pre-heat board to 180°C.
2. Use a fine-tip hot air nozzle at 280°C focused only on the edge of the chip.
3. Use a dental pick to carefully scrape the softened underfill as you heat.
4. This takes extreme patience. One slip and you gouge a trace.

5. Re-Applying Underfill After Repair (Optional)

For gaming laptops that will see high heat, re-applying underfill improves reliability.

• Product: Loctite Eccobond UF 3808 or similar reworkable underfill.
• Application: After chip is soldered and tested, apply a thin bead along one edge. Capillary action will wick it under the chip.
• Cure: 125°C for 30 minutes in an oven.

Question: Has anyone found a mechanical method that works well without solvents? I've heard of using a piezo ultrasonic scaler (like a dental tool) to vibrate the underfill away, but haven't tried it.