Antminer S21 Thermal Maintenance Guide

Step 1: Disassembly

1. Power off the miner and disconnect the AC cord

2. Wait 60 seconds for capacitor discharge

3. Remove the top cover — 4 Phillips screws on the top panel

4. Disconnect the fan cables from the control board headers

5. Remove the fans:

   • Front fan: 4 screws accessible from the front
   • Rear fan: 4 screws accessible from the rear
   • Slide each fan assembly out

6. Disconnect hashboard cables:

   • 18-pin data cables from the control board
   • Power cables from each hashboard
   • Label each cable with its slot number (0, 1, 2)

7. Remove the hashboards:

   • Slide each hashboard out of its guide rails
   • Place on an ESD-safe surface

Step 2: Heatsink Removal

The heatsink assembly is attached to the hashboard with clips or screws, depending on the hardware revision:

1. Identify the attachment method:

   • Clip-on heatsinks: Use a plastic spudger to gently release the clips along each side
   • Screw-mounted heatsinks: Remove the Phillips screws from the back of the PCB (note the screw pattern for reassembly)

2. Separate the heatsink from the PCB:

   • Old thermal paste creates adhesion — do NOT force the separation
   • Gently twist the heatsink side-to-side while pulling upward
   • If stuck, apply gentle heat with a heat gun (60–80°C) to soften the paste

3. Place the heatsink fins-down on a clean surface for cleaning

Step 3: Cleaning Old Thermal Paste

Cleaning the hashboard:

1. Apply IPA (99%) liberally onto the chip surfaces using a lint-free cloth
2. Gently wipe in one direction — do NOT scrub back and forth aggressively
3. Repeat with fresh IPA-dampened cloths until all residue is removed
4. Pay attention to the edges of each chip — paste accumulates in the gaps between chips
5. Allow the IPA to fully evaporate (30–60 seconds)

Cleaning the heatsink:

1. Apply IPA to the flat contact surface of the heatsink
2. Wipe clean with a lint-free cloth
3. Ensure the surface is smooth and free of dried paste residue
4. If residue is stubborn, use a plastic scraper followed by IPA

What the clean surfaces should look like:

• Chip tops: Shiny, clean silicon or metal package surface
• Heatsink contact: Smooth, flat aluminum with no residue

Step 4: Inspecting Thermal Pads

While the heatsink is off, inspect the thermal pads on:

• Voltage regulators (buck converter ICs and MOSFETs)
• Inductor packages that contact the heatsink
• Any tall components that use pads instead of paste

Thermal pad condition check:

• Good: Soft, pliable, uniform thickness, no tears
• Replace: Hardened, cracked, torn, compressed to paper-thin, or missing entirely

If replacing thermal pads:

1. Remove the old pad with tweezers
2. Cut a new pad to the exact size of the component top
3. Standard thicknesses: 0.5mm for close-gap components, 1.0mm for taller components
4. Place the new pad centered on the component

Step 5: Applying New Thermal Paste

Application technique for hashboard chips:

Unlike CPU application where you put a single dot, hashboard chips are small and numerous. The recommended technique:

1. Apply a thin line of paste along each row of chips using a syringe-style applicator
2. Spread evenly using a plastic spreader card (credit card edge works) or a lint-free applicator
3. Target thickness: Paper-thin layer — just enough to fill microscopic surface imperfections
4. Coverage: The paste must cover the entire top surface of each chip, edge to edge
5. No excess: Too much paste is counterproductive — it acts as an insulator. A thin, even layer conducts heat better.

Common mistakes to avoid:

• Too much paste (thick blobs) — actually worsens cooling
• Missing chips — even one uncovered chip will run hot
• Getting paste on the PCB traces between chips — not harmful with non-conductive paste, but messy
• Using conductive paste (liquid metal, silver compounds) — will cause shorts

Step 6: Heatsink Reinstallation

1. Verify heatsink alignment — the flat contact surface must align perfectly with all chip rows

2. Lower the heatsink straight down onto the hashboard — do NOT slide it, which smears paste unevenly

3. Apply even pressure across the heatsink to ensure good contact on all chips

4. Secure the heatsink:

   • Clip-on: Press clips into place along each side until they click
   • Screw-mounted: Install screws in a cross pattern (like tightening a wheel) to ensure even pressure. Do not overtighten.

5. Check for any squeeze-out: A tiny amount of paste squeezing from the edges is normal and indicates good contact. Excessive squeeze-out means too much paste was applied.

Step 7: Fan Cleaning and Inspection

While the miner is disassembled, clean and inspect the fans:

1. Blow out dust with compressed air — hold the fan blade still (letting it spin freely can damage the bearing or generate back-EMF)
2. Inspect blades for cracks, chips, or deformation
3. Check bearing condition:
   • Spin the fan by hand — it should rotate smoothly with minimal resistance
   • Grinding, clicking, or wobbling indicates bearing wear
4. Check the cable and connector for damage

Fan replacement criteria:

• Noisy bearing (grinding, clicking, rattling)
• Wobbling or vibrating excessively
• Reduced airflow (fan spins but moves less air than a new fan)
• Broken blade(s)

To replace:

1. Disconnect the old fan's 4-pin connector
2. Remove the 4 mounting screws
3. Install the replacement fan with the airflow arrow pointing in the correct direction (front=intake toward boards, rear=exhaust away from boards)
4. Connect the 4-pin cable

Step 8: Reassembly and Testing

1. Slide hashboards back into their guide rails in the enclosure
2. Reconnect power cables and 18-pin data cables (matching the slot labels from disassembly)
3. Reinstall fans (front and rear) and connect fan cables to the control board
4. Replace the top cover and secure with screws
5. Connect AC power and boot the miner

Post-maintenance verification:

1. Access the web dashboard
2. Check chip temperatures under load:
   • Normal: 55–80°C depending on ambient temperature
   • Good maintenance result: Temperatures should drop 5–15°C compared to pre-maintenance
3. Monitor for 1 hour to ensure temperatures are stable
4. Check that all 129 chips per board are detected and hashing

# Check temperatures via SSH
ssh root@<miner-ip>
cat /tmp/temp.txt

# Verify all chips active
cat /tmp/freq.txt