The polishing procedure for MTP/MPO connectors is

• Polishing Machine: An automated machine capable of controlling pressure, speed, and time (e.g., Domaille, Seikoh Giken, or KrellTech).
• MPO/MTP Polishing Fixture (Puck): A specialized holder designed to hold multiple connectors simultaneously at the correct 8° angle for APC polish.
• Polishing Pads: A combination of hard (glass) and soft (rubber/PU) pads to achieve the necessary flat angle and controlled radius/fiber height.
• Lapping Films: A sequence of diminishing grit size films (Silicon Carbide, Diamond, Silicon Dioxide).
• Cleaning Supplies: Distilled water, 99% Isopropyl Alcohol (IPA), lint-free wipes, and compressed air or a non-contact cleaning system.
• Inspection Equipment: A fiber inspection microscope (with an MPO tip) and an interferometer for geometry measurement.

1. Cleaning: Thoroughly clean the connector end-faces and fixture after each step using water, IPA, and compressed air. Contamination is the primary cause of polishing defects.
2. Inspection:
   • Visually inspect the connector end-face under a microscope to ensure it is clean and free of scratches, cracks, or embedded debris (meeting IEC 61300-3-35 visual requirements).
   • Measure the end-face geometry using an interferometer to confirm radius of curvature, apex offset, and fiber height meet the required specifications (e.g., Telecordia GR-326).

3. Testing: Conduct Tier 1 testing (Insertion Loss and Return Loss using an OLTS) to ensure optical performance.

Here are the common issues and their typical causes:

1. Contamination and Scratches

Contamination is the most frequent cause of failure in the polishing process and in the field.

• Issue: Scratches or Defects on the End-Face.
  • Cause: Reusing single-use lapping films/pads, a dusty environment, or failure to thoroughly clean the ferrule/fixture between polishing steps. Larger grit from a previous step gets dragged onto a finer film.
  • Solution: Use fresh, single-use films for MT ferrules, maintain a clean workspace, and use specialized cleaning methods (e.g., pressurized water spray) after each step.
• Issue: Contamination in Guide Pin Holes.
  • Cause: Polishing debris accumulates in the guide pin holes and is not cleaned out.
  • Solution: Use specialized swabs or pressurized air/water to clean the guide pin holes as a mandatory step after polishing.

2. Geometry Problems (Fiber Height and Shape)

Achieving the precise end-face geometry is critical for physical contact and performance.

• Issue: Incorrect Fiber Protrusion/Undercut (Pistoning).
  • Cause: In MPO connectors, fibers must protrude slightly (a few microns). If they are undercut (recessed), physical contact fails, causing high loss. If they protrude too much, they can damage the mating connector or break.
  • Solution: Adjust the polishing time and pressure on the final, softer polishing pads. The type and condition of the rubber pad also heavily influence this result.
• Issue: Incorrect Radius of Curvature or Apex Offset.
  • Cause: Worn out or incorrect rubber polishing pads, inconsistent pressure during polishing, or an old/worn polishing fixture (jig).
  • Solution: Monitor radius statistics using an interferometer. Replace aged rubber pads regularly (e.g., every 6 months for moderate use) and verify the polishing fixture is not worn.
• Issue: Angle Error (for APC connectors).
  • Cause: The connector housing is not seated correctly in the interferometer adapter during measurement, or the polishing fixture does not hold the connector properly.
  • Solution: Ensure the APC fixture is correctly installed on the polishing machine and use the proper adapter when measuring with an interferometer.

3. Physical Defects and Process Errors

• Issue: Fiber Breakage.
  • Cause: The initial fiber cleave was poor (leaving a sharp stub), excessive pressure during initial polishing steps, or the polishing machine speed is too high.
  • Solution: Perform an "air polish" after cleaving to remove sharp stubs, ensure the cleaver blade is sharp, and maintain consistent, controlled polishing pressure.
• Issue: Black/White Spots or Pits.
  • Cause: Insufficient cutting force from the initial abrasive film fails to remove rough edges from previous steps, or the previous step was too rough to repair with the next film grade.
  • Solution: Ensure the correct sequence of films is used, and that adequate time is spent on the initial coarse grit steps to properly prepare the surface.

By addressing these common issues, manufacturers can significantly improve yield rates and ensure MPO connectors meet stringent industry performance standards.

Rework Procedure Based on Defect Type

Issue 1: Scratches, Pits, or Embedded Debris (Visual Failures)

• Rework Procedure:
  1. Stop and Clean: Thoroughly clean the connector end-face, the polishing fixture, and the work plate to remove any residual grit.
  2. Return to Fine Polish Step: If the scratches are fine, return to the 1 µm (micron) stage using a fresh lapping film and pad. Polish for an extended duration (e.g., an extra 30-60 seconds) to try to remove the defects.
  3. If Scratches Persist: If defects remain, move back to the 3 µm or 9 µm step to use a more aggressive abrasive to remove the deeper scratches, and then proceed through the subsequent fine and final polishing steps again.
  4. Re-Inspect: After the full polishing sequence, re-inspect the end-face visually.

Issue 2: Incorrect Fiber Protrusion or Undercut (Geometry Failures)

• Rework Procedure:
  1. Analyze Geometry Data: Determine if the fiber is too recessed (undercut) or too high/flat.
  2. Adjust Polishing Parameters: The issue often lies in the final polishing steps using the softer pads.
     • If fibers are too low: the correct rework procedure typically involves restarting the polishing process from the 3 µm stage and continuing through to the final step. or Reduce the time spent on the final or slightly adjust the pressure and rotation speeds. 
     • If the end-face is too flat (not enough radius): Ensure the correct pad is used and potentially increase time on the initial, Glass/harder pads.
  3. Repeat the Final Steps: Rework typically involves re-running the 1 µm and sub-micron final steps with adjusted parameters. It's often difficult to correct severe geometry issues without starting the whole process over.

Issue 3: Angle Error (APC Connectors)

• Rework Procedure:
  1. Return to Initial Steps: The angle is set in the very first step. Return to the 15 µm or 30 µm Silicon Carbide film on a glass plate or hard pad.
  2. Extend Dwell Time: Increase the polishing time in 10-20 second increments until the angle is cut entirely across the ferrule surface.
  3. Proceed Through Sequence: After the angle is confirmed, follow all subsequent polishing steps with the adjusted timings.

General Rework Best Practices

• Clean, Clean, Clean: Thoroughly clean the connector, fixture, and pads with 99% IPA and lint-free wipes after every single film change to avoid cross-contamination.
• Use Fresh Films: Do not reuse films, especially for multi-fiber connectors, as performance can degrade quickly.
• Inspect Before and After Cleaning: Sometimes, cleaning a clean connector can accidentally make it dirty. Inspect after cleaning to verify the end-face condition.
• Consult Manufacturer Guides: Refer to specific process documentation from your ferrule or film supplier ( sales@xytbrands.com ) as their formulations may require slight variations in time or pressure.