AOTG18LVC - Bridge Rectifier Short Circuit and Dried Heat Paste
This article summarises the quality assurance investigation into an AOTG18LVC outdoor unit where the bridge rectifier was found short-circuited and the heat transfer paste had dried.
Issue Summary
- Product: AOTG18LVC
- Serial / reference: E004424
- Manufacture date: 26 August 2010
- Reported fault: Bridge rectifier short circuit
- Additional finding: Heat transfer paste had dried
- Operating history: Unit operated correctly for approximately 3 years before failure
Problem Description
The affected AOTG18LVC had operated normally for approximately three years before failure. Investigation found that the bridge rectifier had short-circuited and the thermal transfer paste associated with the power component mounting had dried.
Similar Cases Identified in Japan
The factory advised that similar failures had previously occurred in Japan.
| Item | Details |
|---|---|
| Models | AO-A250, AO-A220, AO-A360, AO-A22W, AO-J22W |
| Number of reported cases | 5 |
Factory Cause Assessment
The Japanese investigation identified several possible manufacturing causes associated with the installation and tightening of the IPM / power component assembly.
1. Final Tightening May Have Been Missed
- The IPM mounting process used two stages: temporary tightening followed by final tightening.
- Both operations were performed by the same worker.
- The factory considered it possible that the final tightening step could occasionally have been missed.
2. Torque Variation of Motor Screwdriver
- The motor screwdriver used for the IPM mounting process was also used for other screw-tightening processes.
- This created the possibility of variation in the tool's torque setting.
3. Screw May Have Been Tightened at an Angle
- The application of thermal grease could allow the mounting screw to enter or tighten at an angle.
- An incorrectly seated or insufficiently tightened component could reduce effective thermal transfer between the power component and heat sink.
Factory assessment: FGL considered the AOTG18LVC failure likely to be the same type of issue previously identified in the Japanese cases.
Factory Corrective Actions
| Corrective Action | Implementation |
|---|---|
| Temporary tightening and final tightening were separated into different manufacturing process steps. | 24 March 2011 |
| A dedicated motor screwdriver was assigned specifically to the IPM screw-tightening process. | Implemented as part of the corrective action |
| A cover was fitted to prevent workers from changing the screwdriver torque setting. | Implemented as part of the corrective action |
| Periodic inspection of the motor screwdriver was introduced. | Implemented as part of the corrective action |
| Work instructions were introduced to check for angled screw installation and missed screw tightening. | 7 April 2011 |
Action for Existing Product
- Already installed product: Rectify on a case-by-case basis if the problem occurs.
- Stock product: No blanket corrective action was specified in the original report.
- Current production: Manufacturing corrective measures had already been implemented.
Service Guidance
If a similar bridge rectifier or power-module failure is found on an AOTG18LVC:
- Isolate the unit and verify that all high-voltage capacitors are safely discharged before working on the inverter circuit.
- Inspect the bridge rectifier / power module for evidence of short circuit, overheating or physical damage.
- Inspect the heat sink interface and thermal paste condition.
- Check that mounting screws are present, correctly seated and not visibly angled.
- Inspect the mounting surface for evidence of poor contact between the power component and heat sink.
- Where component replacement is required, apply the correct thermal compound and tightening procedure specified for the replacement part.
- After repair, complete the applicable inverter and power-circuit checks before returning the unit to service.
Technical summary: The original QA investigation linked this failure to a known manufacturing risk involving IPM / power-component mounting and tightening. The factory subsequently separated tightening stages, dedicated and controlled the torque tool, introduced periodic tool inspection, and added checks for angled or missed screw tightening.