1. Problem Description
The customer reported abnormal performance after the Main FPC entered production. The observed microphone failures included no audio, low audio volume, and audio noise. The reported defect rate was approximately 5%.
Four defective units were returned for analysis. On all four Main FPCs associated with the microphone failure, the MIC-BIAS and MIC-CLK lines showed abnormally high resistance. The normal resistance was below 10 Ω, whereas the defective units measured in the kilohm range. This abnormal resistance prevented the microphone from operating correctly.
Follow-up testing on the customer's production line confirmed a defect rate of approximately 5%. During verification, the customer damaged the connector while forcibly checking it. The inner solder pins were found to be blackened. After the connector was reworked with a hot-air tool, the microphone function recovered.
The four returned units were identified as follows:
- Unit 1: Low volume from the Main MIC. MIC-BIAS measured 3.3 kΩ, and MIC-CLK measured 350 kΩ.
- Unit 2: Microphone noise. MIC-CLK measured 8.9 kΩ, but the resistance returned to a normal value when the connector was pressed.
- Unit 3: The left speaker resistance was abnormally high at 6 kΩ, and MIC-CLK measured 1 kΩ.
- Unit 4: No audio from the Main MIC. MIC-CLK measured 170 Ω, and MIC-BIAS measured 1.7 kΩ.
2. Failure Analysis
2.1 Unit 1: Low Main-MIC Volume
Unit 1 exhibited low microphone volume. The measured resistance was 3.3 kΩ on MIC-BIAS and 350 kΩ on MIC-CLK.
Visual inspection found no obvious cold solder joints, solder bridging, or other visible soldering defects. The appearance was judged acceptable.
Repeated functional testing with the test fixture initially produced an NG result. When the product was pressed again during a repeat test, the result changed to OK. This behavior indicated an unstable electrical connection rather than a permanent open circuit.
Point-to-point measurement with a multimeter showed 85 kΩ on the MIC-BIAS line and 11 Ω on the MIC-CLK line. The abnormal MIC-BIAS resistance confirmed a high-resistance connection in that circuit.
After the connector area was heated on a hot plate and re-soldered, the resistance returned to normal. MIC-BIAS measured 2.6 Ω, while MIC-CLK measured 0.25 Ω. The recovery after heating and rework confirmed that the failure was related to an intermittent or incomplete solder connection.
2.2 Unit 2: Microphone Noise
Unit 2 exhibited microphone noise. MIC-CLK measured 8.9 kΩ, and the resistance returned to an acceptable value when the connector was pressed.
Visual inspection did not identify obvious cold solder joints or solder bridges.
The functional fixture again produced an NG result during the first retest, while pressing the product during the repeat test produced an OK result.
Multimeter testing measured 10 Ω on MIC-BIAS and 9.9 kΩ on MIC-CLK. The abnormal MIC-CLK resistance was consistent with the microphone noise symptom.
After heating and re-soldering the connector area, MIC-BIAS measured 2.5 Ω and MIC-CLK measured 0.26 Ω. The resistance recovery confirmed an intermittent soldering defect on the MIC-CLK path.
2.3 Unit 3: Damaged FPC and Abnormally High Speaker Resistance
Unit 3 showed an abnormally high left-speaker resistance of 6 kΩ, while MIC-CLK measured 1 kΩ.
Visual inspection found that a contact spring had fractured. The FPC itself was confirmed to be damaged.
Repeated functional testing continued to produce an NG result, and the circuit remained open. The failure was therefore attributed to FPC damage caused by the fractured contact spring, making normal retesting impossible.
2.4 Unit 4: No Main-MIC Audio
Unit 4 exhibited no audio from the Main MIC. MIC-CLK measured 170 Ω, and MIC-BIAS measured 1.7 kΩ.
No obvious cold solder joints or solder bridging were found during visual inspection.
During fixture retesting, component C3801 showed an abnormal result. Pressing the product again changed the result to OK, indicating unstable contact during fixture testing.
Point-to-point measurement showed 54 Ω on MIC-BIAS and 11.2 MΩ on MIC-CLK. The MIC-CLK line was clearly abnormal.
After heating and re-soldering, the resistance returned to normal. MIC-BIAS measured 2.5 Ω and MIC-CLK measured 0.26 Ω. This result again linked the high resistance to an incomplete or unreliable solder connection.
3. Cross-Sectional Analysis
Three good units from unrepaired products were cross-sectioned at the blind-via locations. The copper inside the blind vias was confirmed to be acceptable.
Five NG units that had undergone rework or drop testing were cross-sectioned. The observed defect rate was 20%, and the defective units showed hole exposure at the vias.
Additional cross-sectional analysis was performed on an NG unit after re-soldering. When the heating time exceeded 30 seconds, the hole-exposure defect became more severe. This confirmed that excessive rework heating aggravated the via damage.
4. Process Investigation
Reworked returned units passed testing, and the same result was obtained when the connector was re-soldered on the customer's production line. This initially indicated that SMT soldering defects had produced incomplete solder joints.
Further inspection of the four defective connectors found oxidation and blackening on the pin surfaces. The evidence indicated that connector oxidation contributed to poor solder wetting and incomplete solder joints.
- The defective units came from deliveries made on November 2, involving 1,382 units, and November 4, involving 105 units.
- The material belonged to an earlier pilot-production batch. Because the quantity was limited, it was handled as sample-production material. Records showed that 2,500 board-to-board connectors were issued to the sample-production group in September in a single release.
- The same connector batch, issued on September 22, was used in two production runs: one at the beginning of October and another at the beginning of November. Unlike normal connectors, which were stored in the warehouse, the entire sample quantity was issued at once and stored at room temperature by the sample-production group. This storage condition allowed oxidation to develop on the connector pins.
- SMT production and trial-production records showed that connector pins were found oxidized and unable to accept solder between 23:00 on October 22 and 07:00 on October 23. A total of 134 defective units were identified. The temporary process action was to add two layers of tape beneath the stencil at the connector location to increase the solder-paste thickness.
- SMT return records and finished-product test records confirmed that some units had previously been reworked because of SMT soldering defects.
- The functional test fixture performed unstable continuity testing. During fixture operation, downward force was applied to the product because the test pins moved downward while the upper mold did not provide sufficient clearance. This mechanical force allowed defective units to pass through the test process.
5. Root-Cause Conclusion
The primary cause was the storage and use of a pilot-production connector batch. A total of 2,500 board-to-board connectors were issued on September 22 and consumed in two separate production runs. During the second production run on November 5, connector oxidation caused incomplete solder joints during SMT assembly.
SMT inspection identified 134 open-solder defects, corresponding to a defect rate of 7.9%. FPC testing identified 17 defective units, corresponding to approximately 1%. Subsequent repair of the incomplete solder joints damaged the via copper in some FPCs, causing abnormal resistance, intermittent microphone operation, or open circuits.
The escape mechanism was the functional test fixture. Downward force was applied during continuity testing because the test-pin movement was not coordinated with upper-mold clearance. The fixture pressure could temporarily improve contact or mask an unstable connection, allowing defective products to escape detection.
6. Corrective Actions
6.1 Actions to Prevent Defect Generation
- Sample-production materials must be issued in controlled quantities. Materials should be issued in batches and only in the quantity required for the planned production run.
- The SMT process should use nitrogen reflow for subsequent production involving this connector, reducing the risk of connector oxidation and incomplete soldering. The component placement drawing for this part number must also be marked “Do Not Rework.”
- The blind-via design should be evaluated for conversion to a through-hole solution in cooperation with the customer's development team. This change is intended to improve manufacturing yield and eliminate via damage caused by rework.
6.2 Actions to Prevent Defect Escape
- Add resistance testing for the complete bare-panel PNL to prevent products with abnormally high line resistance from escaping.
- Optimize the finished-product test fixture so that it avoids the connector area. Remove the downward-pressure film and eliminate mechanical pressure on the connector and FPC during testing.
7. Verification of Improvement Results
After the connector was assembled using nitrogen reflow, the solder joints appeared bright and the soldering quality was good. The SMT push-force test also passed, indicating that the revised process improved solderability and mechanical assembly quality.
8. Standardization and Horizontal Deployment
- Update the SMT review documentation. The BM20B(0.8)-30DP-0.4V(51) connector must be assembled using nitrogen production for subsequent builds.
- For all future products using blind vias, engineering design reviews must include an additional inspection point for the blind-via locations. The placement drawing must clearly indicate “Do Not Rework” in the relevant areas.
