Root-Cause Analysis - Product Teardown
Problem Overview
A significant increase in customer returns was observed for a handheld water flosser product that would abruptly stop functioning during use. Initial failure reports were nonspecific, but teardown inspections quickly revealed consistent water ingress damage across returned units, including corrosion and electrical failure on the internal PCBA, battery terminals, and motor connections.This product presented a unique diagnostic challenge due to its internal piston pump architecture, which pressurizes water internally to generate the cleaning jet. As a result, failures could originate from:
-
External water ingress from the environment
-
Internal water exposer from the pressurized pump system(water "outgress" instead of water ingress, a concept that still makes me smile)
Initial Investigation & Challenges
A large population of returned units required confirmation of failure mode, but teardown was:
-
Time-consuming
-
Destructive
-
Not easily repeatable, as the device could not be resealed once opened
Test Method 1: Pump Cycling & Teardown
The first approach involved:
-
Cycling the piston pump system during operation
-
Immediately tearing down units afterward
-
Inspecting for evidence of internal water outgress from the pump into the electronics cavity
While technically sound, this method proved impractical. Once opened, the product could not be resealed, and variability in water distribution made results inconsistent. Unfortunately, this approach did not yield actionable conclusions.
Breakthrough Test Method: Vacuum Bubble Leak Detection
To isolate external leak paths, I developed a submerged vacuum test method:
-
Units were submerged in water inside a vacuum chamber
-
Vacuum was applied to create a pressure differential
-
Trapped air escaping from leak paths produced visible bubble streams
This method proved highly effective and repeatable.
Root Cause Identified
The vacuum bubble test revealed a consistent failure location:
-
A fastener-installed plastic cover near the top of the device
-
The fastener used an O-ring under the bolt head to seal the interface
If the bolt was slightly over-torqued during assembly, the O-ring would bottom out and deform past the bolt head, creating a small but critical leak gap. Because this cover was located in a high-exposure water zone, the gap almost always resulted in water ingress during normal use.
Corrective Action
To eliminate the torque-sensitive failure mode:
-
The O-ring seal was removed
-
The assembly process was updated to use thread-locking sealant (Loctite) on the fastener instead
This change:
-
Fully sealed the interface
-
Eliminated sensitivity to operator torque variation
-
Improved manufacturing robustness and repeatability
Secondary Challenge: High-Volume Return Confirmation
Even after the design fix, hundreds of returned units remained that needed:
-
Rapid failure confirmation
-
Non-destructive (or minimally destructive) testing
-
Documentation for failure trending and quality records
Vacuum chamber testing was effective but slow and resource-intensive.
Rapid Diagnostic SOP Development
Drawing on my experience working as a mechanic, I adapted a classic leak-detection technique:
-
A small access hole was drilled into the housing
-
A hose fitting was attached to allow low-pressure air injection
-
A soap-water solution was sprayed on the suspected leak region
-
Internal pressure caused soap bubbles to form at leak sites
This method:
-
Took under 5 minutes per unit
-
Clearly visualized leak locations
-
Allowed rapid classification of returned units by failure mode
Project Overview
This project focused on resolving a critical drop-test failure in a consumer product driven by a sonic motor with a long, single-piece metal shaft. Unlike comparable designs that use segmented or plastic shaft components to absorb impact energy, this architecture required a rigid metal shaft to maintain acoustic performance—introducing a unique mechanical vulnerability under axial shock loading.
The product repeatedly failed during qualification drop testing, necessitating a full root cause analysis (RCA) and design remediation prior to production release.
Failure Mode Observed
During drop testing, units exhibited rapid performance degradation or complete functional failure after as few as 1–2 drops in early builds. Later iterations showed partial improvement but still failed to meet reliability targets.
Observed symptoms included:
-
Sudden loss of motor efficiency
-
Increased friction and audible noise
-
Complete motor seizure in severe cases
Design Solution
A new flanged bearing architecture was developed to mechanically block axial shaft migration.
Key design changes:
-
Flanged bearing added to the top of the motor
-
Physically prevents the shaft from translating downward
-
-
Metal reinforcement plate added beneath the lower bearing
-
Protects plastic housing if slip occurs
-
-
Adhesive bonding and shaft knurling
-
Increases shaft–bearing retention
-
-
Optional laser welding
-
Validated to withstand ~1500 N axial load for future revisions
-
This solution preserved the acoustic and performance requirements of the sonic motor while eliminating the failure mode.
Validation & Results
Post-remediation testing showed a step-change improvement in robustness:
-
Units survived >8 head-first drops with <20% performance loss
-
No catastrophic housing failures observed
-
Teardowns confirmed:
-
Shaft slip occurred without damaging bearing races
-
Impact energy was absorbed by the flange and reinforcement plate
-
This confirmed the root cause was properly identified and mitigated.
.
Engineering Impact
-
Enabled the product to pass drop-test qualification
-
Preserved sonic motor performance without compromising acoustics
-
Delivered a manufacturable solution compatible with existing motor assembly processes
-
Informed future design standards for long-shaft motor architectures
