Finding Weaknesses Before the Customer Does
Co-authored with Mike Vella
One of the most effective analytical tools in reliability and quality engineering is Environmental Stress Screening (ESS).
ESS is a process designed to force latent defects to reveal themselves. It does this by applying controlled environmental stresses to hardware, accelerating the transition from hidden weakness to detectable failure.
The goal is not to test whether a product meets specifications. The goal is to flush out defects that already exist but have not yet developed into failures under normal conditions.
Burn-In Is ESS at the Part Level
When ESS is applied at the component or part level, it is often called burn-in. This term is especially common in electronics, where early-life failures are frequently driven by manufacturing defects, marginal solder joints, or material inconsistencies.
Whether we call it ESS or burn-in, the intent is the same: expose weak units early, before they reach higher assembly levels or the field.
Common Stress Types
The most frequently used stresses in ESS include:
- Temperature cycling
- Random vibration
- Mechanical shock
- Humidity
- Functional testing
The choice of stress should always reflect how and where the product will be used. ESS is not about punishment. It is about relevance.
Two Critical Rules of Effective ESS
There are two conditions that must be respected for ESS to be effective rather than destructive.
1. Do not exceed the design limits
The applied stresses must stay within the product’s qualified design envelope. ESS is meant to accelerate failure mechanisms that already exist, not create new ones.
2. Apply the most severe stresses at the lowest hardware level
More aggressive environments should be used at the lowest practical level of assembly, such as parts or subassemblies. As hardware complexity increases, screening environments should become less severe.
This approach ensures that failures occur where they are least expensive to fix, both financially and operationally.
A Quick Mechanical Example
Consider a machined mechanical assembly such as an actuator or rotating shaft assembly.
At room temperature and under light load, a unit may appear to function perfectly. However, small issues can remain hidden, such as:
- Marginal bearing fits
- Residual machining stresses
- Slight misalignment between mating components
- Fasteners that were torqued near the low end of specification
Now apply temperature cycling and random vibration as part of an ESS program.
As the assembly expands and contracts, interference fits may loosen, preload conditions may change, and marginal clearances can shift. Vibration can then amplify these effects, revealing fretting, abnormal noise, increased torque, or early bearing distress.
These are not failures created by ESS. They are defects that already existed, exposed under controlled conditions.
Finding these issues at the subassembly level allows for inexpensive corrections, such as adjusting fit tolerances, updating torque specifications, or improving assembly controls. Discovering them in the field often leads to warranty claims, downtime, and loss of customer confidence.
Why ESS Works So Well
Environmental Stress Screening shifts failures from the field to the factory. That shift matters.
Finding a defect at the component or subassembly level is far cheaper than discovering it after final assembly, shipment, or customer use. ESS helps organizations move failures left on the lifecycle curve, where learning is faster and consequences are smaller.
Final Thought
ESS is not a substitute for good design, sound manufacturing processes, or effective supplier controls. It is a complement to those efforts.
Used correctly, ESS becomes a powerful reliability filter, catching problems early and protecting customers from failures they should never experience.
Authors’ Biographies
Ray Harkins is the General Manager of Lexington Technologies in Lexington, North Carolina. He earned his Master of Science from Rochester Institute of Technology and his Master of Business Administration from Youngstown State University. He also teaches 60+ quality, engineering, manufacturing, and business-related courses such as Quality Engineering Statistics, Reliability Engineering Statistics, Failure Modes and Effects Analysis (FMEA), and Root Cause Analysis and the 8D Corrective Action Process through the online learning platform, Udemy.
Mike Vella served as Senior VP Operations at the Suter Company, an employee-owned food producer located in Sycamore, Illinois for 12 years. Prior to joining Suter, Mike was the Vice President and General Manager of TI Automotive’s Brake and Fuel Group in North America. He is a Fellow with the American Society of Quality and an instructor with the Manufacturing Academy, developing training resources focused on quality, problem solving and statistical analysis.
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