Yield Plunges Before Mass Production Even Starts?
I still remember several years ago, a batch of our new products was just about to enter mass production. Process parameters were all tuned, and tests showed no issues. Then, suddenly, a call came from reliability verification, their tone extremely tense: "Bro, for your batch of products, the HTOL (High Temperature Operating Life) report is out, and early failures are a bit high. We might have to hold it!" My heart sank. I thought, "Oh no, everything was smooth sailing before, why the sudden reliability issue?" At that time, there was less than a month left until mass production. If this were to be held up, the entire shipment schedule would be delayed, and my boss's face would surely be hotter than the temperature inside the HTOL oven. Later, we discovered that there was a problem with our Accelerated Life Test (ALT) parameter settings, which almost derailed the entire project.
Where Was the Problem? Damn It, How Can We Wait for Lifespan?
Simply put, how do we know if a product will last ten or eight years? It's impossible to actually wait ten or eight years for verification; my boss would definitely strangle me to death first. So, we have to use an "accelerated" method. Think about it: if you put something in an oven, won't it break down faster than at room temperature? Yes, that's the core concept of Accelerated Life Test (ALT). By increasing stress (like temperature, voltage, humidity), we make products "fail" prematurely, and then extrapolate these "accelerated" results back to the product's lifespan under normal operating conditions. The most commonly used mathematical model for this is the "Arrhenius Model," which tells you that as temperature rises, chemical reaction rates increase, and product aging also accelerates.
So, the key is that we don't just arbitrarily raise the temperature. Instead, we must accurately calculate the "acceleration factor" using the Arrhenius model to correctly convert short-term test results from the lab into a real-world lifespan of ten or eight years. If this factor is calculated incorrectly, all the data becomes useless.
How Is It Actually Done? Getting the Acceleration Factor Right Is Key
Frankly, the Arrhenius model is essentially a formula. But in practical operation, you don't need to become a mathematician to derive it; you just need to understand its core principle and input parameters. The most crucial one is "activation energy (Ea)." Different failure mechanisms have different activation energies. For example, electromigration failure in typical semiconductor components might have an activation energy around 0.6 eV; whereas, thermal degradation in some plastic materials might have a different activation energy.
For example, suppose your product's target lifespan is 10 years, and its operating environment temperature is 55°C. If you perform HTOL at 125°C, and your failure mechanism's activation energy is 0.7 eV. Calculated using the Arrhenius formula, this 125°C test yields an acceleration factor of approximately 100 times. In other words, running for 1000 hours at 125°C is equivalent to running for 100,000 hours (about 11.4 years) at 55°C. This allows you to evaluate whether the product can achieve a ten-year lifespan in a short period.
Therefore, the key is to first identify your "primary failure mechanism" and then find the corresponding "activation energy." This activation energy is usually not something you conjure out of thin air, but rather based on industry experience, literature, or your past product failure analysis results.
The Most Common Pitfall: Randomly Picking Activation Energy, Leading to Catastrophic Failure
The most outrageous incident I encountered was when a colleague, either rushing for time or simply not understanding the concept, directly took a "generic" activation energy and applied it to Arrhenius. What happened? After running for 500 hours at 125°C, the defect rate looked acceptable, with a CPK of 1.08 barely passing. But a year after the products were actually shipped, customer complaint calls started pouring in. We went back to analyze and discovered that the activation energy applied at the time was completely wrong, underestimating the true acceleration factor.
Simply put, you might have thought that 500 hours at 125°C had accelerated the product by 5 years, but in reality, it might have only accelerated by 2 years. This meant that the test at the time was insufficient to screen out early-failure products. Therefore, incorrectly determining the activation energy completely undermines the foundation of the entire accelerated life test, ultimately leading to a skyrocketing product defect rate, with DPMO directly jumping from 6210 to something like 20000. It would be a miracle if customers didn't flood your phone with complaints.
One Thing You Can Do Today
Go back and check what the activation energy is for your product's "primary failure mechanism."