Flying into Lincoln and driving north through Nebraska farmland, I pulled out my phone to change the time zone before landing. The phone beat me to it. Somewhere behind the display glass, a location sensor quietly figured out where I was, synced to the network, and fixed itself.
There was a time when knowing how to change the time on a phone was a small life skill—you dug through settings, switched off automatic time, scrolled through city names. Now it just happens. The hardware works invisibly, never asking for thanks.
(Note to self: that's what good components are like.)
I'm a quality inspector at a medical device company. Over four years, I've reviewed 200+ unique components per year—resistors, capacitors, sensors, optoelectronics—against our incoming quality specifications. In our Q1 2024 quality audit, one component kept surfacing in debates: the Vishay TSAL6100.
This is the story of why that small infrared LED sent me to the Vishay facility in Columbus, NE, and why I now refuse to accept the phrase "functional equivalent" without asking one question.
The Blood Pressure Monitor That Started the Fight
We were developing a home blood pressure monitor. Not the cuff kind—a new optical design that uses infrared light to measure blood flow through the skin. The device shines IR light into the wrist, reads the reflection, and an algorithm converts that signal into systolic and diastolic pressure.
The heart of this design was an infrared emitter: the Vishay TSAL6100, a 940 nm LED with one of the highest radiant intensity ratings in its class. Our lead engineer specified it because the TSAL6100's datasheet shows an unusually narrow intensity distribution. An optical blood pressure sensor depends on a stable, repeatable light source. If the IR output drifts—in brightness, wavelength, or duty-cycle behavior—the pressure algorithm starts guessing.
In a design review, our sourcing manager proposed an alternative. The unit price was 18% lower. "Same wavelength. Same voltage. Same output," he said. "Functional equivalent. Drop-in."
The lead engineer was skeptical. The datasheet did look similar—identical headline spec numbers. But "functional equivalent" is a phrase that never sits right with me.
I asked: "Equivalent under what conditions?"
Silence. Then the team voted to test the cheaper part anyway.
A Visit to Vishay Columbus, NE
While the test lab worked through the samples, my long-planned visit to Vishay's Columbus, NE facility finally came together. I wanted to understand why components from that site performed the way they did.
What I saw changed how I think about specifications.
Vishay's Columbus, NE operation has a deep history in precision foil resistors and strain gauges—the metal elements inside medical pressure transducers and load cells. Those components measure physical force by detecting microscopic changes in resistance. A strain gauge inside a pressure sensor has to respond to changes of a few parts per million. The manufacturing team in Columbus treats every batch like a science experiment. They don't just test a sample at the end of the line. They pull test articles from each lot and stress them through temperature cycles, humidity exposure, and long-term drift measurements.
One engineer showed me a foil resistor that had been thermal-cycled hundreds of times. "We expect the drift," he said. "We design for the drift." Most datasheets describe a component as it leaves the factory: clean, new, untested. The people at that facility talk about components as materials with memory—things that age, shift, and settle. The only question is whether you understand the shift or get surprised by it.
I flew back on a Wednesday. On Friday, our test lab called with a result I didn't see coming.
The 43% Problem
We had run the "equivalent" part through 500 thermal cycles from -40°C to +85°C, the same qualification procedure we use for any component in a medical product. The result: radiant intensity dropped 43%.
The control group—the Vishay TSAL6100s—dropped 3%.
Everything I'd read about component sourcing said that if the datasheet matches, the parts are interchangeable. In practice, I found that's dangerously wrong. Because the datasheet only tells you what happens at 25°C, at 0 hours, under ideal conditions.
The failure wasn't visible at room temperature. It wasn't visible when we flashed the LED manually. It only showed up after temperature stress—exactly the conditions a blood pressure monitor experiences on a shelf in Phoenix, in a mail truck in Wisconsin, or on a wrist during winter.
The worst part: the blood pressure readings from the failed units still looked plausible. The algorithm had silently compensated for the dimming IR signal—for a while. But the margin between a correct reading and a misleading one had nearly vanished.
We rejected the lot, re-specified the TSAL6100, and ran the full qualification again. The monitor passed all tests. The four-week re-run pushed our certification timeline to the edge. The total cost, including lab time and engineering hours, was roughly a $22,000 redo—plus the delay to product launch.
I only fully believed in testing every replacement part after that failed experiment. A lesson learned the hard way.
A Thought Experiment: The Transparent Smartphone
Here's why this matters beyond one blood pressure monitor.
Concept videos of transparent smartphones are everywhere. A phone with a fully see-through body, glass panels on every side, internal parts visible to the naked eye. I find them fascinating, but they also make a perfect stress test for thinking about component reliability.
A transparent phone would expose its IR emitters and sensors to direct sunlight, which is saturated with infrared. The internal temperature inside a glass chassis could hit 60°C during fast charging, then drop below freezing during a winter morning commute. An IR emitter that loses 43% of its output under temperature stress would never work reliably in that device. The concept would quietly fail—not dramatically, but consistently. Face unlock would glitch. Proximity sensing would misbehave. The phone would feel "unlucky."
The "a part is a part" thinking comes from an era when components were simpler. Today, optoelectronics like the TSAL6100 are engineered for specific aging behavior, specific temperature slopes, specific duty cycles. You can't swap them by matching three numbers on a datasheet.
Whether you're building a blood pressure monitor or a transparent smartphone, the component that looks fine on paper is the one that kills your product. The component that matters is the one that remains stable when the environment stops being nice.
Three Questions Before You Swap Any Component
Since the TSAL6100 incident, I've built a short protocol for any proposed component change. It takes about 20 minutes to run, and it has already saved us from repeating this mistake.
1. What's the distribution, not the headline number? Datasheets report minimums, typicals, and maximums. They rarely report lot-to-lot variation. Ask the supplier for process capability data—Cpk on the critical parameters—and compare it with your current part. If the data doesn't exist, that's your answer.
2. What happens at the edges—and one step beyond? Your product will not live at 25°C. It will live in warehouses, in delivery trucks, on human bodies. Get thermal cycling data, humidity data, and long-term aging data. Or run the test yourself. It costs less than a recall.
3. What have they seen fail before? Every reliable manufacturer keeps failure records. At Vishay, they openly walked me through a humidity-triggered drift failure in a foil resistor—how they caught it, fixed it, and re-qualified the process. That honesty was worth more than any marketing document.
Every component is a claim. Per FTC advertising guidelines, claims must be substantiated. In engineering, substantiation means data. If a supplier says "this is equivalent to the TSAL6100," they should be able to prove it under the conditions that matter to your product.
An informed customer asks better questions. I used to think that phrase was corporate nonsense. Now I understand it: the customer who asks "under what conditions" is the customer who never has to issue a recall.
Changing Time on a Phone, One Last Time
On the flight home from Columbus, NE, I checked my phone. The time was already correct. The sensors had synced it automatically—invisibly, silently, perfectly. It did exactly what it was supposed to do.
Good components are like that. You never notice them until they fail.
Our blood pressure monitor passed certification and is now heading into production. There's something satisfying about a product that works so quietly that nobody thinks to ask what's inside it. Nobody will ever know it contains a Vishay TSAL6100. They'll just know that the readings are right and the battery lasts.
The next time someone tells you a cheaper component is "equivalent," ask my question: Equivalent under what conditions?
Not ideal, perhaps. But better than a $22,000 redo.