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DI-145 Data Acquisition Starter Kit Developer's Diary

4/25/2011 Note #24 A Tale of Bad Resistors — A follow up report

You may recall that we had a rather alarming problem with the first few DI-145 boards from our first production run. We've taken the issue up with the resistor manufacturer with disappointing results. We sent them several bad resistors that were lifted off DI-145 boards that would not calibrate properly. We also sent them an entire DI-145 with one channel that failed because of a faulty resistor so they could verify this in-place as it came from the assembly house. Their response was:

"Based on the analysis of the returned parts, short-time overload testing and analysis, the only failure mechanism observed was due to an electrical over stress (EOS) condition."

Hmmm. Let us understand this:

  1. Parts arrive from the manufacturer on a reel.
  2. The reel is sent to an assembly house along with thousands of other components. 
  3. Finished DI-145s arrive from the assembly house.
  4. Boards are placed on a fixture that applies ±10V for calibration.
  5. Boards that fail have only defective 1 M Ohm resistors.

How exactly does this translate into an electrical stress event? The answer is that it doesn't. We followed up with the vendor and explained that, including a rundown on the test process that all the boards were subjected to without exception. We also explained that when the defective resistors were replaced the board operated without further events. The vendor responded that we must be mistaken. Sigh...

We're giving the vendor another opportunity to provide an explanation for the failure rate, or at least a better fib that shows more imagination. However, we suspect that this company is headed to vendor purgatory under the heading, "supplier of last resort."

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