smd capacitors- ICT measuring

Have you verified the measurements on the bench with an LCR meter? Chances are you are experiencing aging with these ceramic caps. We have similar issues with X5R and X7R caps. You will see the effects of aging with returns from the field as well as new production depending on the age of the parts. It's a natural phenomenon and there is little you can do about it. You must operate within the constraints of the components being used. You will need to widen your tolerance. See below for the guidelines we follow at my company.

Dielectric guidelines (Note: This is not a summation).

Y5V -- -20% of nominal value, -part tolerance, -machine tolerance X7R -- -9% of nominal value, -part tolerance, -machine tolerance X5R -- -12% of nominal value, -part tolerance, -machine tolerance

These values indicate a potential drop in capacitance of -9% for an X7R cap over 3 decade hours (10,000 hours or 14 months). The aging process occurs immediately after curing at the mfg. You also have to factor in the part tolerance and the machine tolerance for the given range of capacitance being measured. You can correct this by curing the parts at your facility (baking at elevated temp for predetermined amount of time). Another method used for de-aging is to apply a solder iron to both sides of the capacitor. This will realign the crystalline material in the dielectric and you will be good to go for a few more months. The better option depending on your application may be to just widen the tolerance of your tests to except parts that have aged.

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Is this a new assembly/new test fixture/program? Or is this an existing assembly that is suddenly exhibiting this failure?

Capacitors measured in circuit will not necessarily measure their correct values. Active components in circuit will be affected by the circuit that they are in.

If this is a new program, the program should be checked against a known good golden board from your customer. Run the golden board through the test, and note the values that are measured on these capacitors in the circuit. Compare this to the boards that you are building.

If this is an existing product that has suddenly changed performance, things are a little stickier. First, verify that no one has modified the settings in the tester. Operators/test engineers with access will sometimes adjust capacitor readings in circuit to maintain higher yields. Second, verify the component on the bench out of circuit against manufacturer specifications. Third, measure the component installed in the board with your LCR meter on the bench. Next, check to see if other components have changed in this circuit. And IC from a different manufacturer, for instance, could have different impedences on input/output which are active during the ICT testing of this component. Finally, if no other evidence exists, enlist customer support in testing the boards with the "failing" capacitors in their final assembly, and determine if the components are really failing in application.

Cheers, ..rob

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