Is your Inventory Expired?

Happy New Year everyone! Hope you all had a wonderful holiday and your 2019 is off an running. In my last article, I discussed that the capacitance value of Class 2 MLCC degrades over time, the majority occurring early in their ‘life’. You should discuss with your engineering team about a lower limit, estimate the capacitance loss using the aging chart, then calculate a shelf life for your inventories of Class 2 MLCC. But how do you know if they are Class 1 or Class 2?

It starts with the manufacturer datasheet. Unfortunately, the datasheets don’t make it simple. Rather than expecting it to be clearly in the description, you need to look at the device code and the temperature characteristics. Class 2 codes all start with X, Y, or Z. All others are Class 1 codes. Another method is to look at the how the capacitance change is measured. Class 1 MLCC is measured in PPM and Class 2 are measured as a percentage. Temperature characteristics are usually listed on the second page of the manufacturer datasheet. You can see below that the capacitance change in the temperature characteristics is measured in 15% and the dielectric material is listed as X7R, both pieces of information telling us this is a Class 2 device.

Before starting White Horse, I was with an EMS and we did have issues with an assembly failing, and the root cause was that the capacitors were old and, when tested, had lost enough of their value to cause problems in the application. When I first heard this, I thought the engineer was making up excuses to shift blame to materials, so I turned to The Oracle of Engineering JT Rickenbach, and he verified the explanation. So, yes, it’s a real thing. It may not have an affect on all of your products, so it’s best to discuss with your engineering teams.

When you set those shelf lives, monitor your inventory levels against your production forecast. You might want to sell some of them before you have to do a lot of rework and baking.

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