Capacitors for high mechanical/thermal stress environments

[Barry Hardhouse]

Hi All,
Apologies for covering old ground.
I'm designing a power converter (battery charger to be precise) that will be subject to significant thermal cycling and likely some flexing/vibration of the board. It will obviously need some large and low-ESR capacitors in the 10's-100's uF range.
I was planning to make extensive use of large automotive-grade MLCCs, like this:
https://spicat.kyocera-avx.com/product/mlcc/chartview/12105C475K4T2A
I was actually expecting these to have the flexible end terminals, but on receiving the samples they do not.

Question: Is using large MLCCs wise in this application? Should I be sticking to smaller (0805 and down) ceramic and using (for instance) solid electrolytics in parallel? Should I be considering only the MLCCs that do have (visible) flexible end terminals?
I've no first-hand experience in this field, so any advice will be appreciated.

[berke]

They make "high bending strength" capacitors exactly for that reason, e.g. Samsung CL10A226MP8ZUN#

[Barry Hardhouse]

Thanks. "High bending strength" is a phrase new to me - sometimes knowing the right words to google is a good place to start.

[berke]

Thanks. "High bending strength" is a phrase new to me - sometimes knowing the right words to google is a good place to start.

NP.  Some MLCC datasheets/app notes have general guidelines about capacitor placement and orientation to minimize the risk of cracking but it seemed like common sense to me.  For more specific advice mechanical engineers can take PCB and mount/enclosure models and estimate displacements given some indication of the expected forces.

Regarding capacitor types also see this thread https://www.eevblog.com/forum/beginners/calling-capacitor-experts/msg5297002/?topicseen#msg5297002
where there is a discussion of MLCCs and some knowledgeable peoplpe.

[brabus]

If you are really needing 100's of uF, I would consider some adequately rated electrolytics.
Long gone are the times of the capacitor plague: there are some pretty neat capacitors specified for extended temperature ranges.

Unfortunately MLCC's seem to be an ideal solution to all problems, but for some applications (e.g.: high capacity bulk) they are far from that. If you work with 24V or even only 12V, you must take into account the voltage-capacity derating and end up with 63V to 100V rated MLCC's, in a Flexitherm(R) package: a quite bulky, very expensive and overall fragile solution.

MLCC's still find their ideal spot in the nF to the uF range for supply decoupling and POL bulk. I use Flexitherm(R) myself for our automotive* projects, but generally from 1206 footprint and upwards. I never actually needed to go for the flexible technology for 0603 and 0805 caps - never had a failure with those, to be honest.


*most of our projects are in the railway sector, so the lifetime requirements and vibration and temperature cycling specifications are quite harsh.

[Barry Hardhouse]

Thanks for the input folks. Pretty much as I expected, which is reassuring at least.
I'll read that other thread with interest, too.

[jonpaul]

Use TH axial o radial leaded parts, not SMD.

j