PCB flexion and cracking of SMD capacitors

[treez]

Hi,
For the following nine SMD ceramic capacitors on a PCB, which capacitor would you say is most likely to suffer cracking?
I am talking about cracking due to board flexion. This board gets screwed down to a metal heatsink, with a bit of thermal conductive rubber pad between board and heatsink.

 

[asdf44]

See figure 8 in this document discusses exactly this:
https://www.digikey.com/Site/Global/Layouts/DownloadPdf.ashx?pdfUrl=9A23B3687C14472482403C3540CC7267

Flexing the long way is the worst

In your diagram A is worst, B is next. Everything away from the edges is better.

 

[c_mitra]

Assume the screws are nicely fixed; then major flexing can occur only on the long axis. Components that are mounted along this length shall experience the greatest stress.

I presume that components mounted along the length (long axis parallel to the length of the board) are most likely to suffer cracking.

Near the edges, there is some relief (temp will be highest in the centre) and out of all the components mounted longitudinally, G will see the largest stress.

But because of its size, this is also most securely fastened.

 

[FvM]

This board gets screwed down to a metal heatsink, with a bit of thermal conductive rubber pad between board and heatsink.

Screwed down is the critical point. With permanent force applied, board settling and long time capacitor failure may occur.

You should use a soft thermal gap filler instead of a rubber pad.

 

[treez]

Thanks for this
https://www.digikey.com/Site/Global/Layouts/DownloadPdf.ashx?pdfUrl=9A23B3687C14472482403C3540CC7267
...figure 1 of this suggests to me that we would be best off screweing in our centre screw first, because then that will lead to concave bending, which is better for the capacitor?

I wish soft-termination caps were more common, and that lead-frame MLCC's were cheaper.

Also, its strange that the document doenst say anything about flex cracking and capacitor size...because i believe the bigger a capacitor, the more susceptible it is to flex cracking.?

Also, to reduce depanelisation cracking, i believe a slot can be routed along the board edge to mitigate this.....the slot extending 5mm further (in each direction) than the edge of the capacitor under threat.?

 

[c_mitra]

.because i believe the bigger a capacitor, the more susceptible it is to flex cracking...

I am not so sure.

Bigger size also means physical strength. But how does it scale?

Flexing causes bending stress; for a given amount of flexing, the stress will be now proportional to the area of contact with the board.

But the stain will be depending on the length of the component along the direction of flexing.

But see post #4; perhaps plastic clamps along the edges will be the most preferred solution?

 

[cyberrat]

All the above and ISTR that it was shown that by having the pad narrower than the capacitor ends, cracking was reduced.
However its been so long since I read the article on it that I have no idea who it was by (cannot remember).

 

[asdf44]

Yes size makes a big difference. I've been told by sales reps to stay 1206 and below when possible.

We've had big problems with 18XX and larger ceramics failing. None with anything smaller.

In my experience soft term is quite common at 1206 and above.

At this point I avoid large ceramics in favor of film if at all possible. When you take into account the horrendous C versus V curve of ceramics the size difference isn't as big as it seems.