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cap size and copper thickness vs part size questions

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CW

#37690

cap size and copper thickness vs part size questions | 7 November, 2005

How large of chip capacitor can we use reliably? I��ve heard they tend to crack. I��ve also heard if you pre heat the larger chip caps (1812 and above) before vapor phase reflow, they won��t crack.

If I use 2 mil copper for a power supply board, am I limited to what size caps I can use, or what pitch of parts I can use? Specifically, are 0603 caps, and parts with .65mm pin spacing?

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Rob

#37692

cap size and copper thickness vs part size questions | 7 November, 2005

Hi CW,

Larger chip caps can be susceptable to cracking, but mainly this is due to handling - flexing of boards is the main culprit (the part being mechanically joined at both ends to the board). PCB's usually flex easiest along one axis, and larger sizes should be mounted in line with this to avoid flexing.

Regarding vapour phase reflow, its a very gentle slope (2 degrees a sec?) with a peak of around 230-232C, way below what a reputable ceramic supplier can handle (i.e. Murata, Kemet, AVX etc.) so you shouldn't be seeing too many issues there.

Regarding your layout, I can't be of much help except that as you go down case sizes obviously the capacitance & voltage becomes lower, and the dielectric more inferior as you cannot fit so many plates (layers) inside a package.

Good luck,

Rob.

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CW

#37699

cap size and copper thickness vs part size questions | 7 November, 2005

hi Bob, Thanks for the info.

I understand your comments, but I am looking for specific guidance,

If I am putting chip capacitors on a .063 FR4 board, what size package should we stay away from to avoid cracking? (I realize this is dependent on how the board is handled. Do you even see that type of failure data?)

I��m not sure I understood your comment on orientation. Is my figure below correct?

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As far as my second question, it isn��t really related to capacitors. Let me state it differently:

If I use 2 mil copper on the outside layers of a PCB, am I limited to what pitch of parts I can use? Specifically, will using 0603 caps or parts with .65mm pin spacing cause problems?

Thanks a lot!!

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Rob

#37706

cap size and copper thickness vs part size questions | 8 November, 2005

Hi CW,

Regarding orientation, yes, your diagram is right - see the following link for confirmation.

http://search.murata.co.jp/Ceramy/image/img/A18X/C2EB3C.PDF

Regarding information on bending strength you want to talk to your local Murata office or distributor and get your hands on the following:

"Chip Monolithic Ceramic Capacitor Bending Strength Technical Data"

Unfortunately the version I have is from 1995 (Murata Ref: C05E), and the technology has moved on a lot since then, otherwise I would send it to you.

For your last request, I sorry I'm not an expert in that area, there are more qualified people on this forum who should be able to help you.

However: it is not recommended to use anything over 1206 size (3.2mm x 1.6mm) on solid metal PCB's due to the expansion & contraction during reflow, that leads to the cracking of ceramic capacitors - using a thick copper layer may replicate this somewhat. I think that this will be more of a problem with lead-free solders as there is still a slight elasticity in Tin/Lead solder after reflow.

Regarding minimum pitch (pad spacing)I think this would depend on what power your PCB will be handling & what approvals it holds.

Sorry that I can't be of more help,

Rob.

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