Author Topic: Questions about surface mount cap types  (Read 1649 times)

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Offline OilsFan

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Questions about surface mount cap types
« on: March 01, 2016, 06:38:57 am »
Building my first SMD board and I've spec'd 2x 0805 and 2x 1206 sized caps for ease of soldering. They are all decoupling caps to ground around adjustable power regulators. In through hole I'd just use electrolytic but I get confused as to what you use in SMD. Is the ceramic cap the bog standard of the SMD world?
 

Offline Simon

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Re: Questions about surface mount cap types
« Reply #1 on: March 01, 2016, 06:44:16 am »
You use the same in SMD as you would in through hole. The only change is the termination type and the sizes. If you need a 1+uF bypass cap it makes sense to use an electrolytic. I just use a 100nF ceramic and a 3.3-10uF electrolytic just like in the good old days of through hole.
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Offline OilsFan

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Re: Questions about surface mount cap types
« Reply #2 on: March 01, 2016, 07:03:55 am »
I need 0.1uF and 1uF. Actually looking at the datasheet for OnSemi LM317 it's saying use a 0.1uF Disc or a 1.0uF Tant on Cin and use a 25uF electro or 1.0uF Tant on Co. Since I used 0805/1206 footprints maybe I should look for some tantalum caps.
 

Offline Simon

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Re: Questions about surface mount cap types
« Reply #3 on: March 01, 2016, 07:32:05 am »
Yes they do tantalum capacitors in SMD. I'm not sure why you can't jyst use alumunium electrolytic. Bear in mind you can parallel capacitors if that makes life easier.
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Offline OilsFan

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Re: Questions about surface mount cap types
« Reply #4 on: March 01, 2016, 10:36:46 am »
Yes they do tantalum capacitors in SMD. I'm not sure why you can't jyst use alumunium electrolytic. Bear in mind you can parallel capacitors if that makes life easier.

Well I guess I could use them but  I'm just not familiar with that footprint. Looks like they have sizes like B, C, D, E, F, G. Not easy to tell what footprint from the information at Mouser. I'm using Diptrace and I can find footprints for SMD CAP_POLC, D, E, and G.  Hmm..
 

Offline Alex Trofimov

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Re: Questions about surface mount cap types
« Reply #5 on: March 01, 2016, 10:55:02 am »
The good side of ceramics is their low ESR, which reduces ripple. But the same low ERS may cause instability if they used as out caps with some switch-mode controllers.

The bad thing about ceramics is that they degrade in capacitance with increasing on applied DC voltage.

It's better to get an extra care if you want to use ceramic ones when the datasheet doesn't recomment them.
In most cases SMD tantalum or polymer are adequate.

 

Offline T3sl4co1l

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Re: Questions about surface mount cap types
« Reply #6 on: March 01, 2016, 11:47:55 am »
Selecting footprints:

Read the datasheet.  Regardless if they are lettered or numbered, there will be a drawing, usually with a table of dimensions for each size.

Read the footprint.  View it in the browser, open it in an editor, whatever; determine what the respective dimensions are.  Compare the copper area to the datasheet recommended sizes (and also download IPC-7351B and see what recommended pad sizes are for the various component types -- manufacturers sometimes botch this).

And also simply reflect on the dimensions yourself, and figure if those footprints would be reasonable to form a solder fillet, if they would be good for hand-soldering if that's what you'll be doing (usually you want some extra soldering space on the ends of the pads, for the iron tip to touch), if they aren't excessive and blobby or prone to shorting to exposed PCB areas (helps if you have the parts in hand to inspect).

No, it's not a straightforward process, and no, I don't know of any supplier that actually provides coherent, high quality, good looking libraries for any EDA software.  KiCAD libraries stink; Altium libraries stink!  The best are only what you personally cultivate, it seems.

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Offline Simon

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Re: Questions about surface mount cap types
« Reply #7 on: March 01, 2016, 12:44:16 pm »
The problem is that each footprint has a half dozen names. as Tim suggestsyou just have to get into the nitty gritty. If you want to standardize on a name format yourself that reflects what you are buying then you can rename footprints and libraries.

I tend to use all my own libraries, mostly made up of vetting the kicad ones and copying them into my own library folder and some I have to draw myself if they are awkward.
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Offline OilsFan

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Re: Questions about surface mount cap types
« Reply #8 on: March 01, 2016, 08:51:00 pm »
OK thank you. It's good to know I wasn't just overlooking something obvious with the SMD electros. I already ordered 3 boards from OSHPark with the 0805 footprint for caps but no big deal, I can rework it for next go round. Knew I would make some mistakes.

I think one video Dave is missing is a real good primer on SMD component types. I know he has done a few on the size of them etc but I haven't seen one that has a real good explanation as to why you might pick one SMD component type over another. Maybe lay something out on a breadboard with through hole then show us what the direct SMD replacements might be and why. Like resistors for instance, we commonly work with 1/4w ones but SMD 0805 and smaller are not even 1/4w. There is a lot more to look out for when selecting components.
 


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