SMT component minimum height

Many SMT components have maximum height specs, but that can get one into trouble. My little 14-parameter center-of-the-beehive monitor, has five sensor ICs with tiny holes, to let air in for measurement. To keep bees from covering sensor holes with waxy propolis, my board will have a clear thermo-plastic cover. The sensor heights are 0.8 to 0.9mm. So, to keep the cover from covering the sensor holes, I placed five 1.4mm tall parts on the PCB. These should keep the plastic 0.4 to 0.6 mm above the sensor holes.

AVX spec'd their 1uF, 25V 0805 caps at 1.4mm tall, but, oops, they measure 0.77mm thick. OK, Arrgh, the spec was for maximum height. What we need is a minimum height spec!

Poking around in my inventory, I found TDK C2012X7R1H225K125AC 2.2uF,50V 0805 caps, that measured 1.34mm thick. TDK's spec is nominal, 1.25 +/- 0.25 mm.

But if I use them, 3.3V-bus capacitance will be 11uF, instead of 5uF. The sensor power has a 50mA current limit, making a 11uF 3.3V / 50mA = 0.7ms turn-on risetime.

--
 Thanks, 
    - Win
Reply to
Winfield Hill
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So don't connect some

NT

Reply to
tabbypurr

If you need a part for spacing, why bother using it for anything else?

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07-1-ND/5419048

1.4 mm tall and $0.14 at qty 100. Is that too expensive?

min, 1.4 mm max.

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-ND/3471727

It looks like to get anything thicker you will need to bump up to the 1206 size. I expect you can get the 1 uF you want if you let the voltage rise. Use the Digikey search feature. It works great if you know how to use it.

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  Rick C. 

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Reply to
Rick C

Hi Win, I have no idea. But bee season is here, and it's time to let the bee's find all the mistakes in your design. Bee-ta testing. :^) George H.

Reply to
George Herold

This is a good suggestion, find and add tall parts, even if that's their only function.

--
 Thanks, 
    - Win
Reply to
Winfield Hill

Inductors for height, that's a great idea.

That;s a good find.

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 Thanks, 
    - Win
Reply to
Winfield Hill

If there's room for dummy parts why not add an even cheaper spacer which is available in exact sizes? Then performance would be more similar among all units. Heat will conduct to the shield and radiate to the sensors - inverse square.

Reply to
Tom Del Rosso

"Tom Del Rosso" wrote in message news:qbp636$6aa$ snipped-for-privacy@dont-email.me...

Or cut-off chunks of PCB glued or soldered on; they could even be fabbed as part of the build, on breakaway tabs. :)

On a related note, it's a powerful design and construction method, building things by layer -- obviously, 3D printing is usually done this way, but it's been around for a long time in mechanical builds. Some of the first magnetrons for example were stacked punchings, furnace brazed. Heat exchangers are typically stacked plates, alternating, and furnace brazed. You can't fab just anything with a stack of plates -- each individual layer preferably should be a connected shape -- but that can be worked around in the 3rd dimension (if at some expense to layer count / design complexity, of course).

So, you can do interesting mechanical, even pneumatic/hydraulic, things with stacked PCBs, and judicious use of something to bond the layers together.

Tim

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Seven Transistor Labs, LLC 
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Reply to
Tim Williams

Presumably because it then requires another fabrication process, $$$. Makes adding another p&p part look cheap.

NT

Reply to
tabbypurr

h

ong

-

as

That's not a bad idea, but it may be a fair amount of handling. The spacer doesn't need to go through assembly, so doesn't need to be PCB material ev en. A plastic spacer could be used. The trick is to find something *very* economical to make. Perhaps a plastic washer if you have plenty of room o n the board. Or a Nylon nut? This would be held in place by the shrink pl astic.

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  Rick C. 

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Reply to
Rick C

Well there has to be a step to attach the cover anyway. If components are holding it away from the board then something else must hold it in place.

Reply to
Tom Del Rosso

When he described the cover as thermo-plastic and the requirement for multiple spacers around the device, I assumed he was talking about something similar to shrink wrap that will be applied and heated to fit after the boards are fabbed and working.

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  Rick C. 

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Reply to
Rick C

Might try these Mill-Max circular test points. The -3 is .055in. Not cheap but if you only need a few per board might not too bad.

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Digi-Key, Mouser and Heilind carry them.

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Chisolm 
Republic of Texas
Reply to
Joe Chisolm

Usually, you'd see the plastic cover with punched holes, and matching holes in the PCB, with little plastic standoff/snapins. For standoffs, there's adhesive bump items, too.

Oddly, surface mount inductors seem cheaper (unless I just don't know where to shop) than plastic snaps.

Reply to
whit3rd

Hydraulics, with PCB? Never heard about that kind of usage before. Can you elaborate?

Regards

Klaus

Reply to
klaus.kragelund

Use resistors, but only put solder at one end

the trick would be getting the right amount of solder to promote a 90 degree tombstone perhaps a lifting pad under the belly of the resistor would help.

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  When I tried casting out nines I made a hash of it.
Reply to
Jasen Betts

cute idea but too unreliable.

Reply to
tabbypurr

A variant solution would be to mount the plastic cover with double-sticky foam tape; 3M sells it, with permanent adhesive, in a variety of thicknesses. 1.4 mm is circa 1/16 inch.

Reply to
whit3rd

Kemet 1uF 25V X7R arrived, measure 1.24mm, very nice! Off to the assembly house.

Thanks, Rick!

--
 Thanks, 
    - Win
Reply to
Winfield Hill

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