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Old 23rd May 2006, 02:33 PM   #1
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Default SMD Caps

on the topic of SMD and XO's and filters -- can anyone recommend decent SMD capacitors for filters -- I've used some of the film types from Digikey about 5 years ago -- and wasn't happy with the result -- for now i am just as happy to use polystyrenics here -- with the soldering station cranked down a bit.

can anyone comment on the PEN or PPS variety. It interests me that these devices are now specified in 2% tolerance.
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Old 24th May 2006, 04:43 PM   #2
BWRX is offline BWRX  United States
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The PPS caps would probably be worth a try in my opinion. I've never used them so my opinion is only speculation.
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Old 28th May 2006, 04:26 PM   #3
sbrads is offline sbrads  United Kingdom
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I've no faith in SM ceramic caps that have been hand soldered. We use them a lot at work on military and commercial gear and they're OK if a flow solder process is used or if they're hand soldered onto a preheated board, but woe betied the home assembler who solders high value caps (>.47uF) onto a cold board. Failures are extremely common after a little thermal cycling because of the permanent physical stresses exerted on the caps after being soldered onto a cold board once the cap and board are at the same temperature.
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Old 29th May 2006, 04:19 PM   #4
AndrewT is offline AndrewT  Scotland
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Hi Brad,
can you give details of what to do?
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Old 29th May 2006, 06:26 PM   #5
sbrads is offline sbrads  United Kingdom
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For hand soldering higher value ceramic capacitors, the only reliable way seems to fit the SM caps first so that the board is as flat as possible and can be heated up on an electric hot plate while soldering each component. 150 deg C should take out most of the stresses. Not sure though how you would do this at home without the right gear, I suppose if you're only fitting a few components, just putting the board in an oven and removing it for performing the soldering quickly should be OK.

Another thing that kills these caps is bending the board even a small amount after fitting. They don't usually fail straight away, just after a few thermal cycles.
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Old 29th May 2006, 06:29 PM   #6
poobah is offline poobah  United States
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Andrew,

You could ease this problem by heat soaking the board at 80 C... the quickly swipe one end of the caps in question with a hot iron.

Geeze... there are even guys that do reflow in toaster ovens here. No real reason it can't work... practice (and sheer guts).

How's Scotland today?

EDIT: Cross-post redundancy error...

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Old 29th May 2006, 07:22 PM   #7
Mad_K is offline Mad_K  Norway
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Hand soldering of SMD ceramic caps is no problem. It is allowed according to IPC A-610D. Even for mil.spec. boards. I do some assembly and rework/modification on industrial instrumentation and we also have a pretty rigorous temp cycling programme. Shure, if the operator (person who did the soldering) did a poor job, they will fail..
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Old 30th May 2006, 06:42 AM   #8
AndrewT is offline AndrewT  Scotland
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Hi,
Sounds like soldering ONE end of each SMD (diodes, resistors and caps) then heating in a low temp oven (100degC), just as well I'm not married, and then quickly soldering the other ends will help.

Is it the soldered joint that fails or the component's internal structure/connections?

Scotland is nice and sunny although some guy up North keeps sending down a d..m.d cold wind to ensure we keep our caps on, (notice s m d just to prove we're still on topic).
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Old 30th May 2006, 06:55 AM   #9
Mad_K is offline Mad_K  Norway
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This is really not an issue for diy electronics. Who is going to put heir boards through a potentially disruptive thermal cycle? For what purpose? You put a little bit of tin on one pad, heat it up, slide the component over, solder the other side, resolder (adding a little liquid flux is nice) the first side and you are finished! I fully agree on the fact that this method puts a little more thermal strain on the components, but nothing to shout out in a diyaudio forum about..
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Old 30th May 2006, 07:01 AM   #10
sbrads is offline sbrads  United Kingdom
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The dialectric cracks (I've seen it on a microscope) and then it carries on working for a while until a bit of flexing (e.g. vibration or clumsy assembly of the pcb) or thermal cycling makes the crack worse and the cap goes leaky. The leakage may be enough to cause extra dissipation in the cap and that makes the leakage worse etc. etc. If there's a lot of current available then it generally goes short circuit before going up in smoke and if you're really unlucky the board gets so hot it burns a hole clean through it.

If you're lucky the cap just goes leaky in a circuit that can't provide enough power to warm the cap up so the circuit just never performs properly.
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