Hello,
In my current project, I'm getting strange results from a passive RC filter made out of SMD parts. It's acting like each of the capacitors has a 45K resistor in parallel.
Can anyone comment the resistance of SMD capacitors?
Thanks, Nick
Was the board properly cleaned?
-- Service to my country? Been there, Done that, and I've got my DD214 to prove it. Member of DAV #85. Michael A. Terrell Central Florida
Would help if you said what type of SMD capacitor. Biggest effect on SMD stuff, is how well the board is cleaned after assembly, but even with flux residue present, 45K is 'silly'. I have had artefacts of a few meg ohm, with boards that have not been adequately cleaned, but 45, is in a different 'class'.
Best Wishes
Not bloody likely. Did you remove the capacitor and measure it? What kind of capacitor is it.
--
Boris Mohar
Most, if not all manufacturers quote ESR or dissipation factor on their datasheets.
45k in parallel with what capacitance?How did you measure or estimate the 45K?
--
"Electricity is of two kinds, positive and negative. The difference
is, I presume, that one comes a little more expensive, but is more
durable; the other is a cheaper thing, but the moths get into it."
(Stephen Leacock)
As mentioned earlier, you might be running into a cleaning issue. Flux contamination between the terminations is a consideration. Run or spray some flux remover underneath one of the parts, dry it thoroughly w/warm air and see if the resistance significantly increases. Legitimate "no clean" fluxes, such as the rosin based & synthetic resins when used in cored solders, are usually never a problem. If per chance you've used an OA (water soluble organic acid) flux, that wasn't properly removed, you're in for trouble. Was liquid flux dispensed at the sites during rework / touchup? Excess liquid flux of any time, can remain unreacted and cause leakage. Do you have vias underneath the caps? They can trap flux, water from the cleaners, etc. They also reduce the distance between the end terminations, making the part more susceptible to leakage. Any chance these are bottomside glue parts? If they are, then start looking for voids & flux / contamination entrapment in the glue dots. Some adhesives are much worse than others in this respect.
Another thought is that the parts may be cracked. SMD ceramics are very susceptible to cracking from very hot soldering irons and board flexing. Thin boards and large packages are particularly at risk. Cracks can be very difficult to see - especially thermal cracks which are often underneath the end terminations. Flex cracks are typically diagonal, and visible at the side of the cap, running from part way up termination metal right down to the bottom of the package. If you must handsolder a ceramic smt cap, use the lowest temp iron tip that'll do the job - 500F - 600F will usually work fine, but may struggle on a termination that's connected to VCC or GND planes.
Everyone,
Thank you for your suggestions. Removing the cap, cleaning, and putting on a new cap has fixed the problem.
Nick
"nickjohnson" wrote news: snipped-for-privacy@m7g2000cwm.googlegroups.com...
Maybe the capacitor was internally broken? If it is a multilayer cer. capacitor and there is a tiny crack between 2 or more layers you can also measure some parallel resistance. A 4u7/500V ceramic multilayer cap can explode quite good when there is a small internal crack. :)
Al
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