How might over temperature during refow affect film capacitors? - Page 1
Forum section: electronics / general

author: AndersJ
author_rank: Frequent Contributor
timestamp: November 02, 2019, 07:41:36 am
content: I have problems with a switched mode power supply,where a film capacitor seems to misbehave in the feedback loop.I use film caps due to microphonic issues with ceramics in previous generations.It is a 10 nF, 63V cap, size 1812.The capacitor seems to leak slightlty, varying slowly,causing a few hertz of very low (single digit mV) ”ripple” on the output voltage.I also see, in a few cases, a small sensitivity to signals from a trace between the pads,where the signal transitions seem to couple capacitively to the capacitor.What might happen to a film capacitor if its max specified reflow temperatureis exceeded during the soldering process?

author: forrestc
author_rank: Unknown
timestamp: November 02, 2019, 07:54:21 am
content: Quote from: AndersJ on November 02, 2019, 07:41:36 amCould you be more specific about which switchmode IC/architecture you're using and which film capacitor and in which location in the circuit?Switchmode power supplies are often sensitive to various parameters of the capacitor beyond the voltage and capacitance. For example, film capacitors typically have a higher ESR and ESL than a ceramic capacitor which can definitely make a switchmode supply act differently.

author: AndersJ
author_rank: Frequent Contributor
timestamp: November 02, 2019, 08:03:00 am
content: It is a boost converter using the LT3571.The design is almost identical to the example on the first page of the data sheet.The cap in question is the 10 nF across the 1M resistor.The instabilty is in 20% of the products.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 02, 2019, 08:47:01 am
content: At only 10 nF there is no big problem getting ceramic caps with C0G dielectric instead of cheap X7R. These are closer to an ideal capacitor - not micro-phonic, low leakage, low loss. Chances are the C0G ceramic could be still be cheaper than the film version.How the film capacitors behave is hard to tell. There is a chance for delamination, possibly cracks developing, maybe more sensitive to humidity.Anyway, I would not take the risk.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 02, 2019, 08:59:28 am
content: SMT stacked PPS film caps can have leakage problems, indeed. It's a flux and cleaning problem, AFAIK. Don't know that it causes any significant excess noise at frequencies relevant to control loops though.Seems more likely there's an outside influence you haven't accounted for..? How have you proven that the problem is due to varying capacitor leakage?Tim

author: AndersJ
author_rank: Frequent Contributor
timestamp: November 02, 2019, 12:52:59 pm
content: I have not proven anything.I do however know there is coupling from the trace mentioned earlier.Not sure about the leakage, but see no other explanation,since a capacitor replacement removes the ripple.

author: jmelson
author_rank: Super Contributor
timestamp: November 02, 2019, 11:59:00 pm
content: Quote from: Kleinstein on November 02, 2019, 08:47:01 amRight! I used to use 250 V film caps in a servo amplifier, and mounted them by hand to avoid temp issues.When big C0G caps became available, I switched, and they are very good.Jon

author: wraper
author_rank: Unknown
timestamp: November 03, 2019, 12:19:07 am
content: Using huge film cap is plain stupid. You can get 10n C0G, easily available starting from 0603 size. Much cheaper and not finicky. Basically has most of best properties of ceramic and film capacitors combined.

author: forrestc
author_rank: Unknown
timestamp: November 03, 2019, 02:06:33 am
content: Quote from: AndersJ on November 02, 2019, 12:52:59 pmIt sounds like you might have a layout problem instead of a capacitor problem. I agree with others though that moving to a c0g might solve some of your problems.Can you share a screenshot of the layout? "Couping from a trace" seems like a layout issue and the capacitor change might be changing things just enough that you're seeing the effects more with the film than with the ceramic.

author: AndersJ
author_rank: Frequent Contributor
timestamp: November 03, 2019, 05:39:09 am
content: The problem remains when the trace between the pads is taken out.Its effect is marginal, and occurs in very few cases.