Electromigration and/or dielectric erosion in FR4 substrates
Stefen
thin FR4 between DC voltage potentials. I have a couple of data points
that FR4 can have punch through and arcing at voltages far below the
dielectric breakdown specified for the materials.
1.) 48 Vdc punched through 4-5 mils of FR4 within 6 months continual
usage
2.) 35 Vdc punched through 0.4 mils of FR4 in about 630-650 hours
3.) FR4 type was not specified, but these are credible data points.
This (in part) appears to be dielectric erosion exacerbated by
Electro-migration of the copper traces/planes. More information would
be great.
I am concerned by the data points because I am looking at ~125 Vdc on 4
mil thick FR4 and the product is expected to last 24x7x365.25x10
(613620 hours) minimum.
If any one could help, there doesn't seem to be a lot of information
available. Thanks.
Boris Mohar
Your data points were probably wet. Literally speaking that is.
Regards,
Boris Mohar
Got Knock? - see:
Viatrack Printed Circuit Designs (among other things) http://www.viatrack.ca
Mac
100V/0.004"=31,250 V/in = 375,000 V/ft.
I wouldn't think that you would experience dielectric breakdown across
4 mils. Even 4 mils of dry air. But I haven't ever built a board like you
describe.
--Mac
mrob...@worldnet.att.net
>I am concerned by the data points because I am looking at ~125 Vdc on 4
>mil thick FR4 and the product is expected to last 24x7x365.25x10
>(613620 hours) minimum.
70 years, wow! Must be Western Electric or military.
Matt Roberds
Pooh Bear
Why not use Kapton as a substrate instead ?
Can you actually *get* 0.1 mm thick FR4 ?
Graham
Stefen
Close, but it is 10 years with 100% utilization (all day every day),
not 70 years:).
Terry Given
look harder. 24*7*365*10 = 70 years. there are *NOT* 7*365.25 days in a
year. To put it in perspective, the "normal" working year is 2,050 hours.
Cheers
Terry
Terry Given
I have come across anecdotal evidence of low-voltage breakdown of FR4 as
well. But I looked into the product in question (a commercial UPS), and
found some pretty shonky layout/design/workmanship.
Firstly, the PCBs were all made somewhere in China, for the lowest price
obtainable (100,000 per annum) no doubt using the cheapest, nastiest
material known to man. Looking at half a dozen units, I saw significant
variation in the LPI solder mask coverage and screen-printed overlay
registration. I would suspect the process is neither clean nor consistent.
The design itself had acute angles, potentially allowing etchant to
accumulate. bad CAD man. The copper width/thickness was also inadequate,
and the high-current low voltage traces were discoloured. Standard 125C
Tg (glass transition temperature) FR4 was used.
For our MIL products we stuck with a large, reputable PCB manufacturer
with stringent process & quality control. We used high Tg FR4 (150C
IIRC), and designed tracks for a 10C temperature rise (and considered
dynamic thermal resistance too). And we very carefully cleaned then
conformal coated our PCBs.
One faulty PCB was found thru hi-pot testing (17,000 commercial units)
but that had a clearly visible lump of contaminant within the PCB
material itself. Which came from China, but was cheap :)
Cheers
Terry
Rich Grise
Syntactically, it's close: "twenty-four seven, three hundred sixty-five
(and a quarter) days a year" sounds almost natural, since "twenty-four
seven" means "continuously" in the vernacular.
Oops! Is the vokda kikcing in alradey?
If 7-11 stores are open 24 hours a day, howcome the doors have locks?
Cheers!
Rich
Robert Baer
Because, the stores were supposed to open at 7AM and close at 11PM.
Stefen
I am sufficiently embarrassed.