[sdiy] Tantalum manufacturing / assembly failures OT?

[Richie Burnett]

Thank you.  That presentation is *very* interesting indeed.  The failed 
capacitors I witnessed weren't from Kemet but they are based on the same 
technology from a similarly well known brand manufacturer.

The bit about what they consider an acceptable voltage de-rating factor is 
very interesting!  So basically a 25V "rated" device is only good for 12.5V 
service if you want an acceptable failure rate at initial power up and in 
the following 1000 hours of service.  And this is before you take into 
account any degradation due to exposure to reflow temperatures during 
assembly / rework, and any further degradation due to possible electrical 
design issues such as voltage transients, current ripple, etc. if they were 
to exist.

I'm used to generously spec'ing the voltage ratings on ceramics because of 
the nasty voltage-coefficient effect in high-k ceramic caps, but had no idea 
that Tantalum caps needed that much safety margin on their voltage ratings. 
I guess I'm more used to aluminium electrolytics that run quite happily near 
their rated voltage, and are even characterised to tolerate significant 
over-voltage surges for many seconds without damage!

-Richie,


-----Original Message----- 
From: Ullrich Peter
Sent: Wednesday, November 14, 2018 9:16 PM
To: Richie Burnett ; SDIY List
Subject: Re: [sdiy] Tantalum manufacturing / assembly failures OT?

Hi Ritchie!

We had some failures with Tantalum capacitors but not that much.

The Austrian Kemet Sales Manager once gave us a presentation about derating 
you should take care of.
I found this presentation online - you can have a look at it here at 
Digikey:
https://www.digikey.com/en/ptm/k/kemet/derating-guidelines-for-surface-mount-tantalum-capacitors/tutorial

Ciao
Peter

________________________________________
From: Synth-diy <synth-diy-bounces at synth-diy.org> on behalf of Richie 
Burnett <rburnett at richieburnett.co.uk>
Sent: Wednesday, November 14, 2018 18:46
To: SDIY List
Subject: [sdiy] Tantalum manufacturing / assembly failures OT?

Since we're discussing Tantalum capacitors, has anyone else who manufactures
things in reasonable quantities, (either hobby, SDIY or in their day job,)
had problems with SMD Tantalum capacitors failing short or partially shorted
due to excessing heat during reflow?

I've had a bunch of boards made by a well known UK PCB fab and population
company, and several off them had 100uF/25V Panasonic SMD Tantalums fail
short-circuit within about 10 seconds of powering up the board.

Just to rule out a design issue, the two capacitors that were failing only
saw 15VDC from a well regulated power supply, and charge slowly through a
resistor so there is no significant inrush current or AC ripple current
either.  And they weren't installed with incorrect polarity!  The two
devices that failed on each board are right next to a D2PAK SMD power device
that is clearly discoloured from excessive temperature during assembly, so I
can only suspect that the use of a hot-air pencil during assembly is the
culprit!

I haven't used SMD tants that extensively in electronics, so would be
interested to hear others experiences, either on list or via PM.

-Richie,

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