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From:
Reliability Resources <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Reliability Resources <[log in to unmask]>
Date:
Mon, 14 Jul 2008 07:35:26 -0700
Content-Type:
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text/plain (323 lines)
Yes,

Flexure, stackup or sintering processes during capacitor manufacturing, or
other variables can cause the cracking. 

It's not always thermal.

 A Root cause F/A should determine what really occurred.

 

 

Clayton Bonn

Reliability Resources

 

Ph: 714-903-1005

Cell: 714-612-9359

e-mail:[log in to unmask] 

www.reliability-resources.com

From: Igoshev, Vladimir [mailto:[log in to unmask]] 
Sent: Monday, July 14, 2008 7:23 AM
To: Reliability Resources; TechNet E-Mail Forum; Guy Ramsey
Subject: RE: [TN] Ceramic caps

 

Thank you gentlemen,

 

Those were almost my first thoughts, but they failed during operation (when,
as I was told there was no fluctuation in the temperature) and for some
strange reason the failure mode was "a short"???

 

I was suspecting some problems with the caps themselves (material,
assembly). Would it be a legitimate doubt?

 

Regards,

 

Vladimir

 

  _____  

From: Reliability Resources [mailto:[log in to unmask]] 
Sent: Monday, July 14, 2008 10:10 AM
To: 'TechNet E-Mail Forum'; Igoshev, Vladimir
Subject: RE: [TN] Ceramic caps

 

Yes we have. Here are some of our findings.

 

*Usually due to excessive change in temperature

- Reflow, cleaning, wave solder, rework

-  Inability of capacitor to relieve stresses during transient conditions.

 

* Maximum tensile stress occurs near end of termination

- Determined through transient thermal analyses

- Model results validated through sectioning of ceramic capacitors exposed
to thermal shock conditions

 

* Three manifestations

- Visually detectable (rare)

- Electrically detectable

- Microcrack (worst-case)

 

*Variations in voltage or temperature will drive crack propagation

 

* Induces a different failure mode

- Increase in electrical resistance or decrease capacitance

 

Corrective Actions (Manufacturing)

 

* Solder Reflow

- Room temperature to preheat (max. 2-3oC/sec.)

- Preheat to at least 150oC

- Preheat to maximum temperature (max. 4-5oC/sec.)

- Cooling (max. 2-3oC/sec.)

- - In conflict with profile from J-STD-020C (6oC/sec.)

- Make sure assembly is less than 60oC before cleaning

 

* Wave soldering

- Maintain belt speeds to a maximum of 1.2 to 1.5 meters/minute

 

* Touch Up

- Eliminate

 

Corrective Actions (Design)

 

* Orient terminations parallel to wave solder

 

* Avoid certain dimensions and materials (wave

soldering)

- Maximum case size for SnPb: 1210

- Maximum case size for SAC: 0805

- Maximum thickness: 1.2 mm

- C0G, X7R preferred

 

* Adequate spacing from hand soldering operations

 

* Use manufacturer's recommended bond pad dimensions or smaller (wave
soldering)

 

* Smaller bond pads reduce rate of thermal transfer

 

Cracking parallel to the electrodes is due to stackup or sintering processes
during capacitor manufacturing

 

* These defects can not be detected using in-circuit (ICT) or functional
test

- Requires scanning acoustic microscopy (SAM)

 

* With poor adhesion, maximum stress shifts away from the termination to the
defect site

 

* No correlation between failure rate and cooling rates (0.5 to 15C/sec)

 

Flex Cracking of Ceramic Capacitors

 

*Excessive flexure of PCB under ceramic chip capacitor can induce cracking
at the terminations

 

* Pb-free more resistant to flex cracking

- Correlates with Kemet results (CARTS 2005)

 

* Rationale

- Smaller solder joints

- Residual compressive stresses

- Influence of bond pad

 

* Action Items

- None

 

Clayton Bonn

 

Reliability Resources/DfR Solutions

 

Ph: 714-903-1005

 

Cell: 714-612-9359

 

e-mail:[log in to unmask] 

 

www.reliability-resources.com

 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Igoshev, Vladimir
Sent: Monday, July 14, 2008 6:08 AM
To: [log in to unmask]
Subject: [TN] Ceramic caps

 

Hi TechNeters,

 

I came across an unusual (at least to me) failure mode of ceramic caps.

They crack along the component body in either normal (from top all the

way through to the bottom), or parallel (from one side all the way

through to the other side) direction to the board the caps are mounted

on.

 

Has anyone seen something similar?

 

Thanks a lot in advance.

 

Regards,

 

Vladimir

 

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