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July 2008

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Subject:
From:
Brian Ellis <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Brian Ellis <[log in to unmask]>
Date:
Mon, 14 Jul 2008 17:32:23 +0300
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In addition to this impressive list, I have also seen cleavage of 
ceramic capacitors along the component due to cleaning in ultrasonic 
baths. This was a very tricky because the reduction of capacity was 
quite small because the capacitor still looked normal; the two halves 
were held together by the solder meniscus. The problem only came to 
light after the boards were put into service and the top half of the 
capacitor fell off.

What happened was that there was a mechanical resonance, the Z axis 
being a quarter wavelength at one of the three ultrasonic frequencies. 
Of course, this was a fairly large capacitor by the wavelength in the 
ultrasound was much longer then than most now, so the same problem could 
arise with a smaller component today.

Brian

Reliability Resources wrote:
> 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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