There are many factors relating to thermal shock cracking in ceramic
capacitors. I will list but just a few considerations for ceramic
capacitors but wave soldering thick chips can be an interesting science
fair project, interesting academically but doesn't pay the kids college
tuition except of course when I was consulting and it was an excellent
source of income.
1) Wave temperature
2) PWB surface immediately prior to entering the wave
3) Chip size
4) Vendor
5) Chip thickness, all things being equal crack sensitivity is
approximately a square function with thickness
6) Dielectric (composition) used and grain size
7) Electrode metal used, some vendors supplying PdAg electrodes have mixed
ceramic particles in the electrode ink to promote grain growth bonding the
ceramic layers together during sintering. I have not observed significant
grain growth in nickel electrodes though.
8) Internal side, end and cover layer margins or thickness
9) Electrode distribution within the structure
10 Electrode shape

John Maxwell



At 09:06 AM 8/17/01 -0400, you wrote:
>Last thought on this topic:
>
>Over the past 18 months, CALCE has seen a significant increase in ceramic
>capacitor failures (thermal shock cracks, flex cracks, manufacturer flaws)
>among its customers. In terms of pure speculation, its possible that the
>huge demand for capacitors during the boom may have lead to some relaxation
>in process control.
>
>What this implies is that your process was borderline to begin with, and
>then the addition of caps with larger flaws or a lower fracture toughness
>simply lead to the introduction of failures where there were none before.
>
>Best Regards
>Craig
>
>
>-----Original Message-----
>From: TechNet [mailto:[log in to unmask]]On Behalf Of Bartkowaik Christine
>Sent: Friday, August 17, 2001 6:56 AM
>To: [log in to unmask]
>Subject: Re: [TN] To wave or not to wave, that is the question...
>
>
>Kevin:
>
>We have no problem with 1uf caps (TANT). Thus I suspect:
>1-      thermal shock
>2-      too long a time in the wave >2 s
>
>I would not suspect moisture if the boards have already been thru a reflow
>oven and have not sat long.
>
>Chris
>
>         -----Original Message-----
>         From:   Kevin Stokes [SMTP:[log in to unmask]]
>         Sent:   Thursday, August 16, 2001 4:26 PM
>         To:     [log in to unmask]
>         Subject:        [TN] To wave or not to wave, that is the question...
>
>         Old problem: mounting ceramic capacitors to the bottom side of an
>assembly and your customer wants you to wave them on.
>
>         We are having problems with 0805 0.22 uF and 0.1 uF caps going
>through wave and cracking during the process.  We are following all of the
>standard protocols (preheat, etc.).  My understanding is that 0805s are
>typically capable of handling this process.
>
>         Anyone got any ideas short of moving the cap to the top or doing a
>double sided reflow?
>
>         Kevin
>
>         Kevin Stokes
>         Reliability Manager
>         Kimball Electronics Group
>         (812) 634-4207

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