Hi Daan,
Preheat rates is one of the most confusing guidelines ever stated by MLCC 
vendors. If you ask them specifically where the numbers come from,,, well 
don't be surprised by the answers. Basically those guidelines are vapor (or 
perhaps vapour) especially when you consider that billions are wave 
soldered each year and exposed to rates of change exceeding 100C/sec with 
no cracking. I would look first at board handling for 0805 sized chips and 
possibly fillet size and cooling rates. If parts were larger cooling rates 
become more important. For those that want to rush to point out in some of 
my much earlier publications that I repeated the party line of 4C/sec, it 
was just that, a party line.

Basically you can't crack 0805 sized MLC capacitors during reflow during 
the heating stage. Cool down is not very likely unless you are using 
hurricane cooling fans and huge bulbous solder joints.

John Maxwell

At 05:32 PM 9/4/2003 +0200, you wrote:
>Hi Technet,
>
>I just saw a specification for the soldering profile for a simple
>0805-sized ceramic chipcapacitor. It mentions a maximum preheat ramp,
>both for wave and for reflow soldering, of 2 degrees Celcius per second.
>
>In the beginning of the slope, for some of our products, we go slightly
>higher than this.
>One of our customers now thinks that the preheat ramp being steeper
>than the component suppliers specification must be the reason for
>malfunctioning of this specific part.
>
>I won't annoy you with all the discussions we had and the
>investigations that we've done, I just hope to get an answer to the
>following question: is it possible that a preheat ramp of 3 °C/s results
>in failures (but without any visible damage) of 0805 ceramic
>chipcapacitors ?
>
>Best regards,
>
>Daan Terstegge
>Thales Communications
>Unclassified mail
>Personal Website: http://www.smtinfo.net
>
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