http://www.avxcorp.com/docs/techinfo/cracks.pdf
Tells everything you ever wanted to know about how the cracks occur, and
what you can do to prevent or minimize it.
There are many other articles in the AVX website. Check 'em out. 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of John Maxwell
Sent: Tuesday, January 31, 2006 11:42 AM
To: [log in to unmask]
Subject: Re: [TN] Large SMT physical failure prevention

Phil :
In my former lives capacitor cracking paid the mortgage and put the kids
through school. I have a few general comments concerning cracking
capacitors as I do not have your specific board in hand.
1) Avoid board edges during layout
2) Avoid mounting holes
3) V grove board separation was very profitable for me in conjunction
with item 1 above
3) Never wave solder these parts
4) Do not mount near large heavy components
5) Do not mount near connectors
6) Do not mount perpendicular to board edges (see 3 above)
7) Keep parts isolated where the assembly is handled especially when
inserted into test systems
8) Analyze bending when placed into totes especially when mounted into
slots
9) Analyze when the assembly is installed into final product
10) Pay attention to natural flex or bend zones when the assembly is
mounted
11) Small assemblies were particularly profitable due to all of the
above
12) Keep solder fillets small
13) Do not force cool after reflow
14) Ramp up cracking is a myth but cool down is a real problem
15) Watch where people put their fingers and thumbs while handling
assemblies, big flat smooth capacitors are preferred by operators

John maxwell

>Good morning,
>
>We have some designs that are calling for large, 1825 and 2225 
>components.  We have seen some mechanical failures where the parts have

>cracked.  If we need to use these monster components, how do we ensure 
>we reduce or eliminate the mechanical failures.
>
>Our failures were seen on a 16 inch by 17 inch 0.062 thick board.  On 
>these we have changed our handling methods.
>
>On smaller 2 inch by 2 inch boards is this a don't care?  I know that 
>rapid temperature ramp up or down in the reflow process could be a 
>killer.  Something to work out with our CM.
>
>Phil
>
>
>

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