The configuration of the caps is only a small part of crack prevention.
Cracks in MLCCCs can be caused in a multitude of ways. Just check the
AVX website. They have visited this problem a thousand times if they
have done it once (I know because I have had them help me with this
problem), and they have listed all of the sources for cracking, and how
to determine the source of cracking by the crack location on the
capacitor body.
Read the information they have at
https://www.avx.com/docs/techinfo/soldmlc.pdf#search='avx%20cracking%20c
eramic%20capacitors'. This is just one of the documents that John
Maxwell and the other multi-layer ceramic chip capacitor experts have on
the AVX website. They have several excellent papers on the website. Be
sure to read all of them.
The answers to all of your questions regarding detection, prevention,
package size substitution, etc. are there to mitigate cracking.
Free. No charge.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Dehoyos, Ramon
Sent: Tuesday, June 06, 2006 11:08 AM
To: [log in to unmask]
Subject: Re: [TN] Finding cracked capacitors, fix the issue
It seems that finding after the fact cracked ceramic Caps is a
chore. The emphasis should be put on avoidance. This subject was talked
about before. One of the main avoidance steps is to lay the length of
the caps perpendicular to the length of the board.
Ramon
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Steve Gregory
Sent: Tuesday, June 06, 2006 10:03 AM
To: [log in to unmask]
Subject: Re: [TN] Finding cracked capacitors, fix the issue
Good Morning Werner,
You bring up an interesting point about wave soldered capacitors that
reminds me of something that I seen on a board long ago. All the passive
bottomside pads were thinner than the terminations of the components.
Like this:
________________________
| | | |
___________| | | |_________
| | | CAPACITOR | | |
| PAD | | | | PAD |
|___________| | | |_________|
| | | |
|___|________________|___|
Would this type of layout give a more reliable connection? I've only
seen it one time, and I imagine that if it were more reliable, everybody
would be doing it by now. But like I said, I've only seen it once and it
was probably 10-12 years ago. I wonder if they were experimenting to try
and solve a capacitor cracking problem?
-Steve Gregory-
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Werner Engelmaier
Sent: Tuesday, June 06, 2006 8:45 AM
To: [log in to unmask]
Subject: Re: [TN] Finding cracked capacitors, fix the issue
Hi Ioan,
The keywords are your addition to the Subject Line: "Fix the Issue."
Unfortunately, the way chip components are solder-attached to PCBs harks
back to the requirements for wave soldering-you need large PCB soldering
pads that can be touched by the liquid solder wave for heat and material
transfer. The consequence of this are large solder fillets and very thin
solder joint gaps between the CC and the PCB. The large solder fillets
act like the jaws of a vice both during cooling from the soldering
process and during any kind of PCB flexure, thus putting the CCs under a
lot of stress; the thin solder gaps provide no compliance for the solder
attachment, thus reducing the capability of the SJs to absorb some of
the deflection/warpage deformation as well as thermal cyclic loads. The
large fillet/thin SJ gap geometry is about as bad an attachment scenario
as you can imagine.
This is explained in more detail in my Reliability Column in Global SMT
&
Packaging magazine, "Lead-Free (LF) Wave-Soldering: Can (Should) It Be
Eliminated," Global SMT & Packaging, Vol. 5, No. 9, October 2005, pp.
46-50.
As I pointed out in my column, the solder attachment geometry can be
dramatically improved if one uses reflow rather than wave soldering.
This not only essentially eliminates CC cracking, but has a number of
other advantages as well, particularly with lead-free soldering. The
soldering pads need to be only the size of the termination, eliminating
the fillets and increasing the SJ gap between CC and PCB.
Werner
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