Arturo -
If the capacitors are Z5U dielectric or similar and the capacitance value is
near the upper end for the 0805 size, they will be more sensitive to thermal
shock and high temperature gradients than other dielectrics. Too rapid a
heating rate, can cause microcracking, which is often an internal problem
and becomes a latent defect.
After looking at your placement equipment for excessive placement pressure,
check your reflow profile, specifically the ramp rate to your reflow spike
temperature. Additionally, I do not recommend using a soldering iron on
multilayer ceramic (MLC) capacitors, because of the almost instantaneous
heat transfer when the soldering iron touches the capacitor. Only hot air
should be used for rework on MLC capacitors because it provides a more
gradual heating rate.
Ed Valentine
Electronics Manufacturing Solutions
8612 Mourning Dove Road, Raleigh, NC 27615
Phone: (919) 270-5145, Fax: (919) 847-9971
Email: [log in to unmask]
Website: http://www.ems-consulting.com
----- Original Message -----
From: "Arturo Urias" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Thursday, November 02, 2000 2:20 PM
Subject: [TN] Cracked capacitors question
> Dear subscribers:
>
> I have seen in a few of our products some cracked capacitors. The cracks
> are very small and it is usually very difficult to see them, even under a
> high maginification microscope. When we reflow one side of the capacitor
> with the solder iron, the capacitor completely separates. In a few cases I
> have seen the cracks even before we touch it up with the solder iron, so I
> know that somehow it is happening before touchup. This problem has been
> experienced on boards that do not go through the wave. There are other
> components like resistors and even other capacitors that do not show the
> same problem, even on the same product. The capacitors are 805s.
>
> Any ideas as to what is causing this?
>
> Thanks!
>
> Arturo Urias
> National Instruments
>
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