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June 2005

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Subject:
From:
John Maxwell <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, John Maxwell <[log in to unmask]>
Date:
Tue, 14 Jun 2005 13:35:20 -0700
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Ioan,
I'll add a few comments to the thread.
At 06:38 AM 6/14/2005, you wrote:

>1. The ceramic caps will be more prone to internal cracks when on
>lead-free. To prevent that it is necessary to eliminate the fillets and
>make the joints thicker. I've been through major fighting here to get the
>designers stick to IPC-7531, now I have to make them change the CCs...
>Well, that's my problem. On the other hand, besides overprinting, are
>there any other solutions to increase the height of the solder joint (I
>assume applications where stencils thicker than 6 mils are not an option)

Perhaps reduce solder fillet size and use somewhat smaller pads. Large pads
and fillets have been a problem in the past and will only become more
troublesome in the future.


>2. Recently Ingemar made a statement on the tantalum caps, that they
>should be a stay-away. Why?

Tantalum capacitors have dielectric flaws when shipped directly from the
factory. They are made from clean dirt but when dealing with <100 nM thick
dielectric layers any contamination like a small bit of nickel results in
very thin dielectric layers resulting in localized leakage current and
depending on current density joule heating. Unfortunately the dielectric
has a couple of stable phases, amphorous and crystalline, the amphorus
being a good dielectric and crystalline an excellent current conductor.
That is why tantalum caps were first developed with MnO2 cathodes, as flaws
heat up due to joule heating of the cathode converts to an insulator as
oxygen is robbed from the cathode layer in effect scabbing over. Polymer
cathode parts do not scab and are limited to low voltage parts in part of
no self heal mechanism being present other than vaporization of the polymer.

Tantalum pent oxide also has another interesting characteristic, it is
electrostrictive or grows in the direction of the applied field and this
movement coupled with impurities like nickel or other non valve metals can
lead to "inrush" current failures. It is really a dV/dt and not a current
phenomena.

Now that I am back consulting I just love lead free soldering, ceramic and
tantalum caps

John Maxwell

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