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October 2006

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Subject:
From:
David Greig <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, David Greig <[log in to unmask]>
Date:
Thu, 19 Oct 2006 19:41:11 +0100
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Hi Werner

Many thanks for an insightful answer.

Might it be possible to gain an increased thickness of solder between a chip components metallization and the PCB by designing
with a pad that matches the underside metallisation and having a stencil aperture that is larger than the PCB pad (perhaps not
larger at the 'toe' for tombstoning reasons).

Best Regards

David Greig

  _____

From: [log in to unmask] [mailto:[log in to unmask]]
Sent: 19 October 2006 13:07
To: [log in to unmask]; [log in to unmask]; [log in to unmask]
Subject: Re: [TN] Tombstoning and nitrogen


Hi David,
What happens with all chip components and large pads is, that most of the solder goes for the fillets and very little winds up
between the chip and the PCB pad. Thus, the stand-off height, one of the prime reliability parameters, is very small and you get
crack initiation underneath the chip very early. What the fillets do in this case is provide functional life because of the
larger volume through which the cracks need to propagate for functional failure. Out in the fillets the cyclic strains are
smaller than underneath the chip, because of a much larger 'effective' solder joint thickness.
However, you would be much better of if the solder were underneath the chip instead of the in the fillet, thus preventing any
premature crack initiation in the first place.

Werner


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