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July 2000

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Subject:
From:
Werner Engelmaier <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Mon, 10 Jul 2000 09:48:15 EDT
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Hi Graham,
You wrote:
>We are looking at an application using a ceramic BGA (429 balls, 1.27 mm
>pitch), in a high reliability, harsh environment application (in a vehicle).
>I have two questions:
>1) on FR4, say an 8 layer board, will I need an underfill for this
application?
I am assuming that this BGA is about 1-inch-square; such a large ceramic
component on FR-4 will give you inadequate fatigue life even in relatively
benign environments, e.g. telecommunications. In automotive applications it
is likely that not even solder columns are adequate.  You can verify this
with your design details with the analysis recommended in IPC-D-279.
>2) the IC's manufacturer is using balls made of 46% Sn, 46% Pb, 8% Bi.
> Why this alloy, and are there any precautions I should know building with
>this?
This alloy makes little sense in this application; commonly, high-melt
alloys, such as 10/90 Sn/Pb, are used so that a maximum solder joint height
is maintained--I do ot know what the Liquidus of this alloy is (it is likely h
igher than eutectic Sn/Pb but much lower than 10/90 Sn/Pb).

Werner Engelmaier
Engelmaier Associates, L.C.
Electronic Packaging, Interconnection and Reliability Consulting
7 Jasmine Run
Ormond Beach, FL  32174  USA
Phone: 904-437-8747, Fax: 904-437-8737
E-mail: [log in to unmask], Website: www.engelmaier.com

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