Hi Charlie,
I will not go into all your questions, because I do not have experience with
all of them.
On the one's I do:
>5. Did you find that your oven profile changed drastically from that used
>for boards with, say, 200+ pin leaded devices?
The larger--in a thermal sense--a component is, the slower it will reach
solder reflow temperature, with the solder joints closest to the center of a
BGA heating up the slowest. The PCB will typically reach soldering
temperature before the component, but that depends on the mass of the BGA.
The only way to determine an adequate reflow profile is to instrument an
assembly with thermocouples and measuring the temperature profiles at various
locations. Your slowest heating soldering pads need to reach minimum
acceptable reflow temperature with some margin to assure consistent properly
wetted solder joints.

>6. What about solder mask defined pad areas as opposed to SM opening bigger
>than the pads?
Solder mask defined (SMD) solder joints show shorter cyclic lives than otherwi
se equal non-solder mask defined (NSMD) solder joints. The SMD solder joints
contain stress concentrations which have been show to reduce to between 70
and 33% of the NSMD solder joints, depending on severity of the test
conditions as well as the severity of the stress concentration.

>7. With this large of a device (40mm x 40mm) I am concerned about CTE
>mismatch over the operating temp range specified as well as the process
>temp range to which it will be exposed.  What board materials would you
>recommend?  Would you spec one of the higher Tg materials?  Why?  I will
>need to provide cost justification for wanting this more expensive and for
>some shops, more difficult to process, material.
The concerns about the reliability threat from a CTE-mismatch can be assessed
using the information in ANSI/IPC J-STD-013 "Implementation of Ball Grid
Array and Other High Density Technology", and IPC-D-279 "Design Guidelines
for Reliable Surface Mount Technology Printed Board Assemblies". The Tg of
the BGA and PCB materials comes into play only during processing; from first
principles, the closer the BGA and PCB materials are in Tg and CTE above Tg
the better.

Werner Engelmaier
Engelmaier Associates, Inc.
Electronic Packaging, Interconnection and Reliability Consulting
23 Gunther Street
Mendham, NJ  07945  USA
Phone & Fax: 973-543-2747
E-mail: [log in to unmask]

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