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October 1997

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Date:
Mon, 20 Oct 1997 16:42:37 -0400
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Hi,
1. The BGA lands you refer to are commonly known as 'solder-mask defined
(SMD)' in contrats to non-solder-mask defined (NMSD)'.
2. I do not know where you got that " the spheres collapse during reflow the
extent to which they spread latteraly is controlled by the solder resist".
This is simply not true. The reason the solder ball attachment of flip chips
is called C4 (Controlled Collapse Chip Connection) and now for BGAs C5
(Controlled Collapse Chip Carrier Connection) is that the solder balls
collapse controlled by the surface tension of the molten solder, the weight
of the component, the size and shape of the attachment pads, and the solder
volume; the solder mask has nothing to do with the control of the collapse.
However, for SMD pads, the solder mask etches deform the solder ball
geometries to have stress concentrations.
3. Solder mask defined (SMD) solder joints show shorter cyclic lives than
otherwise equal non-solder mask defined (NSMD) solder joints. The SMD solder
joints contain stress concentrations which have been shown to reduce the
cyclic lives to between 70 and 33% of that of the NSMD solder joints,
depending on severity of the test conditions as well as the severity of the
stress concentration.
4. Hence, you want the solder-mask opening to be large enough that you don
not get any touching of the solder-mask and the solder balls. Typically this
is accomplished with a 31 mil solder-mask opening and 25-mil-diameter copper
pads.

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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