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Controlled C-4 Solder Joint Elongation for Improved Thermal-Mechanical Stress Relief

IP.com Disclosure Number: IPCOM000100166D
Original Publication Date: 1990-Mar-01
Included in the Prior Art Database: 2005-Mar-15
Document File: 2 page(s) / 82K

Publishing Venue

IBM

Related People

Spalik, J: AUTHOR

Abstract

Thermal expansion coefficient differences between parts which are soldered together require solder joints that can flex during thermal mechanical cycling. (Image Omitted). When solder balls are used in a soldering process, as in controlled collapse chip connector (C4) or surface mount array, joint flexibility is severely limited by the resulting oblate-shaped connections. A process which controls the shape of the elongated solder connection is automatically accomplished during a single soldering cycle in the following manner.

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Controlled C-4 Solder Joint Elongation for Improved Thermal-Mechanical Stress Relief

       Thermal expansion coefficient differences between parts
which are soldered together require solder joints that can flex
during thermal mechanical cycling.

                            (Image Omitted)

.
 When solder balls are used in a soldering process, as in controlled
collapse chip connector (C4) or surface mount array, joint
flexibility is severely limited by the resulting oblate-shaped
connections.  A process which controls the shape of the elongated
solder connection is automatically accomplished during a single
soldering cycle in the following manner.

      Two Sn/Pb alloys are utilized having differing melting points
to allow complete wetting of electrical connections before the second
alloy melts.  The first alloy, in ball form, is of proper size to
bridge connection pads.  The second, in disc form, of height slightly
less than the ball diameter, is used to lift or elongate the joint as
it tends to achieve a spherical shape, as seen in the illustration.

      The principle utilized here is based on the fact that a
flattened solder ball will return to its original shape due to high
surface tension, and the minimization of surface energy achieved as a
sphere.  By using solders that melt at sufficiently different
temperatures, and by simultaneously using flattened large balls with
a higher melting point and small spherical balls of lower me...