Solder self-alignment is an important phenomenon enabling cost-effective optoelectronics assembly. In this study, the wetting of Sn-rich solder to under bump metallization (UBM) pads is identified as a critical factor affecting self-alignment accuracy. Incomplete wetting of solder to the metallization pads is responsible for chip-to-substrate misalignment larger than 1 μm, while fabrication tolerances, such as solder volume variation and pad diameter deviation, only account for misalignments in the submicron range. To quantitatively investigate the effect of incomplete wetting on self-alignment accuracy, a three-dimensional (3D) model based on a force optimization method was developed. With the input parameters of incomplete solder metallurgical wetting area, position and diameter of metallization pad, volume of individual solder bumps, coefficient of solder surface tension, mass of the chip, external forces acting on the chip, and initial pick-and-place position of the chip before assembly, the model predicts the assembled position of the chip in terms of the misalignments in the X-Y plane and the rotation angle along the Z axis. The model further confirmed that incomplete wetting of solder is the most critical modulator among the undesirable factors affecting solder self-alignment accuracy.

References

References
1.
Tan
,
Q.
,
Lee
,
Y. C.
, and
Itoh
,
M.
, 2005, “
Soldering Technology for Optoelectronic Packaging
,”
Passive Micro-Optical Alignment Methods
,
R.
Boudreau
and
S.
Boudreau
, eds.,
CRC Press
,
FL
, Chap. 5.
2.
Itoh
,
M.
,
Sasaki
,
J.
,
Uda
,
A.
,
Yoneda
,
I.
,
Honmou
,
H.
, and
Fukushima
,
K.
, 1996, “
Use of AuSn Solder Bumps in Three-Dimensional Passive Aligned Packaging of LD/PD Arrays on Si Optical Benches
,”
Proceedings of 46th ECTC
, pp.
1
7
3.
Hayashi
,
T.
, 1992, “
An Innovative Bonding Techniques for Optical Chips Using Solder Bumps That Eliminate Chip Position Adjustments
,”
IEEE Trans. CHMT
,
15
(
2
), pp.
225
230
.
4.
Lin
,
W.
, and
Lee
,
Y. C.
, 1999, “
Study of Fluxless Soldering Using Formic Acid Vapor
,”
IEEE Trans. Adv. Packag.
,
22
(
4
), pp.
592
601
.
5.
Kallmayer
,
C.
,
Oppermann
,
H.
,
Engelmann
,
G.
,
Zakel
,
E.
, and
Reichl
,
H.
, 1996, “
Self-Alignning Flip-Chip Assembly Using Eutectic Gold/Tin Solder in Different Atmosphere
,”
Nineteenth IEEE/CPMT International Electronics Manufacturing Technology Symposium
, pp.
18
25
6.
Lu
,
J. C.
,
Busch
,
S. C.
, and
Baldwin
,
D. F.
, 2001, “
Solder Wetting in a Wafer-Level Flip Chip Assembly
,”
IEEE Trans. Electron. Packag. Manuf.
,
24
(
3
), pp.
154
159
.
7.
Qi
,
Y.
,
Zbrzezny
,
A. R.
,
Agia
,
M.
,
Laml
,
R.
,
Ghorbani
,
H. R.
,
Snugovsky
,
P.
,
Perovic
,
D. D.
, and
Spelt
,
J. K.
, 2004, “
Accelerated Thermal Fatigue of Lead-Free Solder Joints as a Function of Reflow Cooling Rate
,”
J. Electron. Mater.
,
33
(
12
), pp.
1497
1506
.
8.
Nguyen
,
L.
,
Walberg
,
R.
,
Zhou
,
L.
, and
Koh
,
T.
, 2003, “
Board Level Reliability of Components With Matte Tin Lead Finish
,”
ECTC
, pp.
346
352
.
9.
Sattiraju
,
S. V.
,
Dang
,
B.
,
Johnson
,
R. W.
,
Li
,
Y.
,
Smith
,
J. S.
, and
Bozack
,
M. J.
, 2002, “
Wetting Characteristics of Pb-Free Solder Alloys and PWB Finishes
,”
IEEE Trans. Electron. Packag. Manuf.
,
25
, pp.
168
184
.
10.
Evans
,
J. W.
, 2007, “
A Guide to Lead-Free Solders. Physical Metallurgy and Reliability
,
1st ed.
,
Springer-Verlag
,
London
, pp.
6
7
.
11.
Heinrich
,
S. M.
, and
Lee
,
P. S.
, 1997, “
Solder Geometry Prediction in Electronic Packaging: An Overview
,”
Advances in Electronic Packaging
,
ASME
,
New York
, Vol.
2
, pp.
1371
1381
.
12.
Patra
,
S. K.
, and
Lee
,
Y. C.
, 1991, “
Quasi-Static Modeling of the Self Alignment Mechanism in Flip-Chip Soldering- Part 1: Single Solder Joint
,”
ASME J. Electron. Packag.
,
113
(
4
), pp.
337
342
.
13.
Patra
,
S. K.
,
Sritharan
,
S. S.
, and
Lee
,
Y. C.
, 1995, “
Minimum-Energy Surface Profile of Solder Joints for Noncircular Pads
,”
ASME J. Appl. Mech.
,
62
, pp.
390
397
.
14.
Brakke
,
K. A.
, 1994,
Surface Evolver Manual
,
Version 1.94
,
Geometry Center, University of Minnesota
.
15.
Rayasam
,
M.
, and
Subbarayan
,
G.
, 2006, “
A Three-Dimensional Solder Shape Model Incorporating Top Pad Inclination and Misalignment
,”
ASME J. Electron. Packag.
,
128
, pp.
291
293
.
16.
Josell
,
D.
,
Wallace
,
W. E.
,
Warren
,
J. A.
,
Wheeler
,
D.
, and
Powell
,
A. C.
, IV, 2002, “
Misaligned Flip-Chip Solder Joints: Prediction and Experimental Determination of Force-Displacement Curves
,”
ASME J. Electron. Packag.
,
124
, pp.
227
233
.
17.
Lin
,
W.
,
Patra
,
S. K.
, and
Lee
,
Y. C.
, 1995, “
Design of Solder Joints for Self-Aligned Optoelectronic Assemblies
,”
IEEE Trans. Compon., Packag., Manuf. Technol., Part B
,
18
(
3
), pp.
543
551
.
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