IJUP08 - Universidade do Porto

Characterizing the intermetallic formed during ball attach process
Paulo Pereira a,* , Rúben Santos a , Maria M. Barbosa a , Cátia Almeida a
a** Dep. de Engenharia Metalúrgica e de Materiais, Faculdade de Engenharia, Universidade do
Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal
* pereira.paulo@fe.up.pt
Continuously miniaturization of electronic components requires the most advanced
technologies in which electrical response is a crucial factor to success.
In this study we characterize the interface of an attachment between the copper conductors
of a memory and a solder ball which allows further bonding to printed circuit modules, see
figure 1a). , This connection, made of diferent layers, has a capital importance to ensure a
good electrical and mechanical performance.
The attachment uses a 10 m thick intermediate layer of nickel deposited over a copper
pad and protected by a thin gold film. The solder, with a spherical shape and composition
of Sn-1.0Ag-0.5Cu, is placed on the gold film and the assemblage is heated up to the
brazing temperature.
At the interfacial zone, between the nickel and the solder ball, a continuous layer of an
intermetallic compound of Sn-Ni-Cu is formed. This intermetallic layer has 1 to 2 m
thickness as seen on figure 1b). Some particles of this compound are also observed at inner
zones of the solder ball (see arrow on figure 1b), indicating a preferential copper and nickel
diffusion path inside the ball.
By an AFM analysis it was possible to perform nanoindentation tests, shown on picture
1c). The indentation size variation between Sn braze alloy and intermetallic layer clearly
states the hardness increase at the interface.
Figure 1. a) SEM image of the ball attach bonding; b) SEM image of interface showing the intermetallic
compound formed between the Ni (darker layer) and the Sn braze alloy (lighter area); c) AFM image of the
indentations in the braze alloy and intermetallic compound.
Acknowledgement
The materials characterized in this work were kindly supplied by Qimonda S.A.
** The authors are undergraduate students of the 4th year of Master in Metallurgical and
Materials Engineering.
59