Eutectic Au/Sn 80/20 solder is especially
beneficial for flip chip applications where the usage
of flux has to be avoided. Also it is a lead free
alternative for applications where high temperature
resistance is indispensable. The process steps
involved in the fabrication of AuSn bumps by
electroplating are described in detail elsewhere
[1, 2]. First the plating base consisting of a
Ti:W(N) and an Au layer is sputtered. The
sputtering is followed by lithography of thick photo
resist as plating mask. After electroplating of the
Au socket and the Sn cap the resist is stripped. In
the last process steps the Au and the Ti:W(N)
layers are etched.
The bumps are accomplished by a reflow
process which leads to a smooth bump surface and
prevents it from oxidizing. After the manufacture
the bumps consist of an Au socket with an eutectic
solder cap on top and a z-phase layer between.
It is possible to calculate the shape of AuSn
bumps prior to manufacture provided that the
reflow process is under control. This means that it
is possible to predict the height of the total bump,
the height of the eutectic solder cap and the
remaining Au socket beneath after the reflow when
plating certain amounts of Au and Sn [3].
Kallmayer et al. proved that it is possible to
control the reflow process of AuSn solder bumps
which had a solder cap height of at most 20 μm.
This was possible by using a certain heating rate
during the reflow process. Too fast heating lead to
poor results [4].
For the assembly of large GaAs dice to ceramic
substrates for an optical switch the usage of large
volume solder bumps was necessary in order to
compensate a certain unevenness of the surface of
the very complex ceramic substrate. However, even
though a small heating rate was used, the reflow of
those AuSn large solder volume bumps often lead
to an inhomogeneous dissolution of the Au-socket
as shown in Figure 1 on the left hand side. In our
previous work [3] we proved that this phenomenon
is due to the coexistence of two liquid phases
reacting with each other when the reflow peak
temperature of 285 °C is reached. We have shown
that it is possible to prevent the inhomogeneous
dissolution by implementing an additional ageing
step prior to reflow. A SEM micrograph of a cross
section of a bump after ageing and reflow is shown
in Figure 1 on the right hand side. There is no
inhomogeneous dissolution visible anymore, the
interface between the Au socket and the eutectic
solder cap is very planar.
This work aims at presenting a solution how to
control the reflow process regardless of
composition and volume of the solder bumps, i.e.,
regardless of the tin layer thickness deposited in the
galvanic process. Phase transformations are
investigated using SEM/EDX and DSC analysis.