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Figure 3: Stress-strain diagrams for a) medium and b) high-purity copper as a function of the deposit thickness.
affected part, even if each load cycle Chemistry solutions to improve consist of copper traces and dielectric
remains well below the part’s limits reliability layers. The reliability of the package might
of mechanical strength. This effect is Optimization of the reliability may be improved by considering the copper
referred to as fatigue fracture, or as be obtained by improvement of the lines and the surrounding dielectric as
metal fatigue, because the effect is individual materials with regard to strongly interconnected, therefore, as a
especially visible in metal parts. If the ductility. The mechanical properties whole, and not anymore as an individual
mechanical loading takes the form of an of the individual copper material were material. Such interconnection would
externally imposed strain, as opposed shown to be impacted by the purity of ideally result in the formation of a
to imposed stress, it is called strain- the deposit, which can be controlled composite of the various materials. In
controlled fatigue. by proper design of the organic plating this case, the mechanical properties of
One of the most useful tools to additives [2]. Figure 3 compares the the composite, and no longer those of the
analyze the strain-controlled fatigue properties of medium- and high-purity individual materials, should determine the
behavior of materials is the Wöhler copper obtained from tensile tests at whole package.
experiment, in which pieces of the different layer thicknesses. The respective Composite formation depends on
mater ial a re subjected to cyclic ductility values at a given thickness were creating strong adhesion between the
loading of defined strain amplitude significantly larger in the case of high materials. For this purpose, stacks of
and the number of cycles until failure purity. In general, a strong increase of the electrodeposited copper and polyimide
is counted. These experiments are ductility and correspondingly improved dielectric both in the absence of, and in the
repeated for different strain amplitudes reliability was observed with increasing presence of different adhesion promoters,
and the results plotted in a strain range purity of the copper. were prepared and characterized in
over cycles to failure plot, called the Despite optimization of
Wöhler diagram. Figure 2b shows the individual properties
idealized curves for two hypothetical of the deposited copper,
copper deposits: a hard and a ductile a s c a l i ng i s s ue wa s
material. In the low-strain regime, the identified for both high-
deformation is predominantly elastic, a n d m e d i u m - p u r i t y
and the strain a given sample can materials. In this context,
sustain at a given number of cycles is a strong decrease of the
proportional to the tensile strength of ductility was observed
the material. In contrast, fatigue life in with decreasing deposit
the high-strain regime is determined thickness (Figure 3). This
by the ductility of the material. As results in an increasing
shown in Figure 2b, a more ductile susceptibility to fracture
material will withstand more cycles upon decreasing the RDL
of a large given strain amplitude L/S. In order to overcome
than a hard material with high tensile this scaling issue, a
strength. Therefore, under the strain holistic approach may be
imposed onto RDLs during thermal adduced. As schematically
cycling, high ductility will lead to depicted in Figure 1, Figure 4: Normalized ductility of copper-dielectric stacks in the absence
(UF 3) and presence of adhesion promoters (AP1, AP2), as well as
improved device reliability. multilayer RDL packages corresponding normalized peel strength between the materials of the stack.

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