Nondestructive testing of defects in adhesive joints

temperatures exhibited by the 50 nm thick film is similar to the behaviour observed earlier for pure
P4VP [8] indicating that the film consists of small particles separated by small distances. With
further increase in P4VP content, the negative TCR part diminishes, giving rise to a near zero TCR.
It is also interesting to note that even at 50% P4VP, with an increase of silver deposited, films show
electrical characteristics as that of the films on pure P4VP [8]. Further, when 150 nm thick silver is
deposited on a polymer blend with only 25% of P4VP, the films show desirable electrical
characteristics in contrast with the very high room temperature resistance observed for films on pure
PS even at 300 nm of silver [7].This indicates blending has positive effect on electrical properties on
silver films deposited on PS. It was shown through X-ray photoelectron spectroscopy (XPS) studies at
various electron take off angles (ETOA) that silver clusters are formed at a depth of a couple of nm
from the polymer surface [7,8]. It is known that the formation of subsurface particulate structure is
subject to certain thermodynamic [4] and deposition conditions [3]. While the thermodynamic
conditions are met for the deposition of metals on most of the polymer substrates, deposition
conditions used in the present study are similar to those used in our earlier studies. Therefore, it is
reasonable to assume that the particles are formed just a couple of nm below the polymer surface.
Table 1 gives the resistance data and particle size estimated from the X-ray line width measurements
for the silver films of different thicknesses deposited on the PS/P4VP blends of various compositions.
It is seen that the resistance of silver films at room temperature lie at a few tens of MΩ/ , at certain
thicknesses even for the PS content of 50-75%, in contrast with the behaviour of silver on pure PS.
The particle size increases with increase in PS content at a fixed silver thickness, as expected. It is
seen that as the P4VP concentration increases there is a regular decrease of resistance at a fixed silver
thickness. The plot of logarithm of these resistances with blend concentration gives linear fit as shown
in figure 2. Through this fit, one can estimate the resistance of the film at a particular blend and for
the given conditions and thickness.
Conclusions:
1. Deposition of silver on polymer blends coated substrate held at 457 K provides an approach to
produce stable island films with reasonable control over their electrical resistance.
2. Higher thickness films show almost zero TCR near room temperature, a desirable property for most
of the devices. Low thickness films show a negative TCR, characteristic of island films.
3. Silver particulate films deposited on PS/P4VP blends show better electrical properties compared to
those on pure PS, even at a P4VP content of 25%. The blends PS/ P4VP (50:50, 25:75) seem to be
better with regard to their electrical behaviour.