Electrical Conductivity

Further observations made from the Figure 3.10 are given below in Table 3.5. It can be seen from Table 3.5 that electrical conductivity close to that of a commercial silver flake filled ICAs was achieved with the use of 4.8µm MPS but with a higher volume fraction. This higher volume fraction may have a negative impact on the printability and the mechanical strength of the ICA. However, according to Genovese (2012) high concentrations of deformable spheres can accommodate each other at rest and squeeze past each other during flow, increasing ϕmeff than for flakes or solid mono-sized spheres and reducing ηr in Equation 2.1 resulting in a lower viscosity. Further, previous studies conducted on the rheology of the Ag-MPS filled ICAs demonstrated (i) good aperture filling during stencil printing and (ii) mechanical strength similar to that of a commercial flake filled ICA at as high as 50 vol% of Ag-MPS (Nguyen et al. 2010;

2011; 2013). This shows that the processability and mechanical strength are not major concerns for ICAs loaded with as high as 50 vol% of Ag-MPS. However, if rheology modifiers such as SiO2 (silica) are used, the printing may be difficult at 50% (Redei 2014).

Table 3.5 Maximum observed conductivity of formulated ICAs

Table 3.5 also shows that the conductivity of an ICA formulated using silver flakes is lower than that with 4.8µm Ag-MPS. This may be because the silver flakes used in the study are coated with surfactants to reduce the tendency for clustering or agglomeration, whereas the Ag-MPS used are not coated with any surfactants. The presence of surfactants on the surface of flakes may increase the contact resistance between silver flakes and could therefore be a cause of their lower conductivity.

Furthermore, Table 3.6 also shows that the conductivity of the ICA formulated using silver flakes is lower than that of the commercial ICA, H20E, filled with silver flakes.

The lower volume fraction of silver in the ICA formulated compared with the

Chapter 3: Feasibility Study - Use of Silver Metalised Polymer Spheres as Conductive Filler in ICA

0.1 1.0 10.0 100.0 1000.0 10000.0

0 30 60 90 120 150 180

Log Resistance (Ω)

Temperature (°C)

4.8_15A 30_13A FS-34 Commercial ICA

commercial ICA could be one reason for their lower conductivity. Another reason could be that 353ND and H20E contain same resin but in different quantities, further the curing agents is also different in both these matrices, some of the constituents of the commercial ICA and 353ND are not listed in the material safety data sheet (as given in Table 3.2). Therefore due to different curing agents, these matrices will have different curing reactions. H20E contains a reactive diluent, the presence of a reactive diluent generally leads to a faster rate of cure and a higher crosslink density than without one.

Thus different curing agent and presence of a reactive diluent may impart different shrinkages to H20E than 353ND thus different conductivities (Klosterman et al. 1998;

Lu et al. 1999). The effect of shrinkage of epoxy matrix on the conductivity of the ICA is further investigated in Chapter 6. Figure 3.10 shows a steeper rise in the conductivity of the ICA containing 4.8µm Ag-MPS and silver flakes as compared to 30µm Ag-MPS.

The reason may be that as the volume fraction of filler is increased beyond the percolation threshold the number of parallel paths increases more in 4.8µm Ag-MPS and silver flakes filled ICAs as compared to 30µm Ag-MPS because of the smaller size of 4.8µm Ag-MPS and silver flakes.

The resistances of all the four ICAs during cure were monitored and its variation with temperature is plotted in Figure 3.12. The volume fractions used in this experiment were the ones showing maximum conductivity. The plots in Figure 3.12 show that before cure the commercial ICA, H20E, has relatively very high resistance as compared to all other ICAs formulated using 335ND.

Figure 3.12 The variation of the resistance with temperature during thermal cure

As the temperature is raised above room temperature the resistance of all the ICAs remain nearly constant until 100°C, after 100°C the resistance of ICAs made using 353ND starts decreasing but the decrease in resistance is small with further increase in temperature. As the temperature is increased beyond 135°C the decrease in resistance increases and this decrease in resistance continues till 150°C. In an adhesive matrix, cure begins with the formation and growth of the linear polymer chains and as the cure proceeds these chains begin to branch and then crosslink, forming a cross-linked network. The transformation from a viscous liquid to an elastic gel marks the first appearance of the cross-linked network and is called gelation. The formation of elastic gel does not inhibit the curing process. On further curing this elastic gel converts to a glass state this is called vitrification. This marks the end of cure. The initial decrease in resistance around 100°C can be associated with conductive filler packing on heating i.e.

as the adhesive viscosity falls on heating it is squeezed from intra filler spaces lowering the resistance. The large decrease in resistance after 135°C can be explained that as temperatures increases more chains get cross-linked and the adhesive transforms from a viscous liquid to an elastic gel forcing the filler together. The subsequent decrease in resistance can be associated with the transformation of elastic gel to a glassy state called vitrification where on further development of cure, larger force on the particles act, forcing them further closer. Any solvent if present in the adhesive matrix may evaporate during curing. This may also the affect ICA conductivity. However, different trend is seen in the case of H20E. During the curing of H20E the resistance suddenly increases at around 140°C and then decreases drastically. This indicate the presence a constituent which either decomposes on heating at around 145°C or initiates rapid cure at around 140°C. The decomposition of this constituent or rapid curing may be one of the reason for better final conductivity of the commercial ICA than other ICAs. This observation needs further detailed investigation and is out of scope of the present study.

Further, ICA samples at volume fractions close to percolation threshold have been found to bleed. Pictures of uncured and cured ICA samples are shown in Figures 3.13 and 3.14 respectively. Comparison of these images shows bleeding of adhesives below 40 vol% of filler. The bleeding makes the width and thickness of the cured sample non-uniform along the whole length of the printed trace. If the spreading/bleeding of the resin displaces the filler particles with it then it would affect the long range particle to

Chapter 3: Feasibility Study - Use of Silver Metalised Polymer Spheres as Conductive Filler in ICA

particle connectivity and may increase the percolation threshold and reduce the conductivity.

Figure 3.13 Uncured 4.8µm Ag-MPS filled ICA at different volume fractions

Figure 3.14 Cured 4.8µm Ag-MPS filled ICA at different volume fractions

On the other hand, if bleeding/spreading adhesive does not displaces the filler particles with it, instead the particles come closer upon curing then the resin bleed/spread may increase the effective volume fraction of the particle in the adhesive increasing the particle to particle connectivity thus lowering the percolation threshold and increasing the overall conductivity. However, effect of bleeding on particle displacement within adhesive and thus on conductivity needs detailed investigation and is out of scope of the present study. Moreover, it can be observed from the Figure 3.14 the resin do not bleed on the Cu/Ni/Au metallisations even at volume fractions below 40 vol%. The reason could be that the adhesive made larger contact angle with gold compared to FR4 substrate. Thus choosing the suitable substrate which makes larger contact angle with the adhesive matrix the bleeding may be controlled (Petrie 2006).

7.74%, 2303

1.30%, 411

22%, 1706

31%, 2333

0 500 1000 1500 2000 2500

0% 10% 20% 30% 40%

Conductivity (Ωcm)-1

Vol% of Silver

4.8 _15A 30 _13A FS_34 Commercial ICA