Silver-filled adhesive

Figure 12.10. Micrographs of devices fabricated using gravure printing technology. Left—shows the interdigitated transistor gate fabricated by an ink composed of nanoscale metallic particles. Right—Channel fabricated when using a silver-filled adhesive to print the transistor source and drain.

Thermal conductivity and expansion are important properties of adhesives used in electronics. Both properties influence the performance of computer chips. Generally, the chip has a protective cover which is attached by an adhesive. The adhesive bond must be maintained during thermally induced movement in the chip. The chip is bonded to its base with an adhesive which must also take thermal movement and, in addition, transfer heat from the chip. Two epoxy adhesives were used in the study silica filled epoxy (65 and 75 wt% SiO2 epoxy) and epoxy containing 70 wt% Ag. Figure 15.6 shows their thermal conductivities. The behavior of both adhesives is completely different. The silver filled adhesive had a maximum conductivity at about 6()"C whereas the maximum for SiOz filled adhesive was 120"C. The Tg of both adhesives was 50 and 160 C, respectively. Below its Tg, the thermal conductivity of the adhesive increases at the expense of increased segmental motions in the chain molecules. Above the Tg the velocity of photons rapidly decreases with increasing temperature and the thermal conductivity also decreases rapidly. 

Silver migration will occur whenever a silver-filled adhesive, coating or ink is in close proximity to another conductor in a circuit, provided that a DC voltage potential (the silver-filled polymer being the anode), and a film of liquid water, exist on the surface separating the silver compound from the other conductor. 

Licari, et al. (6) showed that by placing a drop of deionized water across a 20 mil gap between a conductor and a silver-filled adhesive, and then applying a one volt DC potential, silver dendrites began to grow across the gap within 30 seconds. Bridging, causing an electrical short, occurred within 3-4 minutes. Within eight minutes, silver particles had completely filled the gap. 

Summer A, Friesen D, Matthews M. Viscosity and Its Effect in Silver-filled Adhesives. Ablestik Laboratories Technical Paper Aug. 1998. 

Some discoloration of silver-filled adhesives may occur due to surface oxidation of the silver. This is considered more a cosmetic issue than a reliabiUty problem however, the tarnish can be removed by subsequent exposure to an argon plasma. 

High-modulus adhesives are likely to have high bending stresses, high deflections, and low radii of curvature. A comparison of typical and ultra-low-stress conductive adhesives was reported using 250-mil square silicon die (20 mils thick) bonded to 10-mil-thick copper leadframes." A conventional silver-filled adhesive had a deflection of 3 mm and an ROC of 1,070 mm while a low-stress adhesive showed a deflection of 0.01 mm and an ROC >125,000 mm." ... 

Summer, A., Friesen, D., and Matthews, M., Viscosity and its Effect in Silver-Filled Adhesives, Ablestik Laboratories Technical Paper (Aug. 1998)... 

In practice, several techniques can be used to maximize the dissipation of heat. Of course, solder and other metallurgical attachments provide the lowest thermal resistance but, where adhesives must be used, those having the highest thermal conductivities should be selected. Some silver-filled adhesives, formulated with unique suspensions of silver, resin, and carrier fluid are reported to have thermal conductivities of 20 W/m-Kf l approaching those of ceramics and some metals. An adhesive having a high silver loading is produced once the carrier fluid has been removed, and the adhesive is fully cured. 

To test for silver migration, a constant DC voltage is applied between a silver-filled adhesive and a nearby conductor, such that the adhesive is the anode, and the voltage gradient between the conductors is on the order of 1 volt/ mil. Moisture vapor condensed on the intervening surface can permit silver ions to migrate toward the cathode and form a conductive path of metallic silver which short-circuits the device. Migration has traditionally been more of a problem in hybrid than in monolithic ICs, because the adhesive fillet may approach other conductors on the hybrid substrate. 

Increasingly, silver-filled adhesives are being used to replace tin, lead, and silver-based solders. A number of reasons account for this increase in use of conductive adhesives ... 

Two silver-filled adhesives, Johnson Matfliey JM 7000 and JM 7800, have respective elastic modulus of 10 and 5.8 GPa, 7g 250 and 210°C, thermal conductivity 1.1 and 1.6Wm K , volume resistivity 2x10 and 5 X 10 n cm. According to the manufacturer, the thermal stresses, determined by measuring the radius of curvature of silicon chips, are far from the level of stress where the risk of cracking and delamination increases considerably. In 1994 Johnson Matthey proposed a low-stress cyanate adhesive (JM 2500) exhibiting a modulus of elasticity of only 0.4 GPa. The low stress properties of this material were demonstrated by the large radius of curvature (Im)ofa 15x 15 mm die bonded to a 0.15 mm thick leadframe. This value remained constant after KKX) thermal cycles from — 65 to 150°C. 

Curves of Figure 19 compare the data published for (a) boron nitride [37,40] (b) aluminium (c) diamond-[37-39] (d) aluminium nitride [37-42] (e) crystalline silica. It can be seen that, at 45 vol.%, the maximum thermal conductivity achieved with diamond powder is 1.5 W m K, while crystalline boron nitride at 35 vol.% affords 2.0Wm K. The thermal conductivity of silver-filled adhesives was studied by using silicon test chips attached to copper and molybdenum substrates [43]. The authors outline the importance of the shape factor A, related to the aspect ratio of the particles, to achieve the highest level of thermal conductivity. Another study reports the variation of the effective thermal resistance, between a test chip and the chip carrier, in relation to the volume fraction of silver and the thickness of the bond layer [44]. The ultimate value of bulk thermal conductivity is 2 W m at 25 vol.% silver. However, the effective thermal conductivity, calculated from the thermal resistance measurements, is only one-fifth of the bulk value when the silicon chip is bonded to a copper substrate. 

The electrical conductivity of gold- and silver-filled adhesives is addressed in Section 6.3.2 and the qualification standards in Section 6.4.2.9. Although this latter directive limits the thermal ageing conditions to 150°C, different studies have been published on the behaviour of organic adhesives at higher temperatures [133,134]. The volume resistivity of commercial adhesives is of the order of... 

Volume conductivity measurements indicate that the composition of the electroplated substrates is of prime importance with an increase in resistivity covering 5-6 orders of magnitude. The conductive adhesives have volume resistivities in the range of 6.5 X 10 -1.6 X 10 " fi cm but the values measured for the joined assembhes extend from 9 X 10 " to 1 X 10 fi cm. The general trend observed with all adhesives is that the increase in resistivity is related to the ease of oxidation of the plated substrates, in particular when nickel and aluminium are compared to noble metal plating. Other studies support these results indicating that the best silver-filled adhesives have a volume resistivity of 5 X 10 fl cm approaching the value of solders, typically 2 X 10 fi cm. The contact resistance at the interfaces with the metallic conductors is, however, more important than the bulk conductivity [155-157]. 

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