Pharmaceutical Sciences Claire Jarry and Matthew S. Shive
11.3 Electrically Conductive Adhesives Yi Li, Myung Jin Yim, Kyoung-sik Moon,
and C.P. Wong
11.3.1 Introduction
Electrically conductive adhesives (ECAs) are composites of polymeric matrices and electrically conductive fillers. The polymeric resin, such as an epoxy, a silicone, or a polyimide, provides physical and mechanical properties such as adhe- sion, mechanical strength, impact strength, and the metal filler (such as, silver, gold, nickel, or copper) conducts elec- tricity. Metal-filled thermoset polymers were first patented as ECAs in the 1950s [1–3]. Recently, ECA materials have been identified as one of the major alternatives for lead-con- taining solders for microelectronics packaging applications.
ECAs offer numerous advantages over conventional solder technology, such as environmental friendliness, mild pro- cessing conditions (enabling the use of heat-sensitive and low-cost components and substrates), fewer processing steps (reducing processing cost), low stress on the substrates, and fine pitch interconnect capability (enabling the miniaturiza- tion of electronic devices) [4–7]. Therefore, conductive adhe- sives have been used in f lat panel displays such as liquid crystal display (LCD), and smart card applications as an interconnect material and in f lip-chip assembly, chip-scale package (CSP), and ball grid array (BGA) applications as a replacement for solder. However, no currently commercial- ized ECAs can replace tin-lead metal solders in all applica- tions due to some challenging issues such as lower electrical conductivity, conductivity fatigue (decreased conductivity at elevated temperature and humidity aging or normal use con- dition) in reliability testing, limited current-carrying capa- bility, and poor impact strength. Table 11.2 gives a general comparison between tin-lead solder and generic commer- cialized ECAs [8].
Depending on the conductive filler loading level, ECAs are divided into ICAs, ACAs, and NCAs. For ICAs, the electrical conductivity in all x-, y-, and z-directions is provided due to high filler content exceeding the percolation threshold.
For ACAs or NCAs, the electrical conductivity is provided
TABLE 11.2 Conductive Adhesives Compared with Solder
Characteristic Sn/Pb Solder ECA
Volume resistivity 0.000015 Ω cm 0.00035 Ω cm
Typical junction R 10–15 mW <25 mW
Th ermal conductivity 30 W/m °K 3.5 W/m °K
Shear strength 2200 psi 2000 psi
Finest pitch 300 mm <150–200 mm
Minimum processing temperature 215°C 150°C–170°C
Environmental impact Negative Very minor
Th ermal fatigue Yes Minimal
only in z-direction between the electrodes of the assembly.
Figure 11.16 shows the schematics of the interconnect struc- tures and typical cross-sectional images of f lip-chip joints by ICA, ACA, and NCA materials illustrating the bonding mech- anism for all three adhesives.
11.3.2 Isotropically Conductive Adhesives ICAs, also called as “polymer solder,” are composites of polymer resin and conductive fi llers. Th e adhesive matrix is used to form a mechanical bond for the interconnects. Both thermosetting and thermoplastic materials are used as the polymer matrix. Epoxy, cyanate ester, silicone, polyurethane, etc. are widely used thermosets, and phenolic epoxy, maleim- ide acrylic preimidized polyimide, etc. are the commonly used thermoplastics. An attractive advantage of thermoplastic ICAs is that they are reworkable, i.e., can easily be repaired. A major drawback of thermoplastic ICAs, however, is the degradation of adhesion at high temperature. Another drawback of poly- imide-based ICAs is that they generally contain solvents.
During heating, voids are formed when the solvent evaporates.
Most of commercial ICAs are based on thermosetting resins.
Th ermoset epoxies are by far the most common binders due to the superior balanced properties, such as excellent adhesive strength, good chemical and corrosion resistances, and low cost, while thermoplastics are usually added to allow soft en- ing and rework under moderate heat. Th e conductive fi llers pro- vide the composite with electrical conductivity through contact between the conductive particles. Th e possible conductive fi ll- ers include silver (Ag), gold (Au), nickel (Ni), copper (Cu), and
carbon in various forms (graphites, carbon nanotubes, etc.), sizes, and shapes. Among diff erent metal particles, silver fl akes are the most commonly used conductive fi llers for current commercial ICAs because of the high conductivity, simple process, and the maximum contact with fl akes. In addition, silver is unique among all the cost-eff ective metals by nature of its conductive oxide. Oxides of most common metals are good electrical insulators, and copper powder, for example, becomes a poor conductor aft er aging. Nickel- and copper- based conductive adhesives generally do not have good resis- tance stability, because both nickel and copper are easily oxidized. ICAs have been used for die attach adhesives [9,10].
Recently, ICAs have also been considered as an alternative to tin/lead solders in surface mount technology (SMT) [11,12], fl ip-chip [13], and other applications and a large amount of eff ort has been conducted to improve the properties of ICAs in the past few years.
11.3.2.1 Improvement of Electrical Conductivity of ICAs
Polymer-based ECAs typically have lower electrical conductivity than Sn/Pb solders. To enhance the electrical conductivity of ICAs, various methods have been used and signifi cant improve- ment of conductivity of ICA has been achieved.
11.3.2.2 Increase of Polymer Matrix Shrinkage In general, ICA pastes exhibit insulative property before cure, but the conductivity increases dramatically aft er cure. ICAs achieve electrical conductivity during the polymer curing process caused by the shrinkage of polymer binder. Accordingly, ICAs with high
Silver flakes Substrate pad
IC electrode Polymer matrix
(b)
Substrate pad IC Bump
Conductive ball Polymer matrix
(d)
IC Bump
Substrate electrode
Polymer matrix
(f) Silver
flakes
Conductive spheres Contact
pads
Substrate
Substrate Polymer matrix
(a)
(c)
(e)
Component or IC
Polymer matrix
FIGURE 11.16 Schematic illustrations and cross-sectional views of (a,b) ICA, (c,d) ACA, and (e,f) NCA fl ip-chip bondings.
cure shrinkage generally exhibit higher conductivity [14]. With increasing cross-linking density of ICAs, the shrinkage of the polymer matrix increases, and subsequently, the resistivity of ICAs decreases. For epoxy-based ICAs, a small amount of a mul- tifunctional epoxy resin can be added into an ICA formulation to increase cross-linking density, shrinkage, and thus increase elec- trical conductivity.
11.3.2.3 In Situ Replacement of Lubricants on Ag fl akes
An ICA is generally composed of a polymer binder and Ag fl akes.
Th ere is a thin layer of organic lubricant on the Ag fl ake surface.
Th is lubricant layer plays an important role for the performance of ICAs, including the dispersion of the Ag fl akes in the adhe- sives and the rheology of the adhesive formulations [15,16]. Th is organic lubricant layer, typically a fatty acid such as stearic acid, forms a silver salt complex between the Ag surface and the lubri- cant. However, this lubricant layer aff ects conductivity of an ICA because it is electrically insulating. To improve conductivity, the organic lubricant layer should be partially or fully removed or replaced during the curing of ICA. Short chain dicarboxylic acid is one of the suitable lubricant removers because of the strong affi nity and more acidic of carboxylic functional group (–COOH) to silver. With the addition of only small amount of short chain dicarboxylic acid, the conductivity of ICA can be improved sig- nifi cantly due to the easier electronic tunneling/transportation between Ag fl akes and subsequently the intimate fl ake-fl ake contact [17].
11.3.2.4 Incorporation of Reducing Agent in Conductive Adhesives
Silver fl akes are by far the most used fi llers for conductive adhesives due to the unique properties of the high conductiv- ity of silver oxide compared to other metal oxides, most of which are insulative. However, the conductivity of silver oxide is still inferior to metal itself. Th erefore, incorporation of reducing agents would further improve the electrical conduc- tivity of ICAs. Aldehydes are one of the reducing agents that can be introduced. Obviously improved conductivity was reported in ICAs due to the reaction between aldehydes and silver oxides that exist on the surface of metal fi llers during the curing process [18]:
+ → −
R-CHO Ag O2 R COOH + 2Ag (11.3) Th e oxidation product of aldehydes, carboxylic acids, which are stronger acids and have shorter molecular length than stearic acid, can also partially replace or remove the stearic acid on Ag fl akes and contribute to the improved electrical conductivity.
11.3.2.5 Low-Temperature Transient Liquid Phase Fillers
Another approach for improving electrical conductivity is to incorporate transient liquid-phase metallic fi llers in ICA formulations. Th e fi ller used is a mixture of a high-melting-
point metal powder (such as Cu) and a low-melting-point alloy powder (such as Sn-Pb or Sn-In). Th e low-melting-alloy fi ller melts when its melting point is achieved during the cure of the polymer matrix. Th e liquid phase dissolves and wets the high melting point particles. Th e liquid exists only for a short period of time and then forms an alloy and solidifi es. Th e electrical conduction is established through a plurality of metallurgical connections in situ formed from these two powders in a poly- mer binder (Figure 11.17). Th e polymer binder with acid func- tional ingredient fl uxes both the metal powders and the metals to be joined and facilitates the transient liquid bonding of the powders to form a stable metallurgical network for electrical conduction, and also forms an interpenetrating polymer net- work providing adhesion. High electrical conductivity can be achieved using this method [19,20]. One critical limitation of this technology is that the numbers of combinations of low and high melting fi llers are limited. Only certain combinations of two metallic fi llers, which are mutually soluble, exist to form this type of metallurgical interconnection.
11.3.2.6 Low-Temperature Sintering of Nanosilver Fillers
Recently, nanosized conductive particles were proposed to be used as conductive fi llers in ICAs for high-performance and fi ne-pitch interconnects. Although the nanosilver fi llers in ICAs can reduce the percolation threshold, there has been concern that incorporation of nanosized fi llers may introduce more con- tact spots due to high surface area and consequently induce higher resistivity compared to micron-sized fi llers. A recent study showed that nanosilver particles could exhibit sintering behavior at curing temperature of ICAs [21]. Typically, applicav- tion of nanofi llers increases the contact resistance and reduces the electrical performance of the ICAs. Th e number of contacts between the small particles is larger than that between the large particles as shown in Figure 11.18a and b. Th e overall resistance of the ICA formulation is the sum of the resistance of fi llers, the resistance between fi llers, and the resistance between fi ller and pads (Equation 11.4). In order to decrease the overall contact resistance, the reduction of the number of contact points between the particles may be eff ective. If nanoparticles are sintered
Component
Substrate
Metallic network Polymer binder
FIGURE 11.17 Diagram of transient liquid phase sintering conduc- tive adhesives.
together, then the contact between fi llers will be fewer. Th is will lead to smaller contact resistance (Figure 11.18c). By using eff ec- tive surfactants for the dispersion and eff ective capping those nanosized silver fi llers in ECAs, obvious sintering behavior of the nanofi llers can be achieved. Th e sintering of nanosilver fi ll- ers improved the interfacial properties of conductive fi llers and polymer matrices and reduced the contact resistance between fi llers. Th erefore, an improved electrical conductivity of nanosil- ver-fi lled ICAs can be achieved.
= + +
total btw fillers filler to bond pad fillers
R R R R (11.4)
11.3.2.2 Reliability Enhancement of ICA Interconnects
One of the critical reliability issues of ICAs is that contact resis- tance between an ICA and nonnoble metal fi nished components increases dramatically during an elevated temperature and humidity aging. Th e National Center of Manufacturing and Science (NCMS) defi ned the stability criterion for solder replace- ment conductive adhesives as a contact resistance shift (increase) of less than 20% aft er aging at 85°C/85%RH for 500 h [22].
However, most currently commercialized ICAs cannot pass the reliability test on nonnoble metal surfaces such as Sn/Pb, Sn, Cu, Ni, etc. However, some of the improved ICAs have been devel- oped recently to satisfy the reliability requirements.
11.3.2.2.1 Mechanism Underlying Unstable Contact Resistance Simple oxidation and galvanic corrosion of the nonnoble metal surfaces are the two possible mechanisms for unstable contact resistance of ICAs. Studies have shown that galvanic corrosion at the interface between an ICA and nonnoble metal is the domi- nant mechanism for the unstable contact resistance [23,24]
(Figure 11.19). Under the aging conditions (for example, 85°C/85%RH), the nonnoble metal acts as an anode, and is oxi- dized by losing electrons, and then turns into metal ion (M − ne− = Mn+). Th e noble metal acts as a cathode and its reac- tion generally is 2H2O + O2 + 4e− = 4OH−. Th en Mn+ combines with OH− to form a metal hydroxide then further oxidizes to metal oxide. Aft er corrosion, a layer of metal hydroxide or metal oxide is formed at the interface. Because this layer is electrically
insulating, the contact resistance increases dramatically. A gal- vanic corrosion process has several characteristics: (1) occurs only under wet conditions, (2) an electrolyte must be present, and (3) oxygen generally accelerates the process, and (4) dissimi- lar metals should be present. Based on the mechanism of unsta- ble contact resistance of ICAs, several methods can be applied to stabilize the contact resistance and improve the reliability.
11.3.2.2.2 Low-Moisture Absorption
Moisture in polymer composites has been known to have an adverse eff ect on both mechanical and electrical properties of epoxy laminates. Eff ects of moisture absorption on conductive adhesive joints include degrading bulk mechanical strength, FIGURE 11.18 Schematic illustration of particles between the metal pads. (From Jiang, H.J., Moon, K., Lu, J., and Wong, C.P., J. Electron. Mater., 34, 1432, 2005.)
Nonnoble metal
Ag flake
Polymer binder Electrical conduction
Ag
(Before corrosion)
Nonnoble metal Ag
Ag flake
Polymer binder
Condensed water solution Metal hydroxide
or oxide Electrical conduction
(After corrosion)
FIGURE 11.19 Metal hydroxide or oxide formation aft er galvanic corrosion.