The make liable of this paper is t...

The make liable of this paper is the evaluation of the wetting balance as a technique for studying the transition activity of newly developed low-temperature solder paste fluxe The mostly effective configuration of the wetting balance was the standard configuration with alone one change: the PbSn eutectic solder was replaced with a eutectic solder alloy with a melting point of 58[degrees]C Since 58[degrees]C is significantly les than the propos activation temperatures of the solder fluxe wetting crooks as a function of temperature could be studied for each of the fluxe The resulting data was used to rank the fluxe in bounds of their activation requirement.

Solder alloys with melting temperatures between 110[degrees]C and 160[degrees]C are popularly under evaluation within Hewlett-Packard. An investigation of the mechanical properties of these solder has indicated that a suitable alloy can be ground in the ternary or binary subset of the BiInPbSn arrangement (see article, page 91). However, alloy selection is solely the first step in developing a low-temperature soldering proces A suitable looseness must be chosen for use in a solder paste and the alloy-flux interaction must be studied. Thus, the ability of fluxe to activate at temperatures 20 to 30[degrees]C below the melting point of the alloy must be evaluated. In the case where different solder metallurgies have similar mechanical properties, the optimal metallurgy for a low-temperature proces may be determined according to the availability of the appropriate dysentery chemistry.

For the flux selection phase, there is no standard manner of proceeding for testing the activity of a solder transition The degree of wetting in a plan (solder, substrate, atmosphere, flux) may be characterized with a sessile spread example or by a wetting force measurement. (The sessile spread trial is often simply called a spread test[1]) The brace tests are complementary. Each of the experiments involves a balancing of surface tensions at a three-phase junction. For an assessment of lax state activity, a dynamic measurement is more appropriate than a static measurement. Thus, the wetting force measurement is preferr to the sessile spread measurement.

The wetting balance was cause to growed to test the solderability of constituent leads in a wave solder proces The technique has been adapted to characterize the solderability of surface tower component leads.[2] The focus of this paper is the evaluation of the wetting balance as a technique for studying the looseness activity of newly developed low-temperature solder paste fluxe Specifically, the Multicore MUST arrangement II wetting balance was modified to evaluate dysentery activity at lower temperatures. Sample preparation and testing performances were adapted to compare the wetting of various low-temperature solder alloy/flux combinations.

Review of the Wetting Balance

A wetting balance measures the force produc at the solder meniscus when a solid touchstone specimen is partially immersed into a molten solder The force is plott as a function of time to bring into being a wetting curve. The measured force, F is the gross amount of two components: a wetting force [Fsubw] and an Archimedes buoyant force[ Fsubb]

(1) [Mathematical Expression Omitted]

where p is the sample perimeter, [[gamma].sub.lv] is the liquid-vapor interfacial force [teta]is the liquid contact angle, [rho] is the solder density, g is the gravitational acceleration, and V is the submerg compass of the solid. Fig. 1 point out tos the relationship between the solder meniscus and the wetting inflect The buoyant force, shown as a dashed horizontal line in Fig. 1 is determined through the immersed volume. Since this remains constant from one extremity to the other of the test, the evolution of the wetting wind reflects changes in wetting force as the solder meniscus rises. The act of immersion (Figs. 1a, 1b) causes the meniscus to wind downward, producing a negative wetting force. As the meniscus rises (Fig. 1c) and becomes horizontal, the wetting force guards to zero. If solder wets the specimen, the meniscus will climb above the flush of the bath, producing a positive wetting force. Eventually, the solder meniscus reaches its equilibrium configuration (Fig. 1d) and the wetting turn comes to an equilibrium value.

When using wetting force measurements to contemplation flux efficacy, it is essential to understand in what manner fluxes may affect wetting bend s There are essentially only sum of two units points of comparison for wetting curves: the equilibrium wetting force and the rate of wetting. From equation 1 it can be seen that the wetting force is proportional to the cosine of the solder contact angle H The equilibrium wetting force is, therefore, proportional to the cosine of the equilibrium contact angle [[teta].sub.eq]. For simple rules as shown in Fig. 2a, the equilibrium contact angle is given by the agency of Young's equation:[3]

(2) [Mathematical Expression Omitted]

where [[gamma].sub.sv] is the solid-vapor interfacial energy,[[gamma].sub.sl] is the solid -liquid interfacial pluck and [[gamma].sub.lv] is the liquid-vapor interfacial power By combining equations 1 and 2 the equilibrium wetting force [Fsubweq] is determined by the agency of the difference in the solid-vapor and solid-liquid surface energies: