2005 Articles

Regardless of flux, silver-containing alloys can achieve acceptable wetting at high temperatures.

A few Pb-free alloy candidates can be used as replacements for Pb-containing alloys. To make the most appropriate selection for a particular application, solder properties such as wetting characteristics should be understood.

Intrinsic wetting ability of solder alloys is an important performance property that directly affects the integrity of solder interconnections. This wetting ability also controls production yield and throughput when using a dynamic soldering process, such as wave or reflow soldering. The wetting balance test, sometimes called a meniscograph test, is used by some industry segments to test wettability of components and is possibly the most versatile quantitative method.^1-4 The wetting balance test is classified⁵ in ANSI/J-STD-002 as a “Test Without Established Accept/Reject Criterion.” This test method is recommended for engineering evaluations only, not as a production pass/fail monitor. It is difficult to compare studies directly due to the use of different test conditions and analysis in each study.

The wetting balance test measures the forces imposed by the molten solder upon the test specimen as the specimen is dipped into and held in the solder bath. This wetting force is measured as a function of time and automatically recorded. A typical wetting balance curve is shown in Figure 1. Initially, the force is negative, indicating that the solder has not yet begun to wet to the specimen and, in fact, shows buoyancy effect. The force exerted by the solder approaches zero as the solder begins to wet to the specimen. One commonly used performance measure is the time to cross the zero axis of wetting force. This point indicates the transition from nonwetting (F<0) to wetting (F>0). Wetting time t_z [s] is the time counted from the moment at which the specimen and solder bath first make contact till the moment when the contact angle is equal to 90° (only the buoyancy force acts on the specimen and the non-wetting state passes into the state of wetting).

FIGURE 1: A typical wetting balance curve.

Maximum wetting force – F_max [mN/mm] – is the measured force F_max [mN] per unit area between specimen surface (metallic portion only) and the molten solder. Solder coating quality of the immersed surface is estimated visually after withdrawal of the specimen from the solder bath.

The basic solder process depends on wetting for the formation of the solder/substrate contact. According to the classic model of wetting described by Young-Dupre equation (1) the liquid (molten solder) would spread over the solid (substrate) until all three surface tensions (gLV, gLS and gSV) are in a balance as shown in Figure 2 of the schematic of the thermodynamic equilibrium.

FIGURE 2: Schematic of the thermodynamic equilibrium during wetting.

gSV = gLS + gLV Cos ? (1)

where

gLV is the surface tension between molten solder and vapor (or flux)

gSL is the surface tension between substrate and molten solder

gSV is the surface tension substrate and vapor (or flux)

? is the contact angle of the molten solder on the solid surface (substrate).

The driving force for the spreading of the solder would be imbalance in the surface tension (surface energy). The use of various fluxes would affect the gSV (cleanliness of the surface would be affected by the aggressiveness of the particular flux). The gSL and gLV are constant at a fixed temperature for a particular composition.

The variation in test temperature influences the surface tension and contact angle of the solder. This is of particular importance because it permits the determination of the optimum temperature condition at which the specific alloy/flux couple could be used to achieve acceptable wetting performance of the system.

Total wetting force would be affected by all three surface tensions. The surface tension between substrate and solder (gSL) decreases with increasing temperature, thereby negatively affecting the wetting force (Figure 2). A decrease in contact angle would show a positive effect on the wetting force because of the greater cosine value (resulting value of the flux/solder surface tension would be affected due to the gLV Cos ?). Therefore, the increase of maximum wetting force (F_max) as result of the temperature increase means that the effect of the contact angle is stronger than that of the surface tension.

The wetting parameters would be highly variable depending on the choice of alloy/flux system.

Procedure

Solderability tests of lead-free alloys of interest had been carried out by the wetting balance method. A Minisco ST50 solderability tester was used for wetting balance. Copper test coupons that were 25.5 x 12.78 x 0.28 mm were cleaned in isopropyl alcohol, dipped in 10% flouroboric acid solution, rinsed with tap water, flushed with DI water, then dried. The solder pot was heated to 260°C. The copper coupons were dipped for approximately 5 sec. to a depth of 10 mm in a flux. Excess flux was removed by touching the end of the coupon to clean filter paper. The coupon was then positioned in the machine. The coupon was immersed in the solder to a depth of 5 mm for 10 sec. at a dip rate of approximately 21 mm/s. The machine automatically corrects for buoyancy and calculates wetting force at every 1/8th sec.

In this study, wetting time (t_z) and maximum wetting force (F_max) were measured for several lead-free alloys (Table 1). Five no-clean fluxes were used to study wetting characteristics of the different lead-free alloys (Table 2 shows characteristics of the fluxes used in this study).

According to generally accepted criteria per ANSI/J-STD-003, the typical wetting time should be less than 1 sec. for reflow operations and less than 2 sec. for wave soldering processes. As a rule of thumb a wetting force is expected to exceed 250 µN/mm within 2 sec. of wetting time. This is considered a good wetting characteristic corresponding to a good quality soldering process. Some researchers suggest that adequate soldering could be achieved even when the wetting force is 150 µN/mm.

The effects of both solder temperature and flux chemistry were studied during this investigation.

Results and Discussion

Figures 3-8 are acquired wetting balance curves for each alloy at two different temperatures (245° and 260°C) with various fluxes. Eutectic tin/lead (Sn63Pb37) solder alloy was used as a reference. Figure 9 illustrates the acquired wetting balance curves for this alloy with the same fluxes used for lead-free solders of interest and at two temperatures. One temperature is typical for eutectic SnPb alloy (235°C) and another is the temperature used for the Pb-free alloy in current study (260°C).

FIGURE 3: Wetting characteristics of SAC 305 solder alloy tested at different temperatures with different fluxes.

FIGURE 4: Wetting characteristics of SAC 405 solder alloy tested at different temperatures with different fluxes.

FIGURE 5: Wetting characteristics of Sn96.5Ag solder alloy tested at different temperatures with different fluxes.

FIGURE 6: Wetting characteristics of Sn99.3Cu solder alloy tested at different temperatures with different fluxes.

FIGURE 7: Wetting characteristics of SACX0307 solder alloy tested at different temperatures with different fluxes.

FIGURE 8: Wetting characteristics of Sn99.3Cu0.7+Add solder alloy tested at different temperatures with different fluxes.

FIGURE 9: Wetting characteristics of Sn63Pb solder alloy (as a reference) tested at different temperatures with different fluxes.

Comparing the relative wetting time performance among the Pb-free solder alloys, it was observed that the wetting performance (the maximum wetting force (F_max and zero wetting time) are affected by the solder composition, temperature and flux used.

During testing, fluxes were falling into two categories (based on their influence on the wetting characteristics of the solder). IF2010F and EF2202 fluxes appeared less effective compared to EF 3215, EF4102 and RF800.

Some fluxes clean (remove the oxide, corrosion products, etc.) the surface of a substrate much more efficiently, thereby affecting the interfacial energy between the substrate and molten solder that would alter the surface tension (gSL). For all the tested solder alloys, some fluxes (EF 3215, EF4102 and RF800) improved the wetting characteristics of the Pb-free solder alloys. This could be attributed to an ability to clean surface of the substrate more effectively.

Based on the wetting force and wetting time data (Tables 3 and 4), it is clear there are 1) temperature dependencies (wetting characteristics improve with increasing temperature due to greater metallurgical reaction, surface tension change and fluidity of the molten solder) and 2) solder alloy composition dependencies.

Pb-free solder alloys used in the present work have a melting point of around 215-225°C, which is a little over 30°C higher than that of eutectic lead-tin (T_m = 183°C), so its use should be able to be implemented into existing soldering processes with minimal process modification. The temperature used to attach components with eutectic SnPb solder is close to 220°C (industry accepted criteria for the testing of the eutectic SnPb solder is 235°C). The present investigation showed that all Pb-free solders investigated could be used at 260°C with any of the fluxes used in this study.

However, to choose a reflow profile that would not cause damage to components or boards, the lowest possible temperature would be required. The decrease in the test temperature to 245°C from 260°C revealed that some Pb-free solder alloys would not perform as required. Silver-containing alloys appear to perform better, even when a solder alloy with lower silver content was used (i.e., Sn99.0Cu0.7Ag0.3) at a lower temperature (245°C).

At 260°C, the test results indicate all silver-containing Pb-free alloys tested in this study, regardless of the flux used, have a wetting time less than or slightly higher than 1 sec., while alloys without silver were more flux-dependent. This demonstrates that all tested silver-containing alloys can achieve acceptable wetting at this high temperature independent of the flux chosen. Even at 245°C, all Pb-free silver-containing solder alloys tested exhibited shorter wetting time and higher maximum wetting force

In this investigation, wetting behavior of several Pb-free alloys is characterized. The results from wetting balance tests were used to determine the difference in wettability between different solder alloys coupled with several different fluxes.

As expected, the process temperature affects the wetting performance (an alloy’s wettability increases with increasing temperature) of the Pb-free alloys included in this study. Flux effectiveness plays an important role in Pb-free solder alloy wetting performance. For non-silver-containing alloys, more aggressive fluxes were required, even at high temperatures. The alloy system and the composition of the alloy predominantly control the wetting performance at a given temperature/flux.

Based on the results of the wetting tests, it could be assumed that silver-containing Pb-free solder alloys exhibit better pad coverage and plated hole fill. Silver-containing alloys exhibited superior performance.

References

1. Humpston, Principles of Soldering, ASM International, 2004, pp. 13-18.
2. R.J. Klein Wassink, Soldering in Electronics, 2nd ed. Electrochemical Publications, 1989, pp. 37-40.
3. F.G. Yost, Mechanics of Solder Alloy. Wetting and Spreding, Van Nostrand Reinhold, New York, 1993, pp. 12-19.
4. H.H. Manko, Solders and Soldering, 3rd ed., McGraw-Hill, 1992, pp. 5-10.
5. ANSI/J-STD-002, “Solderability Tests for Component Leads, Terminations, Lugs, Terminals and Wires,” Test Without Established Accept/Reject Criterion.

Anna Lifton is senior research scientist, Ronald Bulwith is manager, Technical Services Group and Louis Picchione is application engineer, all with the Analytics/Technical Services Laboratory at Cookson Electronics Assembly Materials (cooksonelectronics.com); alifton@cooksonelectronics.com.