Pb-free hot air leveled solder coatings: is HAL viable for Pb-free assemblies?

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Until recently more than 80% of all PCBs produced worldwide were hot air leveled. Today, still more than 60% of all PCBs undergo HAL. Yet most studies conducted so far used PCBs with finishes of ENIG, immersion silver, immersion tin and organic solder preservative (OSP). Pb-free hot air leveled PCBs invariably have been excluded.

Numerous Pb-free alloys can be eliminated from consideration for use in HAL and assembly (wave soldering and paste) for reasons of cost, availability of alloy constituents, compatibility with other solder alloys or soldering processes, or reliability considerations. Those remaining can be sorted into two main groups: SnAgCu alloys and SnCu alloys. Although there is a substantial difference (44[degrees]C) between the melting points of SnCu-based alloys and SnPb37, the actual difference in the process temperatures these two alloys are used at, both in assembly and HAL, is 10 to 20[degrees]C. SnPb alloys are usually run at temperatures of 250[degrees] to 260[degrees]C, whereas SnCuCo alloys are processed at temperatures in the range of 260[degrees] to 270[degrees]C and SAC305 at 255[degrees] to 265[degrees]C in HAL, wave and SMT soldering equipment. Table 1 details the physical properties for some Pb-free and SnPb alloys.

When choosing a Pb-free alloy, either binary or tertiary, several considerations should be kept in mind. First, a binary alloy is much easier than a tertiary alloy to keep in spec. Second, the 10[degrees]C difference in melting temperature means little in a through-hole wave soldering application but that same 10[degrees]C may be significant in a reflow oven, where dwell times are longer. Also, as both alloys are compatible with all other board finishes, there seems to be little justification to spend extra money on a silver-containing alloy for wave soldering and HAL.

Wetting Balance Tests

It would be expected that the ideal Pb-free replacement solder, in order to yield results comparable to SnPb37 solder, would wet copper surfaces as well as SnPb37. Wetting balance tests were conducted on the SnCuCo alloy to determine if an optimal wetting temperature existed. The test was used as a means to compare the wetting characteristics of the SnCuCo alloy to that of SnPb37. Testing was conducted on a Metronelec wetting balance.

Wetting balance tests were first conducted at 250[degrees]C for SnPb37 (its recommended operating temperature) to establish a baseline to which the Pb-free alloy could be compared (Figure 1).

SnPb37 Alloy

* Temperature: 250[degrees]C.

* Maximum wetting force: 0.32 mN/mm.

* Time to achieve maximum wetting: 0.241 sec.

* Average force at 1.125 sec.: 0.32 mN/mm.

Next, wetting balance tests were conducted on the SnCuCo alloy at temperatures of 250[degrees]C, 255[degrees]C, 265[degrees]C and 275[degrees]C (Figure 2).

Howard Stevens is vice president sales and marketing at Metallic Resources Inc. (metallicresources.com) and Nimal Liyanage, Ph.D., is director of new business development, PC Fab at Metallic Resources; [e-mail].

Howard Stevens and Nimal Liyanage, Ph.D.