SMTA International Conference Proceedings


Author: Christopher Hunt et al.
Company: National Physical Laboratory
Date Published: 9/22/2002   Conference: SMTA International

Abstract: Lead-free soldering is still in its infancy with technical and cost issues posing major challenges for the industry. Many of these arise because of the need to minimize any increases in soldering temperatures.

It is expected that soldering in a nitrogen atmosphere might overcome some of the technical barriers and provide soldered products comparable to those using conventional eutectic tin-lead solders processed in air. But quantitative data regarding the soldering behavior of lead-free solders under various atmospheres are sparse.

Previous studies at NPL have examined the wetting balance behavior of Sn/Pb, Sn/Ag/Cu, Sn/Cu, Sn/Zn and Sn/Ag/Bi, under a range of residual oxygen levels between 10 ppm and 21% and at superheats between 20 to 60C. In general the data show that reductions in solderability, particularly when the superheat is reduced, can be recovered by inerting.

There was some difficulty in making comparisons between the alloys since some of the data were obtained with copper coupons and some with copper wire, using the globule technique. In the present work additional data using globule testing for Sn/Pb and Sn/Ag/Cu is presented, enabling easier comparisons between the alloys. Comparisons between both varieties of wetting balance tests can also be made.

The overall conclusions remain the same. For many of the challenging conditions and poor solderability associated with lead-free solders nitrogen offers an attractive alternative to stronger fluxes and high processing temperatures.

Key words: solderability, nitrogen inerting, wetting, leadfree.

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