Abstract
The solder surface produced on bare printed circuit boards by hot air solder leveling (HASL) has been used extensively as a solderable finish for many years. The recent conversion to lead free solder in the electronics industry has led many people to predict the eventual demise of this process. The purpose of this paper is to report that lead free HASL or LF HASL is not only seen as a way of continuing to use the HASL process in the lead free era but also to give the reader a chance to reconsider this reliable surface finish.

Introduction
The work reported here involves the use of a tin-copper-nickel proprietary alloy known in the industry as SN100C. The SN100C alloy is widely used in both wave, selective and hand soldering in the electronic assembly industry. SN100CL, the HASL version of SN100C, is now being widely adopted in the PCFAB industry as the de-facto standard for LF HASL.

SN100CL is a solder alloy of the following composition: 99.3% tin, 0.7% copper, 0.05% nickel and a nominal of 60 ppm germanium. The mechanism by which the nickel addition works is not fully understood. It seems that nickel facilitates nucleation of the Cu6Sn5 intermetallic so that the alloy can freeze as a eutectic. Without nickel, the solidification of Sn-0.7Cu is dominated by primary tin dendrites, which are the cause of the dull, grainy, cracked finish that is typical in most lead-free solders. The benefits of the eutectic solidification of SN100CL are high fluidity close to its melting point, a property that is useful for the HASL process and a smooth bright shiny surface comparable to the surface seen with SnPb (Figure 1)1. SN100C/SN100CL has a melting point of 227○C and is nominally of a eutectic composition.

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This article originally appeared in Global SMT & Packaging issue 8.3 - March 2008.

Keywords : Lead-Free, Hot Air Solder Leveling, SN100CL

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