Hot Air Solder Leveling in the Lead-free Era



Hot Air Solder Leveling in the Lead-free Era
Paper discusses best practice for lead-free HASL lines and the properties expected with a properly applied HASL finish.
Materials Tech

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Authored By:


Keith Sweatman
Nihon Superior Co., Ltd.
Osaka, Japan

Transcript


Although the advantages of Hot Air Solder Leveling (HASL) in providing the most robust solderable finish for printed circuit boards are well recognized, the conventional wisdom was that it would have no place in the new lead-free electronics manufacturing technology.

The widely promoted view was that HASL would be replaced in the lead-free era by Organic Solderability Protectants (OSP) and immersion silver with perhaps a minor role for immersion tin. This view was reinforced by some early trials of lead-free.

An unexpected advantage was that the thickness of the lead-free HASL finish was more uniform than typically obtained with tin-lead so that it could be used in applications previously excluded to tin-lead HASL because of concerns about coplanarity.

Many Chinese printed circuit board shops have installed lead-free HASL. As problems have been encountered with OSP and immersion silver finishes electronics manufacturers have looked to lead-free HASL as a solution.

This paper discusses current best practice on the operation of lead-free HASL lines and the properties that can be expected of a properly applied lead-free HASL finish.

Summary


Although the advantages of Hot Air Solder Leveling (HASL) in providing the most robust solderable finish for printed circuit boards are well recognized, in the years leading up to the implementation of the EU RoHS Directive in July 2006 the conventional wisdom was that it would have no place in the new lead-free electronics manufacturing technology. The widely promoted view was that HASL, which had been the most popular printed circuit board finish in North America, Europe and most of Asia outside Japan during the tin-lead era, would be largely replaced in the lead-free era by Organic Solderability Protectants (OSP) and immersion silver with perhaps a minor role for immersion tin.

This view was reinforced by some early trials of lead-free HASL in which the tin-silver-copper alloy, then promoted as the universal lead-free replacement for tin-lead, was used as the coating alloy. The aggressive dissolution of copper by that alloy and its non-eutectic behavior made it difficult to use and to get satisfactory results. In the meantime, however, in Europe a microalloyed tin-copper alloy with low copper dissolution and eutectic behavior was evaluated and found to yield promising results. A smooth mirror-bright finish could be achieved on existing equipment with process temperatures that existing laminate materials could accommodate.

An unexpected advantage was that the thickness of the lead-free HASL finish was more uniform than typically obtained with tin-lead so that it could be used in applications previously excluded to tin-lead HASL because of concerns about coplanarity, e.g. pads for BGA, CSP and fine pitch QFP. By July 2006 there were nearly 200 lead-free HASL lines running in Europe and that number has continued to increase since then. In response to demand by European customers many Chinese printed circuit board shops have installed lead-free HASL and there are now lines operating in South East Asia, India and North America.

As problems have been encountered with OSP and immersion silver finishes electronics manufacturers have looked to lead-free HASL as a solution. In the more than 5 years in which the lead-free HASL process has been used in commercial mass product much has been learned about the operation of the process on the optimization of results. In this paper the author will report on current best practice on the operation of lead-free HASL lines and the properties that can be expected of a properly applied lead-free HASL finish

Conclusions


Lead-free HASL provides a printed circuit board with a corrosion resistant finish that can assure solderability over long periods of uncontrolled storage and through multiple lead-free reflow profiles.

•The lead-free HASL finish is superior to the tin-lead HASL finish in terms of uniformity of coating thickness.
•The main quality requirement for the achievement of the full potential of the lead-free HASL finish is a coating thickness that provides full coverage of the intermetallic layer that forms on a properly wetted copper surface.
•Best results in the HASL process are obtained with lead-free solders that come as close as possible to matching the eutectic behavior of tin-lead solder.
•A lead-free alloy formulated for minimum copper dissolution makes control of the solder bath composition easier and reduces the extent to which board reliability is compromised by reduced copper thickness.
•A lead-free alloy formulated for minimum growth of the intermetallic layer ensures maximum solderable shelf live and best final solderability in multi-stage soldering.
•While the vertical HASL process is still the most widely used the horizontal HASL process provides the most uniform finish and the high throughput required to meet the growing demand for this process.

Initially Published in the IPC Proceedings

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