HOT AIR SOLDER LEVELLING – A SOLUTION TO LEAD-FREE SOLDERABILITY PROBLEMS
Authors: Keith Howell and Keith Sweatman Company: Nihon Superior Co., Ltd. Date Published: 5/22/2009
ICSR (Soldering and Reliability)
Abstract: When electronics assembly was based around the use of tin-lead solders, hot air solder levelling (HASL) was the most widely used printed circuit board finish in North America and Europe. In the period leading up to the implementation of the RoHS Directive, however, HASL was widely dismissed as an option with immersion silver in particular being recommended as the finish that would best meet the need for a reliable lead-free solderable printed circuit board finish. Where other considerations might preclude the use of immersion silver, organic solderability protectants (OSP), immersion tin, and electroless nickel/immersion gold (ENIG) were identified as the remaining options. While immersion silver and OSP have indeed established a position as the dominant lead-free printed circuit board finishes in the North American market with ENIG and immersion tin holding on to their specialist market niches in Europe, where leadfree implementation is very much more advanced, HASL has retained much of its market share with more than 300 lines currently running the lead-free process. As volumes have increased European printed circuit board shops and their customers have looked to Asia and there are now more than 100 lines running the lead-free HASL process in China, India, and South East Asia. Even in North America there are at least 20 lead-free HASL lines in operation. A key factor contributing to this success has been the development of lead-free alloys more suitable for the HASL process than the high silver alloys that were used in early trials. Along with improvements in the design of HASL machines these alloys make it possible for the process to operate at lower temperatures than were expected. When properly applied the HASL finish is the most robust of all solderable finishes offering a shelf life of more than a year even without special storage conditions and solderability that can survive multiple lead-free reflow profiles. The fact that the uniformity of coating thickness is better by a factor of two than that which is possible with tin-lead HASL has meant that the range of use of the lead-free process has expanded to complex boards with fine pitch area array packages. In this paper the authors will review the features of the leadfree HASL process that have resulting in it retaining a substantial and growing share of the market for solderable printed circuit board finishes in the lead-free era.