Pan Pacific Symposium Conference Proceedings


IMMERSION TIN AS A HIGH PERFORMANCE SOLDERABLE FINISH FOR FINE PITCH PWBS

Author: David Ormerod
Company: Dexter Electronic Materials
Date Published: 1/25/2000   Conference: Pan Pacific Symposium


Abstract: ABSTRACT “A new high performance immersion tin incorporating a co-deposited organic provides a highly solderable PWB surface finish. The process is described and includes solderability data after accelerated aging and extended processing. Studies of SIR, metallic dendritic growth and ionic contamination are included. A final section shows the benefits for fine pitch assembly, including the yield improvements seen at Reflow, ICT and final inspection. The reliability questions are answered and conclude that the flat solderable tin process provides a cost effective and yield-enhancing planar alternative to Nickel-Gold, HASL and OSPs.” “Nothing solders like solder” – so it is written. The application of molten Tin/Lead onto clean copper gives an excellent electrical and mechanical interconnection. The copper/tin/lead metallurgy is very well understood and documented, and the solder joints formed have a well-established track record for reliability. However, the use of the 63/37 tin-lead Hot Air Solder Level (HASL) process, which has been the finish of choice for several decades, continues to decline. HASL is a cost-effective finish but has several technical drawbacks related to the difficulty of application and to the assembly limitations with fine pitch technology. For this reason, it has already been superseded by a range of flat, so called “planar”, finishes which offer significant benefits with regard to the improved paste screening, the control of solder joint volumes and the fast, accurate placement of small components

These selective planar finishes include several metallization processes such as Electroless Nickel-Immersion Gold and Electroless Nickel-Electroless Palladium which are not surprisingly at the high end of the cost spectrum. At the opposite end resides a whole group of substituted Imidazole type coatings that form the basis of the Organic Solderabiliy Preservatives (OSPs). These OSPs are relatively cheaper to apply than HASL and, in many cases, provide adequate solderability for the assembler. It is not surprising, however, that the unrelenting trend of miniaturization continues to challenge not just HASL but in fact all the finishes. The explosive growth of the Ball Grid Array technology, which has critical planarity needs, has seen its solder joint reliability plagued by incidences of interfacial stress fractures associated with Nickel/Gold . On the other hand, the use of mixed technology in the assembly process, (i.e. surface mount components on both sides of the printed circuit board together with through-hole interconnects), demands that the surface finish has a high tolerance to multiple heat cycles. This, in turn, is now challenging the OSPs which, in many cases, have limitations in withstanding the multiple thermal excursions, particularly when less aggressive and/or low solids fluxes are applied (an increasing trend).

As a direct result, there has been a growing demand for further alternatives, and a new generation of thin immersion metallization processes has entered the market. These include both silver and tin technologies, both of which offer lower cost-of-use relative to gold and palladium. The modified immersion tin process offers a very novel solution to this problem, largely because it provides a very comparable copper/tin/lead metallurgy to that of HASL when used with tin-lead solder pastes and solder-wave. This paper goes on to review the case history of one such modified proprietary flat solderable tin process, and the benefits demonstrated to the assembler.

Flat solderable tin, or any other new finish, has to unequivocally satisfy the stringent requirements of planarity, solderability, ionic cleanliness and surface insulation resistance. It is important that the new process can be readily integrated into the PWB fabrication operation. Coupled with this, an ability to deal effectively with the emerging volume production and assembly of fine pitch and area-array technologies represents the ideal. It is also extremely important to demonstrate that the tin finish, whether soldered or unsoldered, will not whisker and will not be subject to any corrosion, dendritic growth or any other mechanical or thermal breakdown which could conceivably affect the long term performance or reliability of the device.



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