NON-PRECIOUS METAL COATINGS FOR FINE PITCH ASSEMBLY AND DIRECT CHIP ATTACHMENT
Author: Steve Beigle Company: Alpha Metals Inc. Date Published: 9/10/1996
Surface Mount International
Abstract: The industry trend towards higher density interconnects on PWB’S has caused board both board fabricators and assemblers to explore alternatives to the traditional hot air solder leveling (HASL) technology. Problems with pad planarity in the assembly of fine pitch devices with high I/O counts are well documented. Organic finishes based on substituted imidazoles address some of the assembly requirements, as do precious metal finishes such as gold and palladium. However both types of solderable finish have limitations or undesirable characteristics. This paper discusses and compares other non-precious metal systems as alternative solderable finishes. Immersion plating P~ based upon bismuth and silver have been developed and the data presented demonstrates that the solderability of these systems is superior to organic and palladium finishes. Test results on hole-fill, solder-spread & meniscograph (wetting balance) are extensively discussed in this study. Potential concerns over joint reliability & diffusion are addressed. The chemistry of these non-precious metal systems also affords benefits to the PWB fabricator. These include increased yields, reduced costs & the elimination of solder mask compatibility issues. Considering possible future industry trends, a full compatibility with lead-free solders. Data on compatibility with solder replacements and wire-bonding technology is also presented.