Non‐precious Metal Finishes for Fine Pitch Assembly

The industry trend towards higher density interconnect on PWBs has caused board fabricators to explore alternatives to the traditional hot air solder levelling (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 processes based upon bismuth and silver have been developed, and the data presented demonstrate that the solderability of these systems is superior to that of organic and palladium finishes. Test results on hole‐fill, solder‐spread and meniscograph (wetting balance) are extensively discussed in this study. Potential concerns over joint reliability and diffusion are addressed. The chemistry of these non‐precious metal systems also affords benefits to the PWB fabricator. These include increased yields, reduced costs and the elimination of soldermask compatibility issues. Considering possible future industry trends, a significant advantage of these materials is their full compatibility with lead‐free solders. Data on compatibility with solder replacements and wire‐bonding technology are also presented.