Authored By:
Don Gudeczauskas, Albin Gruenwald and George Milad
UIC Technical Center
Southington, CT, USA
Summary
The use of ENEPIG (Electroless Nickel / Electroless Palladium / Immersion Gold) has been steadily increasing the past several years and benefits of the finish have now become well known throughout the industry. The finish provides both reliable solder joints and wire bonds. In some Asian countries where mass production is performed at many facilities, dedicated production lines have been installed for plating of ENEPIG using an immersion gold optimized for ENEPIG but not for ENIG. In the North American market, however, many PWB facilities are producing both ENIG and ENEPIG finishes from the same plating line due to lower overall production volumes and desire to use the same immersion gold for both finishes. Most facilities have neither the room for two separate immersion golds nor the desire to tie up capital with the cost of gold for two separate immersion gold tanks.
The challenge for North American manufacturers has been in choosing the proper immersion gold chemistry which can suitably deposit gold for ENIG and ENEPIG while providing a robust finish for soldering, wire bonding, and electrical contact with both finishes. This paper presents results of a comparative study for three types of immersion golds which could be used for both ENIG and ENEPIG deposits in the same production line: standard displacement immersion gold, high efficiency immersion gold which limits nickel dissolution, and a mixed reaction immersion gold which utilizes a mild reducing agent. Comparative results for solder wetting force, solder joint reliability and wire bonding will be presented for ENEPIG. Additionally, plated samples will be examined by SEM for evidence of nickel or palladium damage from the immersion gold plating operation.
Conclusions
Solder wetting balance tests showed that the reduction assisted immersion gold process yielded the fastest wetting times while the standard displacement immersion gold showed slightly lower wetting times. All final wetting forces were excellent regardless of heat exposure from 3X reflows.
Wire bond testing showed very good results from all six samples with higher standard deviation observed for the standard displacement immersion gold with thick deposit. Average wire bond values were well above the 3 gram minimum with average readings all above 9 grams.
Cross section analysis of the plated samples showed some small corrosion spikes in the standard displacement and high efficiency immersion gold with no such corrosion spikes seen in the reduction assisted gold deposit. It is thought that the corrosion spikes, if present in higher amounts, would affect solder wetting and wire bond results and could be used a process check for production considerations.
Future data presentations will include the performance of the three different immersion gold types with gold thicknesses at the high end of the new IPC ENIG specification. Solder testing and cross section analysis will be performed.
Initially Published in the SMTA Proceedings
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