How to Tailor Immersion Tin Plating for IC Substrate Applications

Abstract

Immersion tin finishes are widely used in the printed circuit board (PCB) industry. They provide reliable connections with high solder joint reliability compared to other final finishes. Due to the direct intermetallic formation between copper and solder, the intermetallic compounds (IMC) formed during the assembly exhibit ductile properties and are less prone to brittle fractures than e.g. solder joints formed with nickel finishes such as electroless nickel/immersion gold (ENIG). With the given high solder joint reliability, the immersion tin finish therefore is one of the first choices for the application in the IC substrate industry. It provides already a robust and nickel free finish with high process efficiency for lower cost. But due to the trends to increased I/O counts, reduced lines and spaces, improved signal integrity and further lowering the cost in the packaging applications, the adjustment of the immersion tin process became necessary to cope with the changed technology demands. Topic of this paper is the introduction of an immersion tin plating process specifically designed for IC substrate applications. It is focusing on three major aspects: the plating process itself, the equipment solutions to realize best quality control and process monitoring and finally the performance of the final finish itself in comparison to other common end finishes. The trend to more and more reduced lines and spaces requires the adjustment of the plating chemistry. A good solution flow needs to be ensured to prevent excessive attack of the copper tracks or disproportionate solder mask undercut. The study presented here compares conventional immersion tin plating solutions with a modified process for IC substrate plating. It shows, how the variation of solution viscosity and flow can impact the copper attack and solder mask undercut. Due to the better solution exchange and adjusted plating conditions, the copper dissolution can be significantly reduced and the attack to the solder mask can be minimized. The complimentary horizontal plating equipment supports the specific properties of the plating electrolyte. Insights to the equipment design and adjustments are being discussed to identify the best conditions for the immersion tin plating on fine structures with highest quality demands. Finally, the tin deposit is investigated and compared to conventional immersion tin as well as to other standard finishes such as ENIG, ENEPIG (Electroless Nickel, electroless Palladium, Immersion gold) or OSP (organic solder preservative) regarding its solder wetting performance and solder joint reliability. It can be concluded that the immersion tin finish designed for small L/S applications can validate the benefits known from the long years’ experience with immersion tin. The layer shows comparable solder wetting properties to conventional immersion tin finishes and excellent solder joint reliability when compared to ENIG or OSP with better performance even after thermal aging. The risk for typical IC substrate related defects such as excessive copper corrosion or solder mask undercut as well as missing bumps is lowered compared to the conventional process. The main focus of this paper is to show, that the high demands in substrate industry can be tackled when the plating solution and plating equipment are designed and adjusted to the new requirements in a coordinated approach so that they can complement each other.