Immersion tin captures a starring role.
There are a variety of surface finishes to choose from these day including OSP, HASL, ENIG, immersion silver and immersion tin. When I visit board shops, I often am asked, “Why can’t those assembly guys chose one surface finish, so I can get rid of all the rest?” The fact of the matter is that each surface finish has its strengths and weaknesses. Based on an assembly site’s driving need – whether it be cost, shelf life, appearance, long term reliability, component attachment method or solderability performance – one surface finish may be a better fit than the next.
In the world of surface finishes, immersion tin enjoys a fair market share. It can typically be found on many circuit boards in the telecommunication, printer and automotive worlds. The benefits of immersion tin include good in-circuit test after assembly, tarnish and corrosion resistance in harsh environments, press fit suitability, lower thermal stress vs HAL on the PCB, excellent solder joint strength and it is a halogen-free coating. When not applied properly, occasionally there can be issues with solderability, shelf life and tin whiskers.
Immersion tin whisker resistance, shelf life and solderability performance are based on suppression of copper diffusion. When immersion tin is initially plated, there are three distinct layers viewed on a cross section. These layers include the base copper, a thin intermetallic layer made up of copper and tin and a pure tin layer of ~ 1 micron. Very slowly, over time, copper continues to react with tin, and the intermetallic layer grows. Higher temperature and humidity accelerate intermetallic growth. It is the disparity in diffusion rates between copper into tin and tin into copper that creates stress in the coating, ultimately leading to tin whiskers. Additionally, as the copper intermetallic layer grows, less and less pure tin is available to protect the copper and the intermetallic layer from oxidation. A minimum of 0.1 micron of pure tin is required to protect the copper and intermetallic layer for the last soldering step. Once the intermetallic layer breaks through to the surface, copper oxides form, making solderability very difficult. For these reasons, the industry has accepted a standard that requires a minimum of 1.0 micron of tin be plated for lead-free applications to ensure good solderability.
Writing a specification for a minimum of 1.0 micron and actually plating 1.0 micron consistently are two different topics. Conventional immersion tins are applied in vertical or horizontal equipment sets. The process typically consists of cleaner, microetch, predip, tin plating bath and rinses. Immersion tin baths are very acidic with operating temperatures typically between 63° C and 68° C and dwell times between 20 minutes to 25 minutes. The combination of the time and temperature can create a lot of stress on the soldermask. Also, conveyor lengths and equipment costs become quite large.
There is new technology available that can reduce the requirement of a minimum 1.0-micron tin coating. The use of organic metals can allow for a 65% decrease in tin thickness while maintaining the whisker resistance, shelf life and soldering performance of a more traditional immersion tin coated circuit board. Testing has shown that when using organic metals in the immersion tin process, plating thickness can be reduced to 0.35 micron.
Organic metals are advanced forms of conductive polymers that have metallic properties, even though they are characterized as an organic compound. The material contains carbon, hydrogen, nitrogen, oxygen and sulphate as elements and is synthesized and dispersed in the form of 10-nanometer-small primary particles. In this process, organic metals are used as the copper surface preparation predip prior to the immersion tin deposition and as a post dip on top of the freshly plated immersion tin.
Use of the organic metal in the predip slows the diffusion of copper into the tin, greatly reducing the intermetallic layer growth. As a result, solderability is preserved for greater periods of time. Additionally, the slow diffusion rate suppresses whisker growth. Using organic metals in the post dip provides a coating that passivates the tin, and later the intermetallic layer, preventing oxidation.
Plating a thin immersion tin with organic metals has many side benefits. The organic metal predip now separates the whisker suppression ingredient from the main tin-plating bath, improving control and replenishment for a more predictable whisker resistant coating. The bath temperature and plating time is reduced. The equipment footprint is reduced and corresponding output is greatly improved. There is reduced stress put on the soldermask and less chance for failures like lift and peel.
Immersion tin will continue to grow in market share as a final finish. The coating’s use in press fit applications and in harsh environments is well established. The addition of organic metals in the immersion tin process show great promise to iimprove the process reliability and overall productivity by reducing the required plated thickness to 0.35 microns. PCD&F
Jim Kenny is global product line manager for Enthone Inc., a business of Cookson Electronics. He can be reached at This email address is being protected from spambots. You need JavaScript enabled to view it..
