PCB have many kinds of Surface Finish which affected by price, availability, shelf life, reliability and assembly processing. Due to each surface finish has own benefits, in the most cases, the process, product or environment will dictate the surface finish which is best suited for the application. It is recommended to end-user, designer or assembler work closely with their PCB supplier to select the best PCB surface finish.
The 63/37 tin lead solder has been industry-standard since the inception of the original PCB. If lead-free is not a concern, HAL is a very cost effective, reliable surface finish utilized in the manufacturing of lower technology PCBs. The HAL process can add stress to high layer circuit boards which can make long-term reliability issues. It added stress, along with uneven solder height on dense SMT, are good reasons to replace HAL. Inevitably, tighter design criteria, advancing technologies, and/or environmental legislation will force the replacement of HAL. There are lead free alloys which can replace the conventional 63Sn / 37Pb solder in this process, but there are still capability limitations that exist within this process.
ENIG (Electroless Nickel/Immersion Gold)
The ENIG finish has historically been the best fine pitch (flat) surface and lead-free option world-wide. Benefits to this surface finish are; long-term experience/knowledge of the product and excellent shelf life. The typical Nickel thickness is 75 micro inches and 3-5 micro inches of gold. Disadvantages include; limited availability, higher cost and this being the only surface finish which requires a two-part process. In addtion, if the process is not controlled, quality issues such as “Black Pad” may occur.BGA boards use ENIG offen.
With the pending lead-free legislation impacting PCB manufacturing on a global scale, the immersion silver process is rapidly gaining popularity as the lead-free surface finish of choice. While ENIG presently has a larger market share, over the past 12 months more immersion silver process lines have been installed in PCB facilities than any other finish. Immersion silver has a controlled thickness of 5-12 micro inches and a shelf life of at least 12 months. Silver is compatible with most assembly processes, is cost advantageous, and with its increased popularity, is becoming more commonly available.
OSP have been around since the 1970’s. It is widely believed that IBM was the first major corporation to give this finish credibility. The thickness of the OSP finish is almost unmeasurable .The original formula OSP (Organic Solder ability Preservatives) had a short shelf life of 3-6 months and could only withstand one or two heat cycles. By today’s standards, this would be considered applicable only for lower technologies. The latest OSP formulas are far more robust and are designed for lead-free assembly. They can handle multiple heat cycles and have a one year shelf life.