PCB: The Right Final Finish
Selecting the right final finish should not be a puzzle, but it can seem like one. Often, there is conflicting information about different finishes and their intended applications. The best surface finish meets the needs of the board design and the assembly process.
If you are a contract manufacture, you might be looking for a surface finish that provides good-looking solder joints, is easy to rework, can sit in process for as much time as needed without fear of future problems and has a long shelf life. The OEM, on the other hand, is most interested in cost, reliability and availability. Chemical vendors may have a very narrow point of view highlighting the advantages of their latest system at the expense of all other surface finish options.
Hot air solder level (HASL) came into the industry to replace fused, tin-lead solder. Using solder mask over bare copper (SMOBC) fabricators could protect base copper and have a solderable surface. HASL was introduced just prior to the advent of SMT. Solder-coated copper has taken much of the work out of soldering at the assembly level because the tin-copper inter-metallic layer is already formed. The heat required at assembly is just enough to fuse HASL solder with solder paste. The boards have excellent shelf life and are easy to rework.
However, HASL does not lend itself well to high-density boards. Once SMT pitch goes below 16 mils or co-planarity is an absolute requirement, HASL will not perform well.
The ENIG process came into play in the 1990’s as a co-planar surface finish to replace HASL. ENIG has many benefits including excellent shelf life, easy rework at assembly (but not at the fabricator), low contact resistance, excellent solderability and good solder spread. The downsides include cost of operation, soldermask attack, high demand for shop utilities and a tight process window. ENIG is the most expensive surface finish to apply because pricing is tied to gold values and thickness. All that being said, ENIG usage has risen dramatically over the years and very widely available.
Immersion silver is the most cost effective, metal, surface finish alternative to HASL. It has excellent soldering characteristics and is highly conductive. Baths are stable, with relatively mild chemistries, and dwell times are quick so it is easy to conveyorize. Poised to replace HASL as the dominate surface finish, recent issues related to performance in harsh environments has exposed a weakness of the coating. Initially, studies placed the blame solely on the immersion silver finish, but recent work has shown assembly materials are perhaps a more significant contributor to the problem. The industry has responded with new immersion silver post treatments that either suppress the corrosion reaction or keep the corrosion localized to unsoldered areas.
So, as you can see the finished can be a little puzzling, but with the proper information a knowledgeable decision can be made for the best fit keeping in mind the needs of the board fabricator, assembler and that of the OEM.
Defined Features:
A solder mask defined feature is created over an underlying copper land area where the mask clearance itself defines the feature. Avoid the use of solder mask define features whenever possible. Sometime the mask feature are not as well defined over metal, or worse, they will leave small deposits of mask that prevent the surface finish from adhering in these locations. In addition, without an underlying pad, the surface finish does not get full encapsulation of the pad, making it more difficult to assemble.
To prevent this a thermal tie under the mask clearance on the circuit layer so the edge of the mask clearance are over material, not copper, and you are still tied electrically to the metal by way of the thermal spokes. When this is not possible, for adhesion reason stick with green solder mask, because the green color holds up better that the other colors for “mask defined” features.
For more information on PCB Surface finishes and solder masks, please contact us at 412-828-5322 or click here to contact us online.
