As the need for protective coatings that improve solderability has risen, the number of coatings has also increased. That growth is driving the development of new coatings which have a wide range of traits and tradeoffs.
As fine pitch devices begin to dominate new designs, HASL (hot air solder leveling) is no longer the obvious finish for soldering. Avariety of finishes are now available to fill the need. Organic solderability preservatives (OSPs) fill many needs for low-end boards and a number of options exist for more demanding applications.
Immersion tin and immersion silver have seen fair acceptance, while electroless nickel immersion gold (ENIG) has also been widely accepted. In addition, newcomer electroless nickel electroless palladium immersion gold (ENEPIG) has garnered a lot of interest.
As more options see usage, there’s a need for new standards. IPC-4556, Specification for Electroless Nickel/Electroless Palladium/Immersion Gold Plating for Printed Circuit Boards was started in the latter part of 2008 and will be released in the first quarter of 2013.
IPC-4552, Specification for Electroless Nickel/Immersion Gold Plating for Printed Circuit Boards, is presently being revised and expected to be released in 2014. It was first issued in 2002.
The long timeframe for the development of these two standards is due to the huge amount of data that’s included in these specifications.
“The committee does a number of studies and tests to find out the best thicknesses to get a good solder joint while keeping costs in line,” said George Milad, national accounts manager for Uyemura International Corp., and co-chair of IPC’s Plating Processes Subommittee. “All the data we collect is published in the appendices of the standards. The people who do the studies provide all the documentation.”
Though technical issues are important for all standards, costs are a critical factor for those that involve precious metals. With gold prices soaring well above $1,500 per ounce, making thinner layers can yield big savings over a product’s lifetime.
IPC-4552 has been revised to reduce the amount of gold needed to create coatings that still yield good connections when it’s soldered or aluminum-wire bonded. It’s also good for contact surfaces, such as push buttons. Along with its solderability and durability traits, ENIG is popular because it meets extended shelf life requirements.
The research into ENEPIG is attracting a lot of interest. The protective coating is sometimes called a universal surface because it works well for soldering, for both aluminum and gold wire bonding and for contact surfaces.
“ENEPIG is the most cost effective coating for gold wire bonding. It’s also the only surface finish that yields better solder joints with lead-free than with lead-based solder.” said Milad.
Though committee members are focused on the coatings that use gold and palladium, Milad noted that there are also alternatives that use less costly metals. Immersion tin and immersion silver both work in specific applications. Immersion tin has good solderability, particularly for parts that have leads which are inserted. However, it has a fairly short shelf life.
Immersion silver has good solderability traits as well as respectable contact surface characteristics. However, if it is not protected from the environment, its shelf life is comparatively short.