The evolution of technology manufacturing is always moving towards a higher level of complexity. Newer designs have to be lighter, faster and smaller with every iteration of the next device. Not only do we continue to move towards more complex technology as a printed circuit board manufacturer, but we strive to do so in an environmentally friendly way, working to eliminate potentially harmful chemicals for our processes. To that end there have been recent advances and improvements to the surface finishes used on printed circuit boards. The surface finish of the board is critical to how the components will connect and ultimately function in the final product. The goal then is a surface finish that can accomplish strong connectivity and still meet today’s strict environmental demands.
We detail the most common surface finishes in another blog post called Printed Circuit Board Finishes. There you will find information on the more common finish types used by a printed circuit board manufacturer, including Electroless Nickel Immersion Gold (ENIG), HASL, Lead Free HASL, and more. While you may be familiar with many of these, today we will introduce you to Electroless Nickel / Electroless Palladium / Immersion Gold (ENEPIG).
ENIG and Black Pad
ENIG has been a popular surface finish because of its very flat surface, ability to handle multiple reflow cycles and good shelf life. However, there can be challenges with nickel corrosion that can cause what is called “Black Pad”. Black pad occurs when the nickel under the gold plating corrodes excessively and the component soldered to that pad loses connectivity.
ENIG can be challenging for some printed circuit board manufacturers, as black pad can be difficult to detect when examining the surface finish since the gold completely covers the final surface.
What is ENEPIG?
ENEPIG is a surface finish made of three metallic layers; first a layer of electroless nickel, then a layer of electroless palladium, and finally a layer of immersion gold. The nickel is 150 to 200 micro-inches thick, the palladium is typically 8 to 15 micro-inches thick (although sometimes 4 to 8 micro-inches), and the gold is 1 to 2 micro-inches thick. ENEPIG is suitable for all common types of wire-bonding and suitable for soldering, providing a major advantage over ENIG (rarely suitable for wire bonding) and soft bondable gold (not suitable for soldering) in applications that require both soldering and wire-bonding. Some designers choose ENEPIG to prevent the potential ENIG quality issue called “black pad”, although black pad has become very rare in recent years. ENEPIG can also be used to provide a thinner finish than soft bondable gold in very fine pitch applications, but this use is more common in semiconductors than in printed circuit boards.
Electroless Nickel / Electroless Palladium / Immersion Gold, or ENEPIG for short, has Palladium as one of the key ingredients.
From Wikipedia: Palladium is a chemical element with the chemical symbol Pd and an atomic number of 46. It is a rare and lustrous silvery-white metal discovered in 1803 by William Hyde Wollaston.
While similar to Electroless Nickel Immersion Gold in process and application, the palladium in ENEPIG forms a flat, coplanar, hard surface with several advantages including gold wire bonding, aluminum wire bonding and excellent solderability, earning ENEPIG the nickname “the universal surface finish” (at least that’s what we call it!). These advantages, along with a decrease in the price of palladium, have resulted in recent gains in popularity and demand for ENEPIG.
During the ENEPIG process, palladium is applied to the nickel before gold thus eliminating any problems with gold/nickel corrosion (i.e. no Black Pad). By contrast, the ENIG process simply applies gold to the nickel.
IPC-4556 thickness specification states the following:
• All measurements should be done on a nominal pad size of 1.5 mm x 1.5 mm [0.060 in x 0.060 in] or equivalent area
• The EN thickness shall be 3-6 µm [118.1-236.2 µin] at ±4 sigma (standard deviations) from the mean
• The electroless Palladium (Pd) thickness shall be 0.05-0.15 µm [2-12 µin] at ±4 sigma (standard deviations) from the mean
• The minimum immersion Gold (Au) thickness shall be 0.025µm [1.2 µin] at -4 sigma (standard deviations) below the mean
Recent increases in gold costs and the demand for lead free finishes have driven improvements in the immersion gold processes for both ENEPIG and ENIG. While ENEPIG has been around since the mid-90s, the cost, features, and advantages over ENIG have caused it to gain in recent popularity.
For those interested in diving deeper into the ENEPIG specifications, IPC has provided a detailed specification in the document “IPC-4556 Specification for ENEPIG” which can be purchased directly from IPC. Additionally, there is a standards document (IPC-4552) available for ENIG that may be of interest. Contact your printed circuit board manufacturer for additional assistance.
For additional definitions of terms please visit our Glossary of Terms page.
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