Journal of SMT Article

Is a High Phosphorus Content in the Nickel Layer A Root Cause for Black Pad on ENIG Finishes?

Authors: Dipl.-Ing. Mustafa Özkök, Joe McGurran, and Dr. Kenneth Lee
Company: Atotech
Date Published: 10/31/2013   Volume: 26-4

Abstract: In the electronics world, the "black pad" phenomenon is known as a failure mode related to PWBs with and electroless nickel / immersion gold (ENIG) surface finish. Many publications describe the root causes and theorize the actual mechanism for this failure. However, there are also publications supporting the more traditional “myth” of the black pad failure. Such papers or reports about black pad do not address the real nature of the problem. Claims about high phosphorus content of the nickel layer being the root cause for black pad (without supporting data with regard to the chemical mechanism or without solid measurement data) are more speculation and are not scientifically supported. Such historical speculation, based largely on widespread uncertainty of the black pad failure mode, has resulted in the creation of PWB specifications that limit the maximum phosphorus content of an ENIG nickel layer as a means to reduce the possibility of the occurrence of this defect.

This paper is focusing on one specific question: "Is the phosphorus content of the nickel layer of an ENIG surface finish the root cause for black pad?" This issue is resolved using a purely scientific approach and by showing the related technical data by means of micro sections, solder joint reliability tests and phosphorus content measurements.

The paper will describe and explain the black pad failure mechanism and provide a comparison to other similar features like brittle fracture. Furthermore, the paper will submit examples and results of (ENIG) electroless nickel layers with different phosphorus content, while examining the degree of corrosion. Additionally, results of solder joint reliability tests will be presented that include variations in the amount of thermal exposure.

Keywords: 

Black Pad, ENIG, High Phosphorus, Mid Phosphorus, Low Phosphorus



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