Printed Circuit Design & Fab - September 2008 - (Page 21)

Behind Black Pad Process control is the only solution for consistent success. oVER THE PAST ten years, composite coatings of electroless nickelphosphorous and immersion gold (ENIG) have become established gEorgE as the preferred milAd solderable surface finish for high reliability applications involving complex circuit designs. The ENIG coating has gained market share and demonstrates versatility in a wide range of component assembly methods including solder fusing, wave soldering and wire bonding, as well as the ease with which it transitioned to lead free assembly. The ENIG finish provides a highly solderable flat surface that does not tarnish or discolor. It has a long shelf life, and the precious metal topcoat provides excellent electrical continuity. The nickel serves as a barrier against copper diffusion and prevents copper contamination of the solder during wave soldering and rework operations. About eight years ago, a major OEM brought to the attention of the industry a low level of interconnect failures when ENIG was used as the surface finish. The failure mode is associated with a poorly formed joint at the solder/nickel interface. When the suspect joint is stressed, the connection is easily broken leaving an open circuit with dark corroded nickel, commonly referred to as “black pad.” Initially, it was thought that the cause was the formation of Au/Sn intermetallic, but now it is understood that gold is not part of the intermetallic which is strictly Ni/Sn. It is a phosphorous rich layer that is a natural component of the Ni/Sn solder joint. SEPTEMBER 2008 Subsequent investigations have shown that excessive nickel corrosion during the immersion gold deposition causes this condition, now commonly referred to as black nickel or black pad. The immersion reaction by which the gold displaces the nickel is a displacement or corrosion reaction that does not produce black pad or soldering defects. So what is excessive corrosion of the nickel and how does it occur? Under normal deposition conditions the nickel deposit has an even topography. This type of nickel deposit will never produce a black pad. It is only when an irregular topography with distinct crevices between the domains occurs, that corrosion may initiate and cause black pad. Irregular topography can be caused by a contaminated incoming copper surface or inadequate pre-treatment in the front end of the ENIG line. Additionally, the nickel bath itself could create the irregularity during the course of deposition. The nickel bath is constantly plating and needs to be replenished to maintain. This operation must be carefully controlled to ensure the desired outcome. A compromised deposit can occur from by-product build up if the bath is operated beyond its recommended bath life. Another cause is a higher than normal deposition rate that can result from having a high temperature and/or pH, operating outside the recommended range. A well-controlled nickel bath is the key to the elimination of this defect. Since the black pad occurs during the gold deposition step, what is the role of the gold bath, if any, in creating the defect? Ideally, for every two atoms of nickel metal oxidized to nickel ion, one gold atom is reduced to gold metal. The nickel released into the gold bath over time should follow the stoichometry of the chemical reaction as shown in Equation 1. If the amount of nickel produced exceeds the calculated value, the immersion gold bath is labeled “aggressive.” Aggressive gold baths are more prone to producing black pad on a compromised nickel surface. Ni Ni++ + 2e 2Au + 2e 2Au+ Eq. 1 Over time, two major developEquation 1 ments in ENIG deposition have occurred. The first is the awareness of the suppliers and manufacturers of the criticality of the process control in the ENIG line. Shops that implement process control and are ISO 9000 certified stay clear of this problem. This is a buyer beware situation in that often you get what you pay for. The second major development is the IPC-4552 ENIG specification. The document specifies 2 to 4 micro inches of immersion gold. It is now clear to all that the gold is only there to protect the nickel until it is soldered. These changes have gone a long way toward eliminating the occurrence of black pad. Like any other chemical process in the manufacture of a PCB, if the process is not controlled and run to vendor specification, the results lead to defective product. In today’s sophisticated electronic manufacture process, it comes down to process control as the only insurance for continuous success. pCd&f GEorGE Milad is the national accounts manager of technology at Uyemura International Corporation (UIC) and can be reached at gmilad@ uyemura.com. printEd CirCuit dESign & fAB 21

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Printed Circuit Design & Fab - August 2008 Contents Our Line Market Watch Around the World Happenings ROI Positive Plating Off the Shelf Marketplace Ad Index EMC for the Real World Final Finish Forum Design for Green: Laminates A Systematic Approach to Increasing Layer Count The NTI $100 Million Club Printable Nanocomposites BGA Bulletin

Printed Circuit Design & Fab - September 2008

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