Black pad on ENIG PCBS: the discovery of "extreme dewetting" prompted a search for the source--in Shenzhen.

Early in 2005, SMC formed a steering committee to discuss and define implementation of Pb-free and RoHS production capabilities at its Lexington, KY, facility. One major decision was the use of electroless nickel immersion gold (ENIG) finish on bare PCBs. This finish was requested by customers and recommended after onsite tests were completed on boards with OSP, immersion silver, Pb-free hot air leveling (HAL), and ENIG. Despite its slightly higher cost, ENIG was chosen for its longer shelf life, ability to withstand multiple temperature cycles, and for superior surface mount component placement and solderability because of its coplanarity. Also, ENIG is used by many board shops and is an established process. This increased the committee's confidence level that ENIG-finished boards could be provided within standard lead times and quality parameters.

Late in 2006, while visually inspecting ENIG-finished, RoHS-compliant product in the final quality control area, extreme dewetting was discovered. Accompanying the dewetting was a black, grainy pad surface identified by IPC-certified QC technicians and manufacturing engineers as a defect known as black pad. This identification was a result of intense inspection and a well-defined product sampling plan. In April 2005, members of the quality and manufacturing teams attended a Pb-free/RoHS training seminar at the American Competitiveness Institute (aciusa.org), where this and other defects were discussed. Based on this training and subsequent research, this incidence of black pad was determined to be a bare PWB, batch-related problem. Therefore, the entire lot code of PWBs was quarantined and inspected. The inspection included a rigorous visual evaluation using magnifications up to 40X and tests whereby suspect assemblies were placed in an environmental chamber and cycled over several days to determine if the condition worsened when stressed by extreme heat and humidity. In parallel, engineers gathered more information on the black pad phenomenon to better the defect's impact.

According to George Milad and Gerard O'Brien of the IPC Plating Process subcommittee, "Black pad is a low-level nickel corrosion defect formed during the immersion gold deposition step. It takes a compromised nickel surface and prolonged dwell in the immersion gold bath for it to occur." Black pad is well known to fabricators and EMS suppliers, and as Milad and O'Brien added, the subcommittee "was convinced that with good definition and a proper specification, the black pad phenomenon could be virtually eliminated."

IC Tests

Samples were sent to a third-party, and through use of ion chromatography, visual inspection and sheer force tests, the defect was confirmed. Several indicators of black pad were revealed, including:

* Ion chromatography indicated the presence of unbound phosphorous on the surface of the pads. While phosphorous is found in both the nickel and gold plating baths, it should become bound in these materials and not exist freely on the board surface.

* Visual inspection revealed the black grainy surface typical of hyperactive corrosion reaction of the nickel surface during the gold deposition process.

* Sheer tests revealed reduced mechanical strength of the solder joints. While sheer tests are somewhat subjective, they do indicate the possibility of a compromised nickel surface that is not apparent visually.

The environmental tests completed at SMC were inconclusive, as engineers could not detect a significant change in wetting. However, the external report, coupled with the staff's quality technicians, ensured defective product could be easily identified and contained.

Once the nature of the defect had been established, efforts were then focused on identifying the scope and root cause of the problem. Samples were sent to the board shop that supplied the PWBs, and SMC issued a corrective action based on information provided by the third-party analysis. Instrumental in the failure analysis were members of SMC's International Procurement Office, located in Shenzhen, China. The IPO team, comprised of purchasing managers and engineers, quickly visited the fabricator and provided the SMC team with specific plating process documentation, written and oral reports of its plating process control and quality policies. The IPO team also had experience with this defect and validated information provided by the fabricator. The investigation determined that a process deviation between the nickel and gold plating process compromised nickel deposition. In response to the corrective action initiated by SMC, the fabricator determined the probable number of assemblies affected, thus accurately defining the scope. Immediate action by SMC's quality and manufacturing teams, as well as cooperation from the supplier, contained the defect and prevented escapes to the customer.

Although black pad could occur again, ENIG will continue to be the finish of choice for RoHS assemblies, and SMC will continue to diligently inspect all ENIG-finished PWBs to ensure this and other issues are contained.

Bibliography

1. http://members.ipc.org/ipclogin/ipcmembers/IPC/Route/0605/0605plating.asp.

2. rohmhaas.com/electronicmaterials/CBT/vol5/papers/electroless_nickel.pdf.

Bryan Gillespie is a customer engineer at SMC (smcems.com); [e-mail].

[ILLUSTRATION OMITTED]