Customer issue: The customer called the Helpline to resolve their solderability issue with a wave soldered through-hole printed circuit board (PCB) connector. Connector samples were taken to be evaluated using surface and intermetallic formation analysis, and solderability testing.

The connector samples were supplied along with the profile used during the customer's wave soldering process. The analysis of the surfaces and intermetallic formations were conducted using Scanning Electron Microscopy (SEM) with Energy Dispersive Spectroscopy (EDS). The EDS revealed that the two rows of leads were not of the same type. One was made of brass plated with gold. The other was made of zinc plated with nickel.

Further investigation of the surfaces presented no confirmation of contamination on the leads. What piqued some interest were traces of copper and oxygen along with the nickel that plates the leads. The oxygen levels were consistent with normal oxidized nickel plating, and the copper was resolved to be beneath the nickel layer.

There was no verification of the existence of any nickel-tin intermetallic formations. Though the nickel-tin intermetallic forms faster than copper-tin intermetallic, image magnifications from the SEM could not resolve if any copper-tin intermetallic existed.

Evaluation criteria for solderability testing was based on IPC J-STD-002B. All of the leads that were nickel plated failed the wetting balance test even though the surfaces of the leads were clear of defects. The test demonstrated that solder did not wet to the bodies of the leads, but only to the tips. Images of a nickel plated lead before testing (Figure 7-1) and after testing (Figure 7-2) are shown here.

The wave soldering profile was reviewed and showed that the low preheat temperature for the top sides of the PCBs may not have been enough to overcome the large thermal mass of the connectors.

Conclusions
The wetting balance test revealed that the nickel-plated connector leads had solderability issues. This problem was most likely due to the bare nickel oxidizing when not properly stored. Nickel surfaces that have oxidized will be difficult to solder and may require aggressive fluxes and better process controls. In addition, the topside preheat temperatures may have been insufficient considering the large thermal mass of the connectors. Surface contamination and defects were not evident in the analysis. Any intermetallic issues could not be resolved due to small differences between the two intermetallic regions.

Recommendations
The use of gold as a final finish was recommended to aid in the prevention of nickel oxidation. Connectors currently in stock should be processed with an organic coating or tin-lead surface finish, and stored in a nitrogen rich environment. The reliability of assemblies is a concern, therefore testing and lot-tracking of the connectors’ solderability was recommended.

Improvements to the wave solder profile and the use of aggressive fluxes were also among the recommendations to help the current assembly process.