Customer issue: The customer requested numerical solderability analysis of wire spools, which were a nickel alloy (nickel 29) with eutectic tin/lead (63/37) plating.

Ivestigation Technique:
There were seven wire spools provided to the EMPF in clear plastic bags. As per the customer’s request, quantitative solderability testing was performed according to their end user's specification and visually inspected by following EMPF procedure AP0500 before and after performing solderability testing.

Wetting Balance Test

• Inspection Equipment: Olympus model SZX12 optical microscope
• Dipping Mechanism: Kester® KWB-1000 Wetting Balance was used for the testing according to the designated test method, as well as EMPF laboratory procedure AP0270-2.
• Flux removal material: 98% Isopropyl alcohol (IPA)
• Solder: 63/37 tin/lead alloy solder from Kester®
• Flux: Un-activated, Rosin Flux Type R Multicore SM/NA 92MP1701QT

Three 4cm sections were taken from each spool and dipped in flux followed by solder per the above parameters. The numerical results were evaluated against the criteria given to the EMPF by the customer.

Results

• All of the spool sections failed the evaluation criteria.
• The test results of the spool sample (Figure 5-1 red line) indicated abnormalities when compared to an acceptable base-line (black line).
• The visual appearance of the soldered sections indicated that the solder had wet the surface (Figure 5-2). All of the other spools had a similar appearance.
• An unidentified black residue was observed on spool 6 and may have been on the others in trace amounts. Its presence may have contributed to the solderability problems.

Conclusions
A component that demonstrates good wetting will allow solder to adhere well and quickly, as indicated by the sharp upward direction (after one second) of the black curve in Figure 5-1. It is typically unacceptable if there is no upward curve. The appearance of the gradual red curve may be attributed to either the activity of the flux or the component's oxidation level. The more active the flux is, the easier it is to solder with. Conversely, the more oxidation that is present on a lead, the more difficult it is to solder to.

In this instance, the sectioned spools did not meet the end user's specification. Results for similar spooled wire have been better in the past, and are represented by the black curve in Figure 5-1. However, the visual inspection did indicate that the solder wetted to the surface of each sectioned spool.

Recommendations
The presence of black residue on one of the spools could have affected the analysis, and should be investigated further since the underlying substrate is a nickel alloy. If the plating surface of the spools was porous, it would have been possible that the nickel was oxidized, thus producing an un-solderable surface. To confirm the uniformity of the plating surface, Scanning Electron Microscopy (SEM) imaging of the spool surface along with a cross-section is recommended. The EMPF can perform such an analysis.