A customer contacted the EMPF helpline concerning a solder wetting problem that was observed during a lead-free, SMT wave soldering process with a gold plated PCB.
A customer contacted the EMPF helpline concerning a solder wetting problem that was observed during a lead-free, SMT wave soldering process with a gold plated PCB.
Test Methods:
The EMPF was tasked with assessing the solderability of the board. Samples provided by the customer were stored in a nitrogen box prior to quantitative solderability testing (Wetting Balance testing). This test measures the time to reach the maximum wetting force and is a routine test the EMPF performs.
First, appropriate pads were identified and removed from the board by sectioning. Then, the pads were fluxed and tested in the KWB-1000 Wetting Balance per the parameters listed in table 3-1. Any residue flux was removed with IPA (isopropyl alcohol) before final inspection.
The wettable perimeter and cross-sectional area were determined for each sample. This information, along with the immersion distance, was used to calculate the volume and maximum theoretical wetting force based upon the formula shown below. The final units are normalized based upon the wettable perimeter and reported in terms of μN/mm.
F max. theor. = t P cos(α) – (d V g)
Fmax theor = maximum theoretical wetting force, μN/mm
V = volume, mm3
t = surface tension of the solder, (0.4 joules/m2)
g = gravitational constant, (9.81 m/s2)
P = wettable perimeter in mm
α = wetting angle (assumed to be 0
d = density of 63/37 eutectic tin/lead solder at 245ºC,
(8110 kg/m3) degrees for perfect wetting)
Acceptable solderability can be established through evaluation of wetting balance curve properties: wetting time, wetting force and general shape of the curve (See Figure 3-1). J-STD-003A provides suggested evaluation criteria based upon these properties. This suggested criteria has been established as a two tier evaluation format with Set A being more stringent. Components meeting Set A suggested criteria are applicable to a larger soldering process window than components meeting Set B suggested criteria.
Results:
The pads tested did not meet Set A, but did meet Set B evaluation criteria. Wetting was slow with significant wetting forces occurring much later in the test with the standard RMA flux and ROL1 flux activity (Trials 1205 and 1206, Table 3-2 and Figure 3-1). Wetting was positive with the wetting forces at five seconds greater than those observed at two seconds. (i.e. F5>F2 as prescribed in Set B evaluation criteria). Visual wetting was acceptable. Analysis of a pad with a highly active water soluble flux (ORH1 flux activity) improved numerical wetting with results meeting both Set A and Set B evaluation criteria (Trial 1207 in Table 3-2 and Figure 3-1).
* F(max at 50%) is 159.6 μN/mm for the PCB pads examined. F¬2 is wetting force at 2 seconds from start of test.
To is time to buoyancy corrected zero (cross-over time) F5 is wetting force at 5 seconds from start of test.
The PCB samples displayed marginal wetting as established by the evaluation criteria within J-STD-003A. Wetting was slow when the standard RMA flux was utilized. Visually, the pads wet with the given standard activity flux (ROL1). Improved results were obtained with a more active water soluble flux (activity ORH1), suggesting an oxide or similar hard-to-remove tarnish was present, hindering wetting.
Conclusions/Recommendations:
The results were obtained with a eutectic Sn63/Pb37 solder and mildly activated rosin based flux. These results indicate that these PCBs may not be appropriate for a process which is not as forgiving as the one which had been used by the customer. As a result, a less active flux and/or lead free process may provide less than marginal results.
The EMPF recommends further wetting balance testing to confirm the observations. In order to determine a root cause, EMPF recommends analysis of the surface of the PCB by Scanning Electron Microscopy with Energy Dispersive Spectroscopy (SEM/EDS) and Auger Spectroscopy. X-ray Fluorescence Spectroscopy (XRF) is also recommended to confirm plating composition and thickness. Micro-sectional analysis with subsequent SEM/EDS analysis of failed assemblies is also recommended.
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