All wet: protocols for solderability testing.

A customer contacted the EMPF Helpline concerning a solder wetting problem observed during Pb-free SMT wave soldering of a goldplated PCB. The EMPF was asked to assess board solderability. Customer samples 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.

Appropriate pads were identified and removed from the board by sectioning. Then, the pads were fluxed and tested in a KWB-1000 wetting balance per the parameters listed in Table 1 (online). 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 on the formula below. The final units are normalized based on the wettable perimeter and reported in terms of [micro]N/mm.

F max. theor. = t P cos([alpha]) - (d V g)

Fmax theor = maximum theoretical wetting force, [micro]N/mm

V = volume, [mm.sup.3]

t = surface tension of the solder, (0.4 joules/[m.sup.2])

g = gravitational constant, (9.81 m/[s.sup.2])

P = wettable perimeter in mm

[alpha] = wetting angle (assumed to be 0)

d = density of eutectic SnPb37 solder at 245[degrees]C (8110 kg/[m.sup.3]) 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 (Figure 1, online). J-STD-003A provides suggested evaluation criteria based on these properties. These suggested criteria have been established as a two-tier evaluation format, with Set A more stringent. Components meeting Set A suggested criteria are applicable to a larger soldering process window than components meeting Set B suggested criteria.

Results. Tested pads did not meet Set A evaluation criteria, but did meet Set B. 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 2 and Figure 1). Wetting was positive, with wetting forces at 5 sec. greater than those observed at 2 sec. (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) revealed improved numerical wetting with results meeting both Set A and Set B evaluation criteria (trial 1207 in Table 2 and Figure 1).

The PCB samples displayed marginal wetting as established by J-STD-003A evaluation criteria. Wetting was slow when the standard RMA flux was used. Visually, the pads wet with the given standard activity flux (ROL1). A more active water-soluble flux (activity ORH1) showed improved results, suggesting an oxide or similar hard-to-remove tarnish was present, hindering wetting.

Conclusions/recommendations. Results were obtained with a eutectic SnPb37 solder and mildly activated rosin-based flux. The results indicate that these PCBs may not be appropriate for a process that is not as forgiving as the one the customer used. As a result, a less active flux or Pb-free process may provide less than marginal results.

We recommended further wetting balance testing to confirm the observations. To determine a root cause, we recommended SEM/EDS and Auger spectroscopy analysis of the PCB surface. 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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Table 2. Summary Results of Wetting Balance Testing

Sample  Trial No.  [T.sub.0]   [F.sub.2]  [F.sub.5]

Pad 1   1205       1.85         59.1      294.19
Pad 2   1206       1.6365.725  290.32     Fail
Pad 3   1207       0.57        482.41     488.98

                                   Acceptance Criteria
                                           Set A
                                        [F.sub.2] >/=    [F.sub.2] >/=
                                        [F.sub.(max.)]/  [F.sub.(max.)]/
Sample  Trial No.  [T.sub.0]</= 1 sec.  [micro]N/mm      [micro]N/mm

Pad 1   1205       Fail                 Fail             Pass
Pad 2   1206       Fail                 Pass             Pass
Pad 3   1207       Pass                 Pass             Pass

                                       Acceptance Criteria
                                           Set B
                                                          [F.sub.5] >/=
Sample  Trial No.  [T.sub.0] </= 2 sec.  [F.sub.1>2] > 0  [F.sub.2]

Pad 1   1205       Pass                  Pass             Pass
Pad 2   1206       Pass                  Pass             Pass
Pad 3   1207       Pass                  Pass             Pass

F(max at 50%) is 159.6 [micro]N/mm for the PCB pads examined. [F.sub.2]
is wetting force at 2 sec. from start of test. [T.sub.0] is time to
buoyancy corrected zero (cross-over time). [F.sub.5] is wetting force at
5 sec. from start of test.