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NOVEMBER 2025 I SMT007 MAGAZINE 85 ples" along with a clearly defined checklist of defect criteria to guide their evaluations. Finally, thorough documentation of process parameters, such as bath temperature, dwell time, immersion rate, flux type, and any preconditioning steps (e.g., steam aging), should be recorded directly on the traveler to pro- vide valuable context for the results. (Figure 1) Wetting Balance Test Method This is an instrumented immersion test that records net wetting force vs. time, producing a wetting curve (Figure 2). Common metrics include: • t 0 /zero-crossing: Time to transition from buoyancy to net wetting • t wet : Time to a defined wetting threshold • F max : Peak wetting force • Initial slope: Rate of wetting onset Why it's powerful This method stands out because it is quantitative and highly reproducible, capable of detecting sub- tle differences in wetting behavior that simple visual inspections often overlook. Its precision makes it particularly valuable for engineering applications, including alloy and flux development studies, eval- uating surface finish aging, assessing storage or shelf-life impacts, and supporting process optimi- zation. Moreover, it is data-rich, generating measur- able results that enable the establishment of base- lines, guardbands, and trend charts—essential tools for continuous process monitoring and improvement. Important limitations Despite its strengths, this approach also comes with notable limitations. One key challenge is that there is no universal pass/fail criterion—industry standards do not define a single acceptance limit for force or time measurements. As a result, most organizations develop their own internal baselines and control lim- its, tailored to each finish, alloy, and flux. Addition- ally, specialized expertise is required to correctly interpret the data; poor calibration, contamination, or setup errors—such as misalignment or incorrect buoy- ancy correction—can all lead to misleading conclu- sions. Finally, the method demands strict laboratory discipline, including stable temperature control, a ver- ified and calibrated load cell, clean solder and flux, and consistent specimen preparation and geometry. Best use cases This technique is especially well-suited for qualifi- cation and reliability studies, particularly when pre- conditioning steps like steam aging are part of the evaluation. It is also ideal for comparing re-tinning or re-finishing processes and tracking degradation over time. In manufacturing environments, it pro- vides valuable numeric discrimination when more precise performance data is needed than visual methods can offer. Good practice (to keep curves honest) To ensure data integrity and reliable results, cer- tain best practices should be followed. Calibration and buoyancy correction are essential, along with verifying immersion depth, angle, and speed. Stan- dardizing flux type and solder alloy/temperature— and regularly refreshing solder baths to prevent contamination or dross buildup—helps maintain con- sistent conditions. Finally, using replicate measure- ments and reporting key parameters, such as ini- tial time (t₀), wetting time (t wet ), maximum force (F max ), slope, and relevant bath, flux, and aging details, ensures that the resulting curves are meaningful and comparable. (Figure 3) When to Combine Methods Because a strong wetting force does not guarantee a defect-free joint, many teams pair wetting balance (quantitative sensitivity) with dip-and-look (visual K N O C K I N G D OW N T H E B O N E P I L E F i g u re 2 : S et u p fo r wet t i n g - b a l a n c e s o l d e ra b i l i t y te st . ▼