A novel test vehicle and methodology for solderability evaluations.

We recently performed an experiment for a customer interested in comparing Pb-free solder wetting with varying temperature profiles and atmospheric conditions. To deliver an objective measurement of solder wetting (in addition to subjective inspection analysis), a simple wetting indicator pattern was added to the solder stencil in an area with exposed and unused copper on the test vehicle.

This pattern comprised two rows of 22 printed solder deposits. Each individual deposit is 0.025" x 0.050". The deposits are paired in sets of two with decreasing gaps among each pair. The gap between each pair is constant (Figure 1). Two rows are included in the pattern to give two replications of the measurement on each test vehicle. This pattern permits two different measurements to be taken from the test vehicles after processing has been completed. The first evaluation is a count of the number of wetting pattern pairs that shorted together during reflow. As each pair of deposits is spaced farther apart than the last, the number of pairs that bridge can be used to compare solder wetting under different conditions – an increasing number of bridged patterns indicates increasing wetting. Figure 2 shows an example of a wetting pattern after reflow, with four shorted patterns.


The second evaluation is a measurement of the gap between the paste deposit pair that is spaced farthest apart. This gap will decrease during reflow as the paste wets to the underlying copper, and thus a smaller gap is an indication of greater solder wetting. Figure 3 shows an example of a paste deposit gap measurement. (Note: The PCB test vehicle was finished with OSP over bare copper, and the gaps were designed with that in mind. Other surface finishes would require gaps of different sizes because of the expected wetting and spread of solder on the particular surface finish.)


If all the deposits bridge, the count of bridged deposit pairs is no longer valid as a measurement, and there is no gap to measure between the last pair. For example, an ENIG finish permits solders to wet and spread to a much higher degree than bare copper and would require much larger gaps to produce useable measurements. The solder alloy can also affect that proper spacing of this type of pattern: SnPb solders are generally expected to wet and spread to a greater degree than their Pb-free counterparts.

However, when this type of pattern is properly designed for a specific combination of materials and processes, it can be a useful tool for testing the general wetting properties under varying processing environments. This evaluation method can be used on both purpose-built test vehicles and incorporated into an unused area of a production assembly for an easy indicator of the degree of wetting between the solder and PCB surface.

The American Competitiveness Institute (aciusa.org) is a scientific research corporation dedicated to the advancement of electronics manufacturing processes and materials for the Department of Defense and industry. This column appears monthly.

Sidebar: Free ACI Workshops

ACI this month will sponsor a pair of free workshops. On Dec. 11, ACI will team with Aqueous Technologies, Kyzen and Technical Devices to present the Low Stand-Off Cleaning Symposium. The one-day program includes training on practical approaches in cleaning the z-axis using inline and batch processes. On Dec. 3, ACI will partner with Dage Precision Industries for a special half-day workshop on x-ray inspection criteria and common defect analysis, covering solder-related defects such as head-on-pillow, BGA and substrate opens.

Both workshops are free and will be held at ACI in Philadelphia, and lunch will be provided. RSVP to registrar@aciusa.org or at aciusa.org/workshop.