SIMULATION AND INTERPRETATION OF WETTING BALANCE TESTS USING THE SURFACE EVOLVER

The wettability of PCB pads and component terminations, and the variat ion of wettability with time, are important factors in the successful formation of a solder joint. However, reliable techniques for determin ing these properties, under conditions representative oSrhe the solder ing process, do nor as yet exist. The development of wetting balance t ests has provided a technique for the qualitative comparison of surfac e wettability but the test has poor repeatability and it is difficult to relate the rest results to the actual surface properties The small size of the terminations in surface mount technology (S.M.T.) prevents successful use of the conventional solder bath based wetting balance test, The globule wetting balance goes some way towards removing this limitation and is now often used as a means of comparing the solderabi lity of S.M.T. devices, bur is even worse in terms of repeatability an d interpretation. This paper presents results from art evaluation of c onventional and globule wetting balance rests using both computational modelling and experimental techniques. The results from the experimen tal tests show that reasonable consistency in the measured force can b e obtained, but that the insensitivity of this force to the wetting an gle of the component, particularly in the globule block test, preclude s their ability to accurately establish the wetting angle It is theref ore concluded that this test is unsuitable for providing direct quanti tative measurements of the wetting angle of the surfaces of interest, The computational models, through allowing an analysis of the sensitiv ity of the test to the uncontrolled test variables also provide an exp lanation for the significant scatter obtained in wetting balance test results. It is also seen, from comparison with test results, that mode ls which incorporate commonly quoted text book values for the solder s urface tension do not adequately predict the forces empirically observ ed. A technique is therefore also described whereby the solder surface tension under the actual test conditions may be deduced using the res ults from a scanning mode wetting balance test, thereby greatly improv ing the capability of the models to predict the surface tension forces , although significant errors ill the predicted meniscus shape remain.