EFFECT OF HOT SOLDER DIPPING ON PART STRESSES

Hot solder dipping is an accepted method to mitigate whisker growth on electronic components plated with pure tin. To provide the best protection from tin whiskers, the solder coating needs to be up to the package body. The conventional wisdom for solder dipping maintains that dipping up to the package body can cause component damage. If the specific construction differences of ceramic and plastic-encapsulated parts are considered, it appears that while temperature gradients are important for ceramic parts, temperature is the major factor in failure of plastic parts. Finite-element analysis (FEA) is used to determine internal stresses resulting from solder dipping. Transient thermal FEA analysis is used to determine the temperature distribution inside the part and a static structural FEA is used to calculate internal stresses. For a 128-lead plastic part, maximum stresses from dipping the side of the package are 35% of those expected in reflow and 37% higher than that expected if only the lead tip is dipped. For a 184-lead ceramic part, dipping until the package side touches the solder increases stresses by a factor of more than three over dipping the lead to within 10-mils (0.25 mm) from the package body. Plotting the maximum relative stress as a function of maximum semiconductor die temperature shows a strong linear correlation between maximum temperature and stress for plastic – a correlation not observed for a ceramic part.