Wednesday, July 25, 2012 | Ward Gatza, Agilent Technologies, Inc., and Tom Evans, Thomas C. Evans Consulting
Editor's Note: This article originally appeared in the June 2012 issue of SMT Magazine.
This paper was presented originally at IPC APEX EXPO 2012 Technical Conference and published in the show's Proceedings.
Authors’ Note: Prior to committing production boards to vapor phase soldering, we performed an evaluation to assess reliability and evaluate the vacuum soldering option. The reliability of vapor-phase-processed BGA solder joints, with and without vacuum applied, was evaluated by means of a test vehicle circuit board assembly. The test vehicle was designed with daisy chain nets through multiple solder joints. These were designed with all of the solder balls in a chain having a similar distance from neutral point, so this factor could be part of the reliability assessment. The boards were temperature cycled for 8,250 cycles of -5°C to 95°C, by which time all of the outermost daisy chains had failed. The number of cycles-to-failure was analyzed using Weibull plots and characteristic life was calculated.
While convection reflow is the standard method for the surface mount process, thermal variation is always present across a large board; and smaller temperature margins are found within the tighter process window for lead-free soldering. It can be difficult to achieve minimum temperatures in highly-dense areas without over-heating less dense areas on the same printed circuit assembly (PCA). Vapor phase reflow provides an advantage in limiting the maximum temperature on the PCA to the vapor temperature of the heating fluid. Vacuum applied in the vapor phase process allows for reduction in the amount of voiding. The reliability of vapor phase with and without vacuum is not well known and the objective of this study was to evaluate this area of interest.
The Test Vehicle
The test vehicle board was designed to hold a single BGA, as seen in Figure 1, for the purpose of evaluating a number of material or process options.
Figure 1: The test vehicle board.