This paper presents a systematic approach to study the effect of manufacturing variables on the creation of defects and the effect of those defects on the durability of lead free solder joints. An experiment was designed to systematically vary the reflow and printing process variables in order to fabricate error-seeded test assemblies. The error-seeded samples were then inspected visually and with x-rays, to identify different types of defects, and tested for electrical performance. The specimens were put under accelerated thermal cycling test to characterize the durability of specimens and to study the effect of each manufacturing variable on the durability of solder joints. Thus, the response variable for the design of experiments is the thermal cycling durability of the solder joints. Pre-test micro-structural analysis shows that specimens produced under inadequate reflow profiles suffer from insufficient wetting and insufficient intermetallic formation. Statistical analysis of the response variable shows that waiting time, heating ramp, peak temperature and cooling rate have non-linear effects on the response variable. Two variables, in particular (the heating ramp time and the waiting time), appear to have optimum values within the ranges investigated.