Jacky: There�re major four factors that affect the reliability of a solder connection.
1 Design of the connection including the shape and height of the lead and the type and amount of solder. 2 Dissolution of base metals into the solder connection. 3 Formation and growth of intermetalic compounds. 4 Differences in the coefficient of thermal expansion between the solder, component leads and body, and printed wiring board materials.
So, for a large part, reliability is the result of decisions that were made long before a solder connection is looked at in production. Most often, designers evaluate the impact of these decisions on product reliability during design qualification. Then, quality assurance uses the design qualification as the baseline for on-going product assessment using environmental stress screening (ESS). ESS is not a reliability test, because the accelerated testing does not represent operational reality. The closer test conditions are to the actual in-use conditions the more the test with represent the reliability of the product.
So, after that long winded meandering:
1 Test the reliability of your product through design qualification and long term in-use simulation testing. 2 Monitor the affect of changes in your production processes on the reliability of your product through ESS.