Technical Information

IPC-SM-785
This standard was developed by the SMT Accelerated Reliability Test Task Group of the Product Reliability Committee of IPC. This document provides an excellent introduction to issues of solder joint reliability testing and analysis. Some highlights particularly relevant to Event Detectors are quoted below.

Section 4.3.1 Failure Definition

[para 3] A solder joint that fails by fully fracturing typically does not exhibit an electrical open or even a very noticeable increase in electrical resistance. A failed solder joint is normally surrounded by solder joints that have not yet failed and therefore the solder joint fracture surfaces make compressively loaded contact. Electrically, the solder joint failure manifests itself only during thermal or mechanical transients or disturbances in the form of short duration (~ 1 usec) high resistance spikes (>300 ohms). During thermal changes the solder joints are subject to shear, not tensile, loading; therefore, fracture surfaces of fractured solder joints slide relative to each other producing the characteristic short duration intermittents. Therefore, in this context, the practical definition of solder joint failure is the interruption of electrical continuity (>300 ohms) for periods greater than 1 microsecond. [16] ...

Section 4.5 Statistical Considerations

[para 2] Solder joints are not uniformly subject to the same thermal expansion mismatch, and thus the same amount of fatigue damage, because of their distances from the neutral point of the component or connector. Therefore, the total number of solder joints cannot be counted as the sample size. For test purposes and depending on size and symmetry, a given component or connector can be partitioned into two or four samples, if the continuity nets are correspondingly designed. However, treating each solder joint as an individual sample assumes incorrectly that they all have the same failure probability.....

[para 4] Therefore, the partitioning of the test samples needs to include essentially all solder joints into the continuity daisy-chain with each partition having an equal likelihood of failure. ...

Section 7.8 Failure Criteria for Solder Joint Fatigue Tests

The complete document is available from www.ipc.org Other sections of particular interest for the design of accelerated test programs include:

4.0 Surface Mount Solder Attachment Fatigue Behavior and Reliability Prediction
5.0 Design for Solder Attachment Reliability
6.0 Manufacture/ Processes
7.0 Accelerated Reliability Testing
7.1 Reliability Program/Strategy
7.3-7.5 Thermal/Mechanical Cycling /Vibration
7.7 Mechanical Shock Testing

Appendices: References and Bibliography