Fatigue Life of Solder Joints

Jim Shaw, EVP of Engineering at Crystal Group, discusses fatigue life of solder joints, and the impact with the recent introduction of SAC types of solders.

Fatigue Life of Solder Joints

Hi Jim Shaw here from Crystal Group wanted to talk a little bit about fatigue life of solder joints and how the industry has changed a little bit with the introduction of SAC types of solders.  And SAC is tin, silver, copper, types of composites and it’s really to get away from the lead base solders because it’s an environmental hazard, so the industry has been moving towards SAC types of solder.  The importance of deflection in a circuit card is paramount in terms of limiting the amount of strain that a solder joint sees.

Strain on a solder joint really relates to the more strain the faster a crack can propagate and so there’s some laws that have been kicked around for the last 15 years; Miner’s Cumulative Damage Law, as well as the Coffin-Manson Equations for fatigue life.  Both of them have their pluses and minuses in terms of how you characterize fatigue life, but essentially it gets down to the basic premise to keeping deflection to a minimum, keeping stresses to a minimum will help you extend the life of a COTs based architecture or any type of architecture with a solder joint that’s in a rugged environment.

So, the key to making this work really is to limit that deflection, so how do you do that?  You really have very stiff chassis’ that don’t move much, not moving much means lower stress, lower stress essentially reduces the crack propagation that naturally occurs or the creep that naturally occurs in a solder joint.  So, that’s in effect how you maintain optimal life in a rugged environment with a COTs product.