I have been tracking a number of failures of LED's mounted in the 
     lower position of dual LED assemblies.  The failure mode typically has 
     been an open junction.  
     
     Because I have been able to restore LED illumination by heating the 
     lenses on a number of the rejected LED's, I suspected that the failure 
     mechanism was likewise based on the application of heat to the lens.  
     As the lens heated up, the plastic distorted, resulting in open 
     junctions.
     
     Since it was the lower LED which would always fail and the devices 
     were edge-mounted, it was natural to suspect an improper height 
     adjustment of the solder wave.  This condition was verified by 
     observing that the lower LED's indeed did become immersed in the 
     solder.  Based on this observation, the wave height was reduced, and 
     the failure rate dropped dramatically.
     
     Six weeks passed and the failure rate increased again.  I have been 
     assured that the solder wave height has not been readjusted.
     
     Anyone have similar experiences or thoughts on the failure mechanism?
     
     
     Myron Papiz