Latent Shorts
I have been bedevil by unexplained electrical shorts on my latest RF prototype board. The two prototype boards worked OK at first, but slowly over time each unit failed. Probing these failed assemblies, I found that there were shorts at the Direct Digital Synthesizer (DDS) package.
I first thought that I might of damage the DDS with an electrical discharge, but I found that I could repair the part by applying a soldering iron to the shorted pin! The problem was not a damage part, by shorts that grew over time out of the lead-free solder on the DDS fine pitch QFN package pins to neighboring pins. I had these short because the DDS was hand soldered.
These Latent Shorts are a common problem of using lead free solder for fine pitch parts. They are called Tin Whiskers and are enabled by comtamulation on the pcb and the lack of lead in the solder. Even after repair, these latent shorts will return. The only cure is improve board preparation, cleaning and soldering methods.
I first thought that I might of damage the DDS with an electrical discharge, but I found that I could repair the part by applying a soldering iron to the shorted pin! The problem was not a damage part, by shorts that grew over time out of the lead-free solder on the DDS fine pitch QFN package pins to neighboring pins. I had these short because the DDS was hand soldered.
These Latent Shorts are a common problem of using lead free solder for fine pitch parts. They are called Tin Whiskers and are enabled by comtamulation on the pcb and the lack of lead in the solder. Even after repair, these latent shorts will return. The only cure is improve board preparation, cleaning and soldering methods.