Tin/lead Solder Leaching Dendrites

In the portion of Chapter 1 on Flip Chips, Ken Gallio states (1.8.2.1) "tin/lead solder leaching or dissolving gold and form metal compounds, called dendrites, that greatly reduce joint reliability because of high brittleness." What is the composition of these dendrites? Aren't dendrites a surface effect? How do these dendrites make solder connections brittle?

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