Transient liquid phase bonding process and assemblies formed thereby

A process of joining substrates via transient liquid phase bonding (TLPB), the process comprising:

• providing an interlayer of a low melting temperature phase (LTP) between and in contact with the substrates that contain high melting temperature phases (HTPs), the LTP comprising Sn and Bi and the interlayer comprising particles having a core formed of Bi encapsulated in a shell formed of Sn;
• and
• heating the substrates and the interlayer therebetween at a processing temperature equal to or above 200° C. such that the interlayer liquefies but does not cause complete melting of the cores thereof, and the LTP thereof interacts with the HTPs of the substrates to yield isothermal solidification of the interlayer, the HTPs comprising Cu, wherein the processing temperature is maintained for a duration sufficient for the liquefied interlayer to be completely consumed by intermetallic formation, interdiffusion, and precipitation of Bi and a solid bond is formed between the substrates, wherein the concentration of Bi in the LTP is sufficiently high relative to the processing temperature such that Cu6Sn5 will not form when heated to or above 200° C.