Controlled collapse chip connection (C4) structures and methods of manufacture, and more specifically to structures and methods to improve lead-free C4 interconnect reliability. A structure includes a ball limited metallization (BLM) layer and a controlled collapse chip connection (C4) solder ball formed on the BLM layer. Additionally, the structure includes a final metal pad layer beneath the BLM layer and a cap layer beneath the final metal pad layer. Furthermore, the structure includes an air gap formed beneath the C4 solder ball between the final metal pad layer and one of the BLM layer and the cap layer.