Package Builder logs information about its current status, the operations it is performing, and any errors or warnings it encounters. This information is displayed in the Log pane in the UI and is output to the console when running from the command line. It is also stored in a solution-specific log file that can be used for troubleshooting, record keeping, or as a report for an automated build.
The log file is stored in the same directory as the solution file. It named with the format log-<solution name>.txt.
The file is overwritten each time the solution is opened. If you need to archive the log file, you will need to copy it to a new location before opening the solution again.