String Padding (SSIS)
Applies To: SQL Server 2016
The expression evaluator does not check if a string contains leading and trailing spaces, and it does not pad strings to make them the same length before it compares them. If expressions requires string padding, you can use the + operator to concatenate column values and blank strings. For more information, see + (Concatenate) (SSIS Expression).
On the other hand, if you want to remove spaces, the expression evaluator provides the LTRIM, RTRIM, and TRIM functions, which remove leading or trailing spaces, or both. For more information, see LTRIM (SSIS Expression), RTRIM (SSIS Expression), and TRIM (SSIS Expression).
Technical article, SSIS Expression Cheat Sheet, on pragmaticworks.com