Two column join between tables, one column with equal to and other columns with not equal to

Two tables each with 6.4 million records in SQL Server 2012. Let's say A and B. I have to join two tables based on two columns. Lets say A.C1 = B.C1 and also A.C2 <> B.C2. Both C1 and C2 are nvarchar columns. Table B does not have cluster index. I have the query as show below:

Select B.C1...A.C12, A.C2...A.C5 from A ier join B on A.C1 = B.C1 Where A.C2 <> B.C2

This is taking a more than 16 min to 30 min. It does a full table scan on B as other columns from B. When it takes more than 30 mins, the query timeouts. Can you please let me know what is the best way to write this query.