You get synthetic keys when several fields of 1 table are linked to these fields in another table. So,
1) try to reduce the common fields : if you have date, month, year in the fact table, you could remove month and year from it : just the date is enough to do the link with a table composed of date, month, year ....
2) create your own keys
a) you can concatenate them
Field1 & '|' & Field2 & '|' ... as UniqueKey in your fact table
Field1 & '|' & Field2 & '|' ... as UniqueKey, Field1, Field2 ... in your link table
b) use the AutonumberHashXXX that will create a unique integer for each couple given in argument
AutonumberHashXXX(Field1, Field2, Field3 ...) as uniqueKey in Fact Table
AutonumberHashXXX(Field1, Field2, Field3 ...) as uniqueKey, Field1, Field2, Field3 ..; in the link table
But I encourage you to read these two posts by Johk Witherspoon and Henric Cronström:
http://community.qlik.com/message/10279#10279
http://community.qlik.com/blogs/qlikviewdesignblog/2013/04/16/synthetic-keys
Fabrice
