I manufacture pcb in china, and I would like to reduce the cost of PCB manufacturing and assembly even if this will reduce the quality but I don't know what to do either than finding new suppliers. the type of the pcb i use is through hole FR4 and 1.6 thickness could i change one of this factor so reducing the cost?? and also I do assembly for production in other factory which add more cost
You could find a manufacturer and assembler in one. The pcb would need to be re-designed for a lower quality as relaxed tolerances would lead to non-functioning boards. You did nt specify if the pcb is double sided or multi-layer. FR4 is a defacto standard and will cause serious problems if changed without checking the impact. FR4 tends to be flame resistant (UL94V0) and others cheaper materials may not be (CEM1 is normally only UL94H2 and will burn). Thickness rarely changes the cost, but large bulk runs can lower cost per pcb.
There is nothing you could do on the board as you have it to reduce cost. A combined PCB manufacturer and assembler would reduce your overall cost a little.
If you redesign the board for larger pad to hole ratio or single layer will reduce the cost of the board.