Example of Yield Calculations at The PCB Assembly Level

The defect rate for new PCBs is usually calculated based on process observations for existing PCBs. Assuming a PCB with through-hole technology, defects are usually obtained from three sources: incoming materials and components; assembly defects of missing, wrong, or reversed components; and soldering or termination defects. If it is assumed that each component has 2.5 solder connections per PCB, the quality level for multiple component PCBs can be calculated as follows, assuming reasonable PCB assembly process quality:

Solder defect rate DPU = 100 PPM Component assembly defect rate DPU = 500 PPM Incoming component defect rate DPU = 300 PPM

Assuming 2.5 solder connections per component, what is the total process yield at the PCB test level for 100, 500, and 1000 component PCBs?

Solution method 3.Calculating the total yield using power series expansion. In this method, the solution is derived by calculating the total yield by multiplying individual process yields based on1 - DPUcomponent expansion, where DPU is the process defect rate for one component. Note that  the defect rate for the PCB operations should not be used, because some of the values are too high (i.e., the DPU for total assembly defects is 0.5) to ignore the higher-order terms in the power expansion.

The total yield results using all three methods of calculations mentioned above were approximately equal in values. It can be shown that as the number of components increases in the PCBs, first-time yields decrease significantly, assuming that the quality level of the assembly process remains the same. In order to achieve higher first-time yields for complex PCBs of more than 500 parts, the quality level of the assembly process steps has to be improved from hundreds of PPM defects to tens of PPM defects.