A.  DPMO means defects per million opportunity.  What does this mean?  Defects means the number of things wrong observed in the inspection of a part, or board assembly. Users can decide how to count defects.  They may say that a resistor with two leads that are not properly soldered is counted as two defects. But they could also count it as one defect, since it is one part that was assembled incorrectly.  Or, a 24-pin integrated circuit in a 24-pin package might be counted as one wrong defect if it was inserted backwards. Or one wrong defect if one of the pins was not soldered correctly.

Now consider what opportunities mean. It is the number of possible defects that can occur on a product. Again, this is subject to a lot of interpretation. In the case of the two-lead resistor, one might say that if it was incorrectly installed, that was one opportunity for a defect. If one of either leads were soldered incorrectly, there are two more opportunities for defects. If the resistor was ok, but the wrong value, it might be counted as another opportunity for defect (now we’re up to four opportunities for defects).

Now let’s say there are 10 resistors on a board being inspected. That is 40 opportunities for defects. And let’s say that we are INSPECTING 100 boards at one time. The opportunity for defects is now 4000.   And lets consider the IC with 24 pins. Each pin is an opportunity for a defect. Likewise, getting the wrong part is an opportunity for a defect. So is getting the part installed incorrectly or missing altogether. Let’s say there are 10 IC’s on the same boards we’re inspecting. That’s then 250 opportunities per board, or 25,000 opportunities for the 100 Inspected boards. So now, in this lot, so far, we have 29,000 opportunities for defects. To that we can add similar opportunities for the many other parts on the board, as well as some for the board itself. Taken to an extreme, a user may wish to include an opportunity for every hole on the board, because some of them may not have a “through-hole” connection. (Usually though, this is taken into account in the original inspection of the board when they are received.)

So, after all is counted up, a lot like we’re discussing could have 100,000 opportunities for a defect. Remember that this is the number of boards actually observed through inspection. If a sample of a lot is taken, then the opportunities for defects is based on the sample size observed, not the whole lot size.

Now back to DPMO. Let’s say that an inspector examined all 100 boards in this lot, and found that there were two leads (anywhere) that were not soldered correctly. This then amounts to 2 defects per 100,000 opportunities, or 20 DPMO.

It used to be that as a contract manufacturer, we were held to a specific DPMO value for our products for this customer. However, even though the number of defects observed was the same, we could manipulate the DPMO by simply counting more possibilities as an opportunity, thus driving down the reported DPMO, It’s amazing how many things you can find to go wrong if you look hard.

Long and short – Only the boards actually examined are counted in the calculation, and how the opportunities are calculated is up to the user. That’s why we have the place in the routing for them to enter the number of opportunities for each work center. Each work center can have a different number of opportunities, depending on the operations performed at the work center.