Thanks for your responses.
Our customer reported a defect and wanted to know the risk, when we investigated the concern, there were 3 factors involved ( New batch of material, new operator and machine had just been changed over to run the product). Each of these changes is recorded in different places and has always been considered as part of the process.
This made more sense. In the assumption of the following:
1. New batch of material - kept in the qualification and reliability testing records,
PCN and PPAP update.
2. new operator - training and certification records
3. machine had just been changed over to run the product - process certification records, PCP and PPAP plus WI updates.
(
Take note that all automotive customers requires at least 90days of PCN maturity and approval before you can migrate they will scrutinize you if you dont and they will pursue more if you get to have a problem, which i think is your current case)
4. Your product has the traceabilty marked on its surface or has a serial number traceable to lot codes and date codes
If you have done all of the 4 items above and I'm your customer, I can breath....if not, I'll charge you like a raging bull everytime and its going to mark every time i hit you (metaphoric, but you know my point "wink "wink)
Because we were unable to identify the root cause and calculate the possible number of products at risk.
Im not doubting you engineers' problem solving ability however if you are using an
8D format approach, shouldnt you be doing the containment and risk analysis first then you go to root cause analysis, correction and prevention? What I would do is give them a PPM value for potential risk then I can narrow it down to process specific PPM and threat identification. Then I'll do my is-isnot, fishbone, why-why and FTA. (in your case you did not find any RC)
If you cannot find the root cause and you have exhausted your resource in finding it, you may want to use the phrase "guilty until proven innocent". Go the other way around and find information that will justify that the cause of the problem is not within you boundaries. I could have closed the issue at this point by NFF! - this is a long and time consuming process without clear conclusion but sometimes if your lucky you get good lessons learned.
So i guess the customer just asked for CI (continuous Improvement) which is the
"change control system with traceability to identify products affected by a change"
Basically I need advise on how to define 'process variables' and when these are considered a 'change' which requires being recorded and being traced
I suggest you go back to the basics; 1 use JEDEC if your customers agrees to it; 2 get a list of notifiable change from your customer and make it an official boundary for change notice and approval.
Usually they are: manufacturing site, bill of material, process steps (ommision or addition to that effect), dimensions, appearance, function, reliability and/or capability at time 0.
For the system, use the information above as a base line. Modify your product process change procedure to include QA element to the cross-functional review then use your traceability process to ensure that the change cut-off date marked into it. The ECN date of implementation and your product router or process travel cards should be able to pinpoint what were the products processed using the change. If you can integrate this into a single system - great! But seriously all you have to do (or should have done) is to show that you are in control over the change process.
Some companies do send first 3 lot informations together with the shipping documents once the change have been implemented, and I assume that your customer didn't know that you changed something reason why they are running after you.
While your at it, seems to me that you need to review your Product Process Change Notification guidelines as well. Look at it this way, if im giving you an apple all the time for your apple pie and your apple pie is making your children happy, what do you think will happen if i give you a pear tomorrow for your apple pie without telling you before hand?
I hope this helps.