Cpk vs Ppk - Part 4

Case 2

Peel test is part of a process control I used to monitor in the SPC data. This is an important data I need to monitor because I may face high field return from customer if the process is not in control.

The story happen in one of my large scale printer PCA project. The printer was design in such a way the PCA panel will stick on the enclosure of the printer with a 6” long strong double sided tape. Two type of tape width were use for different application.

We apply the double sided tape manually on the PCA after final test. It is a critical process because the PCA will fall down from the enclosure if the operators did not apply the right pressure when putting the tape on the PCA. In order to control the process stability, SPC is the easiest way. The original intention I add in peel test in SPC is to monitor the process stability, unfortunately fall into a Cpk trap and end up a never ending story.

Picture below shows double sided tape dimension to be place on the PCA, for safety reason, both tape have to withstand a minimum 2kg pull force.


From the tape dimension itself, we can sense that peel strength is proportional to the tape width. Smaller tape width will give lower peel strength, larger tape width will give larger peel strength, it is a common sense.

One evening a new engineer call me up and tell me he want to stop the line for smaller tape because SPC data shows the Cpk is not meeting the requirement, i.e. 1.67. I don't know where he get the number and ask him why 1.67. He reply, oh... it is a industrial standard. The process is not meeting the industrial standard, therefore he have to shut down the line.

A day later, I receive a mail from his manager regarding the same issue, he ask why process is not capable for smaller tape whereas no problem for larger tape. He demands a further improvement on the process to bring up the Cpk value for smaller tape size, at least to the level of industrial standard.

Do you ever face the same problem ?

I will show you why we end up two different Cpk in the same process and material property.

Last update : 16/06/08