Parts per million (PPM)

It doesn't matter, it's purely a matter of scale and convenience.
E.G 6ppm is an inconvenient figure to express as a percentage.
Similarly if you have a very tricky process with a 15% reject rate, it would be a cumbersome number to express in ppm.

There is an expression 'horses for courses' which means in a race you would select your horse for the type of race and course conditions.
Similarly you select your units to suit the scale of the numbers.
You don't measure timber in Angstroms

Del

PPM is a way for to push quality to an even greater level. I don't have it around, but I've seen it many times where a percentage is not an acceptable level of quality.

For example if your quality level for take offs and landings at all US airports was 99.9%, then that would equate to (don't remember the exact number ?) like 120 crashes a day.

or with 99.9% acceptable quality level, the US Post office would loose 326 letters every day.

In the delivery room of a hospitable 99.9% would mean 26 babies get dropped EVERY DAY!!

So the Quality engineers of the world came up with PPM as a way of raising the bar.

When you are working with electronics components (or for that matter any product with mass production - say bolts or blades) - every day you produce in terms of tens of thousands.

Let us assume you are manufacturing 20,000 ICs per day.

Can you afford to have a rejection in terms of % say even 1% means 200 ICs failing per day ?

You aim to improve your processes such thet the rejection comes down to say 20 per day. Thus now your process has improved to 0.2%

You further improve and bring it to 15 (0.15%)

These at million levels - say 0.015%, 0.012% etc are not only difficult to understand but don't look attractive way to present.

So as the processes are improved, we change over to the ppm level and say my process defectives have improved from 15 ppm to 8 ppm etc.

As we cross the limit of 6 sigma , we start talking further in terms of ppbs (If I am not wrong motorola is at that level for quite a few years now)

This way of talking about the fraction defectives as we call in SQC (with the base changed ) is only to make the data not only presentable but more decipherable to the management and other stake holders. including the working level staff.

Just talk to your man on shop floor and bost - yesterday our fraction defective was 0.00008 and today it is 0.000063 or in % as .008 and .0063% -

Vs we had a 80ppm and we have improved to 63ppm and you will see the diffce.

All this is provided you are that level- else talk in % only (a defective of 150,000ppm may look bad vs 15% defective )

However when you do your mathematics (statistics) - you talk only in terms of fractions -

fraction defective ,

p=0.000002 or p=0.000025 etc.