To illustrate which uptime pattern from the last post might be more favorable I will add some tool utilization to the same 3 charts:
the combined charts for the 3 scenarios are these:
This example assumes that the productive usage every single day is a flat 80% of the total time, which is a very optimistic assumption since in many factories with moderate to high product mix WIP arrival is highly variant for various reasons.
In my opinion: the tool group with less variability in the uptime pattern is in general a preferable situation – with one big exception: If all of the tool group downtime is scheduled downtime and therefore could be planned. The downtimes could be – at least in theory – perfectly synchronized to the WIP arrival patterns, which would reduce significantly the impact of downtime on the WIP flow and therefore also on the cycle time.
There is a very interesting indicator out there (that tries to measure exactly this) how much of the total downtime of a tool group is planned vs. how much is unplanned. It is actually a ratio:
M – Ratio ( or Maintenance ratio)
To calculate the M ratio of a tool group of interest – just sum up all scheduled down time hours and divide them by the unscheduled down time hours. The data collection timeframe needs to be big enough to capture all typical down events, so I recommend to use data at least from 3 months of history or more.
In the example above the result is 1 or in other words, this tool group has the same amount of scheduled or unscheduled down time. With respect to the synchronization to WIP arrival idea it would be of course good to have a better (higher M ratio). For example: a M ratio of 2 would indicate that only 33% of the downtime is unscheduled.
Before I dig deeper into the M ratio concept and how it can help I like to hear from the uptime experts out there: What are typical M ratio numbers you tool groups achieve ? Or should I ask : What M ratio numbers do your equipment teams achieve ?
Very curious to see the feedback. I will discuss the results in my next post
Factory Physics and Automation blog 
Hello Thomas,
super contribution, super inspiring. And that leads me to the following thoughts:
I am convinced that the industry should strive to no longer accept any unscheduled down time at all. Realistic? No. But it can be formulated as a vision or a step-by-step plan. This then inevitably leads to the M-ratio in the formula being mathematically close to infinity. Everyone has their own idea of what “infinity” means. And mathematicians know many different infinities. But if you turn the formula around for practitioners, everything goes towards zero. And under zero “unscheduled” everyone can imagine something. In any case, better than under “infinitely scheduled”.
Experts, forgive me this nitpicking. But with infinity, there are always questions and discussions.
But not with “zero”. Because zero is zero.
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Thomas,
great input – as always. I will look into the M ratio topic a bit more in the next posts. I definitely want to also venture a bit more into the topic is zero unscheduled downtime really a good thing ?
There is also for sure the cost aspect of achieving near zero unscheduled down time. As for the infinity vs. zero point, I’m with you, but I used the existing M ratio definition (for now).
More to come soon
and thank you again for taking the time to read my blog and comment
Thomas
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M-Ratio in todays wafer fabs is unfortunately too small, often only1-2, which is actually desastrous. TSMC is Said to target for 4:1. don’t know wether they really achieve it.
Another crucial point is the definition itself. Equipment guys subtract the time where a repaired tool is waiting for a testwafer to re-qual and the correct flagging of the tool status is often a mess ….SAP 😦
So I don’t give too much on m-ratio, for me is the short time up KPI more important. What is the % of tools that go down again after planned or unplanned MTNc?
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Hello Konrad, great to hear from you. I do agree with most of your comments, the topic of M ration makes of course most sense if times are measured green to green, not in any form of sub segments.
Typically the transition from up to down and from down to up are pretty solid, since these also mark the change of ownership from the equipment to the operations team.
Thank you for reading and spending the time to comment !
Thomas
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Hi Thomas, this topic is interesting (my passion) to me from a technology / quality / stability point of view (I am not an equipment / Maintenance guy 🙂 and not productivity, but I had deep-dives into the MNT world when I was leading Stability (aka Line yield) teams in two companies. I think, there is a lot of improvement potential lying around but as always not easy to realize 🙂 best regards, Konrad
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