What is Rolled throughput yield? Rolled throughput yield vs First time yield

Rolled throughput yield

We all know that Rolled throughput yield & First-time yield (RTY and FTY) are the two important process yield measurement metrics. These two metrics provide valuable information about how the process is performing or the quality of process output.

But there is always confusion when it comes to the application of these two metrics like when to use First-time yield (FTY) or when to use Rolled throughput yield (RTY). In one of our posts we discussed important six sigma metrics now in this article, we will understand the concept of Rolled throughput yield and first-time yield in detail with a practical example.

This means, in the end, you will be cleared about the application of these two metrics. So let’s start…

 

Rolled throughput yield
Rolled throughput yield

First-time yield –

Before understanding Rolled throughput yield let see first what First-time yield is? FTY is the most common way to calculate process yield and it is the probability of a defect-free output from a process.

It considers only the criteria at the end of the process means what you get at the end of process on the basis of that only we need to calculate FTY, this metric never consider the impact of rework/repair that happened in the process.

It is unit sensitive because it does not take into account whether the output unit is defective or not, it is directly calculated by dividing the number of units produced from a process by its inputs.

It does not capture how many defects were found and corrected within a single unit or in other words, we can say that FTY will not detect the effect of hidden factories, What do I mean by that?

Let’s take an example, suppose we are manufacturing 100 parts so our input is 100 parts and at the output also we got 100 parts but is it possible that all those 100 parts at the output come out of the process without rework/repair or any defect?

definitely not, there are some parts that have a defect and we need to do rework activities to remove the defect issue, this is nothing but the hidden factories. So while calculating, the first-time yield metric does not consider this rework or defect that occurred in the process, it only focuses on the final output of the process.

 

How do you calculate the First time yield?

Now let’s understand how to calculate the First time yield with one practical example, we will also calculate Rolled throughput yield for the same example but before that first calculate this metric.

Suppose we have 4 step manufacturing process and total 200 parts entered into this process means initially 200 parts entering 1st step. To calculate FTY we need to calculate the yield for each step and then multiply those yield values together to get our final FTY.

200 parts enter at 1st step out of which 190 parts leave to the 2nd step and 10 parts are rejected. So FTY for 1st step of process = 190/200 = 0.95

190 parts go into the 2nd step out of which 180 parts leave to 3rd step and 10 parts rejected. So FTY for 2nd step of process = 180/190 = 0.94

180 parts go into the 3rd step out of which 175 parts leave to 4th step and this time only 5 parts rejected. So FTY for 3rd step of process = 175/180 = 0.97

175 parts go into the 4th step out of which 170 parts leave and again this time only 5 parts rejected. So FTY for 4th step of process = 170/175 = 0.97

That’s how we calculated yield for each step in the process hence total process yield = FTY1 *FTY2 *FTY3 *FTY4 = 0.95 *0.94 *0.97 *0.97 = 0.85. Here the process input is 200 parts and output is 170 parts means a total of 30 parts rejected in between the 4 steps of process.

If we know the process input and output then also we can directly find out FTY of process = 170/200 = 0.85 ( While calculating FTY we didn’t consider how many parts reworked or repaired in that 170 parts.

We only focused on what went into a process step and what went out (process output) i.e. 170/200 =0.85). That’s how we can calculate the FTY metric, I hope you got the idea. Now let’s get into the depth of Rolled throughput yield.

 

Rolled throughput yield-

Rolled throughput yield is the probability that a process with more than one step will produce defect-free units. To calculate RTY we need to first calculate the yield for each process step and then by multiplying the yield of each process step, we will get our RTY of the complete process. But RTY considers the impact of rework or repair that happened in the process.

RTY is Defect sensitive that means instead of being based on the yield on produced units, it uses the number of defects found at each process step. It captures how many defects are found in a single unit.

To understand this, come back to the earlier example of 100 parts initially, we have 100 parts as input that are entered into the process and our output is also 100 so we got FTY = 100/100 = 100%.

But out of 100 parts,10 parts have some defect issue and we performed rework activities to remove those defect issues and after a repair, all 10 parts again added to the final output since no parts rejected or scraped.

Here out of 100 parts, 10 parts gone through rework and hence to calculate RTY we considered rework i.e. Rolled throughput yield formula , RTY = {Inputs – (rework + scrap)}/ total units entered in the process , RTY ={100 – (10parts + 0)}/100 = 90/100 = 90% (zero scrap).

Earlier we got process yield = 100% but when we consider rework/repair, we got yield = 90%. That’s how Rolled Throughput Yield adds the consideration of rework while calculating process yield.

 

How do you calculate Rolled throughput yield?

Now let’s understand how to calculate this metric with the same example which we have discussed for first-time yield calculations remember one thing this time we need to add considerations of rework or repair that happened at each step of the process.

200 parts enter at 1st step out of which 15 parts have some defect issue and then 10 rejected and 5 reworked so at the end 190 parts leave to the 2nd step (5 reworked).

So RTY for 1st step of process = {inputs – ( rework + rejected)}/total units entered in the process} = {200 – (5+10)/200} = 0.925. here, defect-free parts at first attempt are 185 parts out of 200.

190 parts go into the 2nd step out of which 15 parts have some defect issue and then 10 rejected and 5 reworked so at the end 180 parts leave to the 3rd step (5 reworked).

So RTY for 2nd step of process = {inputs – ( rework + rejected)}/total units entered in the process} = {190- (5+10)/190} = 0.921. here, defect-free parts at the first attempt are 175 parts out of 190.

180 parts go into the 3rd step out of which 10 parts have some defect issue and then 5 rejected and 5 reworked so at the end 175 parts leave to the 4th step (5 reworked).

So RTY for 3rd step of process = {inputs – ( rework + rejected)}/total units entered in the process} = {180- (5+5)/180} = 0.944. here, defect-free parts at the first attempt are 170 parts out of 180.

175 parts go into the 4th step out of which 10 parts have some defect issue and then 5 rejected and 5 reworked so at the end 170 parts leave (5 reworked).

So RTY for 4th step of process = {inputs – ( rework + rejected)}/total units entered in the process} = {175- (5+5)/175} = 0.942. here, defect-free parts at the first attempt are 165 parts out of 175.

That’s how we calculated yield for each step in the process (Considering rejected parts and the rework at each step) hence total process yield or rolled throughput yield = RTY1 *RTY2 *RTY3 *RTY4 = 0.925 *0.921 *0.944 *0.942 = 0.75.

 

Difference between Rolled throughput yield and First-time yield –

I hope you understood how to determine these two important metrics. Rolled throughput yield and First-time yield both are equally important and used in different situations, most of the time when an organization provides the data to financial institutions they prefer to provide the First-time yield because financial institutions not interested in how many parts are reworked or repaired, they only need final process yield.

 

Rolled through yield vs first time yield
Rolled through yield vs first time yield

 

 

If an organization wants to display better process performance to senior management then also they prefer to use FTY, as FTY value is higher as compared to RTY because FTY never considers the impact of rework or repair. So FTY will not help the business find and correct problems in their processes.

On the other hand, If organizations want to improve a process or they want to capture defects in the process, in that case, they prefer to use rolled throughput yield because Rolled throughput yield considers the impact of rework or repair and it provides great insights of the process or we can say RTY provides better improvement opportunities in the process means it points the way to where improvement efforts are needed.

Both are important metrics so as a quality leader, process engineer, operational manager you need to know these two metrics and you need to select FTY and RTY as per requirements.

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