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Rolled Throughput Yield

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  • #24965

    TY Au
    Member

    Theorethically, throughput and roll throughput yields reveal much more information and hidden phenomenon than that of conventional 1st time / final yield. But in practical, how, in the backdrop of output requirement and demanding delivery schedule, can one really applies this concept albeit all its advantages ?
    To be a bit more specific, our kind of industry require a 100% Visual-mechanical inspection at the end of the process before loading parts into trays. And there are 10 visual defect modes !
    Can anybody share their real life experience as how they go about it ?
     
     
     
     
     
     

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    #59960

    Kim Niles
    Participant

    Dear Ty:
    I’m not quite sure where your frustration is.  Perhaps you are calculating RTY based on the ten defect types as opposed to the different spots where a part can be rejected?  RTY metrics are best applied to determine how many (total) and where in your process your rejections are occurring.   
    I use RTY all the time on projects where I work.  It really puts things in perspective.  Every step may have very high yields but the end result is that all the little amounts of rejected parts at every step can really add up.  Without RTY metrics, managers can get complacent and think everything is fine.  With RTY metrics managers can quickly see where the real problems lie.
    Does this help? 
    Sincerely,
    KN – https://www.isixsigma.com/library/bio/kniles.asp, http://www.KimNiles.com
     

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    #59961

    TY Au
    Member

    Hi Kniles,
    Thanks for advice. But I am afraid I have real problem here.
    In our Visual-mechanical inspection station at the end of the line (after plating and DI-wash), the operators have to look up for as many as 10 defects ie 10 opportunities. They are given a checksheet with all 10 defect modes listed in sequence so that they would put a tick against each detected defect, for example : “stain”. They would look up for the defect in the order as in the list —  once they have seen a particular defect, they would clock it and stop going further down the list and the part is removed to a “reject tray”. As such data collected can never reflect the actual “types of defect” a part may contain. But if we are to strictly adhere to RTY concept “counting the defect modes”, the cycle time would be tremendously long and output/delivery would be affected.
    We could not impose any inspection while the parts are in the process of plating or washing, and, the 100% VMI at the end of the line is specified by customers. Unless we engage more operators to shorten the cycle time at 2-3 folds of the cost.
    I may impose certain adjustment so that I can still proceed with the RTY, but it is best that I tap whatever experience you guys have had so that I don’t re-invent the wheel.
    Thanks again, to all, and Kniles.
    rgds/Au
     
     
     
     

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    #59962

    Kim Niles
    Participant

    Dear Au:
    It doesn’t sound like you are in a position to implement RTY.  You would appear to be inspecting in quality as opposed to using inspection to assure your process is producing quality product. 
    RTY could help you understand where your rejects are really occurring but you would need to inspect at all the areas you want to understand.  If that isn’t possible then RTY is not a useful tool for you. 
    Remember that 100% inspection is only about 80% effective… so with that in mind, I suggest you perform a cost justification analysis to run DOE’s, special test runs, or special inspection on a few lots of material.  Depending upon what kind of reject rates and related costs of those rejections (including soft costs), it may be critical that you get a better understanding of where your rejections are occurring and what you need to do to improve your process or you may be worried about it when you don’t need to worry.
    Good luck either way.
    Sincerely,
    KN – http://healthcare.isixsigma.com/library/bio/kniles.asp, http://www.KimNiles.com

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    #59970

    Mikel
    Member

    Ty,
    2 thoughts –
    1) If you have so many defects you can’t quantify all of them, why would you care about RTY – it is bad, go fix something
    2) Think about some type of detailed failure analysis on at least a portion of the stuff in the defect tray – your pareto will be more accurate

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    #59971

    TY Au
    Member

    Hi Stan,
    Thanks for advice. We DO have that many defects. In term of “yield” we claim to be at >97%, which I knew wasn’t really the case. That was why I wanted to assess the RTY. (there are far too many hidden factories !)
    In fact we have narrowed down to a few defects being the “vital few”, but again the data collected was based on what I described in my previous explanation — 1st-reject-out, hence I was not sure if the data was accurate enough to quantify the reject %, I mean “gut feel” told us the top 3 should be the X, Y, Z, but again….
    And, once you don’t have a benchmark (entitlement), you can’t qualify the improvement or quantify numerically the improvement achieved.
    I would heed what K Nile and you brought up. I would try some lots, going into detail, capture ALL defects which can be found in a piece part, then calculate the RTY. Then I proceed with improvement project (6 Sigma way), confirm the results by subjecting the same number of lot to the same measurement system. (statistically sound ? For sure I can’t go on in this manner in mass production for it eats up too much time ! Once improvement is evident then sorting would become a bliss or even unnecessary…but customers still insist..)
    One other question  —  do you advocate the use of u or c-chart when we are now already doing 100% inspection ?
    Any further input/advice from anyone ?
    By the way, I am working at a disk-drive component MNC in Malaysia, located somewhere 1 hr-drive from Singapore. I am the Senior Manager in-charged of Customer Support / Quality / New Products / QMR-ISO. I am also the 6 Sigma Champion, managing 1 BB and a few GB.
    thansk & regards to all,
    TY Au
     
     
     

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    #60069

    Junjav
    Participant

    I would like to join this discussion, since I may have same questions and more. I would need some  feedback.
    I was assigned to institute the use of RTY and deploy in several plants.
    I am trying to find out the best way, practical in layman’s terms the RTY. There was a good explanation by Jim Johnson about this. But I would like to extedn with this question
    if RTY is 49.7% how can we show the 50.4% of scrap & rework calculating backwards:
    Sample of Jim Johnsons;

    PROCESS

    IN

    REWORK

    OUT

    SCRAP

    True Yield

    A

    100

    5

    5.0%

    90

    10

    10%

    85.0%

    B

    90

    7

    7.8%

    80

    10

    11%

    81.1%

    C

    80

    3

    3.8%

    75

    5

    6%

    90.0%

    D

    75

    10

    13.3%

    70

    5

    7%

    80.0%
     
     
    Thanks for any input
    Junjav
     

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    #60070

    MikeS
    Participant

    Send me your e-mail.  I’ll send you some slides regarding the computation of the T-yield.

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