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Lean Production6 min read

Obstacles and Solutions for One-Piece Flow Implementation

This article delves into the challenges encountered during the implementation of One-Piece Flow, illustrated by a real-world production incident where a critical station's capacity dropped by 30%. The author identifies batch production as the root cause of bottlenecks, leading to excessive work-in-progress at upstream machines while downstream stations remain idle. It explains why One-Piece Flow is crucial for efficient production and helps readers understand the detrimental effects of batch production, enabling them to diagnose similar issues effectively.

That day, I just wanted a chicken leg bento, but the entire production line was jammed.

That afternoon, just as I was about to order a chicken leg bento to reward myself, a call came from the production line reporting that a critical station's WPH (Wafer Per Hour) had suddenly dropped by 30%, from the original 120 to 84. My heart sank; no bento for me. Rushing to the site, I saw upstream machines piled high with work, while downstream machines were idle, practically catching flies. I asked the line supervisor, who innocently replied, "It just suddenly became like this. The Cpk report for that upstream machine came out today, changing from 1.6 to 1.08. To meet specifications, we had to slow it down." After hearing that, I almost vented my chicken leg bento grievances on him. This is typically what "batch production" causes; if one-piece flow is executed well, this situation simply wouldn't occur.

Where's the problem? It's that "batch" that's causing it!

Frankly speaking, the logic of batch production is very simple: produce a large quantity of items at once, then send the entire batch to the next station. Think about it: if you cook 100 bowls of noodles at once, and the noodles in one bowl are spoiled, do you discard just that one bowl, or do you pour out the entire pot of noodles? In a semiconductor factory, we certainly can't just casually discard an entire pot of "noodles." Therefore, if even one wafer has an issue, the entire batch must be stopped for inspection.

So the key is: the larger the batch, the higher the risk, and the slower the reaction time. One-Piece Flow minimizes the batch size, ideally allowing items to flow one by one. This way, if the upstream Cpk becomes 1.08, downstream machines can quickly detect the anomaly, instead of discovering it only after hundreds of pieces have been processed.

How to do it in practice? Keep a close eye on your WIP and Cycle Time.

To implement one-piece flow, the most intuitive method is to start by reducing the WIP (Work In Progress) quantity. When we first introduced it, we began with the bottleneck stations. For example, if your test station can process an average of 100 wafers per hour, but the upstream etch station can only handle 80 wafers per hour, then the WIP in front of your test station should not exceed 80 wafers.

Specific steps can be as follows:

  1. Identify bottleneck stations: First, find the slowest station on your production line; its capacity is the upper limit for your entire production line.
  2. Set maximum WIP limits: Based on the bottleneck station's capacity, calculate the maximum WIP quantity for each station. If the bottleneck station processes 80 wafers per hour, then the WIP quantity for upstream stations should not exceed the bottleneck station's output for 1 hour.
  3. Visualize WIP quantity: Place a sign next to the machine or use a system display to clearly inform operators how many wafers are currently in WIP at that station. If it exceeds the limit, a red light should illuminate.

In other words, you must ensure that each station "only produces the amount needed by the next station." This way, any anomaly will immediately stand out instead of being concealed by large batches.

The most common pitfalls: human factors and equipment scheduling

Frankly speaking, the biggest obstacle to promoting one-piece flow is often not technology, but "people." I remember one time, we finally managed to limit the WIP in a production area, only to find the next morning that WIP had piled up again. Upon inquiring, it turned out that the night shift supervisor felt "it's safer to have more inventory," fearing that if an upstream machine had issues, there wouldn't be enough work to run. This is a typical "just in case" mentality at play.

Another common pitfall is equipment scheduling. If your machines are expensive, everyone wants to maximize their utilization rate, so they schedule a large batch of products to run at once. The result is that upstream machines run very quickly, while downstream machines wait for a long time, ultimately decreasing overall efficiency. In this situation, you might need to sacrifice a bit of individual machine utilization to gain smoothness across the entire production line.

Ultimately, one-piece flow doesn't ask you "not to produce," but rather to "produce just enough."

One thing you can do today

Go check your production line: which station has the highest WIP pile-up?

Want to try it yourself?

Every tool mentioned in this article is available on InsightFab — just upload a CSV to analyze.

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