|Complete monitoring of the line allows quick corrective actions and analysis of line performance.
Quality assurance (QA) has long been the recognized term and method for manufacturing quality processes. But we should question this, as additional benefit can be gained from focusing on Quality Control (QC). These benefits can move quality from a "non value- added cost" to actually supporting lean manufacturing and cost reduction.
It might seem odd that to suggest differentiating Quality Assurance from Quality Control, when many people interchange the two terms to mean the same thing. But the presumption that the two are the same leads to organizations missing the cost reduction opportunity that quality, in particular, Quality Control can bring.
How QA Evolved
Many of us have seen the evolution in action; there is an issue with product quality. Our first reaction is to add another quality check. Each new QA point gives us the warm feeling that we are doing something to remove defects and improving quality.
In fact what we have done is add costs, production delays and further opportunities for process defects. The additional costs are easy to understand along with the additional delays caused by adding another process step. But we have also added opportunities for process defects, as no inspection or test is perfect.
For all quality processes there are four possible outcomes:
1. Bad products fail — a correct outcome.
2. Good products pass — a correct outcome.
3. Bad products pass — an incorrect outcome.
4. Good products fail — an incorrect outcome.
There can be several reasons why we have the incorrect outcomes, but here we will examine the results of these outcomes, which can be to cause "good products" to be scrapped (outcome 4) and "bad products" to be labeled as good (outcome 3).
Once we understand these possible outcomes, we can see that adding more "assurance tests" not only adds to the direct production costs but also to indirect costs related to the incorrect outcomes. For anyone trying to implement "Lean Manufacturing" all this is bad news unless we also question the need for additional "Quality Assurance" operations as our response to quality issues.
From this simple premise we can see the route to lowest cost manufacturing is also the "Lean Manufacturing" route, where we strive to do only those activities that add value or cannot be avoided. This should exclude unnecessary Quality Assurance operations and their associated handling, data recording and later QA analysis.
|Demonstrating a predetermined repair and re-inspection point for that repair on failure at test, with the addition of "loop count monitoring".
Somewhat like a golfer hitting a good shot, if we keep our eye on the ball and focus on the process of hitting the ball, we can predict the outcome. Similarly, we can use Quality Control to predict the outcome of manufacturing and, at the same time, lower the overall cost of manufacturing.
Defining Quality Control
Unfortunately you will still see definitions of Quality Control advocating inspection as the primary feature of QC. See Wikipedia for an example.
The more progressive approach is to implement "control"
methodologies. Control should mean there is only one way a product can be processed through manufacturing and that is the prescribed route with the prescribed skills and resources. Poka Yoke, which we have learned from Japan, has long advocated the mistake- proofing of manufacturing, implemented in simple steps like placing the correct parts in a logical sequence of bins in each assembly cell. Poka Yoke has been very effective in reducing errors within a work cell.
With advances in shop floor connectivity we can now look beyond Poka Yoke in each work cell for the next advances in Quality Control.
Therefore we can cite definition for Quality Control as: "The monitoring and controlling of the manufacturing execution to a specific operations plan at specified operations points with specified resources of defined capability."
There are some fundamental building blocks of control. The first is to have a plan specifying the exact route that manufacturing products need to follow and the resources needed at each point to correctly process the product. While an obvious step, many people can miss the significance of having a plan that will be executed exactly and repeatedly. The plan needs to have a clearly defined flow through the engineering and quality functions, usually starting in the ERP work order level and then on to association with the operations plan for that product type. The plan should also associate the operations and equipment sets with the required instructions and operator skills.
Absolute Routing Control
The most common form of product routing control is the "paper traveller". Routing control done by paper travellers is a simple and efficient solution for low volume, slow moving, manufacturing organizations where work typically progresses in small batches associated with a fixed and linear build plan and usually fixed batch quantities. This solution does, however, have some major drawbacks:
- All actions are recorded manually and only accessible on examining the paper traveller. This means only those who can put their hands on the product batch and its paper traveller can extract the knowledge it contains.
- Progress from one operation to the next goes unmonitored until the final operation is completed and the traveller completed.
- Incorrect process routing is only noted by the diligence of those completing the paper traveller.
- Changes to the required routing for repair and retest can only be managed by holding back the whole batch or more commonly by raising new paper travellers splitting the batch — something that can take time and is prone to error.
Absolute and auditable routing control requires automated monitoring and control of the actual process route in real time to the pre- determined operations plan. The plan should also allow for conditional routing on pre-determined events like a test failure at a specific test process.
Restricting Rework Loops
Each item within the batch can be allowed to follow the predetermined failure route while allowing the test passed items to proceed and each process action is recorded for each production item.
We often will want to restrict the number of test and repair loops a product may take. This requirement is common in automotive and military manufacturing where the number of solder reworks allowed is fixed. We might also want to ensure that products are not locked in an endless cycle of test failure and ineffective repair without some alert being raised, ensuring this cycle is broken.
At each of the operations, workers with the skill level defined in the plan will be required. This must be managed by automated confirmation that the operator at each station is approved for the product task and associated operation. When product is routed to an operations point, the associated skills required for that product must be verified.
|The Valor MSS Quality Management module provides clear repair instructions when failures are detected, and logs all failure drag for tracking and monitoring.
By ensuring the plan contains the skills level and an operator skills table is maintained identifying each operator who then must log onto each operation with a unique ID and password, we can positively ensure that the skills required to process the product are available at that operations point. A simple interlock will prevent work progressing through an operations point that does not have the required skills.
True "Quality Control"
Manufacturing Execution systems like the one from Mentor's Valor Division now offer the "Control" we need to move from Quality Assurance to Quality Control. For sure we will need to continue with traditional assurance steps like 100 percent inspection and extensive test regimens as these are our measure of the effectiveness of our Quality Control.
But MES systems open up the exciting prospect of reducing our costly quality assurance steps as we eliminate defects caused by poor control of the manufacturing processes. By combining a reduction in QA steps with the MES solution's ability to capture and report on all key manufacturing variables, we receive the data we need to intelligently scale our "assurance" steps where we are not able to resolve issues with "Control" alone.
MES solutions containing Valor's Quality Management module can also provide continuous monitoring in real time that can alert the manufacturing team when out-of-control limit defect levels are exceeded, such as a test yield that falls below target, a DPMO (Defects Per Million Opportunity — as defined by IPC), or a repeat of defects on the same product or component.
Within the Valor solution, the trigger level for repeat defects, known in the quality world as "systemic defect", can be set to alarm on these systemic defects. Or, more sophisticatedly, having programmable incident rates such as the same defect 3 times in 10 for example.
Making Alarms Effective
Connectivity to the shop floor also opens up the possibility of making quality alarms effective in controlling our shop floor. This is in contrast to the simple level of assurance an SPC (Statistical Process Control) chart can offer; as each chart is maintained at a single process control point and any corrective actions recorded at that point. The connectivity offered by MES solutions to all process points can be used in two advanced ways.
First, the knowledge of out-of-tolerance conditions and the associated corrective action can be captured and analyzed to prevent future occurrences; analysis of the data from manufacturing can also be used to optimize future manufacturing projects.
Secondly, the alarm can be delivered to the engineering and operations team in real time, hence the process corrective action can be completed at the original source of the defect before there is the opportunity to repeat that defect.
The Valor Quality Management solution allows the alarms once triggered to be delivered to shop floor quality clients for corrective action at that operation, bringing to reality the adage "it is better to avoid a defect than just count them".
Does this mean the demise of Quality Assurance? Of course not! At least, not until we remove the most influential variable in our manufacturing process — the "Human". Even then, we will want to have some form of inspection and test to minimize the risk of product failure, or safety, or poor customer perception.
But we can take giant strides towards minimizing the need for costly added operations that assure us that we have not injected random defects. The route is clear: reducing our non-productive activity by replacing Quality Assurance with MES delivered Quality Control. This is designed to prevent defects and take corrective actions directly at the shop floor operations. This, then, enables "Lean Manufacturing" — reducing our total cost of manufacturing, reducing scrap and wasted effort.
Contact: Mentor Graphics Corp., 8005 SW Boeckman Rd., Wilsonville, OR 97070-7777 503-685-7000 fax: 503-685-7704 Web: