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Critical characteristics drive cost and complexity in process operations. Design FMEAs identify critical characteristics. Process FMEAs assess the ability of the organization to control critical characteristics.

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Quick reference

Critical Characteristics

Critical characteristics are those that have a significant impact on product safety, compliance or performance.  FMEAs can be used to identify critical characteristics and lead to product and process design changes to control or mitigate the failure associated with the characteristic.

When to use

Identification and management of critical characteristics is a normal element in most product development methodologies.  These are identified as part of the creation of the design documentation.  An FMEA is a well-recognized methodology for identifying the critical characteristics.

Instructions

Completing an FMEA provides insight into identification of critical characteristics.  Whenever the severity score for a failure mode is greater than 7, the item parameter that is associated with the failure should be treated as critical.  A severity score of 8 or higher indicates that the product or system may completely fail without warning and may cause regulatory violations or even harm to users or operators.  When a failure mode in a Design FMEA has a high severity score, the product development team should attempt to reduce the severity and make the design more robust.  When a failure mode in a Process FMEA has a high severity score, the process development team must create a process control plan that reduces the probability of occurrence and provides for high detection of possible failure. 

Critical characteristic definitions

There are several variations in the definitions of critical characteristics.  Some definitions focus directly upon safety issues or customer satisfaction.

  • Critical characteristic: A product characteristic or manufacturing process parameter that can affect safety or compliance with regulations, fit, function, performance, or subsequent process of product. Also known as key characteristics or special characteristics.
  • Critical safety characteristic: A critical characteristic related to those product requirements (Dimensions, Specifications, Tests or process parameters) which can affect compliance with government regulations or product function.
  • Critical product characteristic: A critical characteristic that identifies those product parameters and requirements that are important for customer satisfaction with a focus on capability to maximize customer satisfaction.

Hints & tips

  • The identification of critical characteristics by development teams is sometimes an after-thought and done haphazardly.  Using FMEAs provides a rigor and methodology to the identification of critical characteristics.
  • Control of critical characteristics drives many production processes.  The reduction of those characteristics will simplify the process – leading to faster and less expensive processes.
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  • 00:04 Hi, I'm Ray Sheen.
  • 00:06 There's one more concept I'd like to discuss concerning the use of FMEAs, and
  • 00:10 that's critical characteristics.
  • 00:14 We've already looked at FMECAs, failure mode effect criticality analysis.
  • 00:18 But that was really a shortcut way to address risk of product process failure.
  • 00:22 Now, I want to talk about the critical or special characteristics and how we use
  • 00:27 both the Design FMEA and Process FMEA to identify and manage those characteristics.
  • 00:33 One of the things that designers are supposed to be doing
  • 00:36 during the design process is identify special characteristics
  • 00:40 which can have a major impact on product performance.
  • 00:43 These characteristics are often noted in the design documentation, so
  • 00:47 that future designers will know to be careful about making any changes to these.
  • 00:52 In addition, the production or operation folks know that they must closely monitor
  • 00:56 these characteristics because even a minor change could have a dramatic effect
  • 01:01 on the product performance.
  • 01:03 In fact, one of the goals of the design process
  • 01:06 is to minimize the number of critical characteristics.
  • 01:10 This will lead to a more robust product because there are fewer things that can
  • 01:13 create a catastrophic performance impact.
  • 01:17 These characteristics require special attention during both the development
  • 01:20 process and in operations.
  • 01:22 During development, these characteristics are often carefully tested
  • 01:26 to determine the optimum design point and the allowable tolerances.
  • 01:30 Once in production, these characteristics are monitored closely
  • 01:33 with some type of production or process control.
  • 01:36 Often requiring 100% inspection to ensure that the characteristic is within
  • 01:40 the allowable tolerances on each and every unit produced.
  • 01:45 Let's look at some definitions for terms associated with critical characteristics.
  • 01:50 First is the definition of a Critical Characteristic.
  • 01:52 It's a product characteristic or manufacturing process parameter
  • 01:56 that can affect safety or compliance with regulations,
  • 01:59 fit, function, performance, or subsequent process of product.
  • 02:04 Also known as Key Characteristics of Special Characteristics.
  • 02:07 Many organizations will have special marks on drawings or specifications
  • 02:12 to annotate which characteristics are critical characteristics.
  • 02:15 If that item is being outsourced, the purchasing agreement will often specify
  • 02:19 that the supplier's quality control plan must check and
  • 02:22 sometimes record the values of the critical characteristics.
  • 02:26 The next definition is a special case of critical characteristics
  • 02:30 known as critical safety characteristics.
  • 02:33 These are critical characteristics related to those product requirements, dimensions,
  • 02:38 specifications, test process parameters, whatever.
  • 02:41 Which can affect compliance with government regulations or
  • 02:43 product function.
  • 02:45 In some industries, especially government contracting,
  • 02:48 there is a requirement to identify these characteristics.
  • 02:52 A second special category of critical characteristics
  • 02:55 is the critical product characteristics.
  • 02:57 This is defined as a critical characteristic that identifies those
  • 03:01 product parameters and requirements that are important for customer satisfaction,
  • 03:06 with a focus on capability to maximize customer satisfaction.
  • 03:10 These characteristics directly impact the customer.
  • 03:13 Your organization may use one or
  • 03:15 more of these terms, which addresses critical characteristics.
  • 03:19 It's helpful to understand the differences.
  • 03:21 For the rest of this lesson, we'll not differentiate between the different types
  • 03:24 of critical characteristics but just call all of them critical.
  • 03:29 But now let's talk about these in the context of failure mode effects analysis.
  • 03:32 After all, this is an FMEA course.
  • 03:36 Critical characteristics can be associated with failures that are analyzed
  • 03:39 in both the Design FMEA and Process FMEA.
  • 03:43 In fact, the common practice is to designate any product function or
  • 03:47 process step with a failure that has a severity level higher than 7 as critical.
  • 03:52 As we will see when looking at scoring in a later lesson, that the score of 8
  • 03:56 indicates that something very major has gone wrong with the product or process.
  • 04:02 A score of 7 indicates that something major is going to go wrong, but
  • 04:06 the user or operator has an opportunity to take action to prevent it.
  • 04:10 A score of 8 or higher on a Design FMEA will normally carry over to an 8 or higher
  • 04:16 on the Process FMEA at that point in the process where that function is created or
  • 04:20 modified.
  • 04:21 So when a development team does their initial Design FMEA,
  • 04:25 they should review all of the critical functions or items to determine if it is
  • 04:28 possible to eliminate or modify them to a severity score of 7 or lower.
  • 04:33 By doing this, they eliminate the critical aspect of that function or item.
  • 04:37 This leads to a robust design and simplified operational processes.
  • 04:43 However, if they can't eliminate or lower the severity of the design,
  • 04:46 the Process FMEA analysis must consider the process history and
  • 04:50 process control systems to minimize the probability of occurrence and
  • 04:55 to maximize the ability to detect any deviations on this parameter.
  • 04:59 The criticality puts a spotlight on process management and the control plan.
  • 05:04 A high score in either one of those will virtually guarantee that the parameter
  • 05:08 will exceed the mitigation threshold and require some type of change
  • 05:13 in process management or quality control for that parameter.
  • 05:17 FMEAs both design and process are useful in managing critical characteristics.
  • 05:25 They'll help to define which characteristics are critical and
  • 05:28 to assess whether they're being adequately controlled.

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