All About Flex: Specifying a Flexible Circuit
Flexible circuits are custom engineered parts built to a set of engineering and documentation requirements. The IPC?Association Connecting Electronic Industries, a global trade association for participants in the electronics industry, has created a specification document, IPC 6013, which is referenced for many flex circuit applications. This commercial document, in combination with the CAD data and print, is used as the product specification. Most flexible circuitry fabricators’ internal quality standards are based on IPC 6013. This document defines three classes of product specifications, with Class 3 often referenced for high reliability medical, military, and aerospace applications. In other applications, requirements may be specified with customized detailed documents. It is not unusual to see several different documents used to define requirements for a flexible circuit part number. The following is a typical list of documents that specify flexible circuit requirements in the high reliability markets:
The purchase order
Deviations (if needed)
The print and referenced drawings
General requirements’ documents specific to the customer
Industry specifications such as IPC documents
Normally the above documents have the following order of precedence when/if there is a conflict. The purchase order requirements supersede all other requirements. A deviation will supersede the print package. And the general requirements documents will supersede any applicable industry requirements.
The purchase order contains the price, the schedule, terms and conditions and should reference specific documents containing additional requirements. In some cases, the purchase order may contain a specific requirement that is not specified in the other documents or conflicts with other customer documents. For example, the purchase order may waive some of the quality records requirements if an expedited prototype build has been agreed upon.
A deviation is a list of exceptions to requirements enumerated on the print. Many customers have a lengthy and sometimes arduous change/approval process for specifications. In these cases, creating a deviation will allow them to purchase parts to new requirements without waiting for the specifications to be formally changed.
The print is used in conjunction with the CAD data to define the form, fit and function of a part. Most of the part number specific requirements are contained in the print. This usually includes materials, cutline, tolerances, surface finishes, dimensions, feature locations and other elements specific to the part. “Critical to Function” dimensions and requirements are sometimes highlighted for their uniqueness. These features may also be specified to include a first article capability demonstration.
These are usually not specific to a part number but will apply to a broader range of parts. In some cases, a customer may create one general requirements document that all suppliers must adhere to. In other cases, it may only apply to a specific category of product, such as electronic components. Some of the documents may include requirements pertaining to lot control, packaging, testing, quality records, advance quality planning, and other generic specifications.
For flexible circuits, IPC 6013 is the most common document referenced, but there could be a host of other documents, procedures and test methods. Industry documents are usually referenced within the purchase orders, prints or general requirements documents. Customer supplied reference documentation is used by the fabricator to create manufacturing floor work instructions to ensure specifications are communicated and met. Customer prints and specifications are often referenced in these work instructions, in combination with pictures and illustrations to describe a repeatable manufacturing process.