Acceptable Quality Level
Before discussing IPC Inspection Guidelines, it’s important to understand the meaning of the term Acceptable Quality Level (AQL), also referred to as Acceptance Quality Limit. It is an inspection standard that prescribes the range of the number of defective components that is considered acceptable when random sampling those components during an inspection. The Acceptance Quality Level determines the maximum number of defective units, beyond which a batch is rejected. Manufacturers may sometimes set different AQLs for critical, major, and minor defects. Ultimately, an Acceptance Quality Limit is helping to determine how many samples should be picked and inspected among a lot, and, where is the limit between acceptability and refusal for defective product.
IPC Inspection Guidelines
IPC (formerly called Institute of Printed Circuits) is an international trade association serving the printed circuit board and electronics assembly industries. Used to verify the quality of the manufactured printed circuit boards and PCB assemblies, they have developed comprehensive inspection guidelines.
As such they have created a number of guides that are used by manufacturers to provide a standard methodology of inspecting the final quality of the boards.
Two of the primary guiding documents are the IPC-A-600 and the IPC-6012.
IPC-A-600 – Acceptability of Printed Boards
IPC-A-600 (also called IPC-600) – Acceptability of Printed Boards is an inspection criterion, setting the level of acceptance criteria for each class of product. It “…represents the visual interpretation of minimum requirements set forth in the various printed board specifications.” e.g. IPC-6010 series
IPC-600 It is not intended to be used as a performance specification for printed board manufacture or PCB assemblies. It is a tool for the internal and external inspections for printed circuit boards. For instance, IPC-A-600 includes the acceptable condition the gold fingers must have to meet the class requirements, but it does not state the required thickness of nickel and gold for edge connectors.
IPC-6012 – Qualification and Performance Specification for Rigid Printed Boards
IPC-6012 – Qualification and Performance Specification for Rigid Printed Boards “…establishes and defines the qualification and performance requirements for the fabrication of rigid boards.” The IPC-6012 standard establishes the types of rigid boards and describes the requirements to meet during fabrication for three performance classes of boards – Class 1, 2 and 3 & 3A. This “class” assignment is assigned by the designer and usually determined by the end use of the product.
A class 1 product would include boards that would have “limited life” and where the requirement is simply the function of the end use product – your garage door opener for example.
A class 2 printed board would be one where continued performance, extended life and uninterrupted service is desired but not critical. The motherboard in a home PC would typically be class 2.
A class 3 printed board would include end use where continued high performance or performance on demand is critical, failure of circuit board assemblies cannot be tolerated and the product must function when required. Flight controls, defense systems and medical applications are typical areas that would require class 3 (aerospace applications would fall under 3A fabrication).
In general, the difference between the classes is achieved by requiring tighter tolerances and controls that result in a more “reliable” product. In addition, the specification requires production coupons on each panel that represent the boards. They are used for required testing or inspections throughout the manufacturing process and final inspection. The testing of the coupons confirms that the boards meet the specification requirements in place of using product for the destructive testing.
Any PCB manufacturer can claim to be compliant with IPC-6012 and able to manufacture to meet any of the classes. However submission of a sample product and the accompanying coupons to an outside laboratory to measure conformity will provide them with certified test results confirming their capabilities.
Regular samples should also be submitted to assure that conformance is maintained on a continuous basis depending on the class of production being certified. Tests results and reports should be made available as verification that the manufacturer has the capability to meet the 6012 requirements.
There are a number of manufacturers that offer a 6012 compliant product that includes coupon test results, copies of micro-section reports, certificates of conformance for the raw materials used and all of the other necessary accompanying documentation.
Additionally they may also offer product that is produced to meet the standard required but without all of the accompanying documentation, usually at a reduced cost. This type of order does provide the customer who doesn’t need the preponderance of paperwork or micro-section results somewhat of a cost advantage. The panels are usually produced on the identical processing lines as those requiring 6012 documentation, using materials that are also compliant and generally receive similar in-process verifications that the boards/panels are meeting the customer’s specifications.
IPC 6012 lists 101 inspection characteristics that the board manufacturer is responsible for checking, however not all are required for each board type. For instance inner layer inspections are definitely not a requirement for a double or single-sided design nor would wrap plating for those without blind or buried vias. The list is obviously too long and involved to cover here in any detail nor would you as a customer want to spend the time and effort or investment in equipment and training to duplicate what your PCB manufacturer is responsible for doing.