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Friday, July 10, 2009

Manufacturing Printed Circuit Boards to Military Requirements

Made in the USA
Military markets have been perceived as a security blanket for American PCB manufacturers. The increased interest in this market segment is understandable, but the devil is in the details! There are similarities between between commercial and military performance specifications but the difference between compliance requirements is substantial. And it is not just the difference between IPC and Military specification; its also the difference between the Military specifications themselves! Understanding these differences is the key to successful participation in this market.

The following excerpts are provided to illustrate some of the differences in military specifications only, not compliance and/or qualification requirements.

MIL-PRF-55110
MIL-PRF-55110 is the general specification for rigid printed circuit boards that establishes the performance and qualification requirements for rigid single–sided, double–sided, and multi layered boards with or without plated through holes.

Verification of conformance is accomplished through the use of one of two methods of product assurance; QPL (Quality Part Listing) or QPL/QML (Quality Manufacture Listing). Detail requirements, specific characteristics, and other provisions which are sensitive to the particular intended use are specified in the applicable master drawing.
Printed wiring boards are classified by type;
  • Type 1 – Singled-sided printed wiring board.
  • Type 2 – Double–sided printed wiring board with or without plated-through holes.
  • Type 3 – Multilayer printed wiring board with plated holes.

The main body of the specification contains general provisions and is supplemented by detailed appendices. Appendices A and B describe the two product assurance programs that can be implemented by the manufacturer.

  • Appendix A contains the traditional Qualified Products List (QPL) product assurance program.
  • Appendix B is an optional quality management approach using a technical review board concept along with a Qualified Manufacturer List (QML) product assurance program addressed in MIL–PRF–31032, to modify the generic verification criteria provided in this specification.
  • Appendix C provides statistical sampling, and basic test and inspection procedures.
  • Appendix D is optional and can be used when producing printed wiring boards designed to superseded design standards.
  • Appendix D may also be used as a guide in developing a test plan for legacy or existing designs based on the tests and inspections of appendix A.
  • Appendix E is optional and describes an alternative procedure used to evaluate oxidation levels on solderable surfaces. The procedure involves using electrochemical reduction techniques to determine the type and quantity of oxide on plated-through holes.

MIL-PRF-31032
MIL-PRF-31032 is also a general performance specification for printed circuit boards that indicates the general performance requirements for printed circuit boards and the verification requirements for insuring that these items meet the applicable performance requirements. Certification and qualification to this specification allows manufacturers to apply the Qualified Manufacturers List (QML) program to printed boards procured to non-QML documents such as MIL-PRF-55110 and MIL-P-50884.

The intent of this specification is to allow the printed board manufacturer the flexibility to implement best commercial practices to the maximum extent possible while still providing product that meets military performance needs. The principle concepts which allow this flexibility are as follows: The manufacturer forms an internal quality review system or organization referred to in this document as the Technical Review Board (TRB).

The TRB is responsible for the implementation of a quality management (QM) plan. A relationship is established between the manufacturer and the qualifying activity. The process to implement this specification can be summarized as follows:

  • Self-validation: The TRB verifies the implemented QM plan.
  • Certification: The qualifying activity leads a team to validate the TRB’s effectiveness in implementing their QM plan.
  • Qualification: The manufacturer demonstrates capability to produce a printed board technology.
  • Maintenance: The TRB keeps the qualifying activity informed on the status of its QM program.

The main body of this document describes the requirements for obtaining a QML listing. The appendices of this document are intended to aid the manufacturer in developing its verification program.

  • Appendix A describes a quality management approach using a TRB concept.
  • Appendix B contains qualification procedure requirements.
  • Appendix C contains the conformance inspection requirements.
  • Appendix D describes techniques for test optimization.
  • Appendix E provides statistical sampling, basic test, and inspection requirements.

Sectional specification sheets contain the performance requirements and verification methods for printed boards by technology. Detailed requirements, specific characteristics, and other provisions which are sensitive to the particular intended use of printed boards are to be specified in the printed board procurement documentation.

Caveat Emptor
Military orders require certification of compliance. Compliance must be to the specification, the revision, the amendment, and the appendix. If this information is not provided in the procurement documentation, waivers must be agreed upon to identify the conditions of compliance. The manufacturer must complete and document all required tests and inspections per the requirements identified in by the conditions of compliance indicated in the procurement documentation. Provider and User must coordinate activities to ensure compliance. Failure to do so can invalidate the certification with potentially disastrous results!

For copies of referenced specifications and additional information on QPL (qualified part listing) and QML (qualified management listing) programs, visit the Defense Supply Center website at
http://www.dscc.dla.mil/programs/qmlqpl/




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