Estimated reading time: 4 minutes

From the Hill: MIL-PRF-31032 Offers a Rewarding Twist

What Are Your Revenue Priorities Now?
Today’s environmental factors have changed business priorities with regard to printed wiring boards (PWBs) requiring military specifications (MIL-PRF-55110, -50884, and -31032). These factors require re-evaluating and re-ranking revenue streams that until recently seemed very stable. On the positive side, military PWB fabrication is now designated as an “essential” business that must stay open as “nonessential” operations close. This bears consideration for the future.

Overview
If you are fabricating PWBs to military specifications, the master drawing will state: “Fabricate to MIL-PRF-55110, MIL-PRF-50884, or MILPRF-31032.” This sounds very complicated on the surface, but there is a rewarding “twist” if the fabricator is certified to MIL-PRF-31032.

Definitions
1. MIL-PRF-55110
This specification was the first performance military specification for rigid PWBs. The first revision is dated September 1960. It establishes the performance and qualification requirements for rigid single-sided, double-sided, and multilayered PWBs with or without plated through-holes. The standard lists acceptable tests and screening methods for verification. Verification is listed in the associated qualified product list (QPL-55110). MIL-PRF-55110 was superseded by MIL-PRF-31032 as of December 31, 1997; however, legacy fabrication sites not certified to MIL-PRF-31032 can continue to certify to and build MIL-PRF-55110- part numbers.

2. MIL-PRF-50884
This specification was the first performance military specification for flexible PWBs. The first revision was dated October 1971. It establishes the performance and qualification requirements for flexible and rigid-flex PWBs with or without plated through-holes. The standard lists acceptable tests and screening methods for verification. Verification is listed in the associated qualified product list (QPL50884). MIL-PRF-50884 has been superseded by MIL-PRF-31032 as of December 31, 1997; however, legacy fabrication sites not certified to MIL-PRF-31032 can continue to certify to and build MIL-PRF-50884-part numbers.

MIL-PRF-31032
This specification covers the generic performance requirements for rigid and flexible PWBs with plated holes, that will use soldering for component/part mounting. Introduced in 1997, it also covers PWBs fabricated to MIL-PRF-55110 and MIL-PRF-50884. 
MIL-PRF-31032 is segregated into the following six board types (each covering a major design group):

1. MIL-PRF-31032/1: Multilayer Rigid
2. MIL-PRF-31032/2: Single- and Double-Sided Rigid
3. MIL-PRF-31032/3: Multilayer Flex and Rigid-Flex
4. MIL-PRF-31032/4: Single- and Double-Sided Flex
5. MIL-PRF-31032/5: Multilayer Rigid High Frequency 
6. MIL-PRF-31032/6: Single- and Double-Sided Rigid-High Frequency

Key Features of MIL-PRF-31032
Today’s military printed board industry has evolved a great deal since the generic qualification concept was first added to MIL-PRF-55110 and MIL-P-50884. In the past, the DoD drove leading-edge technology, and the commercial industry followed. Most printed board manufacturers were captive facilities building products for specific systems. Today, the commercial industry has much to offer the DoD in the way of technology and cost savings. The qualified manufacturers list (QML) concept of MIL-PRF-31032 attempts to capture these best commercial practices and apply them to military products.
 
The QML concept in MIL-PRF-31032 allows both the printed board manufacturers and the user community to take advantage of best commercial practices while still retaining government oversight to assure printed boards meet the needs of the end-item military user. QML results in quick implementation of new technology into military systems at a higher level of quality and reliability, and at a lower price^[1].
 
Unraveling the ‘Twist’ 
From MIL-PRF-55110 Rev H with Amendment 3:

1. B.1.1 Scope. This appendix contains optional requirements concerning the QML product assurance level  ?(MIL-PRF-31032) for PWBs covered by this specification. This appendix is a mandatory part of the specification when the product assurance level of appendix A is not used.
2. B.3.1 Performance requirements. The performance requirements of the applicable MIL-PRF-31032 specification sheet shall apply to all PWBs procured to the QML product assurance level.
3. B.3.2 Accept/reject criteria. The accept/reject criteria of the applicable MIL-PRF-31032 specification sheet shall apply to all PWBs procured to the QML product assurance level^[2].

From MIL-PRF-50884 Rev F with Amendment 3: 

1. B.1.1 Scope. This appendix contains optional requirements concerning the QML product assurance level  ?(MIL-PRF-31032) for PWBs covered by this specification. This appendix is a mandatory part of the specification when the product assurance level of appendix A is not used.
2. B.3.1 Performance requirements. The performance requirements of the applicable MIL-PRF-31032 specification sheet shall apply to all PWBs procured to the QML product assurance level.
3. B.3.2 Accept/reject criteria. The accept/reject criteria of the applicable MIL-PRF-31032 specification sheet shall apply to all PWBs procured to the QML product assurance level^[3].

To summarize, if the site is certified to MIL-PRF-31032, it can build any parts where the master drawing indicates MIL-PRF-55110 or MIL-PRF-50884. 
 
Two Steps to Implement the ‘Twist’
First, do all inspection and tests per MIL-PRF-31032 for parts requiring MIL-PRF-55110 or MIL-PRF-50884. Second, for the finished product certificate of conformance (CoC), use the following statement: “Product was manufactured to meet MIL-PRF-55110H Amendment 3, Appendix B, acceptable requirements.” Note: Substitute “MIL-PRF-50884F Amendment 3” for  ?flexible products.
 
Summary
The twist with MIL-PRF-31032 allows the fabrication of MIL-PRF-55110 and MIL-PRF-50884 part numbers with a simple two-step implementation process and thus “three military specifications with one stone.” Furthermore, as the reliability of revenue streams has become more important today, the building and selling of military PCBs has moved up a few steps. 
 
References
1. Certification and Qualification Information for Manufacturers MIL-PRF-31032 DLA Land and Maritime: VQE-31032 Revision, August 2013.
2. MIL-PRF-55110H With Amendment 3, March 26, 2018.
3. MIL-PRF-50884F With Amendment 3, March 26, 2018.

This column originally appeared in the May 2020 issue of PCB007 Magazine.