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This month we will dive into the testing of aerospace and military product. These designs require special processing in many cases above and beyond the IPC standards. The main specifications used when testing military product are MIL-PRF-55110, MIL-PRF-50884 and MILPRF-31032. All of these specifications have gone through revision changes over the years so for the sake of our discussion today I will only be referencing the current revisions as of today.
From a testing perspective in today’s electrical test theatre, the question of whether flying probes can be used to test this type of product is quite common. Historically, fixture testers were used for testing military and aerospace product. With the complexity of designs today and the cost of manufacturing fixtures, the requirement for testing on fixtures is not always the case.
As of September 2015, IPC released revision “D” of the IPC-6012 Specification for the Qualification and Performance Specification for Printed Wiring Boards. Historically there was an appendix in the specification for Aerospace and Military Avionics. For electrical test this was important as it stated requirements above and beyond the IPC-Class 3/C standard. With the release of revision “D” of the specification there was also a separate release of IPC-6012DS. This supplemental document titled Space and Military Avionics Applications Addendum to IPC6012D moves the historic appendix to this new document. This should be reviewed for changes from the previous revisions.
The main document reference for standard testing still remains IPC-9252A w/Amendment 1. This document still outlines the methodology and requirements for electrical testing of unpopulated printed wiring boards. (Spoiler alert: There is a revision B forthcoming.) Both the IPC 6012 and military specifications tag this document as reference.
With that said, let’s now explore the military and aerospace aspects of electrical testing. From the aforementioned military specifications, we will first look at MIL-PRF-55110. This specification rests at revision “H” w/Amendment 1. Although this specification is still called out in today’s manufacturing, it actually was retired in December 1997. Although the specification is still maintained for historic builds the notation on the cover states that post December 1997 MIL-PRF-31032 should be used. The main question here is whether I can flying probe my product under this specification or do I have to build fixtures to test? The simple answer is yes. The use of flying probes for testing under this specification is allowed. Requirements are specified in section A.3.7.5 and A.4.8.5. Industry standard adjacency window value is used however unless otherwise specified vertical adjacency must also be used. The used of indirect testing by signature comparison is also allowed. See section A.126.96.36.199 of the specification for reference on this attribute.
MIL-PRF-50884 is the specification around flex and rigid-flex circuits. The specification rests as MIL-PRF-50884 “F” w/Amendment 1. Again, the question regarding fixtures or flying probes? Again, the answer is yes. These requirements are listed beginning in section A.3.7.5. The use of indirect testing by signature comparison is also allowed for production screening. See section A.188.8.131.52 of the same specification.
Now the big one, MIL-PRF-31032. This is the new specification “Printed Circuit Board/ Printed Wiring Board, General Specification For.” This specification currently rests at revision “B” w/Amendment 1. The caveat here is that though this is the general specification there are list of build classifications with different performance requirements. These are referred to as the “Slash Sheets.” This is where it can get tricky. One requirement from the Defense Logistics Agency Land and Maritime is that when a test is performed and the Certificate of Compliance is generated they require not only the specification used but the specific class and amendment level. There are six class levels under the MILPRF-31032. They are: /1C w/Amendment 2, 2B w/Amendment 1, 3B w/Amendment 1, 4B w/ Amendment 2, 5A and 6A. Each of these address a specific build class. A proper Certificate of Compliance for testing product under this specification would have on it for example: MIL-PRF31032B w/Amendment 1 /2B w/Amendment 1. This could be shortened to MIL-PRF-31032B-1 /2B-1 if your system reconciled the notation to the specific revisions of the specification and slash sheet. The requirement is to have the specific revision and amendment levels noted on the Certificate of Compliance. This is important as a yearly report is required by the DLA for all product tested under the Military Specifications. Suitability labs are required to provide this. For an independent or captured test facility knowing just that it is MIL-31032 is NOT enough. The specific revisions and sub class are also required information. If not provided it should be researched prior to performing and certifying the product electrically.
Okay, the question again regarding fixture vs. flying probe? With this large specification I have created Table 1 outlining the specific “Slash Sheets” and allowances therein.
Also remember that when testing military/ aero product the specifications require monitoring of atmospheric conditions at the time of test. These traceability records must be retained for a minimum of three years. AS9100 and medical retention carry longer requirements. Overall the testing of military/aerospace product has become less of a challenge in recent years than historically. The relaxation of the fixture only requirements has made it much more cost-efficient and expedient. Historically minor changes in net design required new fixture tooling which is expensive and time consuming. Changes to flying probe programs are fast and only require time from the front end. Take the time to review the specifications. There are some minor differences outlined in the specifications that differ from the default requirements of IPC-9252A. The performance specifications for military can be found on the Defense Logistics Agency’s website. Remember that customer procurement documents and master drawings may override the general requirements of any specification. Many specifications call out the minimum requirements that meet the specification. Customers may require stronger parameters and simultaneous (fixture) tests when the specifications allow flying probe. All procurement documents and master drawings should be consulted prior to developing the test solution for any given product. This will guarantee the correct solution and prevent costly delays with time to market.