IPC Class III vs. MIL Spec for Printed Circuit Boards

When manufacturing printed circuit boards (PCBs), the boards will be classified into specific groups based on the level of quality. Any circuit board developed for IPC Class III has met the strictest requirements levels based on tight minimum tolerances and very high precision standards for mission-critical PCBs that can be used in industries such as aerospace, military, and medical applications. Reaching these specific standards ensures that the PCB will endure a long lifecycle, can withstand up to harsh environments, and will perform at an exceptional level without failure.

Many manufacturers will also talk about their PCBs meeting MIL specifications. MIL spec refers to circuit boards that are built from scratch based on specific military specifications for the materials, design, layout, and fabrication of the board. The key difference between IPC Class III boards and MIL spec boards is that two different organizations govern and control the standards for each classification: The Institute of Printed Circuit Boards - Association of Connecting Electronics Institutes (IPC), and the Military Performance Specification Printed Circuit Board/Printed Wiring Board (MIL-PRF), along with International Traffic in Arms Regulations (ITAR). There will be specific stringent standards and inspections for each that need to be met.

The numbers for IPC and MIL Spec PCBs indicate the type of circuit board that the standards will apply to during its fabrication. The IPC-6012 refers to rigid circuit boards, IPC-6013 refers to flexible circuit boards, and IPC-6018 refers to high-frequency (microwave) circuit boards.

IPC-6013 Class III (C) meets the same performance requirements as MIL-PRF-31032 and is accepted by government agencies as a commercial off-the-shelf (COTS) equivalent of the latter. If your printed circuit board must meet performance requirements of MIL-P-50884, MIL-PRF-31032, or IPC6013, follow the IPC-2223 design specification recommendations.

In this blog post we will review:

• MIL-PRF-31032 – Covers all PCBs (IPC-6012, IPC-6013 and IPC-6018 equivalents).
• MIL-PRF-55110 – Legacy specification, which was for rigid boards only (IPC-6012 & IPC-6018). You can no longer get approval for 55110 unless you are 31032 approved.
• MIL-PRF-50884 – Specifically covers flex/rigid-flex (IPC-6013).

Printed circuit board manufactured to MIL-spec standards.

Major Highlighted Difference of the IPC-6013 Class III to the MIL-Specs

There are many major differences between the IPC-6013 Class III boards and the MIL-specs. Some of these differences include performance levels, ease of following specifications, up-to-date standards, and foreign materials that are used. One thing to keep in mind is that the IPC-6013 Class III standards are routinely updated while the paragraphs are easy to follow. The military specs are harder to follow between paragraphs and are not user-friendly, while the standards are not consistently updated. Other major differences include the following factors:

Minimum Annular Ring

The annular ring is the copper pad area around the drilled and finished hole. The annular ring's purpose is to establish a good connection with the copper trace and the via. IPC Class 3 and MIL spec standards will define the width of the ring for the hole, how centered the hole needs to be, and other factors.

• IPC Class 3 is 0.002” external and 0.001” internal. Other classes are less stringent. This recognizes designs are becoming smaller and require tighter geometries.
• MIL 0.005” min. for external and .002” min. for internal.

Foreign Material (conductive and non-conductive)

Foreign material represents the type of contaminated material and the amount of this material that may be present which will be accepted for that classification. The foreign material may come from the environment or be residual from the fabrication of the PCB.

• IPC translucent foreign material is acceptable. All other foreign material is acceptable provided it is not closer than the minimum spacing on the drawing.
• MIL must not be conductive, not be >0.031”, not reduce spacing >25%, and not propagate. This results in rejections that in most cases do not affect performance.

Copper Thickness after Processing

During processing of the PCB, the thickness of the copper may be reduced during pre-treatment before the bonding and imaging. The amount of copper to use for the PCB must be considered as the thickness of the copper will be greater than what the finished copper will be after the PCB is completed.

• IPC minimum copper foil thickness after processing is clearly defined.
• MIL not addressed. There is no minimum accepted value for how much copper can be removed during processing.

Coupon Design

A coupon design is used to test the controlled impedance of the PCB. The test coupon is usually manufactured on the edge of the same panel as the built PCB. The coupon design may be integrated into the actual PCB or it may be on the separate section of the panel.

• IPC coupon design is per IPC-2221. Coupon design tries to represent circuit pattern.
• MIL coupon design per MIL-STD-2118. Coupon does not represent the circuit.

Measling and Crazing

Measling occurs to woven fiber laminated materials where the woven fiber bundles have become separated along the intersections of the weave. This problem is often seen as white squares along the base material. Crazing occurs when the glass fibers of the woven laminated materials become separated along the intersections.

• IPC did an extensive study and found that these conditions do not cause any degradation in the performance of the printed wiring board and therefore concluded that “measling” and “crazing” are not rejectable items.
• MIL measling and crazing must conform to sub-surface imperfections requirements. This results in unwarranted rejections that were proven not to have any performance degradation.

Solder Coating

The solder mask, or solder resist, is the color coating that protects the areas on the PCB as it reduces the chances of short circuits from solder bridges and acts as a layer electrical insulation.

• IPC solder finish must have coverage and be solderable.
• MIL solder coating must be 0.0003” min. at the crest on the surface, 0.0001” min. at the crest in the hole and coverage at the “knee” of the hole.

Qualification

A qualification process and plan can be established between the manufacturer and customer of the PCBs. These qualifications can cover a range of requirements including testing samples, materials, and tools and technology that will be used.

• IPC to be agreed upon by user and supplier. Can be pre-production samples, production samples, test specimens (i.e. IPC-A-41, 42 or 43), or based on documentation from testing of specimens furnished on similar product. There is no QPL listing.
• MIL-PRF-31032 requires establishing a technical review board (TRB), submission of PCB samples to be used as the basis of qualifying the type of PCB technology being approved (once every 2 years), ongoing monthly sampling of most demanding MIL PCBs to a DLA-certified lab to maintain approval, ongoing audits, and approval by the DLA to be listed as an approved supplier (i.e. to be listed on the QPL). All PCBs built for customers to MIL certification must be within the approved scope of technology and not to exceed the approved technology by more than 25% (i.e. if approved for 12-layer PCBs, prohibited from building 20-layer MIL PCBs until such approval can be attained).

Rigid-Flex Transition Zone

The rigid-flex transition zone refers to the length of the rigid area of the board as it extends and transitions to the flexible area of the PCB. During manufacturing of the circuit board, the flex coverlays may be extended so that the lamination along the rigid area will capture the flexible area to create a gapless transition.

• IPC allows for visual imperfections in this zone that do not cause any functional degradation.
• MIL does not address this area, defaulting to surface and sub-surface imperfections which do not adequately address anomalies that frequently occur in this zone. This results in unwarranted rejections or indecision.

Cost

PCB fabrication and processes are essentially no different when comparing IPC to MIL. The difference really is on the backend, which is the qualification, ongoing submission to the lab and labor associated with the upkeep/TRB, which can add quite a bit of cost to the PCB.

The costs will be reflective on a wide variety of factors and standards that will have to be met for IPC and MIL Spec. The more standards that must be adhered to will directly reflect in the amount of labor time it will take to create the PCB. This, in turn, will create higher costs to meet testing and auditing specifications. Understanding whether the PCB must meet either IPC Class III or MIL spec will help a customer determine their costs during the design phase.

Summary

There are many factors that are involved when manufacturing a PCB to meet IPC III or MIL spec standards. Determining the circuit board’s role in the application will allow the customer to decide on which classification to follow and what requirements will need to be met by the vendor. It is always ideal to bring the vendor into the project at the design phase so they fully understand what specifications will apply to the PCB.