Military-Grade PCB Design Rules and Considerations
The term military-grade PCBs can only be used in relation to products manufactured to military specifications. The equipment consisting of these boards has to operate faultlessly under extreme conditions. Hence, these boards are highly reliable since they are manufactured with very high precision and minimum tolerance.
A military-grade printed circuit board must be designed from scratch as a military spec or IPC Class 3 part. The design, layout, material selection, and fabrication processes should follow the stringent standards. These PCBs are fabricated by qualified manufacturers who are capable of working with tight parameters and tolerances. The military standard covers the requirements for DoD standards. This includes interface, standard practices, design criteria, test method, and manufacturing process.
We hear from our CAM engineers that usually they receive military-grade PCB designs that don’t match the mil-grade specs. With no other choice, the CAM team informs the designers about the flaws in their design. These PCB designers have to scrap their mil-grade design and start from scratch.
This article is for those mil-grade printed circuit board designers who forget to do their homework before designing their board.
For starters, take a close look at the capabilities of the PCB manufacturers and ensure that they are an ITAR registered company. You can read more about ITAR in one of our blogs, Zoom on ITAR, the International Traffic in Arms Regulations.
Mil-Grade Design Rules
- Specification: MIL-PRF-31032/1, MIL-PRF-31032/2 (MIL-PRF is the performance specification).The MIL-PRF-31032 certifications include additional features like:
- Overall board thickness
- Minimum hole size and aspect ratio
- Minimum line width and space
- Conductive finish and 18” X 24” panel size
- Qualification letters: VQE-18-032408
- Rigid base material: GF: epoxy resin, flame resistant, woven E-glass
- 18” x 24” maximum panel size
- Maximum of 20 layers
- 100” maximum board thickness
- Minimum hole size: 0.007″ (7 mil) laser-ablated plated hole size before plating, 0.01”(10 mil) drilled plated-through hole before plating
- Aspect ratio:
- 6:1 microvia
- 10:1 through-hole
- Minimum conductor width/space: 0.004”/0.004”
- Plasma etchback hole preparation
- Electroless copper hole wall conductive coating:
- Periodic reverse plate copper plating
- Non-conductive hole fill/via plug
- Liquid photoimageable solder resist
- HASL and ENIG surface finish
- Additional fabrication capabilities: Foil lamination, blind vias
- Controlled impedance: Differential, single-ended
- Detailed mil-specs
Next is the IPC standard. Class 3 is the highest IPC standard. Here the electronic assembly is built considering all of the IPC criteria. This includes laminate selection, plating thickness, material qualifications, manufacturing processes, and inspection.
Mil-grade Parameters for a Perfect Board
For best mil-grade board results stick to the following parameters:
- Dielectric thickness between two planes: 3.5 mil minimum
- Annular ring for inner layer: 6 to 7 mil minimum
- Annular ring for outer layer: 6 to 7 mil minimum
- Drill to copper clearance: 7 to 8 mil
Note: These parameters are not mandatory but if followed your boards will be manufactured close to perfection.
- The components incorporated must match the mil-spec grade. In contrast to commercial-grade components that have 5 to 10% tolerance, the mil-grade components have a maximum tolerance of 1 to 2%.
- The boards should be capable of handling the maximum current load.
- The low-frequency components should be isolated from the high-frequency ones. This is implemented since the high-frequency components might interfere with the performance of the low-frequency components by inducing noise. Hence, ruining the quality of the signal.
- Shielding of clock signals by creating a material enclosure. These enclosures are generally made of aluminum.
- Quality heat resistant materials such as Pyralux AP, FR408, and other metallic core components should be used. These materials help withstand high temperatures.
- Thermal compounds should be incorporated for heat dissipation.
- The designer should run pre-layout simulations and impedance to understand the behavior of the PCB in real environments.
- High heat-generating components need more clearance than regular components. The increased clearance protects the nearby components and the whole PCB. Hence, the increased clearance should be incorporated at the design stage.
- As annular ring for component and via drills are one of the requirements for mil-spec jobs, ensure sufficient annular ring is provided in the design.
- The braided wires should be pre-tinned for a better solderability.
- The press-fit components must be soldered to prevent vibration.
- Before PCB assembly, the thermal profiles for wave and reflow soldering processes should be verified. This evades component damage during the assembly.
- The right surface finish material should be chosen so that the PCB performs its functionality in rough environmental conditions. The most popular surface finish materials are:
- Electrolytic nickel and gold
- Electroless Nickel with Immersion Gold Coating (ENIG)
- Lead-free HASL
- Immersion silver
- Electrolytic wire bondable gold
- To protect the PCB, acrylic-based sprays should be utilized for conformal coating of the PCBs.
- Software simulation programs should be utilized to verify the PCB design. This validates the loads at different locations and helps understand the required design modifications.
- PCB routings should be maintained at 45° angles or lesser. This assists in smooth current transmission in the circuit.
- Military and aerospace PCBs should be manufactured in conformance to MIL-PRF-50884, MIL-PRF-31032, and MIL-PRF-55110 standards.
- Define the stack-up on the fabrication drawing for the PCB manufacturing and indicate the required dielectric thicknesses.
The military-grade PCBs undergo uncompromising tests before they are put into use. Before the tests, the designs undergo Design for Manufacturability (DFM). Here, the basic performance and the essential characteristics of the board are tested.
The aforementioned design guidance will help the designers to build their prototype with minimum errors. However, the perfect layout for a military board depends on the application it is designed for.