MILITARY SPEC PCBS – HOW TO CREATE ONE
The term “Military Spec” is sometimes used loosely by people wanting to imply that their product or gadget is tougher than the next guy’s. In other words, the term has taken on a generic role and is applied to products which exceed normal consumer standards.
But in the world of electronics manufacturing, “Military Specification” has very specific and established meaning. It can only be used in relation to products manufactured to military specifications laid down and administered by committees and associations set up to ensure compliance.
Other areas where equipment has to operate faultlessly under extreme conditions also use very high specifications for manufacturing. We therefore find specifications relating to the aerospace industry and the medical industry, among others.
On many occasions and depending on the application of the finished product, the full military specification is not always required. In these cases, an IPC Class 3 PCB may also be specified as an acceptable standard.
A High Bar
To meet Military or IPC Class 3 specifications, a PCB must be designed from scratch as a Military Spec or IPC Class 3 part. The design, lay-out, material selection and fabrication processes must all comply with strict standards.
Fabrication must be done by a qualified manufacturer who will have the correct equipment and knowhow to work within very tight parameters and tolerances.
The Controlling Authorities
Two bodies compile and control the standards applied to the design and manufacture of high grade and high reliability electronics.
On a world-wide basis, the Institute for Printed Circuits, Association Connecting Electronics Industries. or IPC for short, manages the IPC-A-600 Class 3 regulations.
In the United States, electronics destined for use by the US Military, Navy or Air Force must comply with MIL-PRF-31032 (Performance Specification Printed Circuit Board/Printed Wiring Board) and the International Traffic in Arms Regulations (ITAR). These regulations are compiled and monitored by the U.S. Department of Defense.
Commercial vs Military Spec Design Philosophy
Electronics deployed in commercial-use equipment are designed for quick and easy service or replacement of parts to avoid downtime. If a component fails, the entire PCB is replaced.
The operating conditions are more or less known and are not expected to vary much from one installation to the next, so the unexpected is not anticipated or catered for.
In contrast, military and high risk commercial applications leave no latitude for failure of electronic assemblies.
Military Equipment is used as the bench mark because it’s subject to the harshest conditions. It might have to survive being air-dropped into a combat zone and then spend days or weeks operating in the harshest conditions on the planet. Designers must anticipate every possible scenario because failure of military or high risk commercial equipment could very well result in the loss of human life.
The First Essential Component – A Designer
The vital first step is to retain an experienced designer; the thought processes and procedures for Class 3 or Mil Spec PCB design are very different to standard commercial PCB design.
Your designer must know what is required to gain Class 3 or Military Spec certification. They need to know what’s involved in record keeping, practical and virtual testing, fabrication, what specification the components need to have and the inclusion of the PCB into the final product.
Record Keeping and Note Making
From the initial concept stage, the process requires rigorous note taking and record keeping.
Notes made throughout the design stages should include exact component specification, assembly instructions and details such as core thickness, specific PCB laminate and prepreg types, final surface finishes, layer stack-up information and any other information you feel will assist the fabricator during the production process.
Good housekeeping gives you backup, a solid references and full traceability. It also provides material which will be extremely helpful to the fabricator when it’s appended to your design notes and referenced in your Gerber files.
What’s in a Mil Spec PCB?
To the trained eye, there are clear differences between commercial and industrial IPC Class 1 or 2 PCBs and mil spec or IPC Class 3 PCBs. Let’s take a closer look.
Layout – A Totally Different Angle
Mil Spec and Class 3 equipment is designed to work in harsh conditions, so designers anticipate that they will be exposed to sudden jarring or shock extreme heat, dust and moisture and other destructive conditions.
Because of this, the equipment needs to be easy to test and repair in the field; access is the key.
Design for Test
IPC Class 3 PCBs are Designed for Test, that’s an official term meaning a focus has been put on easy access to components so they can be tested in isolation from the rest of the equipment they work with. Test points are brought to the outside edges of the PCBs and components are mounted for easy removal and replacement where possible.
High Quality Components
Specifying the best components is vital to any IC Class 3 project.
All the components have to be totally reliable and operate within extremely fine tolerances. There is no room for average or approximate performance, so it is good practice to be very specific about the components which will be mounted on the PCB. Give manufacturer and part numbers as opposed to a generic description.
Rugged from the Start
Mil Spec PCBs are designed to handle extremes. To do this, they have to be built with reliability and robustness as primary considerations.
Thicker cores and wider layer spacings are preferable. Thin cores create problems with heat dispersion, component isolation, thermal expansion of the PCB itself and basic physical strength.
The Right Materials
Specifying the correct laminate and prepreg materials for the substrate is extremely important. Many factors influence material choice for a specific design, this would include the Tg or Glass Transition Temperature, weave density, Td or decomposition temperature, CTE or coefficient of thermal expansion and thermal conductivity.
Low frequency applications require FR4, D10 (high pressure fibreglass laminate) and Polymide or Cyanate Ester.
High frequency PCBs use Rogers Series RO4003, Duroids, Polymide or other Teflon based material.
The Right Methods
While the right materials and components are essential, there are other factors which need to be taken into account in the design and fabrication of the boards. Here are some of the most important ways in which IPC Class 3 boards differ from the commercial boards:
- Etched Annular Ring requirements for plated (PTH), for non-plated (NPTH) and for internal plated through holes.
- Minimum conductor spacing acceptability.
- Through hole plating wall thickness requirements.
- Drill hole breakout acceptability, in other words, how extensively the drilled hole is allowed to break out relative to the edge of the respective pad.
- Inspection criteria, both during PCB fabrication processes as well as on the finished bare printed circuit board.
Board Thickness to Via Size Ratio
Aspect ratios are important in building Class 3 PCBs.
The ratio of 10:1 must never be exceeded. This is the ratio of the finished board thickness to the hole diameter. If this ratio is exceeded, the reliability and the board’s physical strength will be compromised and the boards will be more difficult and costly to fabricate.
Separation of Frequencies
One of the reasons IPC Class 3 PCBs are larger than the average industrial PCB is to make space for the components and to allow for separation of high frequency and low frequency components.
High frequency units will cause signal cross interference and distortion in low frequency components and vice versa. This in turn will compromise the operation of the equipment.
Clean clock signals are vital for any electronic system, but particularly so for the more sophisticated systems in areas like air traffic control, military systems and equipment designed to be deployed for long periods of time without a break.
In many instances, complex equipment will be running more than one clock signal, so it’s even more important that the individual signals be kept separate and clear of interference from any other signals.
Protection from other waveforms requires shielding capable of completely blocking any potentially disruptive pulses.
Digital and Analogue Signals
Digital and analogue circuits must be kept separate. Analogue circuits use far higher current than digital, generating powerful waveforms which will corrupt the digital circuit’s waveforms.
The most effective method is to isolate the different circuits between their own ground planes. This will prevent any cross-talk which would otherwise happen.
How to Design Echo-free Trace Routing
With any circuit, but especially with high-current and more complex circuits, it’s vital to lay out trace routing to avoid echo.
The surest way to achieve this is to avoid corners of more than 45° and ensure the trace follows a curved route, avoiding angled corners at all costs.
Sharp corners cause problems because, like water, electrical current will not flow smoothly through a sharp bend. When the current reaches the bend, reflections will be generated and a ripple effect will be set up, going against the current. The result in an unclear signal.
Avoiding Heat Build Up
Thermal management needs special attention in high voltage systems which are expected to run for indefinite periods without much attention.
The first step is to include heat sinks where possible and look for opportunities to use existing surfaces to dissipate heat. For instance, ground planes can double up as heat dispersal zones if the copper areas on external boards are exposed, allowing excess heat to escape.
Where possible, metal base PCBs should be used. These have components on one side, with the other side available for thermal control.
Choice of components can also affect heat management. Metal based components are preferable as they tend to disperse heat within themselves. Ceramic based components are more likely to release heat into the surrounding PCB and neighbouring components.
The Goal – Painless Fabrication, Compliance and Certification
The whole point of all the attention to detail and adherence to the IPC or Ministry of Defense regulations is the certification of your product.
If you start with the regulations and stick closely to their requirements, making careful notes and records along the way, you will have a set of intelligence which will smooth your way to certification.
The same documentation will come into play when the owner of the design needs to protect his intellectual property.
Having everything carefully recorded will secure the success of the project and ensure that your product will conform to the IPC Class 3 or MIL-PRF-31032 certification requirements.