Manufacturing / Production Technology, Hardware & Services


Designing PCBs for Mil Spec

29 May 2019 Manufacturing / Production Technology, Hardware & Services

In the field of electronics, Mil Spec – or Military Specification – has a very clear and understood meaning. It denotes equipment designed and made to exacting standards, in accordance with precise rules and regulations.

Professional bodies administer, compile and manage the standards and ensure compliance. Companies producing equipment for the military, aerospace, commercial aviation, deep sea operations or other extreme conditions design and manufacture equipment under these regulations.

Military Spec and IPC Class 3 PCBs

While printed circuit boards (PCBs) may carry Mil Spec certification, they may not be intended for inclusion in military equipment. The term refers to the fact that they’ve been designed and manufactured to very specific and stringent standards.

Equipment destined for use by the United States of America’s army, navy and air force are manufactured in accordance with MIL-PRF-31032 and the International Traffic in Arms Regulations, or ITAR. These regulations are formulated and overseen by the US Department of Defence.

Military Spec for the rest of the world adheres to IPC-A-600 Class 3 regulations handled by the Association Connecting Electronics Industries. Known by the initials IPC, the organisation has changed its name several times since it was founded in 1957, settling on its current name in 1999 (the initials stand for ‘Institute for Printed Circuits’).

What does Mil Spec mean for the PCB manufacturer?

The term Mil Spec applies to everything about the PCB manufacturing process – from initial concept to when the completed PCB is integrated into the equipment it was designed for.

Why should special standards apply to military equipment?

Because nobody knows how, when, where or for how long the kit will be used, it’s built to survive the worst possible conditions imaginable. The boards and their components are designed to survive anything short of a direct hit. They’re expected to handle extreme heat, extreme cold, dust, vibration, extreme humidity and everything in between.

They are deployed in and have to survive conditions far beyond anything PCBs in commercial or industrial applications would encounter. Having said that, there are conditions in civil aviation and in countless industrial and manufacturing environments where equipment needs to be as tough as its military counterparts, so Mil Spec or Class 3 PCBs and componentry are required.

The process begins with the designer

If you’ve never been involved in the design and manufacture of Class 3 PCBs before, the first thing you need is a qualified and experienced designer – not just any designer.

You need someone who really knows their way around the particular sets of regulations which need to be followed – someone who knows what they’re doing at every level, from the concept stage, through design, record keeping, testing, virtual testing, manufacturing and what is involved in the eventual inclusion of the PCB into the end product.

In the beginning – housekeeping

From the outset, it’s required that you keep careful notes on all meetings and discussions relating to the project. A project checklist is available from the IPC website – a great place to start.

The plans for the PCB need to be immaculate. Painfully detailed notes need to be kept as the process unfolds and every little factor must be checked and rechecked before moving to the next stage. Good housekeeping is not only for your backup and reference when you need it, you will have to submit notes with your designs and include references in your Gerber files for the fabricators to reference.

At the very least, your notes must cite such details as the specific components the PCB will carry, with detailed assembly notes, core thickness, specific PCB materials to be used, surface finishes, stack-up information and any other key aspects and vital information which will influence the fabrication and processing of the PCB.

The essential differences

Okay, that’s the housekeeping. Now let’s take a look at some of the key areas where Mil Spec or IPC Class 3 PCBs differ from standard industrial and commercial PCBs (or IPC Class 1 and 2 products) and how this impacts the design and component accommodation.

Layout – a completely different emphasis

Commercial PCBs are usually designed for convenience. If they fail, they, along with their components, are replaced.

IPC Class 3 PCBs are designed for test (DFT). Easy access is essential, so components can be thoroughly tested in isolation from the rest of the unit and, if necessary, replaced as quickly and effortlessly as possible. Disposal of the entire PCB with its components is not a consideration.

Vibration, heat generation and dissipation and unexpected shock, mechanical stresses and unanticipated spikes in current have to be considered and accommodated. For instance, will the unit survive an unexpected blast of searing heat, or the momentary shock of being accidentally dropped? Is the PCB located in the best possible position within the device to minimise the effects of these stresses?

At the layout stage, every eventuality has to be considered; the designer must try and allow for random extremes which might be encountered in the field.

The right components

Apart from withstanding extremely harsh conditions, MIL-PRF-31032 / ITAR or IPC Class 3 components must operate within extremely tight tolerance bands. For a unit to fulfil the precise function it’s designed for, all the components need to be high-quality and deliver almost exactly the performance their ratings say they will deliver.

Extra strength, from the ground up

Best practise in making Military Spec PCBs is to design cores as thick as possible, and definitely thicker than 2 to 3 millimetres. Thin cores will generate problems with heat dispersion, component isolation, physical strength and thermal expansion of the PCB itself.

Aspect ratios

The relationship of finished board thickness to hole diameter in Mil Spec PCBs must never exceed 10:1. Aspect ratios greater than this may well jeopardise the reliability of the board. Apart from this, boards with aspect ratios greater than 10:1 are more challenging, and therefore expensive, to make.

Signal distortion and shielding

Separating high- and low-frequency components is essential. Every precaution must be taken to avoid interference or distortion in Mil Spec equipment.

It is critical that signals of different types are shielded from each other. As a start, digital and analog circuits must be separated. High-current analog systems create waveforms which will interfere with the digital circuit’s waveforms. An effective method of separation is to isolate digital and analog circuits between their own ground planes. This will control any cross-talk which might otherwise happen.

Clock signals are particularly important, especially in more complex equipment which might be running more than one clock signal.

To ensure these critical signals are not degraded by other waveforms in the unit, the circuits carrying them should, where possible, be shielded with aluminium or a similarly effective material.

Echo-free trace routing

As most Military Spec equipment runs on higher-current circuits than standard commercial equipment, close attention must be paid to how the trace on the board is routed.

Turns sharper than 45&drg; must be avoided and, in all cases, the path of the trace must curve as opposed to forming corners. If a corner is formed, problems can occur when the current reaches the bend. Reflections are generated which set up a ripple effect going against the flow, creating an unclear signal.

Thermal management

In high-performance equipment, designing to minimise the generation of heat and the management of the heat produced is critical. Components designed to dissipate heat within themselves, as opposed to dispersing it into the surrounding equipment, are to be preferred.

Where possible, PCBs should be designed with one side accommodating the components and the other side available to disperse heat. Horizontal or vertical heatsinks should be used wherever feasible.

Thermal management can also be aided by the use of ground planes and by exposing copper areas on the external boards to dissipate heat.

The right stuff

Specifying the right materials for substrates is critical.

For low-frequency applications, FR4, G10 (high-pressure fibreglass laminate), polyamide or cyanate ester are suitable, while high-frequency applications require Rogers series RO4003, duroids, polyamide or other Teflon-based materials.

The right methods

Apart from the quality of the materials and components used in fabricating IPC Class 3 and Class 2 PCBs, there are numerous other factors that set them apart. Here are some of the key ways in which they differ:

• Etched annular ring requirements for plated (PTH), non-plated (NPTH) and 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.

Write everything down – you’ll need it

As mentioned at the outset, detailed notes of everything to do with the design and fabrication of Class 3 PCBs are essential for a number of reasons. Specific information on the components needed, specific core thickness, the number of layers, the PCB materials to be used, surface finishes required and detailed assembly notes all form part of these.

Nothing must be left to chance when the order arrives at the fabricator – there can be no room for interpretation or misunderstanding.

The objective – compliance and certification

At the end of the day, the product being designed has to be certified as complying with the relevant regulations. The notes you make and the documentation you compile in accordance with the regulations you need your PCB to comply with, will be used to ensure you meet the standards set.

The same documents will also be used in copyrighting and intellectual property protection, as they record and lay out in great detail the materials used and the desired functions and performance you expect from the equipment you are seeking to protect.

For more information contact Cirtech Electronics, +27 21 700 4900, info@cirtech-electronics.com, www.cirtech-electronics.com



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