ARTICLE

MicroTCA Ruggedisation for MilitaryComms
By Dr Paul Moakes, CTO, CommAgility
www.commagility.com

Introduction

MicroTCA and the Advanced Mezzanine Card (AMC) have proved successfulbuilding blocks for modular switched serial architecture solutions. Unsurprisingly,
the cost and performance benefits found in commercial embedded systems are
attractive to applications spaces with more rugged operating requirements.
To address this, PICMG developed the Rugged MicroTCA specification,
MicroTCA.1, and Hardened MicroTCA specifications, MicroTCA.2 and MicroTCA.3.
Even under these tougher environmental requirements, MicroTCA’s SWaP-C(Size, Weight, Power, and Cost) remain advantageous when compared withalternative solutions.
But what are the challenges when modifying an existing air-cooled AMC design
for a rugged application? This paper discusses the experiences of CommAgility’sengineers when given just that challenge for a military communications project.

Standard AdvancedMC Product
CommAgility specialises in commercial off-the shelf (COTS) high-performance
DSP plus FPGA cards comprising Texas Instruments C6678 multicore DSPs andXilinx Virtex-7 FPGAs, for example the AMC-D24A4-RF4.
Our customer successfully used such a product in the lab for wirelesscommunication baseband processing, and now wanted to take the project to thenext stage with a rugged deployable air-cooled solution for field trials. Beginning
development with a non-rugged COTS solution enables customers to minimise
the capital costs at the start of projects, which typically comprises a feasibility
study period or customer demonstration.
Having invested a great deal of time integrating the COTS card, it was thenimportant when moving to a rugged solution that any architectural changes
which affected interface functionality or software compatibility were minimised.
Fortunately, the AMC form factor lends itself readily to ruggedization and wewere happy to make a proposal that allowed this with minimal impact.

Rugged Specifications

The definition of ruggedization is very much application-specific. There are anumber of guidelines available as reference material: for example theenvironmental specifications for airborne equipment are current contained withinthe US DoD Military Handbook “Electronic Equipment, Airborne, General
Guidelines for”, MIL-HDBK-5400 and general equipment in General Guidelines
for Electronic Equipment Handbook MIL-HDBK-454A. However, these areguidebooks and not a set of requirements.
The PICMG MicroTCA.1 Air Cooled Rugged MicroTCA Specification was released
in 2009. Its aim is to define a version of MicroTCA for use in rugged
environments (specifically systems needing to meet more stringent levels oftemperature, shock, vibration and humidity) that MicroTCA.0 solutions do notsatisfy. It also calls upon base military specifications, including MILHDBK-454A,
to define environmental operating temperatures.
This project also had to meet a unique blend of ruggedization requirements. The
key environmental service requirements were vibration, shock, temperature andhumidity with testing to US Military Standard MIL-STD-810 Revision G“Environmental Engineering Considerations and Laboratory Tests”, referred tohere as Methods. In addition, a conformal coating to MIL-I-46058C “Insulating
Compound, Electrical” was required. This not only allows the card to meet theextended humidity specifications, but also to add electrical resistance.
How CommAgility addressed these challenges within a MicroTCA.1 platform isdiscussed below.

Shock and Vibration
The shock tests are defined in MIL-STD-810 Method 516.6, and there are eight
different procedures defined, which use different ways to cause shock or impact.
The tests determine the ability of a device to withstand general physical abuse
while in operation.
Procedure IV (transit drop) is the most common referenced, however in this caseonly Procedure I Basic Design was required. This includes the “drop” test: 18
impact shocks of 15g comprising shocks in opposite directions along each
perpendicular axis, with each shock impulse having a time duration of 11
milliseconds.
A review of the existing design resulted in no significant design modifications
being required to achieve this level of ruggedization. Of more significance werethe changes required to meet vibrational testing, in meeting these the AMC
became more resilient to shock in any case.
Vibrational testing is more complex than shock testing and is described in
Method 514.6. In this project, Procedure I was required, with an Annex C
tailored Category requiring conformance to an acceleration of 5g over thefrequency range 5Hz to 2000Hz with a displacement of 0.2”.
The weak point of the AMC in this test is typically the backplane connector andlatching handle. An essential modification is therefore to use the additional‘Module to Subrack Retention Device’ specified in MicroTCA.1-XR2. The AMC isfitted with an extended faceplate having an attachment flange with retention
screws. Once the AMC is inserted into the chassis and the latches locked, theretention screws fix the faceplate to the chassis without exerting force on theAMC backplane connector.

Figure 1: AMC fitted with Module to Subrack Retention Device
Our standard card used small form-factor pluggable (SFP+) optical transceivermodules for such tasks as CPRI and RapidIO connectivity. Concerns over thevibrational stability of SFP+ led us to replace these with fixed short form-factor(SFF) transceivers, which could also be sourced at industrial operating
temperatures. Some re-working of the card’s heatsink and PCB for the change inmechanical size and footprint were required to accommodate this.
For the test procedure a vibration test fixture was specially built for the purpose
characterising the AMC prior to chassis integration. The test fixture allowed thefull functionality of an operational AMC to be verified during vibration andenvironmental tests, with a focus on backplane signal integrity.

Figure 2: Vibration test jig in the lab, prior to being sent for conformance testingOperating and Storage Temperature

Operating and storage temperature are defined by two test methods: hightemperature in Method 501.5 and low temperature in Method 502.5. These
determine the safety, integrity and performance of equipment under the effects
of extreme temperatures. The customer application required storage for -40ºC
to +55ºC and operation between -20ºC and +55ºC. These requirements areexceeded by the MicroTCA.1-XT1L specification, which has a lower operating
temperature requirement of -40ºC.
To ensure compliance, we first checked the bill of materials (BoM) datasheets
against the storage and operation requirements of the customer. As a result,several components were upgraded to their industrial operating temperature
equivalents, but no redesign was required. We conducted pre-compliance testingin our own environmental chamber to ensure full functionality before sending theequipment to an external test house.

Relative HumidityOur requirement was required to support a maximum relative humidity (RH) of95%, non-condensing, in accordance with Method 507.5 Procedure I. This
procedure sees how well the equipment can survive in a warm, humid
atmosphere during storage and transit, and during natural environmental cycles.
This exaggerated environmental test with exposure to continuous high humidity
at cycling elevated temperatures, comprises three 24-hour cycles representing
storage and transit conditions followed by three further cycles that replicate thenatural cycles. Ideally, the customer should specify a RH and temperatureappropriate to their operational environment, but it is more common to use oneof the standard test profiles.
MicroTCA.0 has a RH specification of up to 85% RH under normal operation and90% short term. MicroTCA.1 states that the specification will be applicationdependent. After a design audit, and the addition of the conformal coating, wefound that to meet the RH requirements no changes were required for the AMC.
We ran the equipment through our own environmental chamber at theprescribed RH and temperature profile prior to formal certification at an externaltest house where the actual temperature cycling was also extended to 240 hours
for an increased margin of confidence.

Conformal Coating

MIL-I-46058C, Insulating Compound (For Coating Printed Circuit Assemblies), isa material standard which ensures that a coating meets a specific list ofperformance metrics. Despite the specification being made inactive in 1998, it
remains invaluable as the only conformal coating standard with a qualified
product list. Conformal coatings are good practice where long-term exposure tohigh humidity can cause moisture ingress into electrical component packages
where swelling and delamination lead to early life failures.
Compliance is not simply a case of applying a conformal coating: where, how,
and when need to be determined. Firstly, a review of the BoM datasheets wasrequired to identify which components had coating restrictions. This wasfollowed by a PCB review to identify where electrical conductivity was required

on the card, for example, the AMC edge connector. Having decided to apply thecoating by spraying, this enabled us to create a card spray mask.
It was decided to apply the coating after factory acceptance test to ensure any
testing or re-work was not hindered by the coating. A final test after applying
the coating is made to ensure the coating process has not affected cardperformance.

Conclusion
The MicroTCA and AMC architecture is an easily ruggedized form factor andprovides a versatile ecosystem for a wide range of applications. MicroTCA.1
makes ruggedization and compliance to military specifications a natural
progression from MicroTCA.0, allowing low cost development in the lab toseamlessly move to field trial. With our in-depth knowledge of engineering for
aggravated test environments, we have demonstrated that COTS AMCs can beruggedized for field deployment in challenging military projects.

About CommAgility
CommAgility, a Wireless Telecom Group company, is an award-winning
developer of embedded signal processing and RF modules, and LTE PHY/stacksoftware, for 4G and 5G mobile network and related applications. CommAgility
designs the latest DSP, FPGA and RF technologies into compact, powerful, andreliable products based on industry standard architectures. CommAgility’s LTE
software for mobile devices and wireless infrastructure includes physical layerand protocol stack for small cells, physical layer and protocol stack for terminals,
an advanced scheduler for small cells, and IP development in the areas ofadvanced PHY algorithms in multi-core SDR platforms. See
www.commagility.com

About the author
Paul Moakes PhD CEng MIET is Chief Technology Officer at CommAgility. He has
previously held positions at Motorola and Blue Wave Systems. He is co-inventorof two patents in the field of MicroTCA and AdvancedMC. He holds a PhD in
Electrical and Electronic Engineering from Sheffield University and a 1st Class
Honours degree in Electronic Communications and Computer Systems
Engineering from Bradford University.
© Extension Media, 2018. This is the author's version of the work. It is posted
here by permission of Extension Media http://www.extensionmedia.com