Military upgrades march in step with modular design

Anthony Green describes the processes a manufacturer needs to implement to ensure mil-spec reliability for electronic control circuits

The pace of development of electronics in the military and aerospace market brings its own challenges for the manufacturer when undertaking major upgrades to legacy products that already meet regulatory standards.

This case study describes a defence electronics firm with a strength in airborne mission-critical systems, that required two separate electronic control modules for a new AESA radar system to be developed and manufactured.

The main control interface would enable communication with the aircraft’s pilot‑controlled electronics. The front‑end controller module was to emit electrical timing reference signals and manage the clock distribution for the antenna system within the radar. Both were part of an upgrade that would replace an older, more mechanical radar system that was proving unreliable.

Reliability

The customer had designated design for reliability as a top priority. The modules had to be suitable for use in military jet fighters where space and weight is a major consideration; to be able to cope with extremes of temperature, vibration and sudden shock from gunfire; and they had to be compact, lightweight, and electromagnetically shielded to minimise noise amid radar signals.

Once integrated into the avionics of the plane, the boards would deliver high-resolution radar at medium and long range. To achieve the maximum reliability possible the manufacturing design team reviewed and introduced a range of techniques that included careful selection of components and consideration of thermal expansion properties and tin whisker growth.

Careful PCB layout in relation to adjacent metal materials and the addition of conformal coating further mitigated the risk of tin whisker growth.

The design was developed to meet the de-rating guidelines submitted by the customer and the component specifications for voltage, power, frequency and thermal properties. Lightweight, reliable thermal relief techniques were employed that included selective conduction cooling to a cool wall. Potential hot spots were identified by thermal analysis, which lead to revisions to the PCB layout.

Standards

After an assessment of the mean time between failures (MTBF) the team made sure the product would reach the military standard MIL-HDBK-217B. Putting the antennas through failure mode and effects analysis (FMEA) defined which areas were potential reliability risks and lead to a re-design of the protective circuits to defend against threats such as lightning strikes.

By using signal integrity optimisation of critical high-speed signals and then slowing edges of certain signals, the team reduced the potential noise in the radar antenna, which improved reliability of the critical reference clock distribution.

Moving from the design stage, the modules were then manufactured to an IPC Class 3 standard using a tin-lead solder process; conformal coating; edge sealing and corner bonding of ball grid arrays (BGAs); and re-balling and hot solder dipping of components for tin whisker growth mitigation.
The final stage of the process was environmental stress screening, which was carried out by the customer before the modules were moved into inflight trialling. Both the electronic control modules are now in operation.

The manufacturing design team had to work closely with customers and, at times, stray from the script in commoditising the product. They had to look at how the legacy products had worked and re-configure the design to meet 21stC military certification, while looking to the potential for future upgrade of the components, including producing accurate documentation at every stage in the development and production process. Compliance with all regulations had to be achievable and all documentation has to tell a coherent story of equipment’s development and testing.

Co-operation between military and aerospace manufacturers and design engineers can bring products to market on time, on budget, with regulations met.

Anthony Green is director of engineering, EMEA at Plexus

Defence & Security Equipment International (DSEI) 2015 takes place at ExCeL, London, 15-18 September. The companies, systems suppliers and specialist electronics firms attending reflect the importance of electronics to military systems.

Alun Williams

from News http://ift.tt/1Va8X5m
via Yuichun