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Keys to Selecting Military/Aerospace Boards
May 23, 2012 |Estimated reading time: 7 minutes
Mil/aero boards must meet the MIL-PRF-31032-1C fabrication standard and assembly standard IPC 610 Class 3 Rev. C. Beyond complying with these standards, you may likely consider mil/aero PCBs characterized by ratcheted-up specifications. In effect, a “beefed-up” PCB, with either through-hole or surface-mount components, means the EMS provider literally increases board specifications beyond the OEM’s minimum engineering specifications so that the board, and its end product, maintain high-performance and optimum reliability, regardless of environmental, terrain, or temperature conditions. Successfully going above and beyond a mil/aero OEM’s board specification to assure optimum reliability is founded on design and manufacturing experience since no textbook examples exist from which to learn. This extra edge on reliability assurance comes at a time when military and aerospace applications are trending toward smaller, less cumbersome communications gear like personal hand-held communications devices and mil/aero ruggedized versions of tablet computers (Figure 1). Figure 1: Mil/aero applications trend toward small, but ruggedized, versions of tablet computers.
Punching in Extra ReliabilityThree areas for injecting additional reliability are at board layout design, fabrication, and assembly with various key steps at these stages providing an extra 5 to 20% reliability factor, as the examples show in the chart below. In all cases, it is important for the EMS provider to consult the OEM customer to explain the importance of increasing a board specification and to obtain permission to proceed with a revised specification. Figure 2: Going above and beyond the call: Increased PCB reliability can be added at key points at layout design, fabrication, and assembly stages.At design, for example, let’s say a board may be rated at five amperes. However, during pre-PCB layout simulation, it’s best to add a 30 to 40% buffer. Thus, operational amperage is increased to 6 to 7 amps just in case the board is exposed to extraordinarily hot, rugged, and/or hostile environments as it reaches the limits of its original specification. Also, if such a board is specified at six layers, an additional two layers should be considered to provide extra ground planes, if there is a chance of crosstalk occurring between different layers. The reason is to ensure clear signals with no crosstalk or mixed signals. The more solid the ground planes, the better the signals are separated. For example, if there are split planes on one layer, they don’t provide a solid ground for traces to transmit signals. By designing in additional amperage and adding more ground planes, reliability increases by an estimated 15 to 20%. Extra grounding and shielding on critical traces is also important to boost reliability by approximately 15 to 20%. Figure 3 shows how a group of devices are shielded in an RF application using aluminum shield. Particular RF signals demand extra grounding and shielding to protect a digital signal going from point A to point B, for example, and to make sure it doesn’t get distorted by analog signals running on the board. Figure 3: An aluminum shield provides additional protection for a group of circuits. FabricationThe military spec MIL–PRF–31032-1C for fabrication boards has tighter requirements for tolerance toward acceptability of the board. Drill wander on board drilling stage, for instance, is considerably more tightly controlled for mil-spec boards. One can apply these specifications to commercial boards at a slightly higher cost, but this ensures the boards will be tested and will give better yields during in-circuit and functional testing. It also minimizes the chances of defects such as half moons on vias and cracked via barrels.The tolerances are tight in the fabrication process, which result in improved yields. By deploying the military standard as a typical environment for board layout, fabrication, and assembly, greater results occur for both reliability and yields. For example, clean layout design will achieve virtually perfect fabricated boards with tighter military standard requirements and, during fabrication, will assist in providing better yields. Key Steps During AssemblyAt assembly, a number of key steps are enhanced to increase board reliability. In particular, high-reliability assurances are demanded for growing numbers of the small and portable mil/aero applications moving to ultra-fine pitch 0.3-mm micro-BGA and micro-CSP-packaged devices to comply with shrinking size of these boards (Figure 4).Figure 4: Ultra-fine pitch 0.3-mm micro-BGA and micro-CSP-packaged devices comply with shrinking size requirements of mil/aero boards.
Now consider that with the pitch decreasing, ball size is reduced. As a result, standoff height between the board’s surface and chip package is decreased. Hence, the shorter stand-off height reduces PCB-level reliability for a package. Therefore, fine-pitch micro-CSPs and micro-BGAs have difficulty meeting mechanical shock and substrate flexing tests for portable electronics applications. Savvy contract manufacturers and EMS providers are finding that underfill provides the right assembly step to resolve this issue. Underfill is a polymer or semi-liquid applied on the PCB after it has been subjected to reflow. Generally, it is dispensed on a corner or in a line along the edge of the micro-BGA or micro-CSP. Other key steps include applying a special compound called RTV to insulate heat sinks, soldering swages to the pad and PCB, pre-tinning stranded wires, extra spacing between components and the board, and conformal coating. RTV compound after it is properly cured insulates tall through-hole components from colliding with other similar components on the board. But, more than that, it reduces vibration, which can create cracking on the lead solder formations. Applying this technique makes assembly an estimated 5 to 10% more reliable. Flange mounts are normally press fit onto the board for commercial applications. However, with a mil/aero PCB, once flange mounts are press fit, each swage is hand-soldered to the pad and the PCB. This assembly step minimizes vibration and in doing so, the reliability at these points on the PCB is increased by an estimated 10%. Pre-tinning is another significant assembly step. It’s a process that involves dipping stranded or braided wire into hot melted solder, removing it, and drying it so that the solder formation solidifies. In effect, soldering transforms stranded wire into one solid wire to eliminate air gaps and to allow current flow to be more stable. Thus, electromagnetic interference (EMI) effects and attenuation are reduced and reliability at this point is increased by approximately 5 to 10%. Next, if certain components dissipate more than one watt of heat, the components can be mounted higher from the surface of the board with a spacer between the heat sink and the component. This produces the desired spacing, allowing component heat to be thermally dissipated, rather than permitting it to adversely affect the PCB surface causing the solder mask to peel. This spacing also acts as stress relief so that if the PCB and its end-product are smashed or dropped in extreme conditions there will still be sufficient clearance to protect components and keep the product operating. By applying this spacing, an estimated 15 to 20% reliability increase is realized. Further quality and reliability assurance is applied when PCBs with through-hole components undergo wave soldering. But here it’s important to perform a first-article profile. A first-article board is run to inspect it for process and component placement verification prior to running the actual job. This practice run ensures that thermal profiles are properly reviewed, correctly dialed in, and all the kinks have been worked out. If this step is bypassed solder joint cracks in SMT components or cracks on the BGA balls can occur (Figure 5). Figure 5: Thermal profiles must be properly reviewed, correctly dialed in, and all kinks worked out to avoid cracks on BGA balls.
The objective is to get to the correct soldering temperature to make sure there is more than sufficient solder wicking up to the board. This ensures solid and reliable connections and provides an estimated 5% increased reliability. Some mil/aero products are frequently subjected to long and continuous exposure to water, moisture, and extreme humidity and temperature. PCBs in these products can be more reliable by an estimated 10 to 15% by making sure they have sufficient conformal coating applied with high-quality acrylic-based sprays to protect them from such harsh elements. Take, for example, a motherboard for an underwater mine-detection device. This particular military product remains underwater for most of its life. To assure reliable operation, sufficient PCB design and assembly precautions must be taken to protect its PCBs and circuitry from even the smallest amount of moisture. Lastly, another assembly step to increase reliability by an estimated 10 to 15% is to increase solder paste volume by 10 to 15% more per square inch of area unit compared to commercial PCBs. This, of course, would depend on the type of components and is mostly applicable to analog components. Also, for acceptable quality levels (AQL), mil/aero boards must undergo 100% inspection to assure 100% reliability compared to the commercial AQL standard of 0.65. To meet MIL standards, PCBs must have zero defects. About the author: Zulki Khan is the Founder and President of NexLogic Technologies, Inc., in San Jose, California, an ISO 9001:2008 Certified Company, ISO 13485 certified for manufacturing medical devices and a RoHS-compliant EMS provider. Prior to NexLogic, Khan was General Manager for Imagineering, Inc., in Schaumburg, Illinois. He has also worked on high-speed PCB designs with signal integrity analysis. He holds a B.S. in EE from NED University in Karachi, Pakistan, and an M.B.A. from the University of Iowa. He is a frequent author of contributed articles to EMS industry publications.