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Advantages of Mercury's Modified Off-The-Shelf (MOTS) Product Approach

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The number of components treated by Mercury's MOTS service can range from 10-100 BGAs and LGAs per design, and from smaller pin count BGAs to large LGA devices with a pin count greater than 3000. For products with the MOTS service applied, this conversion is built in to MOTS design documents and processes, so that it proceeds without manual intervention should the service be flagged as applicable to a particular design. Component Underfill Another necessary ingredient for durability through thermal cycling is structural component underfill. Mercury has conducted extended ther- mal cycle testing on a wide variety of products, both with and without underfill. These products include the new Intel® Xeon® Processor Scal- able Family (previously code named "Skylake-SP"), which is an LGA with 3647 contacts converted by Mercury to a BGA. The results are clear: structural underfill extends durability significantly, with non-under filled BGA components failing before 1000 thermal cycles, and underfilled components remaining stable well beyond 1000 cycles. Mercury sup- ports a wide variety of underfill materials, and selects the appropriate material based on component material and construction, pad geometry, ball spacing, and other module design parameters. A necessary consideration for underfill, beyond the choice of material, is the component spacing built-in to the module design. With module real estate at a premium, designers are naturally pressured to place com- ponents as close together as routing allows, maximizing usable space. However, when designing a module where BGA underfill is a known op- tion, ensuring proper spacing to support the application of underfill to the placed BGA must be considered and implemented. If module design does not incorporate this requirement up-front, costly design changes and requalification efforts can result, which adds technical and schedule risk to any program. All of Mercury's designs incorporate the necessary spacing to support underfill for all required BGAs on the module design, precisely to ensure that these technical and schedule risks never mate- rialize for our customers. This enables MOTS services and techniques to be applied to any standard product without requiring a re-spin of the module design. Mercury also offers the service to include a customer's specific under- fill via our MOTS+ customization service, which incorporates custom- er-specific requirements above and beyond Mercury's standard MOTS configurations. Gold Embrittlement Gold plated components provide another unique challenge during the solder process. When soldered, gold can dissolve into the eutectic tin- lead solder and create brittle solder joints. This interaction is known as gold embrittlement. SMT RF connectors are a good example where thick gold (50 microinches) is used to avoid fretting and corrosion issues, but when soldered to the PCB, runs the risk of having a weak solder joint that can fail prematurely in a rugged environment. Other devices, such as gold plated ceramic packages for crystal oscillators, can also have excessive gold at the solder interface. To mitigate the risk of gold embrittlement, when producing a MOTS variant of a product, Mercury conducts an analysis to ensure that only 3% gold remains (by weight) after reflow. The gold plating thickness is 3 verified using X-Ray Fluorescence (XRF), rather than simply calculated from the component data sheet, as the actual gold plating thickness has been shown to be up to three times (or greater) in some cases than the vendor's specified thickness. Both the volume of gold on the components leads and the tin-lead volume of the board pad is calculated, taking into account solder flux "burn off" and solder slumping, and the percentage of gold by weight on the component pad after reflow is calculated. When the 3% limit is exceeded, Mercury processes the devices through a sol- der dip and wick process to reduce the gold concentration prior to as- sembly, eliminating any chance of gold embrittlement. Tin Whisker Mitigation Due to a variety of potential health issues, many materials used in the production of electronic products have come under scrutiny. As a result, in 2002, The European Union (EU) enacted the following directives that restrict or eliminate the use of various substances in a variety of prod- ucts produced after July 2006: • 2002/95/EC Restriction of Hazardous Substances (RoHS) • 2002/96/EC Waste Electrical and Electronic Equipment (WEEE) One of the key restricted materials is lead (Pb), which is widely used in electronic solder, the terminations on electronic parts, and printed wiring boards. While these regulations may appear to affect only products for sale in the EU, many suppliers to the aerospace and high performance Industry in electronics are changing their products because their primary market is consumer electronics. Additionally, several states in the U.S.A. have enacted similar "green" laws, and many Asian electronics manu- facturers have recently announced completely "green" product lines. The restriction of lead use has generated a transition by many compo- nent part and printed circuit board suppliers from using tin-lead (Sn-Pb) surface finishes to pure tin or other lead-free finishes. Lead-free tin fin- ishes are susceptible to the spontaneous growth of crystal structures known as "tin whiskers", which can cause electrical failures, ranging from parametric deviations to catastrophic short circuits. Though studied and reported for decades, the interactions and exact physics behind tin whisker growth is not completely understood, and tin whiskers remain a potential reliability hazard. Mercury's MOTS approach is designed to provide overlapping mitiga- tions, based on the superset of recommendations from our customers. The goal is to minimize the tin whisker risk without degrading system performance, without incurring any unnecessary program costs or schedule delays. In specific terms, Mercury complies with the Govern- ment Electronics and Information Technology Association (GEIA) GEIA- STD-0005-2, Level 2B: Standard for Mitigating the Effects of Tin Whis- kers in Aerospace and High Performance Electronic Systems. One of the basic treatments performed on modules designed for rug- ged deployment is the application of conformal coat. Conformal coat provides protection from moisture and abrasion and acts as a barrier against tin whisker growth. Mercury supports multiple coating materi- als, from acrylic to Parylene, but the default for our standard product is Mil-I-46058C and IPC-CC-830 compliant urethane. Experiments have demonstrated that in rare circumstances tin whiskers can tent or even project out of a thin layer of conformal coat, but cannot re-penetrate the coating. While this affords some protection against tin whisker growth,

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