White Paper

WHITE PAPER Enhance product performance under extreme environmental conditions mrcy.com 3 The scope and rate of temperature change for a processing system housed in an unprotected pod under the wing of the aircraft would be extreme. This cycling occurs many times in the life of deployed electronics equipment such as radar or EO/ IR processors. Using a non-eutectic solder under such conditions can result in cracks in the solder joints (see Figure 1) and is simply not a viable option for these systems. To mitigate this concern, Mercury 's MOTS service replaces lead-free BGAs with eutectic tin-lead (Sn/Pb) solder spheres on the product BOM whenever commercially available. Ceramic BGAs may have 90% Pb/10% Sn non-collapsing solder spheres. For BGAs available only with SAC solder spheres, Mercury sends these BGAs to an approved contractor for re-balling. As a result, the solder joints can withstand many hundreds of extreme temperature cycles, as shown in Figure 2. Any land grid array (LGA) components (such as Intel® Xeon® Scalable processors) are regularly converted to BGAs before use in Mercury products, and only leaded solder is selected for this LGA conversion. Conversion of LGAs to BGAs also greatly improves tolerance to vibration by eliminating the socket. The number of components treated by Mercury 's MOTS service can range from 10–100 components per design, and from smaller pin count BGAs to large LGA devices with a pin count greater than 3000. For products designed for MOTS, this conversion is built-in to the manufacturing process so that it proceeds without manual intervention should the customer order the MOTS variant. COMPONENT UNDERFILL Another necessary ingredient for durability through thermal cycling is structural component underfill. Mercury has conducted extended thermal cycle testing on a wide variety of products, both with and without underfill. These products include the latest socket in the Intel® Xeon® Scalable processor family, which is an LGA with 3647 contacts converted by Mercury to a BGA. The results are clear: structural underfill extends durability significantly. Components without underfilled BGA components failed before completing 1000 thermal cycles, while underfilled components remained stable well beyond 1000 cycles. Mercury supports a wide variety of underfill materials, selecting the appropriate material based on component material and construction, pad geometry, ball spacing and other module design parameters. Customers with specific underfill materials required can be accommodated in our manufacturing flow as well. One 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 components as close together as routing allows to maximize usable space. However, when designing a module where BGA underfill is required, proper spacing must be maintained. If a module design does not incorporate this requirement upfront, costly design changes and requalification efforts can add technical and schedule risk to a program. All of Mercury 's OpenVPX module designs incorporate the necessary spacing to support underfill for all required BGAs, ensuring these risks never materialize and enabling MOTS services and techniques to be applied to any standard product without requiring a re-spin of the module design. Figure 2: 84 Ball MOTS SDRAM (tin-lead solder): solder joints after 750 thermal cycles Figure 1: 84 Ball COTS SDRAM (lead-free): solder joint crack after 250 thermal cycles

• Cover