 Precision guided weapons have made a significant impact in recent armed conflicts. These weapons offer increased accuracy resulting in the destruction of a greater percentage of enemy targets along with lower probability of collateral damage. Extending this precision guidance capability to munitions fired from Navy ships would allow precision strikes on enemy targets from a position overthe- horizon. Like many organizations, the military looks for ways to do more with less. Smaller and smarter munitions are becoming the candidates for in-flight guidance, such as the plan to use the Global Positioning System (GPS) guidance in Army artillery shells and in Navy deck gun projectiles. Microelectro-Mechanical System (MEMS) based inertial measurement units (IMUs) provide needed inertial guidance to the munitions complimenting GPS. However, advanced weapon systems must also meet stringent mission requirements in the areas of lethality and especially in the areas of extending gun survivability from 30,000 g to 40,000 g forces for future guided munitions. High speed launchers such a rail-gun will raise the survivability bar to 50,000 g in the future. Therefore, it is extremely important to develop packaging techniques that allow electronic modules to withstand the high-g forces encountered during a gun launch. MEMS-based IMUs can provide the required accuracy and small size at an affordable cost while surviving the high-g environment of gun launches.
A U.S. Navy ManTech program, supported by Office of Naval Research (ONR) and Program Executive Office for Integrated Warfare Systems (PEO-IWS) that focuses on improving the manufacturing technology and supply base for MEMS based IMUs for use in precision guided munitions, was initiated in 2004. Both BAE Systems and EMPF have participated in this program. To date, BAE Systems delivered several prototype IMUs (SiIMU02) to the Navy for evaluation. ACI has been managing this program for the U.S. Navy as well as performing related technical work in the form of IMU and MEMS storage life testing as well as a high-g design and packaging evaluation. The photo above shows a currently developed IMU unit from BAE Systems. The BAE units have survived initial high-g tests and are considered the finalists for Excalibur Low Rate Initial Production (LRIP). It is the desire of both Extended Range Guided Munitions (ERGM) and Excalibur to have a minimum of two viable MEMS IMU suppliers in order to ensure price competition and reduce delivery risk. Therefore, the development of an IMU supply chain is critical. It is also advantageous to have the same IMU suppliers for multiple precision guided munitions (PGM) programs in order to attain higher overall volumes and to reduce IMU pricing. It is expected that the IMUs produced by BAE will meet the reliability demands of DoD precision guided munitions programs. The SiIMU02 is expected to exhibit a higher performance and a more versatile form/fit than the current BAE SiIMU01 product, while retaining all the key advantages offered by the SiIMU01 model. This will make it available to a wider range of insertion opportunities, thus allowing it to be mass produced at a lower cost. Initial test reports indicate the SiIMU02 has shown the desired high-g survivability. This Navy ManTech program has successfully improved the produce-ability and reduced the manufacturing cost of these IMUs.
In order for IMUs to survive shock forces during the gun launch, these components will be extensively tested during the qualification trial. In general, shock is defined as a sudden change that affects the location, velocity, acceleration or forces in a structure. A blast or shock wave due to a near-miss explosion can obviously cause sudden deflection and high strain rates in electronic components and printed wiring boards, but this is by no means the only type of shock loading engineers must be concerned with. To identify the major failure modes of electronic systems under shock, EMPF has been compiling information from government agencies, IMU manufacturers, and research groups. The failures occurred in electronic systems under shock are primarily due to: high stress, which can cause permanent deformation; crack extension or failures; large deflections, which can cause collisions between objects such as adjacent wire-bonds, components, and circuit boards. Even though many packaging failure modes exist, only a few of them have been identified by researchers and users in the high-g packaging community.
Through an investigation with the support from several companies and researchers, two major failure modes were identified:
- Component/soldering pad lifted off from the circuit boards
- Cracked components
For example, two design versions (flip-chip and wire-bond) of digital portion of GPS receiver multi-chip modules (MCM) were tested at an artillery lunch condition of 12,000 g; the packaging limits identified were associated with standard surface mount (SM) components. The SM components (without encapsulation) showed selective failure where the circuit pads lift off the circuit board substrates. Only high mass components, which had sufficient inertial torque at the mounting pads, exhibited these failures. Other organizations have reported that cracks were observed in the capacitors and inductors at g-level reaches above 20,000~25,000 g in gun shock testing and other tests including centrifuge testing.
In order to conduct on-site high-g shock tests, EMPF purchased a Model 23 Shock Test System with a dual mass shock amplifier and a high-g shock accelerometer. This system has been installed. At the start of a shock test, an electric hoist raises the shock table until it reaches the programmed drop height, which was easily preset by the operator using the control system. The test table with mounted samples is then released and will make an impact with a program material to control the shock during on the base. At the same time, the shock pulse will be recorded by an electronic data acquisition system. A seismic base provides a precision impact surface and also isolates high shock loads from the floor and surrounding areas.
EMPF has created a design of experiment (DOE) model that will be used to evaluate the factors such as the component type, component size, lead pitch, component orientation on the board, the board substrate material, encapsulate, and under-fill material. The DOE was generated using a JMP program from the SAS Institute that utilizes algorithms based on providing the optimized design space for a given number of runs. Field test limitations only allowed the experimental design to provide 24 runs with 6 replications for a total of 30 runs, the equivalent of ¼ factorial. The four factors designed into the experiment are:
- Substrate – Two Levels
(FR4 and flex) The differences in material properties will allow for a robust assessment of rigid vs flexural behavior
- Orientation of the PCB in the Mold – Two Levels
( X and Y ) as an orientation to the applied force
- Encapsulant – Three Levels
Silicone, polyurethane, and epoxy resins will be used to determine if the Elastic modulus and other characteristic properties of those resins have an impact. The selection allows for a low, mid, and high range modulus values
- Underfill – Three Levels
A low and a high-modulus underfill will be utilized. The third level will not have any underfill, only the encapsulant.
 The model will allow detection of variability for all the main effects, as well as substrate, encapsulant, and underfill interactions. The design diagnostics show good D, G, and A efficiency, which essentially will capture variability for test points in the extreme ranges, as well as the interior points. The design is randomized, and blocks will be added for assembly and test sequences. The (Figure 1-2) shows a test board used at ACI with populated components. The boards will be subjected to either high-g drop tests at EMPF or later will subjected to air-gun shock testing at US Navy’s Dahlgren facility. EMPF, as the Navy’s Center of Excellence for Electronics Manufacturing, working with BAE Systems, has successfully reached a milestone with the delivery of four (4) cost-reduced, (MEMS)-based IMUs. MEMS-based IMUs are an enabling technology for PGMs. These units have been delivered to PEO-IWS for further evaluation. ACI continues to work with selected suppliers and manufacturing partners to apply electronics miniaturization to high-g DoD applications.
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