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    Flash Memory Reliability – An Insight into the World of Quality Testing



    Quality assurance is a corner- stone of electronics production and is an area where manufacturing companies the device and an electrostatically charged machine Each separate market (consumer, enterprise IT, or industrial) is different.

    Similar to LTOL but at the other end of the temperature scale to accelerate aging and characterize

    can fall short. One of the main causes of failure is a poor electrical connection between components and the PCB; with many hundreds of contacts, the assembly, interconnect technology, and

    printed circuit board all play an import- ant role. It is important for them to withstand frequent temperature changes, shock, vibration, and corrosion.

    For a deep understanding of the quality assurance and component testing process, this article looks to Swissbit.

    The Swiss manufacturer is renowned for durable, long-lasting flash memory modules for industrial applications. The strict testing methods employed at their Berlin plant serve as a detailed example of premium quality control.

    Comprehensive qualification with accelerated environmental testing is the first of a three-stage process of quality assurance. To ensure that a new product meets expectations, a large quantity of sample batches are tested, following standards set by JEDEC, Telefonica, AEC, IEC and others.

    Qualification stage 1 – electromagnetic compatibility

    The first stage includes Electromagnetic Compatability (EMC and ESD) checks.

    EMC testing ensures that the device’s own electromagnetic emissions remain below acceptable limits and that function is not impaired by external radiation. Three models of electrostatic discharge tests are applied:

    • Human Body Model (HBM) – discharge through a charged human body
    • Charged Device Model (CFM) – the discharge of the statically charged device
    • Machine Model (MM) – a replicated rapid discharge between ent EMC standards that components must pass before being considered for production.

    Qualification stage 2 – environmental testing

    The second stage encompasses environmental testing. This is essential to the development of durable products and utilizes accelerated stress testing

    to simulate more than fifteen years of operation under aggressive condi- tions. Testing is done in accordance with JESD94 (a standard written by the JEDEC Solid State Technology Association).

    There are five environmental tests undertaken at Swissbit:

    • Temperature Cycling (TC) •
    [-40°C to 125°C, 1,000 cycles] The rapid and wide temperature cycling, with heating and colling at rates up to 15K/min, places high stress on solder joints and verifies PCB design, soldering process, component packaging, and encapsulation and underfill effectiveness.

    • Temperature Humidity Bias (THB) • [85°C, 85% R.H., 1,000 h,

    supply voltage applied]

    Exposing the device to high temperature and humidity while the supply voltage is applied simulates operation under tropical conditions, provoking humidity-induced corrosion.

    • Low Temperature Operated Life (LTOL) • [-40°C, 1,000 h]

    To prove reliability of the electrical design, the device is operated at the lowest temperature with data patterns that correspond to typical real-world applications.

    • High Temperature Operated Life

    the degradation process.

    • Pressure Cooker Test (PCT) Testing the combined condition of high temperature, high pres- sure, and high humidity shows whether de-laminations occur in the structure, where moisture penetrates, and evaluates struc- tural destruction.

    Qualification stage 3 – additional stress tests

    Further stress tests form the third (and final) stage of qualification. A series of mechanical stability tests determine the robustness of solder joints and device construction. Tests at Swissbit include:

    • Subjecting device to a shock of 1,500g and vibrations with 50g acceleration
      • Impact on concrete and steel plates in free fall from heights between 80 and 150cm
      • Bending and torque tests up to three-times standard requirements
      • Accelerated harmful gas test (H2S, SO2) to check reliability of the open contacting points (plug, screw, and pin contacts)
      • Salt spray test for surface corrosion

    In-depth sample testing

    High quality products, such as Swissbit Flash Memory, are checked throughout manufacture with both invasive and non-invasive examinations. 3D microscopy with 5000x digital magnification is common – here, cross-sectional imaging allows the microscope to example the inner layers of the PCB, the vias, the solder joints, and the internal elements of a semiconductor component at the polished cut edges.

    Superior quality assurance includes analysis techniques that go beyond the usual procedure. Where visual inspections only reach the corner balls of BGA packages, the pins of TSOP packages and the passive parts, a 3D X-ray device allows inspection of the solder joints under the components, including layer information. Infrared microscopy / thermography can detect electrical short circuit hotspots and connections with an excessive current load. Acoustic microscopy (C-SAM, Scanning Acoustic Microscopy) is also a very informative non-destructive technique, using ultrasound at a very high frequency to produce images from inside an object.

    In destructive tests, Swissbit uses a scanning electron microscope to examine cross-sections, allowing impurities and cracks to be examined at chip level. For checking open solder joints, the dye-and-pry test can check several ball joints simultaneously. Here, the BGA is flooded with a low-viscosity red ink that penetrates all cracks and voices between the package and the PCB. Once dried and cured, the BGA package is peeled off the PCB and the PCB inspected; red ink on a pad indicates a bad solder joint where penetration has occurred.

    All these in-depth tests ensure that the design is proven robust enough for harsh industrial operating conditions. When it comes to series production, quality stands and falls with consistent production conditions.

    Leaving no one untested

    As structures become finer, solder paste printing errors are increasingly likely; solder becomes stuck in the stencil, too much solder may be applied, or solder will smear. Discovering assembly defects requires automatic inspection of each board as standard procedure.

    End of line checks with Automated Optical Inspection (AOI) use high-resolution cameras and image recognition to check the correct positioning of components. This makes it possible to determine correct assembly and detect visible PCB defects. Invisible defects, however, require Automatic 3D X-ray Inspection (AXI) that can check for open spots and short circuits, as well as wetting, size, and shape of the solder balls.

    In their creation of memory modules, Swissbit are one of the few manufacturers to employ AXI as standard for a consistently high product quality.

    Final inspection

    A final optical inspection by specialized personnel as well as fully automatic inspection machines is standard (as per IPC-A-620 Class 2 or 3), however, manufacturers seeking world renown for quality should exceed standard expectations. Selecting modules at random for function tests pro- vides some additional testing, but true excellence demands all products be tested in this final manner. As an example, Swissbit’s memory modules are practically tested at this ultimate stage without exception.

    High quality assurance standards belong with the manufacture of above-average products.