Spotlight: Space Enhanced Plastic products for NewSpace – with Texas Instruments

Space enhanced plastic (Space EP) products are radiation-tolerant, plastic encapsulated microcircuit (PEMs) for commercial low Earth orbit (LEO) applications or missions with shorter lifetime requirements.

This article is an introduction to Space EP manufacturer Texas Instruments (TI), a participant in the satsearch membership program, and was produced in collaboration with the company.

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An overview of integrated circuit quality levels

Established quality levels among high-reliability integrated circuit (IC) products help to facilitate the choice of products for a specific application. Product quality requirements differ significantly for different applications such as automotive applications or space missions.

The various quality levels of products include:

• Qualified manufacturers list class Q and class V (QMLQ/V),
• Commercial off-the-shelf (COTS) level,
• Automotive electronics council (AEC)-Q100, and
• Space enhanced plastic (Space EP).

QMLQ and QMLVproducts have ceramic packaging, are hermetically-sealed, feature single-controlled baseline flow and Al bond wires, and are produced to MIL specifications. However, although QMLQ products are military-graded, only QMLV products are radiation-tested.

QMLV products are highly reliable, with life tests per wafer lot, and are ideal for missions requiring longer lifetimes. Compared to QMLV, the additional sub-class of QMLV-RHA products are subjected to additional total ionizing dose (TID) lot testing to qualify as radiation-assured.  COTS and AEC-Q100 standard products are normally offered in plastic packaging, have either Au or Cu bond wires, and are sometimes supported by multiple silicon wafer fabs or assembly test sites thus do not have a single-controlled manufacturing baseline flow.

The rapidly evolving NewSpace industry has led to the use of PEM products, such as COTS and AEC-Q100, which are gaining traction due to their scalable and flexible supply, lower costs and smaller footprints.

However, there are an array of associated quality and reliability risks for such PEMs to be used in space, which has led to a demand for solutions such as radiation-tolerant Space EP components.

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The rigors of space

The radiation challenge

In the NewSpace sector, the requirements for lifetime, reliability, and radiation protection of products are considerably reduced versus QMLV-rated devices. Nevertheless, radiation exposure poses a critical challenge for PEMs, even for LEO missions or missions with shorter lifetimes.

Radiation tolerance is highly dependent on semiconductor and process technology, placing significant burdens on manufacturers who are not experienced in producing components for the challenging environment of space. Lot-to-lot variations in radiation performance can also be a concern.

Generally, COTS/AEC-Q100 products are neither radiation tested nor radiation assured. More often than not, it is difficult to find information about the radiation tolerance, component design, and fabrication process of a COTS/AEC-Q100 product. Therefore, identifying a suitable NewSpace product can be a time-consuming task.

Other potential risks

Significant variations in operating temperature range are also important factors to be considered and tested for when using PEM products in space. The tin plating, commonly used in COTS/AEC Q100 products, may lead to the growth of tin whiskers in harsh environments, which can short the metal leads and cause system failures.

COTS/AEC-Q100 products are not tested for extreme space flight conditions, such as thermal cycling or high vibrations and G-forces during launch. Particularly during thermal cycling, Copper wires with high-temperature coefficients tend to be more susceptible to bond neck breaks, as opposed to gold wires.

The organic mold compound of PEMs can also result in moisture absorption and organic compound outgassing, reducing the reliability of products in space.

Finally, if multiple manufacturing and assembly sites are used, variation in the performance of PEMs under extreme conditions can occur, due to slight differences in fabrication processes and resources used.

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Upscreening of IC systems

Upscreening is an important consideration when selecting an IC for use in space. Semiconductor vendors cannot provide guarantees on similarity between individual wafer lots with respect to radiation performance. Ensuring a flexible supply is usually judged to be more important than reducing lot-to-lot variations in radiation hardness.

In most cases, the radiation protection of a COTS part is not verified on an ongoing basis. Instead, the fabrication process is continually monitored and calibrated to account for drifts over time that impact the electrical performance. For automotive and industrial applications such variations and drifts will typically be identified during these tests in the foundry, and can then be rectified.

However, radiation performance would still be likely to vary, particularly for COTS components, where there are no direct tests to assess whether the protection of an individual component is within acceptable limits. Variability would be expected from fab to fab and from lot to lot, and, in some cases, even from wafer to wafer.

Therefore, engineers looking to upscreen components should make certain that parts are procured from the same lot, and with large enough sample sizes across lots to catch all outliers. But this is extremely difficult when procuring COTS components as it is not often possible to access the full details of the product’s origins.

To ensure the highest level of quality it is important to implement a single control baseline, which means there is a well-defined set of materials used and only one wafer fab, and assembly site, involved.

In addition, to meet quality levels for radiation-tolerant Space EP and QMLV-RHA, radiation lot acceptance tests are used, ensuring that any component leaving the manufacturing site meets the desired radiation hardness.

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Radiation-tolerant Space Enhanced Plastic (Space EP)

Developed as a solution to many of the risks commonly facing ICs in space, radiation-tolerant Space EP products are a unique line of PEM products developed by TI designed specifically for LEO missions or for missions with shorter lifetime requirements.

Space EP products have plastic packaging, either flip chip-mounted (with no bond wires) or gold (Au) bond wires, and do not use packages with high tin content to reduce the risk of tin whiskering. TI’s Space EP products have a single-controlled baseline flow with one wafer fab, one assembly line, and one material set, and have all the documentation to ensure lot traceability.

Most importantly, Space EP products are radiation tested and radiation assured. Different fabrication processes and alternate die designs are also adopted in order to achieve the required radiation performances as follows;

• The optimized material set, with die attached, mold compound, lead frame, and bond wire, is selected to maximize reliability,
• No high tin (>97% Sn) construction, such as terminations (e.g. SnAgCu solderballs and Matte-Sn plating) or internal package components (die bumps or substrate plating), is included, and
• Additional assembly processing is carried out, including 100% temperature cycle or 100% single-pass reflow simulation in lieu of temperature cycle.

Such manufacturing considerations and material uses result in more efficient ICs suited to the harsh space environment.

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Space EP testing and qualification

Compared to COTS/AEC-Q100 products, radiation-tolerant Space EP products are more rigorously tested for space conditions, such as extreme launch conditions and thermal cycling.

The products are subjected to standard parametric testing, with guardbands, to ensure effective performance at typical LEO operating temperature ranges (e.g. –55°C to +125°C). Components are also characterized for total ionizing dose, single event effects (SEE) and neutron displacement damage (NDD).

In addition, Space EP ICs are qualified as space-graded by subjecting them to the following additional testing and qualifications – not usually performed on standard COTS or AEC-Q100 automotive products:

• Wafer lot acceptance tests, using MILPRF-38535 QML Class V as baseline,
• Radiation Lot Acceptance Testing (RLAT) of at least 20 krad TID for each wafer lot, as per MIL-STD-883,
• One-time characterization testing of at least 30 krad TID, as per MIL-STD-883,
• Single event latch-up (SEL) characterization of at least 43 MeV-cm2/mg,
• Neutron displacement damage characterization testing of at least up to 1 × 10¹² n/cm2 (1-MeV equivalent),
• Outgassing qualification for each product, as per American Society for Testing and Materials (ASTM) E-595,
• Qualification to SMC-SO-11, and
• Qualification to Space and Missile Systems Center Standard (SMC)-S-011.

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Examples of Space Enhanced Plastic products

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To find out more about Texas Instruments’ space portfolio, please view the company’s satsearch supplier hub here.