Evaluation of COTS Diodes for Long Term High Reliability - - PowerPoint PPT Presentation

SSL  20 June 2018

Prepared by: Space Systems/Loral, LLC 3825 Fabian Way Palo Alto, CA 94303-4604 USA Prepared for: NASA Electronics Parts and Packaging Program 2018 Electronics Technology Workshop

Evaluation of COTS Diodes for Long Term High Reliability Applications

James Loman June 20, 2018

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Overview 1. We determined it is technically feasible and practical to use some limited commercial parts on large GEO spacecraft with 15 year life 2. We showed the critical risks can be reasonably retired 3. We now have our highest volume EEE part- a diode- as COTS 4. We found a high barrier to entry which limits opportunities for existing GEO programs 5. New design, small satellites at LEO represent better opportunities for use of COTS

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EEE Parts Overview- Large SSL Geo Spacecraft

200K EEE Parts per Large Spacecraft

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Diode Project Project Objective: Use a commercial alternative for the most used Hi-rel diodes without impacting performance and reliability – Must be equivalent form, fit, function, reliability, performance – Must be drop-in replacement (no tray level redesign) Part types usage – Use 16,000 pieces of the most common diode per S/C- replaced half of these – Second most common type- 8000 per spacecraft – Many other opportunities to replace diodes elsewhere, but the return on investment is less and may not be worthwhile as a design change – COTS parts orders of magnitude less expensive

The simplest parts with the most savings and the least impact

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Selected Commercial Diodes Selection criteria utilized – High Volume Commercial Process Control benefits – Same or similar electrical characteristics as current flight diodes – Drop-in replacement package only (no re-design) – Automotive grade preferred = qualification and compliance to AEC Q101 standard

Hi-rel Type COTS TYPE Mfg. Grade V rated I rated Package Fit Most Used Diode

1 A

Automotive 75 V 250 mA Fits ok for all applications

2 B

Commercial 75 V 250 mA OK for some applications 2nd Most Used

3 B

Automotive 200 V 2 A Fits ok for all applications

4 C

Commercial 150 V 2 A Fits ok for all applications

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COTS Vendor Information Amount of info & quality of correspondence varied from vendor to vendor. The goal was to obtain reliability & quality info, qualification & screening process description, product change handling, and other details usually available from Hi-rel vendors Vendors A & B would only communicate through local sales office – Provided general standard answers about their processes but no specific details Vendor C was forthcoming – Provided details about engineering evaluations, summary of environmental testing for the part & estimated failure rate value In conclusion, communication is possible with COTS vendors, but we need to rely on

• ur own evaluation data

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Diodes DC Electrical Parameter Test Results Testing performed on each type: – DC tests at ambient, hot (125 or150 deg C) & cold: (-65°C) on ~250 pieces/ type – Tests were a combination of vendor data sheet tests and Hi-rel tests (where different conditions apply) Testing results summary – No part failed any parameter, except

• One part tested out of spec during initial test, but tested in-spec after rad

testing, so likely it was a bad measurement – Measured values were well within limits – The data compared quite well to the observed Hi-rel data – Some parameters had dual modes indicating mixing of two wafer lots – not a major concern

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DC Electrical Testing Results Summary All diodes performed within their respective Data Sheet limits. Compared to Hi-rel parts, the commercial diodes had tighter distributions and equal or better values for most parameters – Breakdown voltage: higher and standard deviation ~1/3 of Hi-rel parts – Forward voltage: similar to Hi-rel – Leakage current: much lower (>10x less), tighter distribution – A few obvious outliers existed, however all parts well within spec – Forward voltage & leakage notably better at Hot Temp (+125°C)

COTS diodes data shows equivalent or better performance than the Hi-rel parts

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Materials Outgassing Test The diode encapsulation materials were separated and outgassing tested. All 4 types tested passed outgassing measurements with large margin. Max Total Mass Loss (TML) was 0.267% <<1.0% specification All four types had Vacuum Condensable Materials (VCM) << 0.1% specification (typical 0.000%)

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Radiation Performance

Pre- and post-radiation electrical performance measurements – All pass initial electrical limits per SSL SCD (JANS equivalent) – All pass electrical limits in the vendor datasheet – Standard radiation testing method to 300KRad – All diodes were tested and all passed the post-radiation test – Small parameter shifts were seen, but all were well within specification

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Constructional Analysis

Constructional Analysis was performed on 5 pcs from each sample group (total 20 pcs) – External Characterization: (OK)

• Package photographs and dimensions

– Dimensions were measured for each part per vendor datasheet. – All measurements were tabulated for each diode part serial number.

• Package weights were measured and tabulated
• Package, Lead and Lead plating materials were identified

– Radiographic Characterization: (OK)

• Typical radiographs (3 axes), exposure parameters recorded

– Internal Characterization (1 device per part number):

• Plastic encapsulant removal

(OK)

• Photo-documented after removal of package
• Interconnect structures examined

– Leads and/or Lead Frame: Dimensions, materials – Wire bonds: Wire size, material, and bond types

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Constructional Analysis

– Internal Characterization (Continued): (Issues)

• Overall photograph of exposed die and leads (identified Sn)
• SEM photograph of die and leads
• Overall die dimensions
• Photographs of typical structures
• Die bond method and materials (identified Cu wire bonds or lead frame)
• Die metallization system/materials/thicknesses and etch method (unique structures)

– Cross-Sectional Characterization (2 samples per part number): (OK) Sample 1 - Cross-section along the x-axis halfway through the device Sample 2 -- Cross-section along the y-axis halfway through the device

• Photographs of cross-sectioned parts
• Part construction/materials/joining methods
• External lead to package seals
• Leads / Lead Frame material/plating, thicknesses
• Typical diffusion structures/depths
• Isolation methods/structures

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Constructional Analysis- Example

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Constructional Analysis

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Diode Environmental/Qual Test Plan Three best fit qualification documents were compared for plastic/commercial parts: NASA, ESA and Automotive Council (in that order) – NASA PEM-INST-001, ECSS-Q-ST-60-13b, & AEC-Q101 – AEC tests all utilize a 77pcs sample size (3x consecutive lots) – Larger sample size needed to demonstrate equivalent reliability to space grade parts – Obviously “Not Applicable” tests excluded: Cavity parts only tests: hermeticity, wire bonded part tests, die shear, vibration/shock/constant acceleration

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Reliability Life Test Plan & Failure Rate Purpose: – Demonstrate that commercial diode has a failure rate comparable to the currently used space grade (Hi-rel) diode Approach: – Accelerated life testing was used to reduce test time using temperature and voltage as acceleration factors – This approach is well-established and accepted in both the space and commercial industries – To demonstrate failure rate (FR) of 1 FIT (failure in time, 1 FIT = 1 failure/109 hours) without accelerated testing – 1,000 devices would need to operate for 106 hours or 114 years Accelerated Testing Conditions: – Targeted FR = 0.85 FIT @ 67°C operating junction temperature – Test conditions are TA = 150°C, VR = 80% rated (HTRB test) – 538/0 pcs must be tested for 1,000 hrs OR 269/0 pcs for 2,000 hrs

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ROI Considerations Even Hi-rel diodes are a relatively low cost part Savings on a per-part basis may not overcome fixed costs involved unless the volumes are significant (thousands used) – Manufacturing assessment- determine fit, function, pure tin (Sn) assessment – Design assessment- will it work the same over temperature, revisit Worst Case Analysis? – Switching tests are needed for certain applications – Test in breadboard or EM- need to show that it works – Change drawings, bills of material and part numbers – Unit level testing – Brief customers & request contractual relief

High barrier to entry for changing a part in exiting designs

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Test Plan Results- Highest Use Case Commercial Diode (Type 2) – Passed all tests, and better electrical performance than Hi-rel diode – Does not fit physically in all applications due to slightly smaller size – To accommodate other applications, would require board re-layout Automotive diode (Type 1)- Two issues – Several failed during the life test. Unknown if it was intrinsic to the diode or a testing issue – A second issue was found during Intermittent Operating Life (IOL) testing

• Solder joints did not hold up under thermal stress
• The root cause was insufficient strain relief due to their pins configuration.
• No practical solution to this problem seemed worth pursuing

Commercial diode in this case proved more reliable than automotive, but limited application due to size

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Manufacturability Assessment Plan Test Plan – 10 samples of each of diode types – Put through automated manufacturing flow – Temp cycling (simulate oven cures) – Microscopic inspection and photo document – Hand de-solder diodes – Electrically test diodes – Hand solder / de-solder diodes (simulates rework) – Electrically test diodes – Microscopic final inspect and DPA

Testing was completed with no issues

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Application Assessment Plan

DC Analysis: Analyzed the selected commercial diode vs the space qualified diode. The following parameters were reviewed: Breakdown Voltage (Vbr), Forward Voltage (Vf), Average Rectified Forward Current (Ifav), and Power Dissipation (Pd) The highest use case unit allowed us to test up to the maximum current allowed The highest use applications did not depend on dynamic switching characteristics. By serendipity, only the lower volume applications required dynamic testing All the space qualified diodes in the highest volume units were replaced by the commercial diodes Full unit level functional tests over qualification temperature levels were performed

Testing was completed with no issues

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Other issues addressed HALT Test – Decision was made to do Highly Accelerated Life Test – Built confidence and demonstrates performance margins over larger temperature, vibe and combined environments – No issues uncovered Storage Considerations – Humidity concerns due to plastic encapsulation – Performed humidity – pressure HAST test at part level – No issues uncovered Screening of parts – Elected to do small sample screening only, on a per reel basis – Full upgrade screening would negate cost savings, and is not necessary

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Other issues addressed Dynamic Parameter Testing – This has not been performed – Not needed for applications implemented – Would be needed for other applications Vendor Audits are not possible – Rely on our screening on a per roll basis to ensure no changes Obsolescence – Has not been an issue so far, but the parts could be stockpiled if needed “Prohibited” materials - Pure Tin – No whiskers found despite stringent test – Tin/Lead eutectic solder use in-house mitigates any whisker problem – All commercial parts examined used “pure tin” finish, but a very thin matte (not likely to grow whiskers)

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Lessons Learned There were many lessons learned during this project

• 1. “Drop in” replacements for space grade parts are difficult to find. Extensive use of

COTS could require redesign and requalification. It’s probably easier to use COTS in a new design rather than retrofit an old design

• 2. Some vendors of COTS parts produce a very high quality, high reliability part, that can

be suitable

• 3. Due to low cost construction techniques, some COTS parts may not be suitable for

long term use

• 4. Automotive parts are not always better than commercial
• 5. Opportunities abound to provide more affordable systems, but it’s very important to

do a full qualification to avoid reliability problems