Public Lessons Learned Entry: 1803
Lesson Number: 1803
Lesson Date: 2007-7-31
Submitting Organization: JPL
Submitted by: David Oberhettinger
|CSAM Augments X-Ray Inspection of Die Attach (MRO Ka-Band Anomaly)
An in-flight failure of a hybrid amplifier used to derive the Ka-band downlink signal for Mars Reconnaissance Orbiter suggests that X-ray screening alone may not be adequate to determine the quality of coverage of the eutectic die attach. CSAM is a non-destructive internal inspection technique that can complement high resolution X-rays by isolating variations in material properties. When screening RF hybrids and other critical components, consider the use of CSAM to augment high resolution X-ray inspection.
Description of Driving Event:
After 6000 hours of nearly continuous in-flight operation aboard Mars Reconnaissance Orbiter (MRO), the Ka-band circuitry in the primary Small Deep Space Transponder (SDST-1) experienced an abrupt increase in power consumption. It abruptly decreased three hours later, permanently disabling the Ka-band downlink (References (1) and (2)). This was likely due to an internal short circuit within the metal-semiconductor field-effect transistor (MESFET) hybrid amplifier used to derive the Ka-band signal within the SDST, which in turn raised the junction temperature and caused an open circuit in the hybrid. The X-band hybrid amplifier in SDST-1, as well as the X-band hybrid amplifier in the backup SDST-2, may also have been overstressed.
A possible cause of the short circuit is a poor or failed eutectic bond between the hybrid device and the copper-molybdenum carrier. The process of mounting a semiconductor die/chip to a substrate or package is known as die attach. Methods include eutectic bonding (soldering) on ceramic or metal substrates, adhesive bonding, and glass bonding. However, soldering is preferred for high power devices because of its good thermal/electrical conductivity and ability to accommodate thermal expansion. A eutectic bond is formed by heating two or more materials in a joint such that they diffuse together to form a (eutectic) alloy. Manufacturing technology has not eliminated voids and disbonds in the bonding layers that can increase the likelihood of die cracking, increase the chip operating temperature, and weaken the die bonds. This hybrid device failure mode is a risk for other SDST builds, and some flight hardware for the Kepler and Juno missions may be replaced.
The primary method for non-destructive evaluation (NDE) of die attach fabrication is high resolution X-rays. Following the MRO failure, the eutectic (AuSn solder) bond X-rays used to screen the parts were reexamined and revealed no voids between the die and the package (Reference (3)). Subsequently, JPL employed an ultrasound inspection technique known as C-mode Scanning Acoustic Microscopy (CSAM). Unlike X-rays, which depict the integrated density of material between the X-ray source and detector, CSAM technology can examine a specific layer. This distinction is particularly important when looking for disbonding defects such as non-wetting of surfaces or delamination. Figure 1 is an X-ray of a spare MRO hybrid die; it shows 3 void areas on the left side of the chip, as well as an unfilled via, but does not show areas of poor die attach. In the corresponding CSAM image (Figure 2), the 3 large void areas are clearly visible, as well as some smaller ones. The CSAM ultrasound image provides enough detail that, after conversion to a digital image (Figure 3), a more quantitative assessment can be performed that states the total area of disbonds and voids as a percentage of the total die area.
Figure 1. Microfocus X-ray of hybrid amplifier die (S/N D209) with lid
Figure 2. CSAM image of hybrid amplifier die (S/N D209) with lid
Figure 3. Binary image converted from a CSAM image of hybrid amplifier die (S/N D221). The white boxes identify the most critical (i.e., the FET active) regions
The MRO failure investigation suggests that to assure adequate NDE of critical flight hardware, die attach X-rays may need to be supplemented with acoustic images. X-ray imagery can determine that the proper amount of AuSn solder is present between the die and the carrier, but it cannot determine whether the solder has actually wetted the surfaces. Where there is a complete lack of solder, (i.e., a void) the two imaging methods should give similar results.
(1) "Transponder DC input current and power increased for ~3 hours on DOY 14," Jet Propulsion Laboratory Incident Surprise Anomaly No. Z88615, May 31, 2006.
(2) "SDST - 3 Hour Power Surge," Jet Propulsion Laboratory Problem/Failure Report No. Z88763, June 30, 2006.
(3) MRO Small Deep Space Transponder Ka-Band Exciter Anomaly Final Report, Jet Propulsion Laboratory Document No. JPL D-31195, March 16, 2007.
For critical flight hardware, the current X-ray process for screening RF hybrids may not be adequate to determine the quality of coverage of the eutectic die attach. CSAM is a non-destructive internal inspection technique that can complement high resolution X-rays by isolating variations in material properties. CSAM detects the extent of air space-type physical defects in components, including cracks, voids, delaminations, and porosity, that occur during manufacturing, environmental test, and operation.
When screening RF hybrids and other complex RF parts, consider the use of CSAM to augment high resolution X-ray inspection.
Evidence of Recurrence Control Effectiveness:
JPL has referenced this lesson learned as additional rationale and guidance supporting Paragraph 184.108.40.206 ("Telecommunications System Design: Telemetry Links— Spacecraft Downlink RF Carrier") and Paragraph 220.127.116.11 ("Flight Electronics Hardware System Design: Electronic Packaging— Qualification of Electronic Packaging Designs") in the JPL standard "Design, Verification/Validation and Operations Principles for Flight Systems (Design Principles)," JPL Document D-17868, Rev. 3, December 11, 2006.
This lesson also supports Paragraph 7.5.4 ("Safety and Mission Assurance Practices: Electronic Parts Reliability, Application, and Acquisition") in the Jet Propulsion Laboratory standard "Flight Project Practices, Rev. 6," JPL DocID 58032, March 6, 2006.
Documents Related to Lesson:
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Project: Mars Reconnaissance Orbiter
Approval Date: 2007-10-30
Approval Name: ghenderson
Approval Organization: HQ