The goal is to develop new packaging techniques for downhole electronics that will be capable of withstanding at least 200oC (~400oF) while maintaining a small form factor and high vibration tolerance necessary for use in a downhole environment. These packaging techniques will also be capable of integrating a sensor and other electronics to form an integrated electronics/sensor module.
General Electric Global Research Center, Niskayuna, NY 12309 Binghamton University (SUNY), Binghamton, NY 13902
Sensors and electronics systems are key components in measurement-while-drilling (MWD) equipment. Examples of sensors and electronics that can directly impact the efficiency of drilling guidance systems can include gamma ray and neutron sensors, orientation modules, pressure sensors and the all of the associated signal conditioning and computational electronics. As drilling depths increase, more rigorous temperature demands are made on the electronic components in the drillstring. Current sensor systems for MWD applications are limited by the temperature rating of their electronics, with a typical upper end temperature rating of 175oC (~350oF). The lifetime of an electronics system at such temperatures is extremely short (600-1500 hrs). These limitations are driven by the temperature performance and reliability of the materials in the electronic components (active and passive devices) and their associated packages and interconnect methods.
Challenges in this regard exist from the board through the first and second level interconnects. At the board level, this research project will examine the use of polyimide flex as a board material replacement for the plated rigid polyimide boards typically used in standard electronics assemblies and that lack high temperature tolerance. First level interconnection failure due to wirebond reliability will be examined by examining high temperature compatible wire-metallization systems as well as new ways to make connections between a polyimide flex substrate and the active device. The possibility of reducing failures due to the temperature limitations of the plastic packages will be assessed by investigating the alternative of eliminating these packages by directly packaging the dies to the substrate.
Interfaces play a key role in the reliability of an electronic system, and their importance increases with temperature. Structural and metallurgical/chemical incompatibility between dissimilar materials leads to stress concentration at these interfaces, making them the most likely sites for failures to occur. This research project will address options for reducing the number of interfaces and enhancing the design of materials and geometries to reduce the stresses at those interfaces that remain.
The Research Management Plan and Technology Status Assessment have been completed. Coupled thermo-mechanical models of the stress developed in the polyimide flex have been developed. The results of these models have driven the design of validation tests to demonstrate the reliability at temperatures up to 250oC (482oF). The models identified two key areas for validation: copper adhesion to polyimide and via reliability. High temperature storage tests of polyimide flex at 250oC (482oF) with various tiecoat layers have been completed to gage the reliability of the copper adhesion. A Cr tiecoat layer showed no appreciable change in peel strength to the polyimide even after 1000 hrs at 250oC (482oF).
Validation of adhesion tiecoat layers on polyimide has been was completed. Five different tiecoat layer materials were evaluated at various temperatures and in nitrogen and air ambient. Chromium tiecoat layers fabricated at GE Global Research were shown to maintain a high peel strength after 1000 hrs in nitrogen ambient at 250°C. An extensive study of the dependence of peel strength by varying the Cr thickness was performed. From this study, 500 A Cr was downselected as the tiecoat layer for our study based on its superior reliability, ease of fabrication, and polyimide coverage.
Coupons for the testing the reliability of thru vias in polyimide were fabricated and tested. The test specimens have been tested at over 750 temperature cycles from 35-250C without a single failure.
In order to predict the fatigue life of solder interconnected passive devices, characterization of the material properties for five different high melting point solders was undertaken. In anticipation of tensile testing of wire samples, processes for crosssections of each wire was performed to understand the differing microstructures of the solders. These cross-sections indicate the grain structure of the solder which is an important indicator of strength. Nanoindentation was performed to compare the material properties of the solders in wire form. These results showed that the Sn-5%Sb solder to have the highest elastic modulus and good correlation for the results between the paste and wire specimens. Reflow processes for each of the pastes were developed for future reliability testing of the solder with actual components.
A stud bumping process for active devices was developed on flex and tested for reliability after exposure to thermal cycling from 35°C to 250°C. A Design of Experiments was performed on the stud bump process in order to understand the relationship between bonding parameters and reliability. Five coupons underwent testing with all five failing. The die failed at 200, 400, 500, 550 cycles. The difference in the failure tracks with the stud bumping temperature where higher substrate temperature equates to longer life. In addition, a cross-section was performed to determine where the failure occurred on one of the samples. It appears that the failure was at the die level metallization. The cross-section also showed some cracking in the die which may mean that there is too high of a force of bonding. Another set of dies is being assembled for further test using the higher stud bumping temperature and lower die bonding force.
Final Project conclusions include:
Ultimately, the results of this program should be demonstrated through a test vehicle which would be tested at high temperature and vibration. In order to manufacture and assemble such a vehicle, a design for such a vehicle must be made using the recommendations for substrate and interconnect methods from this program. The downselection of packaging methods is based on the conclusions of fatigue and reliability testing of the different packaging methods discussed in the preceding sections.
This study has led some basic conclusions about the design of flex-based packaging methods for high temperature environments. First, Kapton E has been chosen as the primary polyimide material for our application. Kapton E was chosen for its superior CTE match in the x-direction as well as its low CTE in the z-direction which is beneficial for via reliability. In addition, it is a commonly used material within the packaging industry which allows for easier adoption by the oil and gas community.
Two important modes of failure in flex-based packages are adhesion of the copper traces to the flex and fatigue of vias in the flex. From testing in an inert atmosphere (nitrogen), Cr seed layer of approximately 500 A was shown to provide reliable adhesion to the flex for over 1000 hrs at 250ºC. Thermal cycle testing examining via geometries which would enable two-sided board designs and more dense board layouts has shown that for more than 600 vias, no failures have occurred during 1200 thermal cycles. This is significantly more reliable than dictated by recent draft SAE standards on high temperature electronic packaging. Since failures have not occurred, it would be advisable to use the finite element models as a guide to via design. These models indicated that larger diameter vias with thicker Cu lowered the stress in via.
Thermal cycling of active and passive component interconnects has been used to experimentally compare the reliability of different packaging methods. Validation testing of the high temperature solders has shown that the 92.5Pb-5Sn-2.5Ag solder outperforms all other solders in terms of failures during testing. Also this solder was found to be easier to wet to assemble devices than others. For pad materials, NiPdAu metal stack on the polyimide was shown to have higher die shear strength than NiAu pad metallurgy. For active devices, the tests performed by this study indicate that stud bumping of the polyimide substrate is more reliable than wire or ribbon bonding. It should be noted that more tests should be performed to confirm this statistically.
There will need to be further work in order to build a test vehicle for validation of these design recommendations. In order to withstand vibration, a design for substrate constraint will need to be modeled in order to minimize vibration stresses at specific components. This analysis may recommend rigidization of the substrate for lower stresses which will require a lamination material that is suitable for high temperature. If the design for the test vehicle requires a complex interconnect pattern, a multilayer board configuration would be necessary. Additional work on the testing of the solders and stud bumping should be performed in order to gain additional information on the assembly methods and failure modes of these interconnect methods.
Recent studies indicate that as much as 29 percent of the domestic natural gas supply may lie in reservoirs below 15,000 ft. However, due to the temperature, pressure and environmental challenges of drilling such wells, only 1 percent of the wells in the U.S. penetrate below 15,000 ft. Improving the ability of drillers to safely and cost effectively reach these depths will significantly add to the volume of domestic deep gas reserves.
In addition, millions of dollars in cost savings can be realized if the drilling duration of equipment used to drill deep wells can be lengthened. Performance lifetimes for an orientation module, for example, typically range from 500-1500 hours at 150°C (~300°F). Accordingly, the need to pull the drillstring out of the hole for recalibration or repair increases as the temperature increases with drilling depth due to increased electronic component drift and failure.
If the system service lifetime and the maximum operating temperature can be increased simultaneously, the drillstring will not need to be tripped out of the hole as often for recalibration or service, substantially reducing the drilling cost of deep wells.
The project has been completed. The final report is available below under "Additional Information".
Final Project Report [PDF-3.31MB]
H. Schoeller, A. Knobloch, H. Xia, D. Shaddock, C. Kapusta, K. Durocher, and J. Cho, “Adhesive Tiecoat / Polyimide Interactions in High Temperature Flex Based Packaging,” accepted to InterPACK 2007.