R. Wayne Johnson

Reduction of lead free solder aging effects using doped SAC alloys

The microstructure, mechanical response, and failure behavior of lead free solder joints in electronic assemblies are constantly evolving when exposed to isothermal aging and/or thermal cycling environments. In our prior work on aging effects, we have demonstrated that large degradations occur in the material properties (stiffness and strength) and creep behavior of Sn-Ag-Cu (SAC) lead free solders during aging. These effects are universally detrimental to reliability and are exacerbated as the aging temperature and aging time increases. Conversely, changes due to aging have been shown to be relatively small in conventional Sn-Pb solders. Aging effects for lead free solder materials are especially important for the harsh applications environments present in high performance computing and in automotive, aerospace, and defense applications. In the current investigation, we have extended our previous studies to include a full test matrix of aging temperatures and SAC lead free solder alloys. In an attempt to reduce the aging induced degradation of the material behavior of SAC solders, we are also exploring various doped SAC-X alloys. These materials are SAC solders that have been modified by the addition of small percentages of one or more additional elements (X). Using dopants (e.g. Bi, In, Ni, La, Mg, Mn, Ce, Co, Ti, Zn, etc.) has become widespread to enhance shock/drop reliability, wetting, and other properties; and we have extended this approach to examine the ability of dopants to reduce the effects of aging and extend thermal cycling reliability. The effects of aging on mechanical behavior have been examined by performing stress-strain and creep tests on solder samples that were aged for various durations (0–6 months) at room temperature (25 °C), and several elevated temperatures (50, 75, 100, and 125 °C). Four “standard” SAC alloys have been examined in this work including SAC105, SAC205, SAC305, and SAC405. This selection has allowed us to explore the effects of silver content on aging behavior (we have examined SACN05 with N= 1%, 2%, 3%, and 4% silver; with all alloys containing 0.5% copper). The doped SAC solder materials being considered in our ongoing studies include SAC0307-X, SAC105-X, and SAC305-X. In this work, we will concentrate on presenting the results for SAC0307-X (SAC-X), where X is 0.1%Bi. This alloy has been proposed as a lower cost SAC variation suitable for enhancing drop reliability. For all of the solders, variations of the mechanical and creep properties (elastic modulus, yield stress, ultimate strength, creep compliance, etc.) were observed and modeled as a function of aging time and aging temperature. Our findings show that the doped SAC-X alloy illustrates reduced degradations with aging for all of the aging temperatures considered. The stress-strain and creep mechanical properties of SAC-X are better than those of SAC105 after short durations of aging, and approach those of SAC205 with longer aging times. After long term aging, the SAC-X alloy was found to have more stable behavior than all of the standard SACN05 alloys. Analogous tests were performed with 63Sn-37Pb eutectic solder samples for comparison purposes.

Thermal cycling reliability of lead‐free chip resistor solder joints

The solder joint reliability of ceramic chip resistors assembled to laminate substrates has been a long time concern for systems exposed to harsh environments. In this work, the thermal cycling reliability of several 2512 chip resistor lead‐free solder joint configurations has been investigated. In an initial study, a comparison has been made between the solder joint reliabilities obtained with components fabricated with both tin‐lead and pure tin solder terminations. In the main portion of the reliability testing, two temperature ranges (−40‐125°C and −40‐150°C) and five different solder alloys have been examined. The investigated solders include the normal eutectic Sn‐Ag‐Cu (SAC) alloy recommended by earlier studies (95.5Sn‐3.8Ag‐0.7Cu), and three variations of the lead‐free ternary SAC alloy that include small quaternary additions of bismuth and indium to enhance fatigue resistance.

SiC Power Device Packaging Technologies for 300 to 350°C Applications

The challenges of packaging SiC power devices for high temperatures include high operating temperature, wide thermal cycle range, high currents and high voltages. This paper describes ongoing research to develop suitable materials and processes for packaging SiC power devices. Ohmic and Schottky contacts must be protected from oxidation at elevated temperatures. TaSi2:N2 is an effective oxidation barrier, protecting the contacts when exposed to 350 oC in air. For package substrates, silicon nitride ceramics with a thick brazed copper foil are used. A nickel/thick gold or nickel/thick silver surface finish is plated onto the copper foil; depending on the die attach brazing process. With the nickel/thick gold finish, gold-tin eutectic has demonstrated no degradation in die shear strength after 2000 hours at 350 oC or after 500 hours at 400 oC. Work is currently underway with nickel/thick silver and gold-silicon eutectic die attach. No degradation in shear strength has been observed with this system after 100 hours at 400 oC. Gold wire (250 µm) bonding has been demonstrated on both substrate and SiC metallizations. An initial decrease in strength is observed due to annealing of the gold wire, but shear and pull strengths remain high. Polyimide has been used to increase the dc breakdown strength of a test pattern by a factor of ~3x at 300 oC compared to no passivation. The breakdown strength increased with storage in air at 300 oC through 1000 hours.

200 "C Operation of a 500 W DC-DC Converter Utilizing Power MOSFETs

This paper presents results from the operation of the power stage of a 500-W dc-dc converter at an ambient temperature of 200 C. This converter is designed to provide an output voltage of 42-Vdc from a 28-Vdc input. It utilizes an H-bridge configuration composed of eight International Rectifier power MOSFETs (parallel connections of two MOSFETs) and four Motorola diodes to convert the dc input voltage to a high- frequency square wave which can then be stepped up with a transformer. The transformer output voltage is then rectified and filtered to produce the dc output voltage. A phase-shifted pulsewidth modulation (PWM) switching scheme is utilized to control the MOSFETs in the H bridge. This scheme allows zero-voltage turn-on of the MOSFETs to improve the efficiency. The efficiency of this converter when operated on the labo- ratory bench was measured to be 89%. The H bridge and transformer were then placed in an oven and operated over an ambient temperature range of 20 C-200 C. The efficiency varied from 86.1% to 85.4% over this temperature range. The long feedthroughs for oven operation caused the efficiency to decrease from 89% to 86.1%.

Pressureless Sintering of Microscale Silver Paste for 300 °C Applications

High-temperature die attach is necessary to fabricate digital and analog thick film modules for 300 °C applications. Sintered Ag is a promising die attach material, but typically requires pressure during sintering for larger die. Pressureless sintering of a microscale Ag paste has been evaluated with Au and Ag die metallization on Au, Ag, and PdAg thick film metalized substrates. With Au metallization on either the die or the substrate, degradation of shear strength rapidly occurred with aging at 300 °C. Formation of a dense Ag layer and a depletion region near the Au surface was observed with 300 °C aging. This was attributed to the rapid surface diffusion of Ag on Au surfaces at 300 °C. This did not occur with Ag thin film die metallization and Ag and PdAg thick film metallization. After 8000 h at 300 °C, 8 mm × 8 mm Ag metalized die on Ag thick film substrates could not be sheared at 100 kg of applied force. The same was true for 8 mm × 8 mm Ag metalized die on PdAg thick film substrates after 2000 h at 300 °C.

Effect of system and operational parameters on the performance of an immersion-cooled multichip module for high performance computing

Immersion cooling is making a resurgence for use in server applications driven by an increase in chip densities and the need for reduction in overall data center power usage. This paper focuses on pool boiling characteristics from an array of heaters which simulate electronic chips on a vertically-oriented printed circuit board. A pool boiling study was conducted on an array of four bare die using two different dielectric fluids; namely Novec 649 and HFE 7100, both with low global warming potential (GWP). Tests were conducted at two die spacings; 25mm and 10mm under two different pool conditions: saturated and 15°C subcooled representing a start-up transient. Data were collected for increasing and decreasing heat flux cycles. The flux dissipated was found to be 14.6 W/cm2 and 14.2 W/cm2 with Novec 649 and HFE 7100 respectively for the test board with 10mm die spacings under subcooled conditions. These values are recorded at self-imposed maximum surface temperatures that ensured operation well below the critical heat flux. These flux values are quite high especially since they are attained without the need to modify the surface or add heat sinks. In an effort to increase thermal performance, tests were also conducted on dies spaced 25mm apart augmented with two different enhanced heat sinks featuring microporous and microfinned surfaces. Enhanced heat sinks performed better than bare die and the flux dissipated was found to be 18.87 W/cm2 and 18.31 W/cm2 using Novec 649 and HFE 7100 respectively under subcooled conditions. Additionally, these values were achieved at surface temperatures 15°C lower than the surface temperatures recorded by the bare dies for dissipating the same amount of heat. High speed images were obtained to provide a better understanding of nucleation characteristics.

Characterization of Bi–Ag–X Solder for High Temperature SiC Die Attach

Higher operating temperature (200 °C) SiC power devices simplify the thermal management system for power electronics modules in hybrid and electric vehicle applications. However, common lead-free solders have a melting point of only 217 °C-229 °C, limiting their use in this application. In this paper, a Bi-Ag-X solder paste with a solidus point of approximately 265 °C was characterized on direct bond copper (DBC) and CuMo reactively brazed alumina substrates. The Cu and CuMo surfaces were plated with Ni:B and Au. During high-temperature storage (200 °C) of test vehicles on DBC, there was an initial decrease in die shear strength followed by relatively constant die shear strength with further aging (through 5000 h). The initial decrease in shear strength was determined to be the result of NiBi3 intermetallic formation. The formation of the intermetallic layer was limited by the thickness of the initial Ni plated on Cu or CuMo. Similar results were observed for CuMo samples thermal cycled (-55 °C to +195 °C) up to 2000 cycles.

SiC Power Device Die Attach for Extreme Environments

Silicon carbide power diodes and transistors are enabling technology for power electronics capable of operating in extreme environments. In this paper, a AgBiX solder paste has been studied for SiC power device die attach to power substrates for 200 °C use in vehicle and downhole well logging applications. The solder paste has a controlled amount of Sn, which limits the amount of Sn intermetallic formation. This produces a stable solder joint as a function of time at high temperature. The die shear strength remained at ~38 MPa through 2000 h of storage at 200 °C. After 1000 thermal cycles from -55°C to 195 °C, the percent die attach area reduction due to solder cracking was <;10%.

Drop Reliability of Corner Bonded CSP in Portable Products

The use of CSPs has expanded rapidly, particularly in portable electronic products. Many CSP designs will meet the thermal cycle or thermal shock requirements for these applications. However, mechanical shock (drop) and bending requirements often necessitate the use of underfills to increase the mechanical strength of the CSP-to-board connection. Capillary flow underfills processed after reflow, provide the most common solution to improving mechanical reliability. However, capillary underfill adds board dehydration, underfill dispense, flow and cure steps and the associated equipment to the assembly process. Corner bonding provides an alternate approach. Dots of underfill are dispensed at the four corners of the CSP site after solder paste print, but before CSP placement. During reflow the underfill cures, providing mechanical coupling between the CSP and the board at the corners of the CSP. Since only small areas of underfill are used, board dehydration is not required. This paper examines the manufacturing process for corner bonding including dispense volume, CSP placement and reflow. Drop test results are then presented. A conventional, capillary process was used for comparison of drop test results. Test results with corner bonding were intermediate between complete capillary underfill and non-underfilled CSPs. Finite element modeling results for the drop test are also included.Copyright

Investigation and characterization of a high performance, small form factor, modular liquid immersion cooled server model

As the demand for power consumption increases with an expanding global economy, the need for innovative solutions to meet the cooling demands of next generation servers looms large. Particular attention must be paid to ensuring that cooling mechanisms employed are not only efficient from a power consumption perspective, but tightly packed to avoid needlessly expansive server rooms resulting in increased construction costs and energy waste from cooling superfluous spaces. With these concerns in mind, a small form factor, high input power solution is presented that involves no surface modifications, i.e. bare silicon die. In this 150 mm × 300 mm × 38 mm (H × L × W) space, two-phase heat transfer results in roughly 400 Watts rejected from the four-die arrangement and at a level well below the Critical Heat Flux.