Thick Film Substrate Research Articles

Study on the degradation of welding strength in ceramic substrates subjected to temperature cycling

Abstract This article uses AlN and Al2O3 thick film substrates to conduct an in-depth study of the attenuation of welding strength of ceramic substrates under temperature cycling conditions. Using the test method of miniaturizing the substrate to test the transverse shear force, the shear force changes of different substrates with different 62Sn36Pb2Ag solder thicknesses were summarized and compared with the temperature cycle process. The finite element simulation method was used to simulate the Ag/SnPb interface, and the trend of interface stress and strain changing with temperature was obtained. Finally, the study found that during the temperature cycle, the Sn-enriched phase in the solder of the AlN substrate diffuses faster into the Ag layer, so the soldering strength of the AlN substrate decays faster during the temperature cycle.

Enhanced Optical–Thermal Performances of High-Power LED by Plated Copper on Thick Film Ceramic Substrate

Thick printed ceramic substrate (TPC) is a promising technique for the packaging of high-power light-emitting diode (LED). Nevertheless, the incorporation of glass phase and voids in the sintered Ag layer decrease the electrical conductivity and heat dissipation of TPC substrate, which contributes to an increase in the junction temperature of LED. This article demonstrates a novel packaging design to plate copper on the sintered Ag layer of TPC substrate, which was applied to enhance the optical–thermal performances of high-power LED. After plating, the copper layers are well bonded with the Ag layer and the sheet resistance decreases rapidly. With the plated copper on thick film (PCTF) substrate package, the total thermal resistance is 17.45% lower than that with TPC package, while the junction temperature change of LED samples is 8.04 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{\circ}$</tex-math> </inline-formula> C and 9.92 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{\circ}$</tex-math> </inline-formula> C, respectively. Moreover, the surface temperature of LED sample packaged with PCTF is 11.16% lower than that with TPC at the working current of 1600 mA, and the optical power is 4.79% and 3.2% higher at the working current of 400 and 1200 mA, respectively. The results demonstrate that the optical–thermal performances of LED are enhanced by using the PCTF substrate due to its excellent heat dissipation.

High-performance and space-qualified hybrid integrated circuit based on multi-group homogeneous gradient eutectic in quality conformance inspection

The quality and reliability of high integration electronic components has become one of the main issues restricting the advancement of aerospace products. During the development of a military-standard microwave hybrid integrated circuit, the penetration rate of the eutectic welding of the thin film substrates, thick film substrates, and carrier plates in the cavity directly affects the significant performance characteristics, such as gain and working current. By using of traditional process methods, the hybrid integrated circuit cannot pass the experiments (group A, group B, group C and group D) which presents the different types of grouping categories, test parameters, along with acceptance criteria that the quality conformance inspection (QCI) stipulated in the GJB 2438B General Specification for Hybrid Integrated Circuits, which is due to the low rate of finished products. We propose and experimentally demonstrate a multi-group homogeneous gradient eutectic-based method with penetration rate of >92 %, which ensured the grounding performance and reliability inspection requirements of the individual circuit unit. When the hybrid integrated circuit works under the temperature range −55~+125 °C and other environmental test conditions, the delta limits of working current and gain are kept in the extreme range of ±10 mA and ±2 dB which shows high stability and reliability. Then we finally achieved that the aerospace product successfully passed the QCI. Our research progress can expand the process applications of Class K(Space-Qualified) hybrid integrated circuits with the maximum level of assurance for use in extreme environment applications such as space where reliability is paramount.

Enhanced adhesion strength of silver paste on AlN ceramic substrate via sintered nano-CuO

We have optimized a CuO-doped silver paste to significantly improve the adhesion strength of silver thick film on AlN ceramic substrate, which can be widely applied in power electronic packaging applications. Via a combination of scanning electron microscope and X-ray diffraction, we have systematically studied the silver layer morphology on top of AlN ceramic substrate, and the interphases between the silver thick film and AlN substrate. Due to the CuAl2O4 intermediate formed at the silver layer/AlN interface, the adhesion strength of silver thick film can be greatly improved. Particularly, the silver paste with 3 wt% nano-CuO sintered on AlN ceramic substrate exhibits high adhesion strength of 15.3 MPa with good electrical conductivity of 0.9 mΩ/□.

Fe-doped GaN grown on stirring-assisted Na-flux process GaN thick film and its application on Ohmic contact UV detector

Abstract Na-flux method is the most promising approach to grow GaN thick film substrate for GaN homoepitaxy. Here, the influence of solution stirring on grown GaN crystal morphology, crystal quality, dislocation density and impurity concentration was assessed for GaN crystal growth by Na-flux method. It is proved that the crystallinity, surface roughness and dominated orientation of the GaN thick layers are greatly influenced by stirring process, revealing a direct and correlated growth mechanism for the growth of GaN using Na-flux method. Notably, the surface roughness was reduced by three orders of magnitude under stirring (compared to un-stirred sample). This stirring-assisted GaN thick film was then used as seed layer to grow high resistance Fe-doped GaN epitaxial layer by applying HVPE method. On which Ohmic contact was successfully fabricated and an UV-detector with a high detectivity of ~1013 Jones, which was among the highest values, and fast response (rise time of 100 ms, decay time of 160 ms) was obtained.

Evaluation of Reliability of Lead-free Solders in Silver-free Hybrids

Abstract Silver and silver-filled materials are widely used for hybrid substrate conductors, component terminals and conductive attachment materials. However, some concerns exist about using silver, especially for high-voltage hybrid assemblies that must operate in harsh non-hermetic or semi-hermetic applications. This paper discusses the reliability testing of silver-free components attachment materials. The target application temperature for this research was 185°C. Target duration at temperature was 500 hours. The evaluation of Sn100C solder was performed in comparison to solders of Sn96 and Au80Sn20. Two types of Ag-free thick-film substrates were tested with different type of solders to mount surface-mounted technology (SMT) components. The assembled test samples were subjected to thermal aging and thermal cycle tests. At each test interval, shear tests were performed to evaluate the mechanical performance. Cross sections of the solder joint and thick-film interface were used to understand the intermetallic compounds (IMC) formation and failure mechanisms under thermal stress. Tests for conductive filament growth was performed to evaluate the behavior of Sn-based solders in powered assemblies under target environmental conditions. Test results were discussed and suggestions for achieving a reliable, silver-free thick-film hybrid were made.

Fabrication of Aluminum Nitride Thermal Substrate and Low-Temperature Die-Bonding Process for High Power LED

In this study, a low-cost aluminum nitride (AlN) sintering process to produce thick AlN film substrate with a high thermal conductivity is developed. The thermal conductivity of the present produced thick AlN film substrate is about 163.8 W/mK, which is very close to the reported thermal conductivity of the AlN material. Also, a Sn-Bi die-bonding system is developed to die-bond light emitting diodes (LEDs) on the present sintered AlN substrate with a relatively low die-bonding temperature (below 160°C). In this work, to enhance a better wetting at the die-bonding interface, three external forces (10 N, 15 N, and 20 N) were applied on LED chips during the die-bonding process. We found that the 15-N applied force can achieve a better die-bonding interface among three external forces (10 N, 15 N, and 20 N). The LED die-attached on the AlN substrates by 15 N normal force has the best shear strength (41.5 MPa), compared to the shear strength of 36.9 MPa and 31.5 MPa of the LED die-attached on AlN substrates by 20 N and 10 N normal force, respectively. The LED chips die-attached on the AlN substrate by 15-N normal force shows the best thermal resistance (7.3°C/W). The agreement between the thermal resistance tests and the shear strength tests implies that the better die-bonding interface produced a higher shear strength and a lower thermal resistance of the LED chips die-bonded on the AlN substrates.

Multilayer thick-film ceramic for multichip modules with laser microvias

PurposeThe purpose of the paper is to show up the current possibilities by combination of classic thick-film technology with advanced processing. Thick-film hybrid ceramic substrates have been a base for highly reliable devices for space, aerospace, medical and industrial applications since many years. The combination of classic thick-film printing with advanced technologies for fine line structuring provides substrates best suited for packaging solutions with challenging requirements, such as temperature stability and extended product lifetime. Combined with state of the art assembly technologies, thick-film substrates are used in highly demanding industries.Design/methodology/approachIn recent years, several technologies for fine line structuring have been introduced, e.g. fine line printing, photo imaging, etching, laser structuring for local chip fan-out or fine line structuring on single layers. For further miniaturization of thick-film multilayers circuits, after solving the fine line resolution, the reduction of electrical connection of conductive layers through printed insulation/dielectric layer (via) diameters to connect the layers should be addressed.FindingsThe focus of this paper is to show the results of combining fine line structuring with laser microvias and to compare laser drilling in thick-films with different established via forming technologies.Originality/valueThe reduction of via size to 60 µm – smaller than 50% compared to using state-of-the-art printing technologies enables a solution for significant relaxation of current design possibilities.

Resonance shifting by ferrite thick film superstrate

Fritless thick films of Ni-Co-Zn ferrites were fabricated by screen printing technique on alumina substrates. Structural analysis was undertaken using X-ray diffraction and Scanning electron microscopy techniques. A new approach for the determination of complex permittivity (?' and ?'' ) using microwave property perturbation is unveiled. Ag thick film microstrip ring resonator (MSRR) with and without the thick film substrate was used for the microwave transmission studies in the frequency region of 8-12 GHz. The microwave conductivity of the thick films lies in the range of 1.779 S/cm to 2.296 S/cm. The penetration depth is also reported within the X-band of microwave frequencies.

Reliability of Ag Sintering for Power Semiconductor Die Attach in High-Temperature Applications

Low-temperature Ag sintering provides a lead-free die attachment method that is compatible with high-temperature (300 °C) power electronics applications. The reliability of sintered Ag die attach for Si and SiC die has been studied on both thick film substrates for lower current power applications and direct bond copper (DBC) substrates for higher current power applications. Pressureless and low-pressure sintering were evaluated. Sintering with low pressure yielded lower porosity (15–17%) versus pressureless sintering (∼30%). Reliability was evaluated with thermal aging (300 °C) and thermal cycling (–55 °C to + 300 °C) tests. Reliable Ag sintered die attach was achieved with assemblies having Ag-bearing surface finishes on both the die and the substrate. In contrast, the shear strength after 300 °C aging was greatly reduced when Au metallization was used either on the die or on substrate surface. In some cases, low-pressure sintering delayed the failure of the sintered Ag die attach to Au surfaces when aged at 300 °C compared to the pressureless sintering. The reliability with Pd-containing substrate metallizations was intermediate between Ag and Au metallizations. The thermal cycle reliability on DBC substrates was limited by failure at the Cu-to-alumina interface over the wide temperature range, while on the thick film substrates high adhesion was maintained after 1000 thermal cycles.

Component Attachment with Pressureless Ag Sintering for 300°C Applications

With an increased demand for high-power and high-temperature electronics, Ag sintering paste has been considered a promising Pb-free die-attach material candidate for these applications. Extensive research has been carried out investigating pressure and pressureless Ag sintering for die attach. In this work, passive component (chip resistor) attachment with Ag sintering was explored. Due to termination geometry differences between resistors and dies, different processing procedures and parameters were developed. For PtAu terminated resistors, the mean shear force of as-built samples on thick-film Ag metallized substrates was 90 N, but dropped to 18.6 N after 1,500 h at 300°C. Formation of a dense Ag layer near the PtAu resistor termination and a void region near the thick-film metallization was observed in cross sections after 1,000 h at 300°C. For PdAg terminated resistors with a plated Ni/Au finish, the initial shear force results were low due to Ag diffusion along Au metallization surface. For PdAg terminated resistors with Ag thick-film substrates, the initial shear force was ~60 N and remained in the range of 50–70 N during aging at 300°C for 1,500 h. A new thick-film metallization (Au + Ag) was developed to enable the use of thick-film Au interconnect metallization.

Balancing polyelectrolyte diffusion and clay deposition for high gas barrier

The use of clay nanoplatelets in layer-by-layer assembled thin films (50–200 nm thick) has been shown to reduce the oxygen transmission rates of polymer thick film substrates (&gt;12 µm) by many orders of magnitude. In an effort to reduce the thickness and number of deposited layers required to achieve high gas barrier, the pH of the montmorillonite (MMT) clay aqueous suspension was reduced. Growth of a repeating sequence of polyethylenimine (PEI)/poly(acrylic acid) (PAA)/PEI/MMT was reduced when the MMT pH was lowered from 10 to 6. This reduced pH causes the preceding PEI layer to be more charged, which causes more clay to be deposited per layer. Just three PEI/PAA/PEI/MMT quad layers (44 nm thick) exhibit an undetectable oxygen transmission rate (&lt;0·005 cm3/(m2 d atm)) on a 179 µm thick poly(ethylene terephthalate) (PET) substrate. As the clay deposition pH is altered, there is a compromise between high polymer diffusion (at pH 10) and high clay deposition that increases with decreasing pH. Clay at pH 6 provides the best compromise of dense packing and adequate spacing between layers. The three-quad layer film on PET exhibits an oxygen permeability orders of magnitude below silicon oxide (SiOx) and metal thin films (&lt;5 × 10−22 cm3 cm/(cm2 s Pa)).

Component Attachment with Pressureless Sintering for 300°C Applications

Abstract With an increased demand for high power and high temperature electronics, Ag sintering paste has been considered a promising Pb-free die attach material candidate for these applications. A large amount of research has been performed investigating pressure and pressureless Ag sintering for die attach. In this work, passive component (chip resistor) attachment with Ag sintering was explored. Due to termination geometry differences between resistors and die, different processing procedures and parameters were developed. For PtAu terminated resistors, the mean shear force of as-built samples on thick film Ag metallized substrates was 90 N, but dropped to 18.6 N after 1500 hours at 300°C. Formation of a dense Ag layer near the PtAu resistor termination and a void region near the thick film metallization was observed in cross-sections after 1000 hours at 300°C. For PdAg terminated resistors with a plated Ni/Au finish, the initial shear force results were low due to Ag diffusion along Au metallization surface. For PdAg terminated resistors with Ag thick film substrates, the initial shear force was approximately 60 N and remained in the range of 50–70 N during aging at 300°C for 1500 hours. A new thick film metallization (Au+Ag) was developed to enable the use of thick film Au interconnect metallization.

Improving Stitch Bond on Hybrid Thick Film Substrate using Stand-Off Stitch Wire Bond Technique

Thick film technology is based on a paste containing glass frit that is screen printed and fused at high temperature onto various ceramic substrate materials. Softening or melting this glassy frit forms a cohesive layer, binding the conductors, resistors or dielectric materials to the ceramic. The dynamics of the printing process and inherent number of associated variables negatively impact the uniformity of the fired surface on a micro scale, which can lead to variation in the wire bonding process. Other processes associated with thick film substrate fabrication can cause problems as well. Laser trimming is used to adjust the value of printed resistors to meet design requirements. This ablation of printed resistors by high–powered pulse laser leaves a halo of debris and contamination on the ceramic substrate, which can cause wire bond lifting. In this paper, we will demonstrate a way to eliminate these problems using a bonding technique called Stand- Off Stitch bonding (SOS). This wire bond type is formed by first placing a ball bump at the second bond, or stitch, location on the thick film substrate, and then forming a normal wire that terminates on that bump. This places two ball bumps at each end of the wire, similar to a security bond. However, the ball bump is located under the stitch instead of on top. This SOS wire bond technique is compliant with the MIL-STD- 883 for a compound bond, where one bond is placed on top of another bond. With the gold bump placed on top of the gold thick film pad, the bump acts as a foundation for the stitch bond, providing a wider contact area and clean bond surface to secure a reliable stitch bond interconnect. With this change, an abrupt improvement to the resultant destruct wire pull tests can be achieved, promoting a robust, controlled process for wire bond interconnects.

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, $8 {\rm mm} \times 8$ mm Ag metalized die on Ag thick film substrates could not be sheared at 100 kg of applied force. The same was true for $8 {\rm mm}\times 8$ mm Ag metalized die on PdAg thick film substrates after 2000 h at 300 °C.

알루미나-TZP(3Y) 세라믹스 복합체의 제조 및 기계적 특성

Composite ceramics of alumina-TZP(3Y) have good mechanical and electrical properties. So, They have been used as high strength refractory materials and thick film substrates, etc. In this study, Composite ceramics of alumina-TZP(3Y) were fabricated by uniaxial pressing and sintering at 1,400, 1,500, and 1,600°C, and their microstructures and mechanical properties were investigated. As the TZP(3Y) content in composite ceramics increases from 20 wt.% to 80 wt.%, the fracture toughness increases monotonically, which seems to be related to the higher relative density and/or toughening mechanism by means of stabilized tetragonal zirconia phase at room temperature. In contrast to the fracture toughness, Vickers hardness of the composite ceramics shows maximum value (1,938 Hv) at a 40 wt.% of TZP(3Y). The result of Vickers hardness is likely to be due to more dense sintered microstructure of composite ceramics than pure alumina and reinforcement of composite ceramics with TZP(3Y), considering that Vickers hardness of pure Al2O3 is greater than that of TZP(3Y). It is also shown that the ZrO2 particles are l°Cated between Al2O3 grains and suppress grain growth each other.

Static and Dynamic Response of a Cu-Ni Thin Film Heat Flux Sensor

A Cu-Ni thin film heat flux sensor had been fabricated on a 0.05mm thick polyimide film substrate by vacuum coating technology. The overall dimension of the sensor was 8 mm long and 4 mm wide. A thermopile and a thermocouple were arranged on the substrate to measure both heat flux and surface temperature. The thermopile had 18 thermocouple junctions which formed 9 pairs of differential thermocouples and were covered by two different thickness of thermal resistance layers. This research carried out static and dynamic tests of the thin film heat flux sensor. Seebeck coefficient of thermocouple is 19.3761μV/(°C). Sensitivity of the thermopile is 0.010121μV/(W/m2). Steady-state tests of the thermopile and the thermocouple were taken separately. Time constant of the thermocouple is about 0.26s, which is faster than the thermopile of 1.57s.

Characterization of AgBiX™ Solder Paste on Thick Film for 200°C Applications

AgBiX™ (Indium Corporation) solder paste has a solidus temperature of ∼262°C after reflow, which is suitable for passive component, semiconductor and power die attach for 200°C applications. In this work, the paste has been used to assemble SiC die with Ti/Ni/Ag thin film metallization to Ag and PdAg thick film substrates. High temperature storage testing (200°C) was preformed to characterize the reliability of the assemblies. Surface mount chip resistors attached to thick film substrates were also subject to high temperature storage. Comparisons of the performance of die attach and resistor attach on Ag substrates and PdAg substrates are made. EDX and failure analysis was used to understand the role of Pd on the failure mode and lower aged shear strengths with the thick film PdAg conductors.

Pressureless, Low Temperature Sintering of Micro-scale Silver Paste for Die Attach for 300°C Applications

Low temperature Ag sintering technology provides a lead-free die attachment compatible with high temperature (300°C) applications. Previous work with Ag sintering has required pressure during the sintering process or been limited to small area die. In this paper, the pressureless sintering of micro-scale silver paste is examined for larger (8mm × 8mm) area die. Experimental combinations included: Si die metallization (Ag and Au); thick film substrate metallization (Au, Ag and PdAg); and sintering temperature. For Au metallization (die and/or substrate), the initial shear strength results were good with 8mm × 8mm die sintered at lower temperatures (200°C). The shear strength was beyond the limit of the shear test machine (100 kg), corresponding to &amp;gt;15.3 MPa. However, after aging for 24 hours at 300°C, the shear strength dropped significantly. An SEM was used to characterize cross sections of as-built and aged sample. The decrease in die shear strength with high temperature sintering (250°C and 300°C) or high temperature aging is attributed to surface diffusion of Ag along the Au surface resulting in a dense Ag layer adjacent to the Au surface and a depletion layer within the die attach adjacent to the dense Ag layer. Shear failures occurred through the depleted region. For Ag metallization (die and substrate), no decrease in shear strength was observed with 300°C aging. Shear strength of 8 mm × 8 mm dies was &amp;gt;100 kg ( &amp;gt;15.3 MPa) as-built. The shear strength remained &amp;gt;15.3MPa after 3000 hours of 300°C aging.

Mobile Packaging and Interconnect Trends

This paper will focus on emerging and fast growth package solutions to meet mobile products' density and cost requirements. A short review of where package miniaturization and modularization has taken us so far, and where it will lead in the next 5 years. Teardowns of high density systems and packages will be used to illustrate key points. Low temperature Ag sintering technology provides a lead-free die attachment compatible with high temperature (300°C) applications. Previous work with Ag sintering has required some pressure during the sintering process or been limited to small area die. In this paper, a pressureless sintering of micro-scale silver paste procedure is presented for large (8mm x 8mm) area die. Experimental combinations included: Ag metallized Si die, Au metallized Si die, Ag thick film substrate metallization, Au thick film substrate metallization, PdAg thick film metallization and sintering temperature. For Au metallization (die and/or substrate), the initial shear strength results were good with 8mm x 8mm die sintered at lower temperatures (200°C). The shear strength was out range of our shear test machine (100 kg), corresponding to &amp;gt;15.3 MPa. However, after aging for 24 hours at 300°C, the shear strength dropped significantly to 40.38 Kg (6.183 MPa). An SEM was used to characterize cross sections of as-built and aged sample. The decrease in die shear strength with high temperature sintering (250°C and 300°C) or high temperature aging is attributed to surface diffusion of Ag along the Au surface resulting in a dense Ag layer adjacent to the Au surface and a depletion layer within the die attach on the opposite side of the the dense Ag layer. Shear failures occurred through the depleted region. For Ag metallization, no decrease in shear strength was observed with 300°C aging. Shear strength of 8x8cm2 dies was out range of our shear test machine (&amp;gt;100 kg, &amp;gt;15.3 MPa) as-built. The shear strength remained out of range (&amp;gt;15.3MPa) after more than 2000 hours of 300C aging.