;i>efMIDEM A Innrnal of M Informacije ( Journal nf Microelectronics, Electronic Components and Materials Vol. 42, No. 4 (2012), 211 - 224 Innovation steps towards a novel and cost efficient LTCC packaging technology for high end applications Franz Bechtold VIA electronic GmbH, Hermsdorf, Germany Abstract: The emerging of a new MEMS, MOEMS and BioMEMS in the field of Sensors and a new GaN based generation of RF semiconductors in the field of RF Communication create new challenges for the packaging technology. In order to provide adequate packaging solutions for high end applications, manufacturers today are requested for high flexibility, excellent performance and reasonable costs. Requirements with respect to advanced circuit functionality and enhanced thermal management, low dielectric losses, low parasitic effects and crosstalk, SiP integration, thermal management, miniaturisation and reliability have to be fulfilled. Today LTCC provides a proven packaging solution for Standard MEMS, GaAs devices and MCMs. increasing frequencies; increasing power and increasing functionality of the systems are the driving forces to investigate into novel packaging solutions. This paper describes the different steps how LTCC technology can cope with these requirements. Based on a comparison of the different available LTCC systems and a benchmark between them, the packaging performance of several LTCC systems is investigated in detail. Technological features, relevant for miniaturisation, System in package integration and RF performance, like cavities, conductor line resolution, via diameter and flatness are considered. Process capability, relevant for thermal management, SiP integration and hermeticity like brazing of heat sinks, integration of heat pipes, integrated capacitors and resistors are analysed. The impact of novel and low cost materials for metallization, heat sink and brazing will be evaluated with respect to weight and cost. Compatibility with RoHS and REACH regulations will be discussed. The progress beyond the state of the art will be demonstrated by packaging solutions recently developed for industrial applications and in the frame of research programmes. Keywords: LTCC, sensors, RF packaging, System in Package, reliability Inovativni koraki proti novi in ceneni LTCC tehnologiji pakiranja za visokotehnološke aplikacije Izvleček: Združevanje MEMS, MOEMS in BioMEMS na področju senzorjev z RF polprevodniki na osnovi nove GaN tehnologije predstavlja nove izzive za tehnologijo pakiranja. Visoka fleksibilnost, izjemne lastnosti in sprejemljiva cena so zahteve proizvajalcem za izdelovanje ustreznih rešitev pakiranja visokotehnoloških izdelkov. Današnje LTCC predstavljajo preizkušene rešitve za pakiranje standardnih MEMS, GaAs elementov in MCMjev. Povečanje moči, funkcionalnosti in frekvenc zahtevajo razvoj novih rešitev. Članek opisuje različne korake rešitev teh zahtev s pomočjo LTCC tehnologije. Različni LTCC sistemi so medsebojno primerjani in njihove lastnosti natančno raziskane. Opisane so tehnološke značilnosti pomembne za proces miniaturizacije, integracije sistemov v ohišje in RF lastnosti. Analizirane so sposobnosti procesa termičnega upravljanja, SiP integracije in hermetičnosti. Vpliv novih in cenenih materialov za metalizacijo, hladilnike in spajkanja bodo ocenjene glede na težo in ceno. Opisana bo združljivost z RoHS in REACH regulativami. Predstavljene bodo napredne rešitve novega pakiranja za industrijske aplikacije v okviru raziskovalnih programov. Ključne besede: LTCC, senzorji, nizke izgube, nizka teža, RF ohišja ' Corresponding Author's e-mail: bechtold@via-electronic.de 1. Introduction New microsystems, microsensors and microelectronic devices like MEMS, MOEMS, BIOMEMS and GaN based high power/high frequency semiconductors are emerging. These devices show an outstanding performance with respect to miniaturisation, functionality and power. Adequate packaging solutions, encapsulating heterogeneously the different components have to take into account that some of these devices are sensitive to environmental conditions like thermal cycling, heat dissipation and humidity. In addition good electrical stability and good EMC performance together with excellent high frequency and high power performances have to be obtained and hermetic or quasi hermetic packaging is requested in different cases. These requirements create new challenges to the packaging technology with respect to electrical performance, SiP integration, thermal management and reliability. 2. State of the Art LTCC Packaging .solutions Today LTCC provides an established and proven packaging solution for automotive applications, mobile phone circuits, industrial sensors; GaAs based MMICs and RF MEMS. Passive functional elements like miniaturised antennas, filters, couplers, temperature sensors, heating elements, resistors, capacitors and inductors can be monolithically embedded. Depending on the power dissipation and the reliability requirements those packaging solutions cover the whole range from low power quasi-hermetic packages to high power near hermetic packages and fully hermetic packaged high power MMICs. Glob top and transfer moulding protection, adhesive bonded heat sink and cavity shielded MMICs as well as brazing technology for heat sink and ring frame attachment are used. The type of packaging chosen is related to the lifetime, the reliability and the volumes of the different application. 3. Packaging solutions in the low volume industrial market The low and medium volume market for LTCC is mainly represented by customized packages in the field of industrial sensors, medical devices and high end rf modules for telecommunication and satellite communication. Driving forces for all of those applications are the technical performance at acceptable cost for the individual customer solution. In the following example (Fig.1) the complete package of an x-ray detector is shown. Figure 1: KETEK: x-ray detector The key benefits with LTCC is the high miniaturisation, the capability of thermal management which includes a channel for heat transfer and thermal vias for power dissipation, the sspectroscopic neutrality and the full Hermeticity with 0 outgassing. In Fig. 2 different customized packages for X-Ray detectors and pressure sensors demonstrate the benefits of specific shapes; several wire bond levels, integrated fluidic channels and the capability of dedicated material systems. Figure 2: VIA: Customized sensor packages Key benefits shown are specific shapes, several wire bond levels and Integrated fluidic channels Dedicated material systems 4. Packaging solutions in the high volume automotive market Cost together with reliability and Miniaturisation are the driving forces in the automotive business. LTCC in Germany's automotive market today is a business of about 250 Mio €. Typical under the hood applications are Antiblocking, Gear control or Steer by wire systems. Figure 3: TEMIC: Technology platform All of them request heterogeneous integration of highly miniaturised signal electronics together with thickfilm and DCB high power substrates into the same package. Figure 4: TEMIC: Gear control unit 5. Packaging solutions in the high volume mobile phone market In the high volume market of mobile phones a fast evolution of miniaturisation using LTCC packaging technology can be observed. Front end (Fig. 5) is realized today in LTCC. The key benefit of LTCC is the possibility to integrate passive RF functions at substrate level. Using a material system which consists of two different dielectric materials, one for signals with K= 8 and the other with K= 20 (the so called electronic ceramic) enables to integrate a number of such functions like filters, baluns, matching networks and others. Figure 5: Epcos: Schematic of a LTCC frontend, Fig. 6 depicts the top view of a dual-band WiMAX LTCC module. Active components like PA, switch and even some power management (PM), functionality are soldered on the top of the module while the RFIC is connected by means of wire bonds. Figure 6: TDK-EPC: WiMAX SiP The worldwide smallest All-in-one Frontend-Modul for Bluetooth- and WLAN-application has been published by Epcos 2008. With a height of 1,4mm it requests only a small area of 4,5 x 3,2 mm^ on the PCB. This module integrates the power amplifier, the switch, the receive-balun and the bias network with EMW protection. Furthermore, the integrated coexistence filter allows simultaneous operation of WLAN and Bluetooth having all standards of mobile communication and realises all requested functions between WLAN and Bluetooth. Figure 7: Epcos D 601: The world smallest All in One Module The most actual development is an advanced frontend, based on the co-design of Frontend and Antenna by combining flex foil and LTCC. This module combines the advantages of LTCC and Flexible board technology and results in a game-changing 16-band antenna, including Japanese frequency bands in the 1.5 GHz range. It is a single hardware-serving operator worldwide. Figure 8: TDK-EPC: Advanced multi-feed RF front-end 6. RF Packaging solutions in other s The telecommunication market requests also for high end solutions at medium volumes. High power T/R modules, antenna- and phase shifting modules, RF-MEMs Switches are widely applied in active antennas for satellite communication and radio links. Radar detection systems are used in defence applications. Radar sensors have been successfully introduced in the automotive business and industrial applications are following. The following pictures show a 60GHz-band 500-Mpbs transmitter and receiver multi-chip modules (MCMs), in which MMICs and filters are mounted into a LTCC package using flip-chip technique. A Lid protects the IC's and the MCMs are directly bonded with printed wiring boards using ball grid array technique, achieving connections for signals and biasing. Solder Ball(BGA) Figure 9: NEC: Schematic of the NEC 60 TR module The transceiver is applied to the IEEE1394 wireless adapter, which demonstrates 17-mcommunication distance in line of sight. Figure 10: NEC 60 TX and RX Module LTCC is offering again numerous advantages like the rf performance and the capability of passive integration. Another one is the design flexibility combined with the approach of modularity. Ericsson has demonstrated how to develop a number of T/R modules working at different frequencies from 15 to 39, using the same or similar RF building blocks for the realisation of the circuit. Filters, waveguide transition and other RF functions were successfully integrated at substrate level together with buried resistors. The other advantage is the high reliability performance. Figure 11: Ericsson: Modular kit of LTCC T/R modules, 15 to 39 GHz LTCC circuits perform a lifetime of more the 20 years, they are stable under harsh environment and they are capable for full hermeticity according to Mil Std. Those substrates can be sourced from foundries like VIA on the free market in Europe. Figure 12: Thales: 3-bit phase shifters for phased array antenna For space and military applications hermeticity is a must. Weight plays an important role and also cost is a key issue for success. Efficiency in packaging and miniaturisation in order to reduce cost, weight and size are key benefits of the LTCC technology. According to the high energy requested for transmission, power dissipation and thermal management are integral part of the packaging requirements and components like heat sinks and ring frames as well as the attachment of those elements to LTCC are in the focus of this paper. The following picture shows the TNO design of a state of the art packaging solution for a TR module working at K- band for nautical application. Figure 13: TNO: Design of a LTCC Transmit Receive Module The package contains a rather complex stepped cavity structure for MMIC, Circulator and IC integration. The LTCC is adhesive bonded to the Molybdenum heat sink and then populated with SMT and chip and wire technology. A further step towards reliability has to be made if full hermeticity of the packaged dies is required. This is the case in space and defence radar applications. An example is given in the following package of a TR module for phased radar antennas. Molybdenum heat sink and Ko-var ring frame are AuSn brazed to the LTCC substrate. This package type shows extremely good thermal and reliability behaviour and it fulfils the MIL specification for hermeticity (>5x10"8atmcm3/s) Figure 14: EADS: LTCC T/R Package Another approach to achieve full hermeticity and good heat dissipation is shown in the following example, in which a Kovar frame is AuSn brazed to achieve hermeticity and a BGA is used on the backside for PCB assembly. Figure 15: SELEX: RF package Kovar ring and BGA 7. Future ssystem requirements Increasing frequencies, power and functionality of systems are the driving forces to investigate novel packaging solutions. In the following pictures typical T/R modules for x-band and wide band are shown. Figure 16: Thales: Block Diagram Wide Band T/R module Typical functions and sizes to be integrated are: • HPA: 20 mm' • Driver: 5 to 10 mm' • LNA: 5 mm' • MEMS: 2 mm' • Decoupling capacitors and others about 30 mm' in total • Temperature sensor In order to have a smart package, the internal room for components is thus typically estimated from 100 to 250 mm' for wideband and 150 to 400mm' for x-band. The developed package must be compatible with a cold plate cooled by fluid. Power to be dissipated is up to 80 watts peak dissipated power, pulsed with a pulse time from 1 to 20^s associated to a duty cycle of up to 20%. Materials with excellent thermal conductivity are required. These materials must be compatible with high temperature gold tin brazing. Return loss of one microwave transition must be better than - 25 dB and the isolation between the antenna access and the LNA access must be better than -80 dB. The isolation between biasing and /or control ports must be typically 50 dB, which is typically realised by a metallic wall. As sensitive devices are encapsulated a good hermetic-ity must be by enclosure obtained. Typically a 5x10-8 Atmcm3/s (air) is required. The package must withstand -55 +125°C 500 cycles without lack of hermetic-ity or electrical performances degradations. 8. Steps to cope with these requirements The expertise in the technology together with the relevant application know how of the customer is the backbone of the small and medium sized European LTCC Foundries like VIA in order to service the free market with high flexibility and medium to low volumes. 9. Understand materials and their properties Figure 17: VIA: LTCC RF packages transition from Du-Pont 951 towards DuPont 9K7. 10. Overview One of the most mature systems used is DuPont 951. The RF capability of this system is limited to about 10 GHz, dependent from design and application. For RF applications above 10 GHz specific low loss materials are available from different suppliers. RF Properties and characteristics are listed in table 1, in comparison with DuPont 951: Table 1: Table 3: Commercial LTCC material for High F requency 951 DuPont 943 DuPont 9K7 DuPont 9K5 DuPont* A6M-E Ferro Permittivity e < 1 GHz 7,8 7,5 7,1 5,8 5.9+/-0.20 1-20 GHz 7,4 7,4 7,1 5,8 5.9+/-0.20 20-40 GHz - 7,4 5.9+/-0.20 Dielectric losses tan 5 [10-3] < 1 GHz < 2 < 1 < 1 < 1 < 1 1-20 GHz 5 1 <1 < 1 < 2 20-40 GHz <15 < 2 < 2 Insertion loss [dB inch-1] Conductor Ag Ag Ag 1- 20 GHz < 1,4 < 0,3 < 0,3 * Published October 2011, available in Europe end of 2012 And in table 2 physical properties and available tape thicknesses, relevant for RF designs, are as given and compared to DuPont 951 Table 2: Physical Property DuPont 951 DuPont 943 DuPont 9k7 DuPont 9K5 Ferro A6ME Value Value Value Value Value Unfired thickness 50 +3 114 +8 165 ±11 254 +13 51 + 4 127 + 9 254 +13 127 +9 254 +14 127* 254* 127 254 X,Y, shrinkage, [%] 12.7 +0.3 9.5 +0.3 9.1 +0.3 9.1* 15.4 +0.3 Z shrinkage, [%] 15.0 +0.5 10.3 +0.5 11.8 +0.5 11,8* 24.0 +0.3 Dielectric constant, (40GHz) 7.4 7.4 7.1 +0.2 5.9+0.2 TCE a [ppm K-1] (25-300°Q 5,8 4,5 4,4 4,4* 5,6 Environmental Lead-free Lead-free Lead-free Large entities like the big Japanese and Bosch are dealing only with high volume markets. Bosch is focussing on automotive and Epcos on the telecom business. They all have their proprietary material systems either provided by external partners or made by themselves. Constrained sintering and plating are key process for cost and performance reasons. All these processes are dedicated to automate high volume production. A summary is given in table 3: Murata TDK-EPC Kyocera K8.8/K15.1 K8/K20 K9,4/K18,7 for telecom for telecom for telecom K7.7 for automotive Plating K7,7 Cu Plating Pressure assisted K5,7 Cu Pressure assisted sintering K5,7 Cu sintering (0-Shrinkage) K5,2 Cu (0-Shrinkage DuPont 943 for Plating automotive It is obvious, that a deep understanding of materials and processes as well as a good knowledge about the application requirements have to meet together in order to provide proper cost efficient LTCC packaging solutions. 11. Benchmark DP 943/9K7_ In order to understand deeply the different materials, VIA made a detailed manufacturability benchmark between DP 943 and DP 9K7. The impact of thermal co-and post processes parameters on RF characteristics and geometrical properties of LTCC was investigated. Radar front ends also including calibration structures and test vehicles were fabricated using Du Pont Green Tape® 943 and 9k7 material systems. The following parameters have been investigated: • The x- and y- dimensions both after co-firing and re-firing • Warping of the whole substrate • Warping of cavities • Characterization of electrical properties up to 110 GHz • Influence of re-firing on the dielectric constant. The LTCC wafer has been a multi-project wafer, having a particularly complicated package combining several features with unfavourable impact on even and homogeneous shrinkage. Uniformity of the shrinkage is constricted by perforations, clusters with more metal content, cavities or cut-outs. The following Figure shows the test vehicle MPW1 to visualize the local differences of metallization. The left part of the substrate contains 80% of the vias and 70% of the co-fired conductors, while the right side has larger metallization of inner layers and cavities as chip pockets or decoupling features. Both parts of the substrates were measured separately in x- and y-axis in order to quantify possible differences in the local and overall shrinkage. The results are shown in table 4. Shrinkage values are given in % deviation from the mean value. Warping is given in ^m of maximum to Figure 18: VIA: Multiwafer design minimum value using a 40mm long scanning line. Re-firing stability in % expansion after re-firing. Table 4: Investigation measurement DuPont 943 DuPont 9K7 Shrinkage sensitivity Shrinkage around cavity areas -0,175% -0,19% Shrinkage in dense metallisation area -0,02 to -0,040% -0,02 to -0,03% Shrinkage in low metallisation area 0,02 to 0,035 0,02 to 0,035% Shrinkage accuracy Overall substrate area +/- 0,16% +/- 0,13% Planarity sensitivity Warping around cavity -3 to 17^m -3 to 17^m Warping in dense metallisation area 9-15^m 10 - 14^m Warping in low metallisation area 15-22^m 16 - 17^m Dimensional Stability vs. refiring Expansion after 3 refirings 0,07-0,11% 0,07-0,11% Expansion after 5 refirings 0,14-0,17% 0,13-0,15% Expansion after 8 refirings 0,17-0,21% 0,16-0,18% Dielectric Constant vs. refiring (20-60GHz) Deviation of the Dielectric constant after 5 refirings -0,10 to -0,12 -0,02 - 0,02 ous generation 943, process stability, higher accuracy in its shrinkage behaviour and RF stability have been improved. Due to these benefits it allows SiP solutions with complex structures and a high accuracy of co-fired features. 12. Understand the requirements Table 5 gives an overview of the different systems available concerning the aspects of miniaturisation and process capability, which impact cost and performance. Datas are based on the experience of VIA.. Table 5: The 9K7 LTCC material shows excellent RF performance up to 100 GHz and above. Comparing to the previ- Investigation DuPont DuPont Ceram Heraeus 951 9K7 tape CT 700 Available sheet thick- 50 ness ^m 114 127 100 165 254 254 320 320 Shrinkage tolerance % Standard 0,6 0,6 0,6 0,8 Minimum 0.3 0.3 0.3 0.5 Cavities 1,0 1,0 1,0 1,2 0-Shrinkage Capability ++ + ++ ++ Line width ^m 100 100 100 100 Line spaces ^m 100 100 100 100 Dimension Stability Green, ^m 30-70 30-50 <20 <30 Refiring, in % 0,1-0,15 0,1- <0,1 0,1 0,15 Embedding capability lines >30^m + - + +++ resistors ++ - - - - - - - high k capacitors +- - - - - - - - Process sensitivity Warpage ++ + ++ - Waviness + + ++ +- Metallisation impact +- - +- +- Maximum density of metallisation % 75 75 75 75 Line density Width 100 100 100 100 Spaces 100 100 100 100 VIA density Diameter ^m 100 100 150 150 Distance ^m 250 250 250 250 Postfired resistors +++ + - - Embedded Fluidic Channels +++ ++ ++ ++ Chambers +++ ++ ++ ++ Bonding Al ++ ++ ++ + Au ++ ++ +- ++ Al heavy wire ++ - - ++ Cavity bonding Al ++ -- -- -- Au ++ ++ + ++ Soldering PbSn 95-5 ++ ++ - - ++ SnAg ++ ++ - - ++ Brazing AuSn ++ + - - - - GeSn ++ - - - - Laser processing Green +++ +++ +++ +++ Fired +- + ++ +++ RoHS/REACH compat- In pro- Yes Yes In pro- ibility cess cess Lead-free no yes yes yes In order to provide adequate packaging solutions for high end applications, manufacturers today are requested for high flexibility and excellent performance at acceptable costs. Also RoHS and REACH compatibility have to be considered. 14. Carbon-fibre Aluminum Composite (CarfAl)_ This material is very promising concerning weight, machinability and costs. The following pictures are demonstrating that customer designed heat sink and ring frame elements can be easily realized. Solder experiments have been carried out using the same AuSn performs as they are used with conventional Kovar and Molybdenum Components. The solderabilty was excellent and no difference to conventional components could be observed. Hermeticity tested was according to MIL Standard at 5x10-9, comparable to conventional components. After Hermeticity tests some defects were identified: small blisters occurred on the plated surface, which could be eliminated at the supplier side by improvements in the plating process. Figure 19: CarfAl heat sink and ring frame after herme-ticity tests 13. Introduction and impact of novel and low cost materials To improve weight and cost, VIA did investigations in alternative materials for heat sinks, brazing and mixed metal wiring. This work is initially focused on the mature DuPont 951 material system and then expanded to the new 9K7 system. The criterions for materials investigated were availability, thermo mechanical properties, weight and cost. All materials and components investigated were introduced on an existing functional RF design. Hermeticity was tested and compared with standard package performance. 15. Solder performs AuSn brazing using solder performs is well established for high hermetic packages required in space and defence industry. It is obvious that Au is a cost-driving element in these packages. Promising alternatives are high lead containing solder materials, which are widely used for soldering of power devices or sealing of ceramic sensor packages. Additionally SnAg alloyed performs have been investigated.. Figure 20: Preform Pb92.5Sn5Ag2.5 on AgPt, CarfAl ring frame Table 6 shows some promising results: Pre form Liquid. temp. °C Pad Ring Herm. atm Result o rs C o CO < 278 AgPt Kovar Bx1G-9 Excellent AgPd Kovar Bx1G-9 Good AgPd Kovar Bx1G-9 Good Au/AuPd Kovar Bx1G-9 Good AgPt Kovar Bx1G-9 Good r^ < LO C r^ Ol .Q Q. 287 AgPt CarFAL 6x1G-9 Bad AgPd Kovar leaky bad Au/AuPd Kovar 5x1G-9 acceptable AgPd Kovar 2x1G-9 acceptable AgPd Kovar leaky bad < C 221 AgPt CarFAL leaky bad AgPd CarFAL leaky bad Au/AuPd CarFAL leaky bad AgPt Kovar leaky bad C3 U