Integrated Package MCM-D/L/BGA with Embedded Passives
Philippe Poyet, Marc Kavass and Gèrard Teissier,TMW — THALES MICROWAVE, 29, Av. Carnot 91349 Massy Cedex, Phone (33) 1 69 93 40 77, Fax (33) 1 69 93 42 10, Email: philippe.poyet@tcm.thomson-csf.com
Abstract
The always increasing silicon integration leads to components with a higher number of I/Os (>1000), higher clock speed and power. New fine pitch packages like CSP or flip-chip have been developed to fit with these high integrated components. The substrates on which these components are mounted must have the capability to interconnect them at the lowest cost: high routing density, low parasitic elements, embedded passives resistors and capacitors.
THALES MICROWAVE, an European leader in microelectronics, has developed, in the frame of the LAP project, a high integrated and low cost packaging technology MCM-D/L/BGA. This package combines the advantages of each technology: high density and high performances of MCM-D, large size and low cost of MCM-L. This package is obtained by processing MCM-D structure on build-up laminate substrates manufactured by Cimulec and by mounting balls on the back side. NiCr resistors and Ta2O5 capacitors deposited by sputtering are embedded in the MCM-D structure. An electrical demonstrator for a large volume application in the field of satellite television has been designed and manufactured and has allowed to validate the package for an operating frequency up to 2.4 GHz.
Key words: MCM-D process, build-up laminate, BGA package, integrated passives
Introduction
Silicon integration will lead to components with a higher number of I/Os (>1000), higher clock speed and power.
New fine pitch packages like CSP or flip-chip have been developed to fit with these high integrated components.
These high density components will have to be assembled on substrates that must have the capability to interconnect these dice at the lowest cost.
The integrated package MCM-D/L/BGA will allow reaching this goal by mixing the main advantages of each technology: high density and high performances of MCM-D, large size and low cost of MCM-L.
In the frame of the European project Esprit LAP, we have studied the MCM-D process on build-up laminate substrates developed by Cimulec during the FLIPAC project.
Description of the structure
The structure of the integrated package MCM-D/L/BGA is described in the drawing hereafter.
The MCM-L substrate is constituted of a core high Tg laminate with through holes and a build-up structure on each side. Both sides are planarised to obtain a surface roughness compatible with the MCM-D process: stairs lower than 5µm.
Build-up laminate
A test vehicle with dimensions 28.15mm x 28.15mm has been defined with an array of 191 through holes on a pitch of 1.5mm according to the drawing hereafter.
The through holes in the polyimide laminate are filled in on both sides by a rolled film of RCC (Resin Coated Copper) and the planarisation is made by another RCC film. The copper of this second RCC film is completely etched and the resin is lapped until it reaches the same level as the underneath copper.
On the front side, pads are implemented to ensure the connection with the vias of the MCM-D structure. On the back side, assembly pads are implemented to allow the mounting of the balls.
The structure of the build-up laminate substrate is shown in the drawing hereafter.
MCM-D process
The MCM-D stack is processed on the MCM-L substrate. The dielectric is BCB which has a curing temperature of 250°C that is compatible with the Tg of the laminate. A first layer of BCB is deposited to planarise the stairs remaining at the surface of the MCM-L.
The frozen process is:
- internal layer: TiW/Cu by sputtering + Cu/Ni by selective electroplating,
- top layer: TiW/Cu by sputtering + Cu/Ni/Au by selective electroplating,
- dielectric: BCB4024 thickness 6-7µm deposited in a spin coater.
In order to reduce the cost, the size of the MCM-L panel has been increased to 12îx12î which is compatible with our automated manufacturing line.
Passives integration
In order to reduce the parasitic elements for high frequency applications, the integration of both NiCr resistors and Ta2O5 capacitors in the MCM-D structure has been developed.
NiCr and Ta2O5 are deposited by sputtering in the same on-line equipment as the metal layers.
The specific resistance of NiCr deposited on BCB is 50 W/square (50 and 120 W/square on alumina). The values are in the range 50W to 50kW with a tolerance of ±25% without trimming. The thermal coefficient of resistance TCR is lower than 50 ppm/°K. The high temperature stability is better than 1% after storage 1000 hours at 150°C.
The compromise between the deposition time of the Ta2O5 layer and the withstanding voltage of the capacitors has led to a dielectric thickness of 650nm which corresponds to a specific capacitance of 350 pF/mm. For yield reasons, the size of the capacitors is limited to some mm which gives values in the range 10pF to 1nF with a tolerance of ±25% without trimming. The loss angle tangent (tg d) is below 100.10-4. The breakdown voltage is above 30V. The capacitance change is below ± 10% in the temperature range —55°C;+125°C.
Balls mounting
After the realisation of the MCM-D/L substrate, balls are mounted on the back side and a frame on the front side.
High temperature solder spheres Pb90/Sn10 alloy-0.889mm diameter, which are not reflowed during the second level assembly, are attached to the substrate using solder paste Sn62/Pb36/Ag2.
Ball shear strength has been measured around 1kgf, validating the mounting process.
Results
We have deposited one BCB layer and one metal layer on the front side and one BCB layer on the back side of build-up laminate substrates provided by Cimulec.
Good results have been obtained:
- no deformation of the laminate substrate during the MCM-D process (curing at 250°C)
- good adhesion of the BCB layers on the laminate substrates
- good definition of the MCM-D pattern
Some photographs of the test vehicle are shown hereafter:
Electrical demonstrator
In order to validate the developed technology, an electrical demonstrator has been designed and manufactured.
This demonstrator is a direct broadcast switch (DBS) for satellite TV subscribers operating up to 2.4 GHz.
The packaging solution must be compliant with the very demanding electrical characteristics (throughpass attenuation and crosstalk).
An existing solution on PCB contains a DBS 5:4 multi switch making four satellite plus one terrestrial signals available to four subscribers available at the same time. The MCM-D/L/BGA solution, thanks to a higher integration, allows the realization of a DBS 9:4 multi switch.
The module contains seven dice (two analog-switch-ASICs, four digital-switch-controls and one inverter). In addition, 12 pull-up resistors and 32 decoupling capacitors are embedded in the MCM-D substrate. The MCM-D/L/BGA package has dimensions 20mm x 20mm with 115 I/Os.
The structure of the substrate is the following:
- build up laminate
- BCB planarisation layer
- 1st metal layer with resistors (NiCr/TiW/Cu/Ni)
- Ta2O5 capacitor dielectric
- BCB layer
- 2nd metal layer
- BCB layer
- top metal layer (TiW/Cu/Ni/Au)
The photographs below show the two sides of the module.
Conclusion
The integrated package MCM-D/L/BGA with embedded passives is a cost effective solution which avoids the use of an expensive separate package to encapsulate the MCM-D substrate.
This solution has also been proven to reduce the parasitic elements and thus to improve the electrical performances for high frequency applications.
This solution allows to optimize the routing between the MCM-D (critical signals) and the laminate (supplies).
It constitutes a good compromise mixing the advantages of both technologies MCM-D and MCM-L.
Acknowledgements
The authors wish to thank their LAP and FLIPAC Partners for their support in submitting this paper and their technical efforts in the projects, particularly Cimulec which has developed the build-up laminate suitable for the MCM-D process and Hirschmann which has designed and tested the electrical demonstrator.
We recognize the financial support of the European Union, without which this work would not have been possible.
References
“Low cost Large Area Panel processing of MCM-D substrates and packages” ESPRIT project 26261
“Fine Line Interconnection and Packaging” ESPRIT project 26280
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