The very rapid evolution of semiconductor IC technology has put immense pressure on the packaging technologies. The new packages must have increasingly smaller footprints, simultaneously offering improved performance in handling high frequency clocks, very large number of pin-outs and capability to handle and dissipate large heat densities. In other realms there is a need for multilayer solution for miniaturizing the HF circuits in GHz range, while the Micro Electro Mechanical System (MEMS) devices need an one technology solution for packaging sensors / actuators requiring electrical, optical, fluidic and mechanical ‘interconnects’. The Low Temperature Co-fired Ceramic (LTCC) Technology offers and ‘integrated’ solutions to these problems.
Task at hand
C-MET has presently embarked upon the technology demonstration of multilayer ceramic packages using LTCC process for application specific high reliability packaging including the MEMS packaging application.
Infrastructure and Facility
C-MET has built a 1500 sq ft. Class 10000 clean room for housing the complete LTCC process line. Following pictures provide a glimpse of the facility.
The clean room facility runs on auto-control protocol, and is fully backed-up with a 140kVA generator.
Amongst a host of equipment already available with the group, the following equipment has been specifically added for LTCC fabrication:
The Equipment are, but not limited to
LTCC Fabrication Process
The LTCC tapes used by C-MET are delivered as sheets of size 6”×6”. While the circuit area is 4”×4”, the rest is used for registration and process qualification up to stacking. The process starts with via punching of each layer as per individual layer design. If the layer requires any open or buried cavity, it is prepared using the same machine. The alignment holes are also punched using via puncher. Via filling is done subsequently, using stencil printing. This is followed by screen printing of conductor pattern using usual screen printing process. Once individual layers are ready, those layers are stacked in sequence, and aligned mechanically. The stacks are then sealed in a plastic bag are and laminated under prescribed temperature-pressure cycle using isostatic laminator. The individual circuits are then singulated using cutter and fired using a special programmable batch furnace. All the fabrication processes described above, are carried out in our Class 10000 Clean Room. For preparing packages, there are two main post firing processes that remain at this stage. One, processes for preparing BGA, and second, processes for sealing. We have chosen two possible options for sealing processes, viz. Solder sealing and Seam sealing and three BGA formation processes, viz. sphere attachment, stencil printing and electroplating. Following is the brief description of sealing and BGA formation processes.
Solder sealing is done using solder performs. This needs conducting and solderable surfaces on either side. Further, interconnection from lid to base would also require similar pads. As per present plan, these conducting surfaces would be made using a co-fired Ag-Pd paste. Finally, solder sealing is done by reflow after components are placed.
Seam sealing requires brazing of metallic parts to the ceramic substrate. Brazing is a three step process that requires post-fire adhesion layer, post fire barrier layer, brazing layer, each being individually printed and fired. The brazing layer is a low temperature alloy, and is required to be fired with the metallic part. The BGA could then be formed by one of the processes described below. It may be noted here that final sealing process by seam sealing can be done only after components are placed inside the base.
BGA Formation Processes
The three process options selected here have their own advantages and disadvantages. Sphere attachment is done using pre-formed solder spheres which could be attached using another solder paste or through reflow of the same BGA spheres. This process is simple, and cheap, but has limitations in terms of minimum BGA size. The stencil printing process is also cheap, slightly more process dependent but can make BGAs with sizes up to even 100µm but with relaxed pitch. The electroplating process is having more process steps making it costlier, but is capable of even sub-100µm BGA with equal pitch. Through the present project we are attempting an exploration of each one of this process. The materials selection for each process has to be done very carefully so as to ensure subsequently lower temperature steps.
BGA by solder sphere attachment is done by directly placing micro-spheres of high-Pb solder on conducting pads with high alignment accuracy. The solder pads are dispensed with a viscous flux or attachment solder paste prior to this placement. Reflow of these solder spheres or attachment solder paste would produce BGAs.
For good adhesion, one requires Under Bump Metallurgy (UBM) or adhesion / barrier layers before solder deposition. High lead Sn-Pb Solder is deposited by printing solder paste onto the Under Bump Metallurgy (UBM) by stencil printing process and reflowing the solder paste to form a spherical solder bump.
In this case the UBM consists typically of an evaporated adhesion layer that provides conducting path for electroplating, followed by photolithography to open areas of BGA pads. The BGA pads are then electroplated with adhesion enhancing and solder-barrier layers. Plating of the high lead Sn-Pb solder alloy is then done over the UBM followed by removal of resist and underlying evaporated seed layer. The final step is of reflow that forms the solder bump.
At present, the packaging team is engaged in setting-up the principle unit processes for the fabrication of LTCC structures. The important processes that are being set-up include via punching, via filling, screen printing of conductive patterns on green tapes, stacking, lamination and co-firing. BGA preparation by attachment, stencil printing and electroplating are also being set-up. The following optical photographs provide a view of the current status for some of these processes:
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