Challenges of Fan-Out WLP and Solution Alternatives John Almiranez Advanced Packaging Business Development Asia
Introduction to Fan-Out WLP
Introduction World of mobile gadgetry continues to rapidly evolve Products getting thinner and lighter with higher performance Components inside must deliver more performance, memory and sustained power in a smaller form factor Wafer-level packaging is an essential vehicle to achieve these goals
Introduction Wafer level packaging involves thin dies, thin and flexible substrates and tiny passive components, creating many challenges Assembly equipment must adapt to support these challenges, including: Unique substrate handling Efficient die attach Combining passives & multiple die Stacking die (2.5D/3D die attach) In this paper, we will address some of these challenges by providing a packaging technique focusing on die attach
Level of Interconnection 1st Level: Die on Substrate 2nd Level: Component on Circuit Board 3rd Level: Board to Board Unable to deliver the fit, form and function to meet next-generation devices First Level Packaging e.g IC, CSP, BGA Process assembly that involves die attachment, wire bonding and encapsulation. Usually called semiconductor process assembly. Second Level Packaging e.g Card Process assembly that involves solder paste printing, component mounting and soldering. Commonly called PCBA or SMT assembly.
IC Component Evolution The component manufacturers are now fully aware that the evolution is driven by required size, form, device capability and power Solid state transistors integration into one package
Fan-Out WLP Process
Wafer Level Packaging Technique Wafer preparation & sawing Place wafer into dicing tape, singulate the die Metal carrier preparation Clean and remove contaminants from carrier Adhesive lamination Pressure activate adhesive film Wafer reconstruction Pick and place of dies from wafer into the metal carrier Molding Encapsulate carrier in molding compound Carrier removal Remove molded reconstructed dies from carrier Patterning and re-routing RDL to provide metallization to create I/O pads Bumping Bumps are made to form the external I/O terminals Singulation Separate the molded package into its final form
Wafer Reconstruction Wafer reconstruction is the process of die mounting from wafer into a metal carrier All mounted dies are known good dies (KGD) Die attach or pick and place machine receives metal carrier with laminated film Feed with sawed wafer Pick die Inspect and place accurately into an arrayed form.
Fan-Out WLP Challenges & Solutions
Die Mounting Challenges Placement accuracy Typical target +/- 10 μm x-y positional accuracy Die rotation Can be critical for processes such as patterning and redistribution layer, and if die stacking will be required. Nominally, less than 0.15 is required. Die orientation 0, 90, 180, mirror orientation Die handling and packaging Die sizes range from 1mm square to 10mm square depending on application Misplaced Die Top alignment Accuracy relies on the pattern present at the top of dies Die stacking Combining ASIC and memory where the top die to bottom die is critical Rotated Die
Speed vs. Accuracy Speed vs Accuracy have opposing trends and normally cannot merge on a single system High-Accuracy placement machines cannot go faster, High-Speed machines cannot be more accurate by simply going slower cph Speed getting slower Merging of Semiconductor and SMT assembly technologies enables new paradigm in placement equipment This is the foundation of a platform technology / architecture, from the leveling pads, to the spindle/manipulator to handle the device to be placed Placement accuracy getting better μm
Techniques Enabling High-Accuracy Placement Importance of Mapping Each positioning system is unique, thus any absolute vector from encoder position to a particular location can differ from the predicted location Absolute moves over a large area are therefore impossible and the final destination will vary from system to system Solution: A machine map that correlates positioning system imperfections to encoder locations Enhanced Mapping Models behavior of the positioning system and relates encoder coordinate locations to machine coordinates Machine locations are typically defined by the mapping plate(s) Encoder locations are defined by the (X,Y) linear encoders located on the X and Y beams (axis) Plate and grid sizes define the area and mapping resolution Enhanced mapping process certifies measurements modeling
Enhanced Mapping Illustration In an encoder coordinate system, moving from P1 to P2 is purely an X movement When the same movement is done in a machine coordinate system, both X and Y movements are required Due to these spatial differences, both an X and a Y map are generated to translate between the two coordinate systems
Techniques for Accuracy Stability AOI feedback Accuracy is either self-verified or validated through capable automated optical inspection (AOI) systems The challenge: both processes may agree on repeatability but not means, thus a method to align to a common reference is key Appling AOI feedback is essential to refine each placement or spindle bias to a nominal placement coordinate, enabling multi-spindle systems to achieve higher throughputs without affecting accuracy Mold operations or thermal effects on materials High material counts and HVM can only be realized by leveraging the abilities of process feedback from the AOI systems Feedback method supports post process steps requiring up stream corrections to accommodate / enhance final process performance conditions Software utilities enable seamless import of offset data into the placement system
Carrier Handling Challenges Conventional die attach machines are designed to handle leadframe, strip laminate and singulated substrates; maximum width is usually 300mm or 12 inches. They cannot handle larger substrates. The transport conveyor and board support are of fixed design, thus needing to change the whole transport rail if there is a new package size Laminate Leadframe
Increasing Throughput with Large Board Handling Wafer level packaging typically uses 300mm metal wafer format carrier Die size growing to meet the demand for more I/Os, thus 300mm carrier will frequently have to be loaded/unloaded, hampering productivity This can be addressed by transforming the wafer format into a larger panel
Transitioning from 300 to 600mm 300mm metal carrier 600mm glass carrier 300mm metal carrier 600mm glass carrier
Precision Lifter and Board Support Precision Lifter and custom top plate provides flexibility on board support for different board size, different materials and different type Large Panel Support Tooling Thin Laminate, Porous Tooling Spring Support Tooling with Top Frame
Multiple Part Challenges The wafer level packaging will not be limited to die only. Passives such as 01005 chips, interposers and spacers will eventually be part of the process. Also, a carrier may consist of multiple part numbers for more complex circuits. Wafer Innova & Innova Plus - Handles various sizes up to 300mm, capable of feeding multiple wafers Tape & Reel Standard Tape feeders Dual Lane for 0201 and 01005 Matrix Tray Stationary 2 x 2, 4 x 4 & JEDEC Automated Stackable Feeders 2 x 2, 4 x 4 & JEDEC
Die Stacking Process The cross section shows layer of embedded die, epoxy and wafer tape. Due to presence of bumps underneath, the bottom side is not flat. There is a potential surface topography of 100-150 μm. Embedded Die Mold Epoxy Through Mold Via (TMV) 30 50 μm depth 150 250 wide Wafer Tape (Blue Tape) Thickness 100-150 μm Bump Topography (100 150 μm) Cross Section for illustration purposes only
Die Stacking Process The WLCSP die will be placed on top of the die via TMV WLCSP Die Size 15 x 15mm Thickness 0.4 1mm Wafer Tape (Blue Tape) Thickness 100-150 μm Bump Topography (100 150 μm) Cross Section for illustration purposes only
Die Stacking Challenges and Solution Die stacking requires high accuracy between top & bottom components Misalignment may cause loss of interconnection between components High-accuracy placement and local alignment reduces this potential problem Different height of bottom component can cause warping or planarity issues Impact sensing eliminates this issue
TAP - High Accuracy by Top Alignment Process Traditional assembly aligns a device / die for placement based on its bottom features Placement based on top features is limited as the die active surface cannot be imaged while held by a pick tip or spindle/manipulator These barriers can be eliminated through Top Alignment Process (TAP) Die outline and the top active pattern are inspected, setting reference between features Following top inspection, the die is picked and bottom side is inspected, setting the relationship between die outline and spindle position Top to bottom correction is then applied with the outline serving as reference, setting the final offset based on top features and eliminating inaccuracy caused by component shift between inspect and pick
Alternative Solution Wafer Feeder Pick & Place Machine
Placement Capability AMS high Cpk values during 6-hour run
Conclusions
Conclusions Demand for smaller but powerful gadgets or larger but more complex and much more powerful will continue Product and component designs will continue to evolve to meet the market demands The number of thinner, more powerful and ICs will increase The trend of combining system capabilities like ASIC and memory storage will drive higher wafer level packaging complexity The supply chain will have to adapt to these requirements Materials, equipment and processes will also evolve and this emerging packaging technology will drive the semiconductor/packaging industry and wafer fabrication houses