INEMI WARPAGE CHARACTERIZATION PROJECT ACCUMULATES WARPAGE DATA TO ASSIST MANUFACTURERS IN THE DESIGN OF NEXT-GENERATION PACKAGING

• By: iNEMI
• On: 11/27/2019 16:21:10
• In: iNEMI Blog

The ability to predict and manage package warpage at an early stage of product development is critical to ensure product yield and reliability. It is, therefore, important to characterize and monitor package dynamic warpage trends for new package technologies such as 2D, 2.5D and 3D architectures; understand the influence of new solder systems like low-temperature solder (LTS); and derive modeling techniques to enable more robust package designs for PCB assembly.

Electronic Package Dynamic Warpage Trends

iNEMI recently completed the fourth phase of its Warpage Characteristics of Organic Packages project. Over several years, this multi-phase project has accumulated data on warpage behavior for a considerable number of packages with the objective of providing information to electronics manufacturers to refer to when designing the next generation of package technologies and developing SMT recipes. As of today, the project team has collected 15K warpage data points for 56 different packages from 16 package technology types (as shown in Figure 1), including 2.5D silicon interposer packages. The package warpage database created allows us to assess technology trends, identify potential risks of adopting different reflow profiles, and validate the capabilities of various simulation models to improve overall package warpage management across the electronics industry.

Figure 1. Dynamic warpage range for each packaging technology
 

Low-Temperature Reflow Profiles

One of the areas the project team investigated in Phase 4 was the impact of lead-free, low-temperature solders on several different packaging technologies as a function of peak reflow warpage reduction, rate of dynamic warpage magnitude change, as well as shape changes during the solder solidification phase as shown in Figure 3. Low-temperature solder can reduce, but — in some cases — may actually increase package warpage at solder solidification, depending on the package technology/design. The percentage reduction changes with package technology types and package attributes as shown in Figure 3. These results provide high-level information for industry to assess the risk of SMT and identify areas to mitigate the risks of defects like hot tearing for Sn-Bi based LTS paste.
 

Figure 2. Dynamic warpage behavior for SAC reflow or low-temperature reflow for any package type
 

Figure 3. Package warpage change percentage as a function of different packaging technologies

Warpage Modeling Approach

The Warpage Characteristics project also looked at the effectiveness of modeling systems for predicting package warpage. The general assumptions used in modeling can be over-simplified with calibrated material properties to obtain a good warpage prediction on a case-to-case basis. However, such a method is not suitable and sustainable for complex packages (e.g., heterogenous integration package) where new materials and boundary conditions are changing based on assembly process steps. For these comprehensive packaging technologies, the initial design choice and risk assessment based on simulation are critical prior to committing to significant investments in resources and capital. The project team focused on how to refine the modeling framework to be able to capture the actual impact of assembly processes on package warpage (e.g., mold flow, post mold cure). Results showed that a wide range of material models, time-temperature superposition for visco-elastic properties and other boundary conditions need to be considered to ensure that the package's actual mechanical characteristics and stress history are captured. Significant refinement in current modeling techniques to mimic packaging assembly and test process (as shown in Figure 4) is needed to derive a more robust framework for future co-design efforts.  

Figure 4: Modeling multi-physics in assembly process
 

Summary of Project Results & Findings

1. The dynamic warpage data collected covers a wide range of packaging technology from small FBGA packages to large 2.5D packages. Dynamic warpage varies across multiple packaging attributes, including package construction, materials, body size, die size, etc. The data can be used by industry to normalize the understanding of package dynamic warpage and perhaps derive solutions to address package warpage and PCB assembly. 
2. For most packages, package warpage at peak reflow reduces with LTS reflow conditions. The range of warpage reduction varies as well as the package thermal strain behaviour changes with material and design choices. Additional assessment is needed to understand the impact of warpage change during LTS solidification phase on hot tearing defects.
3. Initial FEA analyses using different simulation tools demonstrated consistent results for thermal and visco-elastic stress analysis of molded single and stacked die packages. FEA tools that leverage the PVTC (pressure, volume, temperature and cure) material model to describe the curing process of the mold demonstrated more insight on the evolution of warpage as function of curing percentage. This gives better prediction to the actual experimental result. The team attempted to predict Cu-mold strip warpage but more work is needed to refine the boundary conditions. Hence, continued collaboration and development is needed to derive a reliable modelling framework (e.g., co-design, consistent material database, multi-physics capabilities).

Next Steps

Project Leadership & Participants