Overview
The trend toward smaller device geometries has increased IC functionality and speed. With smaller dies, heat dissipation and the thermal characteristics of the package play a significant role in chip reliability. More and more the critical factor in determining IC reliability is not the IC itself, but the package. At the system level, reliability depends on matching the thermal expansion properties of the die, interconnects, package and PCB to avoid thermal differential expansion due to internal or external temperature changes.

Package engineers must also consider thermal and mechanical stresses associated with the manufacturing process in order to reduce any manufacturing induced defects. During the solder re-flow operation temperatures in excess of 220C are realized. This high temperature, combined with moisture absorption in the package, will lead to delamination and popcorning problems. PakSi-TM allows package engineers to save time and money by determining the thermal and mechanical characteristics of the IC package before a design is committed to fabrication.

PakSi-TM was developed by a team of engineers who are skilled in the area of package design and analysis. This expertise was applied to create a truly state-of-the-art tool with unparalleled accuracy and ease-of-use, for analyzing the effects of hygro, thermal and mechanical stresses on a variety of both custom and industry-standard package styles.


PakSi-TM Simulation Flow
In order to simulate the effects of thermal and structural stresses on a particular IC package there must be a detailed finite element model of the package with all package parameters. To begin an analysis, the model must then be configured according to the design information. The corresponding loading conditions, including power, ambient temperatures, etc. must also be specified. Once this is accomplished, the model can be solved using the very accurate and efficient numerical solver. The results of the analysis can then be manipulated to graphically highlight package behavior under the specified analysis conditions. The simulation results are also available in the form of a detailed text file report.

Package Modeling
PakSi-TM allows design engineers to quickly and easily construct complex, finite element simulation models for various types of electronic package designs. Convenient built-in parametric modeling is available for standard JEDEC packages. With the user-friendly GUI, to define the physical package, all you need to input are the package dimensions and preferred material properties. The material properties can also be obtained from the extensive library embedded within PakSi-TM. To ensure correct data entry, all parameters to be specified for each data entry screen are illustrated on the screen so that the meaning of the parameter is never in question.

The extensive internal database of semiconductor packages covers both leadframe and BGA types of package designs. In addition, the Direct Cad Interface (DCI) to Gerber files can be used to construct non-standard package models and create detailed metal traces in the substrate/PCB. Supported package types include: Ball Grid Array (BGA), Quad Flat Package (QFP), Dual Inline Package (DIP), Small (or Short) Outline (SO), Plastic Leaded Chip Carrier (PLCC), Chip Scale Package (CSP), Flip Chip Package, Quad Flat No Leaded Package (QFN).


Physics-based Numerical Simulation
Once the finite element model for the package to be analyzed is developed, PakSi-TM is able to perform multi-physics stress modeling of the defined package and make a reliability prediction of the defined model. PakSi-TM can be configured, through a series of dialog boxes within the GUI, to perform any of the following analysis on the package:

• Thermal resistance
• Popcorning
• Interface delamination
• Package warpage
• Die cracking
• Thermally induced stress

Thermal Resistance
The temperature at which a package operates determines the service life of a device since excessively high temperatures degrades the chemical and structural integrity of various materials used in the packaging. PakSi-TM can be easily set up to run various temperature related analysis for both single chip and multi-chip modules including Theta-ja, Theta-jb and Theta-jc analysis at different air speeds and power dissipation calculations for the package top, sides and PCB. Heat transfer coefficients assigned to external surfaces are based on experimental results. This produces more practical boundary conditions in the analysis than simulated ones. Detailed package modeling including metal trace and via considerations results in a more accurate prediction of thermal responses.


Popcorning
Most IC plastic package components and organic molding compounds are not totally impermeable to water vapor. At room temperature, epoxy-molding compounds absorb moisture up to approximately 0.5% of their weight when exposed to high relative humidity. During the manufacturing process when the package is heated up quickly during the re-flow soldering operation, the absorbed moisture can evaporate at an explosive rate and thus crack the encapsulation material. This damage is commonly referred to as the popcorn effect. PakSi-TM is able to evaluate and analyze the effects of the moisture content in the Epoxy or Encapsulated Molding Compound (EMC), and material interfaces, the potential for delamination and crack propagation, and the popcorning of the EMC.


Interface Delamination
At each interface of a package, moisture can hydrolyze the epoxy, which in turn degrades the interfacial chemical bonds and produces internal stresses. In general, delamination results from the internal stresses generated in the solder re-flow procedure during the assembly of plastic encapsulated devices on circuit boards. Therefore, delamination may occur between the molding compound and the lead-frame, or between the compound and the die surface. PakSi-TM can be used to analyze and predict the saturation moisture level in the molding compound and at each interface, as well as the overall moisture content inside the package. Based on the re-flow temperature and pre-conditions, it can also determine the initial delamination length and the final propagation length along each interface. The delamination analysis is a subset of the popcorn analysis.


Warpage
Trends in IC packaging are towards mounting larger dies on to smaller and thinner substrates. A typical exaple is the Thin Small Outline Package (TSOP). Since the package thickness is very small, internal stresses can cause external deformation, the so-called warpage phenomenon. Warpage changes the lead coplanarity that significantly influences the asembly process. PakSi-TM provides a powerful set of tools that allows package designers to accurately estimate the Warpage deformation in different parts of a package. The ultimate goal is to choose suitable materials and assembly processes to minimize warpage and the internal residual stresses.


Die Cracking
The first step in package processing is bonding the die to a pad. During the curing process the maximum temperature typically reaches ~180C. Internal stresses are developed during the cycle due to the mismatch of thermal expansion coefficients of different components of the package. If the stress level exceeds the strength limit of the die, cracking occurs in the die. Because of its high-speed computational capability and extensive materials database, PakSi-TM can accurately analyze and determine the onset and propagation of cracking within the die. It provides a useful tool for packaging engineers in determining and/or modifying the package design or process.

Thermally Induced Stress
PakSi-TM is a valuable tool for evaluating the thermally induced deformation and stress in the IC package due to die heating. In the analysis, the temperature distribution in the package is calculated by a thermal analysis. Then the temperature distribution in the package, with or without the PCB, is used as the loading for the deformation and stress analysis. PakSi-TM automatically performs a coupled-field analysis, i.e. a thermal analysis followed by a structural analysis. The thermal results include Theta-ja; Psi-jt; and heat dissipation through the package top, side and PCB. The deformation result is the warpage of the package due to the thermally induced stress condition.


Output Results

	Theta-ja, Theta-jb, Theta-jc, Psi-jt
	Delphi Model
	Thermal Stresses
	Die Cracking Prediction
	Delamination Prediction
	Popcorning Level Prediction
	Solder Joint Fatigue Life Prediction

CAD Interfaces
PakSi-TM can read Gerber, Cadence Allegro, Encore BGA, Mentor Graphics MCM Station and Zuken CR-5000 package designs; Gerber and Cadence Allegro PCB designs.