Milena Vujosevic

Physics Based Requirements for Qualification of BGA Components in Temperature Cycling

This paper discusses a new approach for definition of temperature cycling qualification requirement that accounts for the physics of the deformation process in use condition and in the accelerated temperature cycling test condition. The methodology is used to define solder joint reliability (SJR) requirement for Package on Package (PoP) components. Included in the study is the impact of adhesives on SJR requirements. The approach used is different from standards-based approaches that define the requirements in the way that is often independent of package materials and geometries. Physics based damage metrics and numerical modeling was used to comprehend design, technology, material, and temperature profile and provide an in-depth understanding of package deformation and failure mechanism. This, coupled with a developed fatigue law was then used to translate use conditions to test condition requirement.The study shows that accelerated test will not accelerate all PoP solder joints equally and that requirements for PoP to board interconnects will be different from requirements for top–to–bottom package interconnects. Similarly, for component with adhesives, when requirements are based on physics, they must be different than requirements for component without adhesive and those requirement should be a function of adhesive thermo-mechanical material properties.Given rapid changes in technology, explosion of new devices and new use conditions, manufacturers constantly make tradeoffs between performance, cost and reliability. The qualification process needs to be optimized to meet these increasing challenges and qualification based on knowledge of physics presented in this paper is designed to meet these challenges.Copyright

Accessing adhesive induced risk for BGAs in temperature cycling

BGA components in mobile systems are often found with adhesives applied at the package corners. The primary objective of these adhesives is to improve shock margins to prevent failures of solder joints in the case of sudden drop of the device. Different adhesives with a wide range of thermo-mechanical material properties have been in use with limited understanding and quantification of their impact on the temperature cycling reliability of the BGA. The objective of this study is to comprehend the impact of temperature cycle on adhesives applied in the corner of BGA components and to define a range of acceptable properties of adhesives for appropriate material selection. The study utilizes a computational mechanics based Finite Element Analysis (FEA) and a Response Surface methodology to perform a detailed numerical Design of Experiments (DOE). Results were validated with test data. The results of the study indicate that for many adhesives, accelerated temperature cycling tests can lead to wrong conclusions about adhesive performance in the field. Moreover, the impact of corner glue on SJ reliability in temperature cycling is strongly dependent on assembly parameters (board thickness, substrate thickness, pitch, etc.) which has not been typically accounted for in the past. In addition, impact of any individual glue thermo-mechanical material property (Glass transition temperature, Youngs modulus, Coefficient of thermal expansion) cannot be considered independently of other glue properties. All these interdependencies across geometric and material properties, as well as the temperature range the overall assembly is exposed to have been accounted for in defining adhesive property limits that would not compromise BGA performance in temperature cycling.

Predicting vibration-induced fretting in land grid array sockets in simulated field scenarios

Electrical contacts provide means for a separable connection between two current carrying conductors. Sockets containing these contacts could be exposed to mechanical vibration due to shipping, which can result in micromotion between mating surfaces. Repeated micromotion/fretting could lead to wearout of the protective gold layer and expose the base metal that can oxidize. Traditional laboratory based fretting experiments may not reflect the field reliability risks. In this study, a predictive capability is developed to investigate contact fretting in a socket due to random vibration using a finite element approach. Considering the degrees of freedom involved in the analysis and the resolution needed, a multiscale modeling approach utilizing global models with substructures to track high risk areas and local models to monitor fretting wear on the highest risk contact is employed. This approach allowed the study of micromotion at the contact interface, capturing stick-slip phenomenon, which can influence fretting wear. Predictions are validated through extensive experimentation, which includes matching of fretting risk areas, matching dynamic responses, and matching of fretting wipe lengths and location through image processing techniques.

In Vehicle Infotainment and Advanced Driver Assistance Systems: Advantages of Knowledge-Based Qualification over Standard-Based Qualification for Solder Joint Reliability

In this paper, we proposed a novel approach of applying Knowledge-based Qualification (KBQ) to calculate temperature cycle (TC) requirements for Ball Grid Array (BGA)s in a given automotive electronics module. Realistic use (UC) condition temperature profiles were constructed by combining weather data, driver cabin environment, user behavior statistical data, and component thermal behavior. Individual package geometry, solder ball pin map and material properties were incorporated through Finite Element Analysis (FEA) modeling. Direct translation from use condition to KBQ test condition requirement was achieved by using a physics based damage metric - inelastic strain energy density (ISED). We compared KBQ results to Standard-based Qualification (SBQ) results for In Vehicle Infotainment (IVI) and Advanced Driver Assistance Systems (ADAS) automotive modules to demonstrate and explain why KBQ and SBQ differ and why SBQ requirements can be misleading. We also demonstrate how package geometry and material properties impact KBQ, while they are generally ignored by SBQ.

Shock Risk Assessment of BGA Components: Importance of Physics Based Metrics

This study focuses on the implications that the indiscriminate use of empirical shock metrics can have on shock risk assessment of Ball Grid Array (BGA) components in board level tests. Empirical shock metrics discussed are: 1) shock table applied acceleration, 2) top package acceleration and 3) board strain. Computational modeling and fundamental physics considerations were used to prove that the empirical metrics can lead to wrong conclusions about capability of the BGAs and their performance in the field. These empirical metrics can be considered an indirect indicator of solder joint failure, but because there is no universal (setup independent) correlation to solder joint failure, they are not directly transferable. For a metric to be universal it has to have a unique correlation to solder joint damage, i.e. it has to be based on the physics of failure. For the shock failures, such a metric can be solder joint stress or strain. There are two important implications of this study: 1) when indirect metrics are used, the value determined in the system condition should not be directly used as a test condition requirement and 2) BGA capability when measured by indirect metric will vary as a function of board setup. The indirect metrics can still be used, but they must be scaled to account for structural differences among board setups. For that scaling to be valid, it must be rooted in the knowledge of failure physics and the application of physics based metric. The study provides examples on how empirical metrics scaling should be done and discusses the physics behind it.Copyright

Reliability test induced failures vs field performance: Contact fretting perspective

Connectors, based on the applications, can be exposed to a wide variety of reliability risks. To ensure products meet reliability requirements, qualifications tests are performed in laboratory settings. These tests are accelerated to meet the time-to-market requirements. Incorrect accelerated reliability models can lead to inaccurate field reliability risk assessment. Additionally, a chosen test can generate different failure modes, which are unlikely to be accelerated the same amount in the laboratory settings. In this paper, the authors evaluate some of the existing test methods and requirements vs. actual field vibration data for vibration induced contact fretting. The reliability model for contact fretting is based on a multiscale finite element approach. This model is used to evaluate contact micromotion from the standards based test methods that cover operational vibration in automotive electronics and non- operational packaged shipping vibration. The same model is then used to study contact micromotion in actual use condition. Intent of this exercise is to validate the appropriateness of standards based test in reflecting field reliability risks from a contact fretting perspective.

Negative impact of certain adhesive configurations on solder joint reliability of package on package architecture: A comprehensive experimental and numerical study

In this study we assessed the impact of adhesive configurations on package-on-package (POP) interface solder joint reliability (SJR). This problem has not been previously well understood and we applied both experimental evaluation and computational mechanics modeling to develop insights into the underlying physics. We found that certain adhesive configurations can degrade POP shock performance very significantly - so significantly that the overall package shock performance with adhesive is lower than one without. This is contrary to prevalent expectation of impact of adhesives on shock capability. These shock degrading underfill and corner glue configurations are where the overall height of the adhesive is low, such that adhesive does not contact POP interface. The reason for this loss in POP shock capability is due to overall stiffening of board-package interface and higher transfer of shock stresses to the POP interface. Further, it was found that, POP shock risk can be reduced with appropriate adhesive configuration, in particular by using a high glue height such that the glue covers the POP interface. The impact of low coefficient of thermal expansion adhesive configuration on POP temperature cycling (TC) was also studied; similar impact of glue height was found.

Board level interconnect risk assessment in spherical bend

A modeling based methodology for prediction of the performance of board level interconnects of Ball Grid Array (BGA) components in board flexure tests is presented. Detailed work has been done to comprehend the physics of the problem so that sound theoretical framework is employed. A step-by-step validation of the developed computational model is conducted using a set of carefully designed tests. The model reproduces extremely well all the mechanical parameters measured in testing. The validated model is utilized to understand the impact of various package and board design parameters on BGA performance and risk. The developed modeling methodology is a key part in achieving the larger objective of replacing costly and time-consuming board flexure tests with modeling for BGA risk assessment. This methodology will also be a key enabler in providing comprehensive guidance to customers for board-level performance of Intel BGA components in manufacturing, assembly and test.

Temperature cycling performance of ball grid array packages under thermal enabling load

Presence of a thermal enabling load applied to Ball Grid Array (BGA) components creates a distinctive set of evaluations to be completed for reliability of solder joints (SJ) that connect the BGA to the board. This becomes especially relevant for newer technologies that drive to smaller component form factors. In this comprehensive study, a computational mechanics based Design of Experiments approach was utilized to elucidate the impact of load, load assembly parameters and package design on the reliability of BGA joints. Modeling results were validated with test data showing excellent correlation. Multiple conclusions were derived with important implication for BGA design, definition of accelerated test setups and optimization of system level designs for reliability.

Solder joint height impact on temperature cycle reliability of BGA components with thermal enabling load

Thinner organic BGA packages typically lead to higher warpage before surface mount which, after reflow, causes package corner solder joints (SJ) to have higher stand-off compared to solder joints in the package center. Hence the solder joints at package corners can change from typical barrel shape to an elongated hour-glass shape. Conventional understanding of the impact of SJ height on temperature cycling (TC) reliability states that taller SJs results in longer life. Since that understanding was derived based on the evaluations of BGAs without an enabling load applied, the question was if the same conclusions can be extended to BGAs with an enabling load, where critical SJs are in the package corner. To answer that question, a comprehensive experimental-modeling study was conducted. To investigate the solder joint height impact on temperature cycle reliability of BGA components with thermal enabling load, empirical data was collected on identical BGA packages that were intentionally deformed to create different incoming warpage and as a result different SJ height. The experimental results showed the solder joint shape impacts reliability under temperature cycle condition and that reliability is highly correlated with SJ height. Finite Element Method (FEM) modeling studies was conducted to provide physical explanation of observed behaviors. The modeling results correlate very well with experimental data. The key result of the study is that incoming warpage driven SJ height variations will impact reliability of BGAs with enabling load, that taller SJs will results in shorter life then “normal” barrel shaped joint which is contrary to the understanding held in the past.