Karumbu Meyyappan

Effects of Glue on the Bend Performance of Flip Chip Packages

To combat reliability margin loss in ball grid array (BGA) packages specifically in mechanical shock and vibration testing, companies are exploring the possibility of using glue and complete underfill to mitigate risk to second-level interconnects (SLI). Though glue has been demonstrated to have a positive influence on SLI reliability margin, it can have adverse affects on the rest of the package such as substrate or first-level interconnects (FLI). This paper explains details on how glue modulates the overall reliability of the package. Finite-element modeling (FEM) along with low strain rate bend tests was done to prove the effect of glue on solder joint reliability. Further, shock testing was done to demonstrate how the glue modulates the shock performance. The improvement in SLI reliability was highly dependent on the choice of glue.

Application of shadow moire technique to board level manufacturing technologies

Shadow moire technique is already well-established as a technique for evaluating component warpage. Improvements to the metrology have increased its usefulness for evaluating various board-level manufacturing technologies. This experiment used the technique for two key board-level applications: i) Bare printed circuit board (PCB) warpage during the lead-free reflow: PCB warpage is becoming more significant due to the higher temperature required for lead-free reflow. This work used shadow moire dynamic reflow simulation to explore the effect of three variables - board thickness, glass transition temperature, and vendors - on board warpage. The test data showed clearly that board thickness is the most significant variable; ii) Assembled board warpage under mechanical preload from thermal solutions and under thermal cycling: One of the heatsink design approaches for desktop motherboards is to apply a preload to the CPU area of a motherboard in order to ensure thermal performance. This approach requires a more careful control of the preload. Board warpage is significant and is identified as a good metrology for preload estimation. Motherboards under three different preloads in the CPU area were measured. Both the global board warpage and the local board warpage around the CPU area were measured for preload correlations. The measured board warpage was correlated to the board preload successfully. In addition to these two issues, other examples are discussed briefly to demonstrate the capability of the board level shadow moire technique. The work proved that the metrology has become an indispensable thermal-mechanical analysis for manufacturing technology evolutions

Fundamentals based approach to predict socket stack performance

The field performance of a separable socket/connector is governed by its ability to maintain the target contact resistance throughout its life. Contact resistance stability and protection against field degradation mechanisms is achieved by maintaining a critical normal force on the contact. With growing socket contact pin counts and package complexities there are challenges with achieving and maintaining this critical contact normal force across the array of contacts in the socket. In this effort, the authors characterized the key mechanical and resistance relationship (Force-Deflection-Resistance)through single contact resistance measurements. An analytical model of a socket stack was developed to better capture the contact normal force distribution across the contact array. The contact forces extracted from the analytical model were then converted to contact resistances through the Force-Deflection-Resistance relationship obtained from single contact resistance measurements. Force-Resistance variation from contact to contact was comprehended through a Bootstrapping technique. This fundamental based approach of using finite element, single contact data and statistics was then validated against fully enabled socket measurements using electrically daisy chained test vehicles.

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.

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.

Impact of corrosive environment on contact resistance of infrequently mated connectors

Separable connectors are commonly plated with gold for optimum electrical properties. In aggressive environments, gold provides adequate resistance for corrosion of underneath copper. Corrosion resistance has been historically studied by exposing the connectors to a mixture of corrosive gases through a mixed flowing gas (MFG) test. A few studies in the past claimed that contact electrical resistance can stay stable even in unplated samples if micromotion between the interfaces can be eliminated. However, contact force relaxation and micromotion can disturb the mating interface resulting in resistance increases. To understand the interactions between surface finish plating, end usage and micromotion under an aggressive environment, a DOE was planned with linear edge connectors in a MFG chamber. The effect of end usage was addressed through mated and unmated connector samples followed by electrical testing. The impact of mechanical force was studied by controlling the micromotion through a custom vibration fixture that was designed to operate within the MFG chamber. Results from our studies are expected to initiate efforts towards improving existing industry test standards with recommendation to consider the effects of mechanical retention and end usage in addition to plating material and thickness.

Optimizing gold thickness of land grid array pads for cost, performance and reliability of connectors

Gold plated socket contact tips and substrate lands are commonly used in the electronics industry for optimum electrical properties. Increasing the gold thickness improves corrosion resistance and provides stable contact resistance at lower mechanical forces. However, with the ever increasing cost of gold it is critical to optimize the socket stack for cost, performance and reliability. To demonstrate this balance, a study on the stability of socket contact resistance was conducted at various gold thicknesses of substrate lands ranging from 60 to 400 nm using a single contact test setup. Contact forces were measured with a tri-axial force sensor that provided a means for extracting the coefficient of friction between the contact interface and substrate lands. An empirical model that relates the contact resistance to mechanical force and plating thickness was derived from the Force-Deflection-Resistance trends observed across various gold thicknesses. In this study, some of the plating options considered included a layer of Palladium (Pd) between the Nickel (Ni) and Gold (Au) layers to improve the corrosion resistance. The corrosion resistance was quantified by exposing the substrates to temperature, humidity and mixed flowing gas (MFG) chambers. The experimental findings could be used for optimization of the Gold plating thickness for cost, performance and reliability.

Evaluation of corrosion films on Cu and Ag reactive monitors deployed in data center server environments

Corrosion driven electronic hardware failures are a concern for data center and mission critical server applications. Increasing needs for ubiquitous computing has resulted in an explosive growth of data centers around the world. It has been observed that in emerging markets the contamination either from polluted air or through particulate matter gets inside the electronic systems impacting their long term reliability. The present work focuses on reactive monitoring of these environments using metallic coupons. Surface analysis of the corrosion films showed the presence of sulfide and chloride products for silver and predominantly oxides for copper. Not only did silver correlate well with locations susceptible to hardware corrosion failures they were less prone to polysiloxane contamination than copper and hence a better indicator of the corrosion risks.

Curvature Based Metrics for Interpreting Warpage and Predicting Reliability in Second Level Interconnects

The authors in this paper have proposed a new methodology to predict curvature from depth data. This is a whole field methodology that measures curvature by the method of fitting surfaces through moving patches. Ability to estimate curvature and localized bend modes or shapes can help identify process defects like solder joint non-wets, shorting and also predict board flexure induced stresses in second level interconnects. This methodology has been demonstrated through synthetic range data as well as true experimental data. Authors in this paper have proposed methods to reduce the impact of noise, which is unavoidable in real measurements.Copyright

Impact of Enabling solution design on SLI reliability during Temperature Cycling using Shadow Moire

The enabling solutions used in todays computer systems commonly apply some amount of preload on the socket/package. This load plays a critical role in modulating the reliability performance of the second level interconnects (SLI). The load manifests itself as a board warpage that can be quantified using techniques such as board level shadow moire (BLSM). This paper describes specifically the use of board level shadow moire as a viable and insightful technique for measuring the board warpage induced by a socket to enabling solution (ES) interaction. This paper also outlines thermal cycling experiments that have been conducted to study the impact of enabling solution on the thermal cycling performance. The empirical data from these thermal cycling experiments will be used to establish correlation to the areas of high curvature indicated by BLSM; Hence, validating its benefits