Printed circuit board (PCB) feature sizes are decreasing to support increasing density thrusts for electronic products and packaging. The transition to lead-free products has changed the stress conditions that are generated at the second level interconnects as a result of "stiffer" lead-free solder joints and greater CTE mismatches between the components and the PCB as a result of the higher assembly temperatures. New laminate materials have been introduced to survive the higher lead-free assembly temperatures. The confluence of all these factors has shifted the primary failure mode in mechanical shock testing for BGA joints from solder fractures in tin lead soldered product to laminate fractures of the metal defined PCB pads (or what Intel calls "Pad Cratering") for lead-free product. This paper will review the fundamental drivers that have increased the risk of "Pad Cratering" with the transition to lead-free assembly. In it we will examine and compare the thermal and mechanical material property differences between standard and high Tg FR4 laminate materials after boards are subjected to lead free assembly conditioning. The thermal and mechanical properties will also be compared against the relative "pad crater" response for the test vehicles used in the experiments. This paper will review the metrology methods employed to determine the differences and quantify the results. The paper will also review the effect of tested design changes on "pad cratering" response. The ultimate goal of the project is to identify key thermal/mechanical laminate properties and metrologies which can define limits and quantify a product's susceptibility to "pad cratering". Additionally, we will examine the sources and extent of variation in the properties for the purposes of providing modeling inputs for the development of predictive mechanical models for "pad cratering". This paper is a first step in the development process.
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