Investigation & Resolution of Current Leakage Failure caused by Carbon Black Aggregation in Mold Compound

Abstract

Abstract Encapsulation materials (e.g. mold compounds) are inherently designed to have very high resistivity (volume resistivity of > 1013 Ohm.cm) to avoid any current leakage or shorting of pins. However, multiple ingredients in mold compound, if processed inappropriately, have enough conductivity to cause significant leakage resulting poor yield and/or inferior quality of products. For a Molded Array Process Ball Grid Array (MAPBGA) package, high current leakage (> 1 μA) failure was observed. For standard electronic packages, the allowable leakage is in the range of two to three order of magnitude lower than the leakage observed. Significant current leakage was observed at a specific pin using a standard organic substrate. The problem persisted across multiple substrate suppliers. Electrical and failure analysis did not suggest interlayer dielectric cracking or wire bond related anomalies. Secondary factors such as mold flow direction, mold cap thickness and substrate design aggravated the current leakage issue. Curve trace was able to isolate the problem away from die, and toward package assembly. Further, strategically designed experiments successfully led to the root cause. Current leakage was caused by formation of conductive path between two pins due agglomeration of carbon black in mold compound. Thermogravimetric analysis (TGA) technique was used to characterize the moisture uptake of carbon black and to differentiate between agglomerated and finely disperse carbon black. With the material improvement (change in the carbon black pH to avoid agglomeration), the leakage mechanism was fully understood and the problem was resolved. Surface of carbon black particle was modified by further oxidation process resulting in lower pH, finely dispersed carbon black.