Electrochemical reactions in solder mask of flip chip-plastic ball grid array package

A typical flip chip plastic ball grid array (FC-PBGA) module utilizes a laminate substrate, which has a solder mask layer at the surface and a number of build-up layers. During reliability testing of the assembled chip-laminate modules under elevated temperature, humidity and voltage bias conditions, electrochemical reactions can proceed in the solder mask layer producing various oxidized copper (Cu) compounds and metallic Cu. If metallic Cu dendrites grow from one electrode toward its adjacent electrode, such dendrites can cause electrical leakage. The electrochemical reactions summarized in this paper involve ionic current flow between flip chip attach copper pads, impurity Cl^- ions catalyzing Cu corrosion, the state of the solder mask and/or an absence of oxygen. Under the condition of 130°C, 85% relative humidity and 3.7V bias, Cu²⁺ ions move from an anode (power) or its vicinity toward a cathode (ground). Cu²⁺ ions are then reduced at the cathode to yield metallic Cu. If the reduced Cu forms a dendrite pointing toward the neighboring anode, the electric field will focus at the tip of the dendrite. Subsequently more Cu²⁺ ions will move toward the tip of the dendrite so that the dendrite keeps growing toward the anode. Lack of O₂ under the highly accelerated stress conditions can also promote such dendrite growth since O₂, if present, can be reduced at the cathode where the reduction of Cu²⁺ ions will decrease accordingly as a total number of electrons consumed for all reduction reactions remain constant. Cu dendrites can also grow from unbiased Cu electrodes due to the combined effect of galvanic cells and bipolar circuits.