CO2 SPRAY CLEANING AND OSEE NON-CONTACT INSPECTION FOR WIRE BOND PAD PREPARATION

Abstract

Surface pad contamination is a major cause of poor performance for wire bonding operations. Examples of the wide range of contaminants that can degrade wire bond pull strength include, for example:Halogens and hydrocarbons: plasma etching, epoxy outgassing (dry processing), photoresist strippers, cleaning solvents.Contaminants from plating operations: thallium, brighteners, lead, iron, chromium, copper, nickel, hydrogen.Sulfur compounds: packing containers, ambient air, cardboard and paper, rubber bands.Miscellaneous organic contaminants: epoxy outgassing, photoresist, general ambient air (poor storage).Miscellaneous inorganic compounds: sodium, chromium, phosphorous, bismuth, cadmium, moisture, glass, vapor, nitride, carbon, silver, copper, tin.Human sources of contamination: skin particles, hair, sweat, spittle, mucus, cosmetics, hand lotions, facial make-up and fibers from clothing. As can be seen, there are many types of surface contaminations that may challenge a wire bonding operation, all of which must be removed to insure reliable and strong bonds. In this regard, conventional precision cleaning processes for high reliability surface pad preparation typically involve multiple steps, chemistries, and equipment to accomplish complete decontamination. Moreover, conventional cleaning methods are sometimes non-selective for the surface contaminants and substrates. For example, conventional vacuum plasma using Ar/O2 is typically used to clean bond pads. Vacuum plasmas are usually performed off-line, taking up to 30 minutes to complete, and are non-selective for the organic contamination. The entire organic substrate (i.e., PCB) is etched away during the plasma cleaning process to remove the bond pad contamination. During treatment, secondary organic surface contaminations (plasma treatment by-products from reacted substrate) are produced which can re-contaminate bonding surfaces. Advanced carbon dioxide (CO2) spray cleaning technology provides various methods for consistently preparing bond pads for critical wire bonding operations. A patented hybrid CO2 particle-plasma spray is presented in this paper that has demonstrated efficacy for selectively treating bond pad surfaces to remove a wide range of challenging surface contaminations. Moreover, a novel non-contact surface inspection technology called Optically Stimulated Electron Emission (OSEE) - developed to address surface cleaning and inspection issues that led to the 1986 Challenger Spacecraft explosion - is used to measure the effectiveness of the new CO2 surface cleaning processes. Statistically significant studies have been performed to determine the effectiveness of the selective CO2 particle-plasma surface treatment process for preparing bond pads for gold ribbon bonding operations. One such study compared and contrasted the performance of this new single-step CO2 surface treatment method with that of a conventional multi-step solvent-plasma method. The two treatment methods were used to prepare the surface of a metalized ceramic wafer that simulated bond pad surfaces and treatment areas representative of an actual high-reliability electronic board. The test results of this evaluation demonstrated that the CO2 particle-plasma surface treatment process is statistically similar to or sometimes better than a solvent-plasma hybrid cleaning process. CO2 spray cleaning was determined to be better for some types of contaminants as well – and in particular more relevant mixed-contaminant challenge tests. The CO2 cleaning process demonstrates a lower defect-per-million (DPM) level and an improved CpK. Finally, in this study OSEE surface quality analysis was performed before and after surface cleaning. OSEE analysis provided a reliable non-contact means of determining the proper level of surface pad preparation.