Author: Phil Isaacs & Terry Munson Company: IBM Corporation & Foresite, Inc. Date Published: 1/25/2016
Pan Pacific Symposium
Abstract: Since the conversion to lead-free solders, no-clean flux has become the predominate flux used in electronic card assembly and test (ECAT) manufacturing. No-clean flux performs well and can save steps in manufacturing. If used properly, no-clean flux can be effective, safe and reliable. It is a common misconception that since it is no-clean flux it can be used in any manner and still be safe and reliable. Numerous examples can be shown where no-clean flux, when used improperly, can lead to electrochemical migration and dendritic growth, leading to electrical fails. With this in mind, the question becomes, what is the proper use of no-clean flux? The focus of this paper is to share the conditions in which no-clean flux becomes benign. This paper will present the conditions of no-clean flux residues (from any of the flux and paste suppliers) that cause poor electrical performance and explain what leads to the poor performance. Millions of electronic systems assembled with no-clean flux perform very well in the field. However under some conditions, whether the product has seen early or extended time in the field, it fails to function. Additionally there are cases where causes a battery fails to hold a charge, no matter how many new, good, replacement batteries are installed. The electronics industry is seeing a greater number of product recalls, product returns with no defects found, and hard short failures due to electrochemical migration than the industry experienced with hardware from the ‘70-‘90s. Many of the field failures seen today are not due to software, bad components or abuse of the hardware, but are due to a variety of conditions relating to the use of no-clean flux. These include: shorting dendrites growing under conformal coating/potting compounds; parasitic leakage issues under low standoff components, such as QFNs or Power FETs; dendrite shorting of SMT components and vias near selective solder regions on printed circuit board assemblies (PCBAs). If the board and components are ionically clean, what is causing the leakage and electrochemical migration creating the hard short under/over and around the parts that only saw flux? This paper will explain what can be done on the production floor to ensure that the residues are benign and will remain so. It will also address typical areas to which no-clean flux residues can travel or are trapped and need additional thermal attention and monitoring.