Lead-Free Technology and Influence on Cleaning
- N. C. Lee
- As originally published in the IPC APEX EXPO…
The process cleaning rate theorem holds that the static rate (chemical forces) plus the dynamic cleaning rate (mechanical forces) equals the process cleaning rate. New lead-free flux residues result from more demanding soldering drivers created by high soldering temperature, surface tension effects, and miniaturization. Lead-Free flux compositions require thermal stability, resistance against burn-off, oxidation resistance, oxygen barrier capability, low surface tension, high fluxing capacity, slow wetting, low moisture pickup, high hot viscosity, and halogen free. The static cleaning rate for lead-free flux residues is dramatically different from eutectic tin-lead flux residues. To clean lead-free soils, longer wash exposure time, high cleaning agent concentrations, and high levels of mechanical energy are needed. The purpose of this research paper is to measure the cleaning variability induced by lead-free flux residues and to compare the cleanability of lead-free flux residues to determine the viability of new cleaning agent designs. Introduction The distances between conductors, and the under clearance gaps from the board to the bottom of the components on printed circuit boards, are smaller due to miniaturization. Smaller spacing increases the probability that flux residues or surface contamination will be sufficient to bridge all or most of the under clearance gap between conductors. Flux bridging conductors opens the pathway to form a conductive cell between two points on the board assembly. As a result, higher density board designs increase reliability risks, which are commonly mitigated by cleaning all flux residues and ionic contamination on the surface and under components on the assembly. Cleaning flux residues from under component gaps has become extremely challenging due to the nature of the flux residue, under component clearance from the board to the bottom of the component, time required for the cleaning agent to penetrate the gap, the cleaning agents ability to solvate and break the flux dam needed to create a flow channel, and the mechanical energy needed to deliver the cleaning agent to the flux residue. Flux residues that form a hard shell require longer wash times to dissolve in the cleaning agent, thus requiring increased time to clean these residues under the component gaps. The variability of flux residues from different solder paste manufacturer’s places increased importance on the cleaning agent design. As a response to these cleaning challenges, new cleaning agent designs are needed to better dissolve advanced flux compositions. The most advanced flux technologies fit within the low residue no-clean flux category. For eutectic tin-lead, cleaning product to product variation was not an issue from a cleaning perspective. With the move toward highly dense miniaturized board assemblies and lead-free soldering, flux compositions require higher molecular weight flux vehicles with increased thermal stability. The flux residues from these higher molecular weight flux compositions have a greater degree of product to product variation, form hard resinous barriers, and increasingly difficult to clean. To remove these hardened flux residues under low component gaps, increased wash time, wash temperature, wash concentration, and impingement energy are needed. One of the critical issues from using more aggressive cleaning parameters is material compatibility on board finishes, board laminates, solder joints, labels, and components. The challenge is the need for more effective cleaning agents with improved material compatibility. For aqueous cleaning agent designs, one formulation approach is to build cleaning agents that drive with both hydrophobic (resin loving) and hydrophilic (water loving) properties. A second design feature is to reduce alkaline saponification in an effort to improve compatibility on surface metallic alloys. The goal is a cleaning agent design balance that limits tradeoffs and maximizes performance benefits. The purpose of this research paper is to measure cleaning variability induced by lead-free flux residues and to compare the cleanability of lead-free flux residues under low component gaps to determine the viability of new cleaning agent designs. Most of the leading cleaning agents designed to clean PCBs post soldering are effective at removing these higher molecular weight flux residues from the exposed surface solder leads and pads. This is not the case when it comes to removing flux residues under component gaps. If the cleaning agent is slow at dissolving the hardened flux shell, residue will remain under the component gap following the cleaning process. Therefore, the critical cleaning agent differentiator is the speed at which the cleaning agent dissolves the flux residue. As originally published in the IPC APEX EXPO Conference Proceedings.