reased heat exposure.
The prevention of white residues takes on two highly important process steps: proper heat management from the reflow process, and optimization of the cleaning fluid to adequately remove the residues during the initial cleaning cycle. The organic layer dissolves quickly in the ACC cleaning solvent system, dislodging the inert residues. An aggressive solvent action is necessary to break up the oxide layer at the same time the organic layer is removed. Enhanced fluorinated cleaning fluid with a higher concentration of active solvent is necessary to completely remove the inert layer along the organic layer. If residues remain after cleaning, future cleaning cycles will not remove them.
Cleaning Fluid Enhancement
The current ACC solvent blend is composed of three active solvents, and the balance is fluorinated materials and inhibitors. Only one of the active solvents is the primary solvating agent and higher concentration is required to effectively dissolve difficult residues. Previous fluorinated solvents have been introduced that azeotrope with the solvating agent at higher levels. This enhanced solvent composition is significantly higher in active solvents required to remove white residue around solder.
Board assemblies were tested for residual rosin concentration by UV vis. at various active solvent concentrations per IPC test methods. Increasing the active solvent substantially reduces the residual resin after the cleaning step.
Fluorinated materials are relatively inert and mild, with selective solvency for soils, oils and greases. The materials exhibit limited solvency for soils, oils and greases. The materials also exhibit limited solvency for many higher molecular weight materials such as rosin and synthetic resin-based flux residues. To address the lack of solvency, the chemistry of the cleaning fluid must be similarly adjusted by engineering a composition that forms azeotrope-like properties at higher active solvent levels. These materials can be effectively blended with a variety of other solvents to form specialized cleaning agents for cleaning the challenges of today's electronic assembly environment.
The engineered material composition must have properties that would enhance the performance for removing the flux residues, as well as acceptable environmental and toxicity profiles. The engineered composition must contain the proper ratio and balance to form azeotropes or behave like azeotropes. For clarification, an azeotrope is a mixture of two or more components that vaporizes without a change in composition at the boiling point, and exhibits a boiling point less than any of the individual components. After evaporation, only a small difference exists between the composition of the vapor and the composition of the initial liquid phase. This difference is such that the compositions of the vapor and liquid phases are considered substantially the same.
The most effective active solvent is 1, 2-trans-dichloroethylene (DCE), which has favorable toxicological, environmental and performance properties. Proprietary fluorinated material forms an azeotrope with DCE at higher levels. This property allows the formulation of solvent blends that provides enhanced performance on the removal of many RMA, RA and low-residue resin flux residues. The resultant fluids can be formulated to have no ozone depletion, while maintaining superior cleaning ability, drying ability, low viscosity and low surface tension. In addition, desired blends will exhibit no flash point in keeping with the CFC and chlorinated-based solvents.
When building engineered fluid compositions, flash point must be a consideration. Oxygenated solvents are needed to enhance cleaning and removal of ionic contaminants. When blending oxygenated compounds within the composition, flash point is a concern. To address this concern, a more highly fluorinated compound may be added to the ternary azeotrope-like blend thus retarding the flashpoint. The addition of this highly fluorinated compound forms a unique blend of azeotropes. Being substantially 