 in the boil, vapor and rinse stages remains relatively constant over time.
The limitation, therefore, of the current ACC cleaning chemistry for flip chips is the incomplete removal of solder flux residues. Several design and assembly factors have been found to directly affect the amount of flux residue remaining after cleaning. For a given flux, the initial amount of flux dispensed for the reflow solder operation, peak reflow temperature, length of time between the reflow assembly and cleaning, and most significantly, the gap height and dimensional size of the flip chips make for a difficult cleaning challenge. Flip chips with organic die passivation layers seem to be more difficult to clean than those with inorganic passivation layers. The board complexity of new designs calls for tighter pitch, higher I/O and lower standoffs.
Nature of Flux Residues
White residue around the periphery of the solder bump is a common concern with cleaning flip chip assemblies. These residues are composed of fluxing byproducts consisting mostly of unreacted soluble organic compounds and a small amount of metal salts of tin that are highly stable in inert (Figure 3). The salts are encapsulated in the remaining flux vehicle. Selecting a cleaning fluid that has the strength to dissolve the vehicle and allow removal of the salts in the initial cleaning process is the key to removal of these residues.
Figure 3. Example of white residue.
The formation of metal salts is caused by the active ingredients in the flux chemistry. The fluxing agents react with the tin and lead oxides on the solder's surface. The breakdown of the oxides allows the solder to flow and wet the surfaces to be joined. As heat exposure increases, the likelihood that a visible residue will remain around the solder bumps increases. Analysis of the residue on test boards has shown it to be comprised of tin compounds, perhaps a tin oxide matrix with tin salts of organic acids. The amount of oxide formed and the amount of metal salts formed upon fluxing increases with increased 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 o