1. Field of the Invention
This invention relates to soldering of electronic components and in particular to soldering involving solder fluxes and solder pastes.
2. Art Background
For many applications electronic components are surface mounted on a printed wiring board (PWB). This surface mounting is accomplished by positioning a component on a metallic contacting region of a PWB and introducing a solder paste between the leads and the corresponding contact pads. The assembly is then heated to a reflow temperature at which the soldering material flows and an electrically conductive joint between the lead and the solder contact pad is formed.
The solder paste in this soldering process contains solder balls of relatively small dimension, i.e., balls having a diameter in the range 5 .mu.m to 100 .mu.m, and a vehicle i.e., flux. The solder balls are used to insure the presence of sufficient electrical conductor to produce a low resistance joint between the component lead and the PWB contacting pad. The flux is employed to yield a variety of properties necessary for the soldering process. In particular, the flux is chosen so that it is possible to print the paste onto the contacting pads of the PWB through a stencil without depositing essentially any paste in other regions of the PWB. The paste is also chosen 1) to have good tackiness so that when the component lead is pressed onto the paste, it remains in position and 2) so that at the reflow temperature any solder in the paste that isn't precisely aligned with the contacting pads moves into the desired pad region through surface interactions. The flux also contains a dissolving agent that removes any oxide coating from the solder balls, the contacting pads and from the component lead. The flux additionally prevents reoxidation during reflow by in turn preventing oxygen from reaching the solder surface by diffusion through the flux.
In addition to all the other requirements placed on the solder flux, it is essential that after reflow any residue which remains is either non corrosive and cosmetically appealing or is removable by cleaning. Very few if any fluxes are available that leave no undesirable residue. Most fluxes leave residues which are cleaned with organic solvents. Environmental considerations presently have imposed strong impetus to formulate fluxes so that residue is removable with an aqueous medium. However, the fluxes presently suggested for aqueous cleaning are generally based on a vehicle containing inorganic halide (e.g., chlorides and bromides) as the oxide dissolving agent. These chlorides substantially increase the tendency for PWB components to corrode, provide mobile species which cause leakage current, and thus are not entirely desirable.
The formulation of a solder flux that satisfies all the required properties for the vehicle and that either produces no harmful/corroding residue or yields a residue that is soluble in an aqueous-based medium is extremely difficult to formulate. Additionally, such a formulation that doesn't substantially increase the tendency for components to corrode has been an elusive goal.