Brush/IPA Cleaning - What Are the Risks?
After the rework and repair of electronic hardware, many companies use a brush cleaning with IPA or a Solvent to break up the flux residues and remove dross and solderballs.
This process has generated a large number of reliability problems in fielded hardware, to the point that many manufactures have restricted any rework or repair, even under best industry practices.
What problems can the rework and repair processes create?
Typically, the solder quality is good, but the use of liquid fluxes with cored wire leave residues that spread away from the rework area where the only heat is applied. This means cleaning with a solvent and brush allow for these residues to be solubilized and moved around as does any dross, solderballs, or slivers. The result is contamination relocation leaving corrosive residues in places that may impact performance.
Since the residues are solubilized with the solvent, the brush transfers the residues to nearby components and allows the contaminants to be trapped around other components near the rework and repair area. Rarely is there enough rinsing/flushing to remove the solubilized residues. The thought is the solvent will dissolve the flux residues, but then evaporates leaving the corrosive residues in nearby locations.The compounding factor is that the brush is used for weeks or months.In that time frame, it collects residues from every cleaning step. This creates a concentration of the flux residues on the brush bristles and these transfer to the next sample.
What should be done when using a no clean process to rework or repair an assembly?
If it is a no clean process and the rework materials are no clean, it would chemically and thermally make the most sense to heat activate the flux. Afterwards, the repair with no brush cleaning will allow the flux to transition from a moisture absorbing state to a benign insulative state where all the reflowed flux residues have reached. This localized heating allows the flux to volatilize and transition at 150C for two to three minutes depending on the thermal mass of the parts and PCB. It is important to analyze residues with C3/IC to determine if the time and temperature is adequate.