Cleanliness Requirements: A Moving Target

Abstract

During the last 25 years, major shifts occurred in the electronic assembly industry, such as the transition to contract manufacturing and reduction or elimination of in house manufacturing, the switch from solvent cleaned rosin fluxes to low solids no-clean fluxes and the big shift from leaded solder to Lead-Free solders. The preferred method for cleaning high reliability surface mount assemblies was to employ a suitable solvent batch or inline machine, to clean traditional leaded rosin flux wave solder solvent wash process (Figure 1).1 The fluxes would be reduced from the Printed Circuit Board Assembly (PCBA) with a solvent degreasing process. Visually the board would appear very clean because of the reduction of the amber flux, but when the boards were placed in a water environment the clear flux residue around the leads would turn white. Traditional rosin flux, left a clear film on the board and sealed in the board fabrication and flux activators and visually appeared clean. This is because the solvents used to clean the flux only removed the top 2/3rds off the surface and left a clear film.PCBA cleanliness was monitored using visual inspection and a ROSE (Resistivity OF Solvent Extraction) test system of a process that meet product validation. The ROSE test measured the amount of equivalent m grams of NaCl/sq. cm, by immersing the PCBA in a solution of 75% IPA/25% water.2 This total board average cleanliness reading was a result of the change in the conductivity and the algorithm used to calculate the detectable contamination.3 IPA was selected as weaker solvent that was in the degreasers to soften the rosin and measure the extractable activators and yet safe to labels and ink ID markings.The use of this ROSE monitor for historical rosin-based fluxes with solvent cleaned assemblies appeared to meet the needs of the time, but when the entire chemistry of electronic assembly changed, including fluxes (no solids), laminates, soldermask and not cleaning, this tool was not able to correlate to field performance as a predictor of reliability.Process monitoring of the new no-clean or cleaned processes that passed a ROSE test on the production floor may, or may not, pass during environmental testing, or perform well in the field. As technology has expanded in areas of use, miniaturization and circuit sensitivity, the traditional total board average cleanliness has not correlated to the failure areas that are under a component, between vias, pads, or leads requiring a new definition of cleanliness and how it is assessed.4, , This can be seen in IPC 5702 and 5704 that the IPC recommends that each company determine what level of cleanliness that they require to be included on their print and has not established cleanliness guidelines.6The proliferation of electronics in all aspects of life including medical, wearables, telecom, cell and automobiles is on an exponential growth curve.7, , As electronics complexity increases (Figure 2), the spacing between conductors is decreasing and the circuitry is more sensitive to parasitic leakage caused by the presence of semi-conductive ionic and organic materials.This paper will explore the ever more demanding cleanliness requirements of PCBAs and methods to monitor and assess the cleanliness of electronic assemblies today.