PCB Surface Finishes & the Cleaning Process

 All PCBs that are manufactured require a surface finish to protect exposed copper on the surface which if left unprotected, can oxidize, rendering the board unusable. To address this issue, it is common to surface treat the PCB prior to assembly and reflow. The surface finish not only prevents oxidation of the underlying copper, but guarantees a solderable surface. A cost effective and widely used approach to PCB surface finish is HASL (Hot Air Solder Leveling). However, as circuit complexity and component density have increased, HASL has reached its limitations, necessitating the need for thinner coatings. Thus, coatings such as Immersion Tin (ImSn), Immersion Silver (ImAg), Organic Solderability Preservatives (OSP), and Electroless Nickel Immersion Gold (ENIG) are becoming more widely used.

As most PCBs designed for use in high reliability applications are cleaned or ‘washed’ in aqueous-based cleaning systems, the effect of the cleaning solution on the surface finish is of great concern. Depending on the cleaning process employed, it is possible for stains to appear on the plating or in the worst case, for the plating to be completely stripped from the PCB rendering the applied surface finish useless.

This study was designed to investigate the effect of reflow and various cleaning agent types on ImSn, ImAg and ENIG surface finishes. Unpopulated ZESTRON® test vehicles, with the appropriate surface finish, were used for all trials. In a previous study by the authors, the OSP surface finish was analyzed for integrity within an aqueous-based cleaning system and was therefore excluded from this study.

An inline spray-in-air cleaning process was chosen to assess the surface finish integrity. Three (3) aqueous cleaning agent types were selected for use within this study. Two (2) alkaline cleaning agents, inhibited and uninhibited, as well as a pH neutral cleaning agent, were used. Cleaning system process variables were established and held constant for all trials.

Surface finish assessment following reflow and cleaning was conducted using visual inspection, adhesion test, copper test (ImAg and ImSn), nickel test (ENIG), and the X-Ray Fluorescence (XRF) test. Additionally, baseline tests were conducted on boards with all finish types without exposure to reflow or the cleaning process in order to assess the effect of the reflow process.

Keywords: PCB surface finish, cleaning process, Surface test analysis, copper test, nickel test, adhesion test, XRF test