Authors: Todd Rountree and Steve Stach Company: Austin American Technology Corp Date Published: 2/5/2018
Pan Pacific Symposium
Abstract: Shortly after the electronic circuit card replaced point to point wiring in the 1950’s, the reliability of manufactured circuit assemblies was directly linked to the amount of “freely” ionizable material remaining on the board following assembly because these residues in the presence of moisture and a voltage differential will undergo both chemical and dendritic electrochemical reactions resulting in rapid and catastrophic circuit failure. The resistance of solvent extract (ROSE) test was first developed in the 1960’s as a method to quantify the amount of activated rosin flux residue remaining on a completed circuit assembly. To dissolve the rosin based flux, a mixture of isopropyl alcohol and de-ionized water was selected as the extraction solvent. The ROSE method quickly gained acceptance and was incorporated in the US military specification, Mil-P-28809 in 1971 and required that a sample board be pulled from normal production and a daily ROSE test performed. In 1994 the Mil-P-28809 requirement was dropped for US military contracts and the ROSE test was further defined and incorporated into the generally recognized industry set forth by the IPC. The original ROSE test method remains essentially unchanged since the mid 1970’s although the nature of soldering has changed significantly. In the 1980’s surface mount technology replaced the traditional wave soldering process. Cleaning changed significantly with the Montreal Protocol banning the use of most halogenated hydrocarbons used to clean circuit cards. This led to the introduction of low activity no-clean fluxes. Following the turn of the century, circuit manufacturing underwent another major soldering change, the introduction of lead free soldering alloys. Higher reflow temperatures required new changes to the fluxes, and that again changed the cleaning requirements. In October, 2017 the IPC issued changes to board ionic cleanliness standards effectively replacing mandatory ROSE testing with a new requirement upon the manufacturer to provide “objective evidence” that the cleaning process is producing a clean and reliable assembly. This means, if you have been using ROSE testing, you can continue to do so, or you can substitute other objective evidence of cleaning control. In part these changes were made because of concerns with the validity of the current IPC ROSE test. Some say it is no longer valid because most new flux residues are not soluble in the IPA/water extraction solvent. Others have concerns about contaminated board escapes allowed by sample testing versus 100% testing of all product. The new changes have allowed for much needed improvements in ROSE testing. The product designer and process engineer can now set new contamination limits, use new better suited extraction solvents, and integrate automatic testing into the assembly process. It is time to take a new look at the old reliable ROSE test. This Paper evaluates new automatic ROSE testing protocols that allow circuit manufacturers to completely integrate ionic cleanliness testing into the modern SMT production line without significant cost. This eliminates sampling labor and testing errors. This paper also evaluates the best extraction solvent choices from laboratory experimentation for today’s flux choices, which surprisingly are not a mixture of IPA and water. Different protocols are needed and proposed for fluxes to be removed versus those not intended for removal. These changes are potentially very significant to the industry in that product yields can be improved and better controlled cleaning process will lower product recall and warranty costs.