Addressing the Problems with Ionic Cleanliness Testing on Modern CircuitsAuthors: Todd Rountree, Steve Stach
Company: Austin American Technology Corp.
Date Published: 6/6/2017 Conference: ICSR (Soldering and Reliability)
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. 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. In response to several high profile space and military systems failures in the 1960’s, the US Military developed and incorporated the “Resistance of Solvent Extract” test or as it became commonly known as the ROSE test.
The ROSE test initially was performed manually using a procedure in which a known volume of clean solution of 75% IPA and 25% purified water was sprayed onto the assembly being tested and collected in a clean beaker. The resistance of the soiled solution was measured and compared to standard NaCl solutions. Based upon final resistance, the amount of NaCl equivalent on the test assembly could be determined with some accuracy.
The amount determined to be present was divided by the area of the circuit being tested and an average ionic contamination was determined. A limit of no more than 10 micrograms per square inch, or 1.56 micrograms per square centimeter, was set based upon test data and reliability data from failed units. This remains the default standard today. Automated testing machines became available and replaced the manual method. The ROSE method quickly gained acceptance and was incorporated in the US military specification, Mil-P-28809 in 19711. From that point forward, virtually all US contracts to build electronic hardware required that a sample board be pulled from normal production and a daily ROSE test performed.
Everything got more complicated in the 1980’s with the introduction of SMT assembly design because the flux in solder paste got a lot more complicated. Viscosity modifiers and thixotropic agents were added to improve printability. Tack extending agents and anti-slump compounds are present to keep things in place until the reflow soldering process heats and melts the solder. All these additions make cleaning and cleanliness testing more difficult. In 1990 the Montréal protocol was enacted and stopped the production and use of the primary class of cleaning agents being used. In response the Industry developed a new class of fluxes referred to as “No-clean fluxes”. This gave license to the flux and solder paste formulators to put things in that cannot be cleaned or at the very least were difficult to clean. Cleaning and testing got even more difficult with the introduction of higher temperature “lead free” solders which expose flux residue to a longer and hotter assembly profile.
Through all of this, the ROSE test protocol remained virtually unchanged for 50 years. Confidence in the cleanliness test has eroded significantly because of concerns about dissolving and measuring the “right stuff”. ROSE testing is still required on most high reliability electronic build contracts because there is no better method which is practical in production. Figure 1 shows the evolution of cleaning and testing.
ROSE Testing, Cleaning, Ionic
Members download articles for free:
Not a member yet?
What else do you get when you join SMTA? Read about all of the benefits that go along with membership.
Notice: Sharing of articles is restricted to just your immediate work group. Downloaded papers should not be stored on an external network or shared on the internet.