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Coulometry: A Promising Method of Quantifying Organic Residue on SMC’s (Surface Mount Components) After Cleaning

“Coulometry: A Promising Method of Quantifying Organic Residue on SMC’s (Surface Mount Components) After Cleaning” Buffalo, NY: Allied-Signal Corporation, Buffalo Research Laboratory.

Dr. J.K (Kirk) Bonner and Mr. J.M (Marv) Lewis
Buffalo Research Laboratory
Allied-Signal Corporation
Buffalo, NY 14210

INTRODUCTION

Now that SMT has finally arrived, reflow soldering is becoming much more common. Both infrared and vapor phase reflow systems are now in common use. Solder pastes are routinely used in conjunction with the reflow soldering operation. The pastes are applied to the solder pads either with a screen or a metal stencil, the SMCs mounted on top, the resulting assembly preheated and reflowed and then cleaned.

Solder pastes are complex mixtures, chemically speaking. In addition to containing solder powder (typically 85-92 weight percent of the paste), flux activators, and a suitable binder such as rosin, they also contain thickening agents to maintain a high viscosity during application and reflow, and a high boiling solvent to increase tack time. The thickening agents are normally derivatives of hydrogenated castor oil, the triglyceride of ricinolaic acid. The residues from the paste remaining on the assembly after reflow are difficult to remove, regardless of what cleaning technology is employed.

Concomitant: with the problem of cleaning solder paste residues is the problem of verifying cleanliness. Previously, for conventional through-hole boards, the industry has relied on ionic contamination testing to determine assembly cleanliness. However, SMT brings its own problems to cleanliness verification. There is considerable evidence that the test solution used in ionic contamination testing, viz., a 73% IPA/25% water by volume mixture, will not readily penetrate and remove paste residues under low standoff SMCs. Even if the IPA/water test solution could penetrate under the SMCs, the thixotropic residues are not soluble in this test medium and leave a white residue after being exposed. Further, most pastes employ an RMA flux so ionic readings, even if attainable, are generally quite low. Many people in the industry are using qualification testing by desoldering the components and examining them visibly for evidence of paste residues. This method is subject to all the drawbacks of qualitative testing. Comparisons are very difficult to make.

Coulometry, a method that has already found application in the finishing industry [1,2], offers a way to quantify the amount of paste residues in surface mount components (SMCs) after reflow and cleaning. The method is rapid, simple, reproducible, and quantitative. See Figures 1 and 2 for a depiction of the equipment. Coulometry involves the combustion of sample carbon and its conversion into carbon dioxide (CO2) inside a tube furnace held at 400˚ – 550˚C. Once the sample carbon is converted into carbon dioxide, the carbon dioxide is reacted with ethanolamine to from a carbamic acid [HOCH2CH2NHCOOH], and the carbamic acid is titrated coulometrically. The amount of carbonaceous residue is expressed in milligrams of carbon per sq. meter (mgC/m2) or in milligrams of carbon per sq. foot (mgC/ft2). It is easy to convert the last figure into micrograms of carbon per sq. inch (µgCin2).

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