Issue link: https://iconnect007.uberflip.com/i/935136
72 SMT007 MAGAZINE I FEBRUARY 2018 ing process, and Crow felt this was certainly the case in his factory. With a 22-second mean cycle time, the stencil printer was the gating process on the assembly line because it was stopped every two and a half minutes for clean- ing. As a result, the SMT line would be empty of boards. The line badly needed balancing as no one wants the stencil printer to be the slow- est item on assembly line. To achieve this, Crow started by concen- trating on the screen printing process, which has the largest impact over yield, throughput, quality, and downtime costs. He concentrated primarily on process improvements. Crow began a systematic root cause analysis of the defects. 1. Under Screen Cleaning Crow's initial focus was on the substances that clean the screen mechanically—the "paper" and the under-screen solvents used. After investigating different types of paper, Crow found there were better options out there than the current product. He selected a more porous wipe material that he thought would allow for better wiping. The Micro-Care Stencil easily handled the sharp edges on the SMT stencil, which often shredded old-style stencil wipes, causing defects and rework. The struc- ture of the open paper also captures solder balls better than some of the closed structure fill papers. 2. Solvents Crow then turned his attention to under-screen solvents. IPA (isopropyl alcohol) was problematic because newer solder pastes are comprised of synthetic resins, and the alcohol made the solder viscosity and machinery difficult to maintain. The cleaning properties were not good enough with IPA, as it did not clean and caused some paste bricks to cling to the aperture walls. He even turned off the IPA and sprayed nothing, and experienced the same results. So, he knew he needed to find a better solvent. For the solvent, he wanted something that was efficient without being harmful to the screen printer. He ultimately chose Zestron Vigon UC160, an aqueous-based cleaning medium specifically designed for SMT sten- cil. The water based formulation evaporates more slowly than IPA, but could clean with less wipes. 3. Nanocoating In taking a systemic view of his SMT line issue, Crow had found a better paper and a better solvent, but he was still experiencing defects and downtime. They were still wiping every four to five boards based on the complex- ity of the stencil. Something was missing to make the process improvement complete. That was when Crow met an SMT manu- facturing expert, Chrys Shea, at an industry conference and found she had been conduct- ing the same SMT line improvement programs and had delivered remarkable results. Her scientific approach to solving the SMT line Figure 1: Jimmy Crow delivered terrific results for Rauland: SMT-related defects were reduced by over 50%; throughput was increased by over 20%, and the company saved $1 million per year for three SMT lines.