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28 SMT Magazine • May 2015 Solder paste printing is known as one of the most difficult processes to quality assure in elec- tronic manufacturing. The challenge increases as the technology development moves toward a mix between large modules and small chip com- ponents on large and densely populated printed circuit boards. Having a process for quality as- surance of the solder paste print is fast becom- ing a necessity. This article describes a method to ensure quality secured data from both solder paste printers and inspection machines in elec- tronic assembly manufacturing. This informa- tion should be used as feedback in order to im- prove the solder paste printing process. Introduction This article has its roots in the need to im- prove capacity and quality levels at an electron- ics manufacturing site. Solder paste printing was identified early on as an area that needed to be secured with many of the new demands put onto the process by recent development in the manufactured products' technology level. by Lars Bruno, ericsson ab and Tord Johnson, MTEK ConSulTing AB Solder Paste Printing: Quality Assurance Methodology A. The Solder Paste Printing Process Solder paste printing is one of the most critical processes in electronic manufacturing. The purpose of the process is to apply the cor- rect amount of paste, at the correct position, with the correct form and to do this every time a print is performed. Even though the process can be considered relatively simple, the quality results of the print together with the PCB pro- vide the foundation for the rest of the surface mount process. A good print result is a prereq- uisite for a good soldering result, while a poor print will lead to additional process issues as the product travels through the manufacturing chain. The printing process has the following de- mands and properties: • Solder paste properties: the viscosity drops when the paste is handled • Stencil surface friction: must be relatively high to force the paste to roll instead of skid • Squeegee surface friction: shall be relatively low in order to allow for the paste to roll and release properly when lifted Feature

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