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AUGUST 2019 I SMT007 MAGAZINE 81 If you've been in the SMT industry for any length of time and involved in ordering sten- cils, you may have instructed your stencil supplier to design your stencil to "industry standards" or per IPC standards. These are very loose terms and may be interpreted dif- ferently by different stencil manufacturers. Many parts of the IPC-7525B standard are only recommendations, and the recommendations are typically a range of size reduc- tions and/or area reductions for different types of components. In this column, Greg Smith discusses how his company interprets these guidelines and look at general design principles that should be applied to every stencil. (Source: I-Connect007) Read the column here. increased variability as well as increased man- ufacturing time. Many parameters were taken into consideration when developing the pro- cess, as seen in Figure 1. To validate the implementation of the new automated conformal coating process, a com- parison of the manual hand spray to the new process was performed to ensure there was no degradation between the old and new process. An adhesion by tape test per ASTM D3359 Test Method B was performed using hand-sprayed and auto-sprayed coating samples; all coupons tested yielded a 5B result. A visual inspection of the circuit card assemblies sprayed with the automated-spraying process validated that the coated circuit card met all J-STD-001 and IPC- A-610 requirements. The thickness comparison proved that the new automated-spray process is more consistent that the hand-spray pro- cess. All thickness coupons measured within the J-STD-001 requirements of 0.03–0.13 mm (0.001–0.005 in.), and the variation has been reduced and process capability has been improved from a Cpk of 0.82 with the hand spray to 1.98 with the automated spray. Designing fixtures to incorporate a major- ity of the masking required per the customer drawing has reduced operator masking time by about half. Automating the process resulted in a reduction in overall spray coat cycle time by about one hour, including approximately 10 minutes of manual spraying time per cir- cuit card assembly. The automation of confor- mal coating using a two-component, low-out- gassing polyurethane material has resulted in many significant successful improvements to the overall manufacturing of these circuit card assemblies. Next Steps Process improvements will continue as an ongoing effort with a primary focus on increas- ing component coverage capabilities. SMT007 References 1. J-STD-001G (Joint Industry Standard Requirements for Soldered Electrical and Electronic Assemblies), IPC, 2017. 2. IPC-A-610G (Acceptability of Electronic Assemblies), IPC, 2017. 3. Analytical Report (FTIR Analysis of Coating Samples), Cerium Laboratories LLC, 2018. 4. ASTM D3359-17 (Standard Test Methods for Rating Adhesion by Tape Test), ASTM International, 2017. This paper was first presented at the IPC APEX EXPO 2019 Technical Conference and published in the 2019 Technical Conference Proceedings. Marissa Pati is a senior process engineer at BAE Systems. Ana "Lety" Campuzano-Contreras is a principal manager at BAE Systems. SMT Stencils 101: What Are Industry-standard Stencil Designs?

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