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36 SMT007 MAGAZINE I JUNE 2018 Chan: Our strategy is, do it right the first time (DIRTFT). Early manufacturing engineering is very important; we start from design level to minimize the unnecessary cost and manufac- turing impact. We review the design with the customer in the early stages and provide flex circuit layout design guidelines. We replace or minimize the design to rigid-flex circuits, if there is no special size, weight, impedance, electrical noise, or other limitations. Las Marias: How different is the assembly of flex circuits from rigid circuits, and what are the critical factors to consider? Chan: There are several key factors that must coincide when working with customers that require a solution using flex circuit boards— these are technology through to manufactur- ability and the landed cost. 1. Cost efficiency: Manufacturing and assembly costs are significantly more expensive compared with rigid-flex circuits. 2. SMT process: Special assembly carriers are required to control the flex circuit flatness during the screen printing/ SMT and other processes. 3. Repair/rework: Difficult or impossible to repair/rework. Las Marias: How do you ensure flex circuit assembly success? Chan: Controlling the manufacturing processes can minimize the manufacturing issues. 1. Storage: Requires proper storage conditions. 2. Baking: Requires proper baking before flex SMT. 3. Assembly carrier: Special design assembly carrier required to keep the flex circuitry flat during the screen printing/SMT processes, etc. 4. Solder paste: Selecting low-temperature solder pastes (i.e., ALPHA OM-550) can minimize any delamination or heat damage after reflow. 5. Soldering wire: Selecting the low- temperature soldering wire can minimize any delamination or heat damage during the hand soldering/repair or rework process. Las Marias: What are the best practices to consider when dealing with flex circuit assembly? Chan: Due to the flexible circuit technology, certain design and assembly considerations must be accounted for. Fortunately, the design rules for flex circuits are very similar to the design rules for traditional PCBs. That is, the designer must pay attention to certain design aspects: minimum hole sizes, minimum trace widths, minimum space between traces and pads, mini - mum distances to design edges, flex circuit/ board outline tolerances, distance of copper geometries from flex circuit/board edges, and copper and overall design thicknesses. Traditional PCBs and flex circuits share some, but not all, of the manufacturing process steps. For instance, the flex material—usually copper clad polyimide—is selected, drilled, Harry Chan

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