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August 2015 • SMT Magazine 37 are often run on pallets through the wave solder in order to protect any bottom-side components that can't be exposed to the wave. This issue can also make it difficult to use auto- mated insertion equipment. Use of IPC design guidelines can help the design team to avoid this issue. The recommended clearance for fixturing is 75 mils. However, as products shrink, many PCB designers are going to 35 mils. 3. Exposed Via-in-Pad: Except in very special instances, there should be no open vias in pads, since the via can suck the solder from the component. Unless there are thermal or high-frequency design considerations this should be avoided in design or addressed dur- ing the board fabrication process. For this case, the only option is to have the board fabricator plug the open vias. 4. Thermal Issues with Leaded Com- ponents: Use of through-hole components also drives the use of leaded components, when lead-free packaging isn't available. In a mixed technology PCBA, this means that some components can experience thermal damage during reflow due to the higher tem- peratures required by lead-free components. Where possible, mixing leaded and lead-free technology should be avoided. Even when a datasheet indicates that higher temperatures can be tolerated, damage may occur if the PCBA has multiple passes through reflow as a result of rework. There are two options to minimize potential thermal issues. In some cases, a leaded reflow profile can be run in place of the lead-free re- flow profile. The normal leaded assembly reflow profile is 210–225°C. Comparatively, lead-free solder has a reflow profile between 235–260°C. One option is to use an eight-zone reflow oven, of which the profile can be adjusted for a slower rise-to-peak temperature, eventually reaching 225–230°C. This achieves the melting point re- quired by the lead-free components with leaded solder paste, without damaging the leaded com- ponents. Alternatively, leaded components can be hand soldered as a secondary operation fol- lowing lead-free reflow. 5. Pads Connected to a Ground Plane: If the pads of smaller passive components, such as 0201s or 0402s are connected to a ground plane, the ground plane can act as a heat sink and create tombstoning. 6. Mismatch between Solder Paste and Stencil with Fine Pitch Components: When the designers generate paste layers, they typically base their assumptions on compo- nent requirements without necessarily know- ing which type of solder paste will be used. This isn't a problem for components that are greater than 0.5 mil pitch, but can create issues with components with 0.4 mil pitch and below. For example, if a product has a 0.3 mil pitch mi- croBGA, type III solder paste won't fit through the aperture space on the stencil. The standard formulas for aspect and area ratios don't work on microBGAs. Typically the only option for addressing this is to change the aperture size or the solder paste type. It is recommended that a square rather than round aperture for microBGAs is used. 7. Cleanliness Challenges: Military/aero- space products often have unique cleanliness requirements. This can be an issue for mixed technology PCBAs, since cleaning chemistries that support the RMA flux used with through- hole components may not sufficiently clean residue left by no-clean flux used in SMT. Unquestionably, industries that often have highly specialized quality requirements, com- bined with legacy products and/or qualification processes that limit the ability to make man- ufacturability-driven design changes, present contract manufacturing challenges. The best so- lution is to focus on manufacturability early in the design process and work with supply chain partners who are willing to develop solutions for production issues that can't be designed out of the product. SMT QuALITY & MANuFACTuRABILITY CHALLENGES OF MISSION-CRITICAL PRODuCT continues Amy Yin Chen is an engineering and production manager at naprotek inc. To contact the author, click here. FeAture

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