SMT007 Magazine

SMT-July2018

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JULY 2018 I SMT007 MAGAZINE 73 and dwell time for your board is appropriate for your flux can mean the difference between a good and bad result. Burning off the flux prior to wave can result in bridging. Lifted Components Another common defect is components lifted after wave, which is more predomi- nant on smaller components such as axial or radial components—but just as common on connector s and other components—which are lifted during contact with the wave and are soldered in placement. The most common practice to address is through component lead pre-forming and/or pallet hold downs. Component Considerations Ensuring components such as axial and radial components are properly prepped can avoid most lifting situations. Lead forming or clinching of the leads, which mechanically hold the components in place, are by far the most common. Common with bridging, leads that are too long can also exaggerate lifting, which acts as a lever to push the component out of position. Tooling Considerations Other components such as connectors which cannot easily be retained in place require addi- tional hold downs, which can be in the form of glue or over-clamps as part of the selective solder. When considering over-arms for clamp- ing, the additional thermal mass introduced by these features must be considered in the profile and may potentially require a different flux for better performance. Process Considerations Wave height and the use of lambda versus laminar flow can also contribute to increased occurrences of component lifting. Ensuring wave heights are set to no more than 50% of the PCB thickness relative to the pallet and the use of turbulent flows should be minimized. Other considerations include conveyor vibra- tion, angle, etc. Insufficient Solder Another most common wave defect is insuf- ficient solder and can be categorized as incom- plete barrel fill or incomplete circumferential wetting. Related, but typically more related to contam- ination of the solder, board or component, is de-wetting or non-wetting. For the purpose of this review, we will assume the components are in good condition prior to processing. Best practices to prevent introduction of these types of defects include a well-established incoming inspection process combined with solder dip testing as per IPC-TM-650 for suspect contami- nated or oxidized components. Design Considerations Common design considerations are direct connection of plated through holes to large copper planes which act as a heat sink during wave soldering. To address this, best practice is to provide thermal relief in these areas to allow proper flow during soldering. Thermal Figure 2: The direct connection of plated through-holes to large copper planes acts as a heat sink during wave soldering. The best practice is to provide thermal relief in these areas to allow proper flow during soldering.

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