The larger thermal pad

Exercise caution if vias in pads are used. Open vias act as solder drains funneling solder away from the joint down into the via and to the opposite side of the PCB. In the worst-case scenario, there is insufficient solder on the pad and a "solder bump" on the opposing side. This can result in marginal joints on the topside and printing difficulties and solder shorts on the opposing side. Most pcb manufactures offer several options for closing vias — plugging, capping, tenting — which help keep sufficient paste volume on the pad. Tenting or plugging, the cheaper methods for via closure, are both prone to placement and chemical entrapment issues due to voiding and lack of planarity. Capping is a more robust, more expensive process that eliminates these two concerns.

Stencil design. Appropriate stencil thickness and aperture design are crucial for reliable soldering of QFNs. Follow the QFN manufacturer's design guidelines when they are provided; the general goal is to provide approximately 2 to 3 mils of solder thickness. Excessive amounts of paste can induce large-scale voiding of the thermal pad and component float, where the QFN is literally lifted off the board. Use multiple, smaller windowpane apertures to avoid large bricks of solder paste that can cause larger and greater numbers of voids. Windowpanes also reduce the propensity for solder balling. For the thermal pad, the general rule of thumb for the ratio of aperture to pad is approximately 0.5:1.

Moisture absorption and reflow profile. Current industry standards J-STD-020 and J-STD-033 have been useful for documenting moisture sensitivity levels (MSL) in older component packages, but there are increasing indications that moisture absorption in the thinner QFN package can drive excessive warpage during reflow. In one case study, a military supplier experienced solder separation under QFNs. The QFN supplier admitted that the package was more susceptible to moisture absorption than initially expected. This resulted in transient swelling during reflow soldering, which induced vertical lift and caused solder separation. This was not a popcorning phenomenon since no evidence of cracking or delamination in the component package was seen. To minimize this potential, larger and thinner QFNs should be treated as an MSL of 3 or higher and reflow profiles should be carefully controlled with a slow, steady ramp rate.

Rework and inspection. QFN rework can be difficult, since the thermal pad and any inner row joints are neither accessible nor visible to the repair technician. Mini-stencils, rebumping, or solder performs may be used to replace or add solder volume. Small, portable preheaters can help provide sufficient heat to reliably resolder the larger thermal pad and inner row pads in hand soldering operations.

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