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Placement of 32 pin QFN w/t ground pad

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#60021

Placement of 32 pin QFN w/t ground pad | 30 September, 2009

Hi guys,

Can someone please clarify whether adjustment of Z axis travel is a critical parameter during placement of QFN devices. If so, to within what tolerance? In other words, should a pick and place machine position a QFN down on the board so that pads of the device only make contact with solder paste then, vacuum is released and nozzle moves up. Or is it OK to apply slight pressure on the QFN to make sure it will stay in place and not drop down too early. Solder paste thickness is around 5 mils. The ground pad has solder paste covering 50% of the area. Pads around the perimeter have a 1:1 ration of solder paste to pad area.

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#60072

Placement of 32 pin QFN w/t ground pad | 9 October, 2009

SteveO, I had personal experience with this using a Panasonic MPAV2B. It took a while, but I believe the end result was to have some additional "Z" travel to press the QFN into the paste. Also, the design of the paste on the ground was key to keep the QFN from floating up during reflow. We used a checkerboard pattern to allow for outgassing. Jim

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#60117

Placement of 32 pin QFN w/t ground pad | 15 October, 2009

Thanks Jim. Pressing the QFN into paste was what I thought of, just wasn't sure if it would cause solder paste to be pressed outside the pad area and possibly bridge with an adjacent pad. Do you recall the ratio of paste coverage to the pad area?

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#60125

Placement of 32 pin QFN w/t ground pad | 16 October, 2009

Use multiple small stencil openings [maybe 15 thou square or about the size of a signal pad], where total area approximately 50% of total thermal pad on the board. For more, search the fine SMTnet Archives => thermal and pad

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#60126

Placement of 32 pin QFN w/t ground pad | 16 October, 2009

Many pick and place platforms rely on nozzle spring pressure to determine how hard a part is getting pushed into the paste.

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#60151

Placement of 32 pin QFN w/t ground pad | 21 October, 2009

I agree with Dave. If the device has a lot of power we'll put thermal vias between the solder deposits with annular rings of soldermask to keep solder out of the holes. We've never had to mess with placement pressure. We've been using QFNs for years and they've never given us any grief.

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