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QFN solder paste layers: modifying the stencil layer can make sense--sometimes.

I'm working on my next demo project. As shown, this little board (FIGURE 1) is pretty close to its actual size of 2 "x 1.3".


It's small, but take note of the QFN/DFN parts: The processor in the middle, the Li Polymer battery charger right between the upper two mounting holes, and the RS232 driver in the lower left. I've followed my own paste layer advice and segmented the paste stencil layer to reduce the chance for float or major voids.

I found a footprint in the library for the big processor in the middle. I just had to modify the paste layer. I made the footprint for the charger and RS232 chips from scratch. Neither had anything close enough in the library.

The DFN LiPo charger up top has a slightly different approach to segmenting the stencil layer from the other two. Little squares like I used on the other two chips work just as well, but rectangles like this are effective too. You just want to shoot for 50% to 75% paste coverage.

Another thing to take note of is the marking on the LEDs. The original footprint for the 0402 LEDs does have a polarity mark, but it's one of the types that can easily be misinterpreted or can be difficult to see. The diode symbol put down in silkscreen removes any possibility of ambiguity.

With the PCB finished, it's time to buy the parts and get the thing built. In the land of prototypes, sometimes "close enough" is good enough. That can save money on PCBs and assembly when a particular package version of your part is out of stock. But, it's not universal. Sometimes you can't go that way.

The MCP78338 Li Polymer charger chip comes in 10-DFN and 10-MSOP packages. I used the MSOP version on my larger first PCB pass. Everything worked just fine, so I re-laid out the board to be about half the area as you see in Figure 1. That meant that wherever possible, passives went from 0603 to 0402, and chips went from whatever to QFN/DFN packages.

Unfortunately, the DFN package Li Poly charger seems to be out of stock with long lead times. That got me looking at my options. Option 1 would, of course, be to just wait. Option 2 would be to again lay out the board for the MSOP part in that space. Option 3 is to use the "we'll make it fit" mantra. There are no guarantees at this point, but sometimes it's worth a try.

But ... twas not to be. In FIGURE 3, you can see the footprint of the MSOP part's leads is wider than the DFN lands. I suppose there are still a few really messy and potentially expensive options. I could solder a small wire onto the pads, sticking out from the pads, effectively making them big enough to accommodate the chip. Or I could bend the leads under to approximate a J-lead part. Both very ugly, but might work. Probably too expensive, though.



Ed.: Read Duane's blog each week at circuit.

DUANE BENSON is marketing manager at Screaming Circuits (screamingcircuits. com); His column appears bimonthly.

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Author:Benson, Duane
Publication:Printed Circuit Design & Fab
Date:Apr 1, 2012
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