Larger lands in corners

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Our layout guy recently started making land patterns with th... - Jan 10, 2008 by SteveK

Using larger pads on the corners is fairly common to improve... - Jan 14, 2008 by davef

SteveK

#53143

Larger lands in corners | 10 January, 2008

Our layout guy recently started making land patterns with the corner lands oversized. These are for leadless and various flatpack parts. He claims this is to improve manufacturing. I had seen a few boards with this just as I got out of the CM world. I'm not sure what it is supposed to do. A 100+ lead QFP doesn't float at all, so any supposed increased surface tension from a larger solder volume won't provide any auto centering. We aren't wave soldering, and these aren't additional thieving lands, just larger lands in the corners.

Does anyone know where this comes from? And what the premise of it is?

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davef

#53190

Larger lands in corners | 14 January, 2008

Using larger pads on the corners is fairly common to improve self-alignment of BGA and CSP. For example, "9.3.2.8 Large corner pads and overprint The self-alignment of BGA or CSP can be improved through a design change. By utilizing a large board pad for the corners, the package will have greater tolerane for misregistration, as illustrated in Figure 9.20. This parge pad approach can be enhanced by over printing the large pads with solder paste so that more solder will be available to facilitate the self-centering process." [Reflow Soldering Processes and Troubleshooting: SMT, BGA, CSP and ...; Ning-Cheng Lee; Newnes 2002; 0750672188]

But for leadless and flat packs, it's more difficult to justify, as you say. It could be that the board layout-type perceives that since there's a good reason for doing it with array packages that the rationale must apply to other packages as well.

We'd argue that it's a bad idea from a reliability stand-point, because it could increase the amount of solder to the point that it damages the package.

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