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October 2, 2013

Octagonal Pads for 0402 Chip Caps

Octagonal Pads for 0402 Chip Caps
One of our chip vendors has recommended using octagonal pads for 0402 decoupling capacitors that reside under a 1mm-pich BGA. Is this common practice? Are there any standards that addresses this?

J.M.

Experts Comments

This type of design though not being evaluated by component manufacturer either BGA/0402 but there's patent exists for same and you can opt to try this in sample evaluation. you may google patent- US20080205011
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Subrat Prajapati
Supplier Quality Leader
Ge Healthcare
Subrat has 10 year of extensive experience in PCB assembly process optimizing for quality, process includes screen printing, wave, reflow. He has a copyright in stencil design published in Apex Expo2010 at Las Vegas US.
I have not seen this exact strategy, but it does make sense. The octagonal pattern (effectively a square with the corners cut off) provides added clearance, vs. a square, between the lands and non-common pads while minimizing the impact on the printing, placement and reflow processes.

It's well documented that there is only moderate difference in printing effectiveness between a square and a round aperture of the same dimension, and the octagon is a "compromise" between the two. In fact, it can be shown mathematically the area ratio is exactly the same for a square, a round aperture or an octagon of a given dimension!

The placement process should not care whether the paste deposit is rectangular or octagonal, as long as the size and spacing is correct. The reflow process should be similarly agnostic as to shape. More important is the thermal gradient due to the interconnections with the vias. The sides of the lands attached to the (large) traces connected to the via lands will be cooler than the opposite sides, and as a result you may see shifting of the chips during reflow. The traces are quite large, and I'm not sure why they need to be.

The effect is that nearly half of the 0402 land becomes "solder mask defined" so if the solder mask shifts relative to the metallization, the land can grow or shrink. This effect can be minimized by reducing the trace width. You do have to be careful to maintain adequate mask web thickness to prevent solder from the 0402 from flowing down the via. A reasonable minimum is 0.003"
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Fritz Byle
Process Engineer
Astronautics
Fritz's career in electronics manufacturing has included diverse engineering roles including PWB fabrication, thick film print & fire, SMT and wave/selective solder process engineering, and electronics materials development and marketing. Fritz's educational background is in mechanical engineering with an emphasis on materials science. Design of Experiments (DoE) techniques have been an area of independent study. Fritz has published over a dozen papers at various industry conferences.
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