Rounded rectangles are recommended for pad design

I have come across some documentation that states rounded rectangular pads offer a manufacturability improvement and is recommended by IPC-7351.
some of the information I came across may indicate:

  1. IPC-7351 might say something regarding rounded pads (as well as IPC-2221)

  2. rounded pads may reduce stress concentration due to distributing the solder stress more evenly

  3. improved wetting due to the solder flowing smoothly and evenly

  4. all the images that were in the 610 spec were updated to have rounded corners, square corners in previous revisions showed no wetting in the corners

Do you have any information or thoughts on this? I cannot seem to get specific confirmation on if this is recommended.

Our assembly guys might have more information about this (I’ll ask them) but rounding the corners on rectangular pads is especially helpful when using lead-free solder. Getting it to re-flow requires a higher temperature than leaded and even then it doesn’t flow as well so might not make it all the way out to the corners. Sharp corners can also overheat and cause adhesion issues. Rounding the ends is better for solder flow and no more corners sticking out and getting too hot. Don’t believe there is any downside to rounding the ends.

Our assembly engineering manager Tony Sokhi says:

“It’s excellent how you approached the rounded rectangular pad design. Because they more equally distribute load, rounded corners lower the chance of mechanical failure. Because rounded pad corners assist keep solder paste from building up at sharp corners during the reflow soldering process, the solder joint reliability is increased. This lowers the possibility of solder bridging and inadequate solder joints and results in more uniform solder junctions.”

Rounded Rectangular pads also help with solder paste release in paste stencils. Most likely even if you use rectangular pads, the stencil vendor may already be rounding the aperature corners to help with paste release.

As far as layout goes, those rounded corners can sometimes help with improving electrical clearance. In the case of very high voltages, reducing sharp corners reduces the tendancy of arcing.

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It sounds like rounding is a clear win, with the main reasons against it being tradition, which was a result of quicker/simpler design specifications.

How is the transition handled if a datasheet uses rectangles? Are the rectangles’ corners just clipped off, or are the pads slightly grown first to keep the same amount of metal? Is there guidance on how much rounding is sufficient? (Does a square become a circle, or is the clipping limited to 1/10 the shortest side length, or …)

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