Hi TN-er's,
Here is a thread from a design listserver:

Original question:

>We purchased a canned library from a 3^rd party vendor (****) and are >currently >using part decals from it either as is or as make forms.

>The SOIC'S, PLCC'S and other active components from this library have pad >stack definitions defined using 0VAL instead of RECTANGLE.  Does any one >have any comments as which are preferred and why.

  Here are 3 posted answers:

• "...oval pads [allegedly] allow the solder paste stencil to release more easily and avoid the potential for paste to be trapped in the corners of the apertures."


• "The only valid reason I know of for using oval pads is solderability. The reasoning is this: when solder is in a liquid state it tends to form droplets - when the molten solder paste spreads out on the pad - it spreads in a circular pattern. The potential exists for a non-wetting in the corners of a rectangular pad. This is less likely with oval pads."


• "I've used oval/round pads extensively. I started using them as a captive designer at a design/assembly house and not had any complaints (well, except for the occasional "they aren't IPC Spec" because they weren't squared off) from engineers I've worked with.  Supposedly, after running a bunch of tests, that employer decided that oval pads give a little better solder joint because the round ends mimic the natural tendency of solder to bead when melted (think of melted solder, it makes little balls, not cubes....) . They also used round pads for devices like SOT23's and MELF diodes instead of rectangular or square pads. The only place square pads showed up was pin 1 on THT parts and for large tabs, like TO-220s or DPAKS.  One other nice thing you get is they tend to be a little smaller than square/rectangle pads and you gain that little bit of routing space at the corners of pads, which can make all the difference in the world on a dense board."
• "...a designer has to choose a style and run with it.  If you don't want to defend your style, stick with rectangular.  If you are more of a cavalier then go with round if you want.  Maybe someday, someone will be allowed to perform and report the analysis.  Proposed advantages of round pads over rectangular pads - Better solderability joints due to its natural tendency to bead.  Better solder joint due to the impurities are spread more evenly over the pads as opposed to concentrated in the corners.  Easier routing due to the radius' of the pad. i.e. if you are routing a 45 angle around the corner of a pad the route can be closer than if it were rectangular.  Proposed disadvantages of round pads over rectangular pads - Not IPC approved, although I not sure this is true.  Not as strong as there is less surface copper to adhere to the laminate.  It is different than historical pads.  i.e. QA might not like them as they look different. Unknowns:

What is the effect of round pads for stencils.
What is the effect of round pads for apertures.

For both of these it should be minimal as there usually are round flashes anyway."
My question:
What do the manufacturers approve of, oval/rounded or square/rectangular?

Bob Wilson, Staff Engineer
Electro-Mechanical Design
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Product Development Section
DIRECTV, Inc.
www.directv.com
voice 310.964.4145 fax 310.535.5226