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I'm learning to layout PCBs and lately I came across practice that made me curious. The chip passives' pads are etched with oblong / rounded shape, instead of rectangular shape that is used in all example libraries and even the IPC-7351B standard (you can download LP Viewer for free registration and see for yourself). Here are the examples (I marked the interesting pads with yellow):

The question is: what are these rounded pads good for? Should I use them instead of rectangular ones to make my board look more "pro"?

My first thought was that it might be because it might be better for reflow soldering, but I'm bit puzzled about that reasoning. The one advantage I see with these is more routing space around such rounded pad (no "sharp" edges).

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Rounded pads are better for lead-free surface mount assembly using reflow, because lead-free solder doesn't flow as well as leaded solder, and the higher temperature causes problems with the flux at the corners. They are recommended by IPC. Here is a reference I found.

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    \$\begingroup\$ reference please? \$\endgroup\$ – endolith Nov 20 '10 at 14:57
  • \$\begingroup\$ What do you mean by surface mount assembly? Reflow soldering, wave soldering or hand soldering? I mean is it easier to solder to rounded pads with lead-free solder generally, or using particular technology like reflow or hand-soldering? \$\endgroup\$ – Andrzej H Nov 20 '10 at 15:07
  • \$\begingroup\$ he is stating it is easier with lead free solder and SMT/SMD chips. He is not specifying if it is hand or automated. \$\endgroup\$ – Kortuk Nov 20 '10 at 16:17
  • \$\begingroup\$ I tried to get a reference for this, IPC would like you to pay to see any of their standards, so I am not sure Leon will be able to get your a good reference for this. \$\endgroup\$ – Kortuk Nov 20 '10 at 16:21
  • \$\begingroup\$ I thought lead-free solders flowed worse? Or do you mean the corners are rounded so the solder will cover more of the pad as a whole--"better fill?" \$\endgroup\$ – Nick T Nov 20 '10 at 18:10
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I prefer the rounded pads when I'm etching a board myself. I find that by using a rounded pad and making it a bit larger than it needs to be I have more room for errors in etching or drilling. It's especially useful for through-hole components. If your hole isn't exactly where it needs to be and your pad is circular then a rounded rectangle leaves more area in case the hole is off.

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As Leon noted, 90o corners are undesirable because they heat up faster.

Another downside even if you're not doing reflow work is that the corners are the first thing to lift if you abuse the board during rework, just like the corners of a sticker are the easiest and first things to peel up.

However, I route at 45 degree angles, so an octagon is a better shape than a round pad. It minimizes the space required of the trace around the pad, while simultaneously maximizing the area of said pad for soldering strength, board-to-copper adhesion, and heat dissipation. Here's a diagram in hopes that it will help you see why.The diagram is for a through-hole component, but the same logic applies to SMDs.

trace rounding octagon at even spacing

The 135o angles are better than 90o; but I'm not convinced that going to fully rounded corners is significantly better than 135o. Also, (insignificantly)I like the uniform look that octagonal pads and 45 degree routing produces; I think round pads look out of place.

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    \$\begingroup\$ You can mathematically prove for all the arguments you use that a circle is the most optimal, except for the aesthetic argument, but that doesn't count. \$\endgroup\$ – Federico Russo Jul 24 '11 at 13:28
  • \$\begingroup\$ @FedericoRusso: The difference going from a square corner to a pair of 135-degree corner yields about half the improvement one could ever hope to get by going to a perfectly-rounded corner (since cosine(135deg)^2 is 0.5). Using three 150-degree corners would get about 65% of the improvement (cosine(150deg)^3 is about 0.65). Using multiples of 30 degrees might be pretty; going beyond that probably less so. \$\endgroup\$ – supercat Jan 4 '13 at 18:10

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