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One can sometimes observe rectangle pads in the corners of some IC's footprints:

enter image description here

What are they for?

Null
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Łukasz Przeniosło
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3 Answers3

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They are there to allow for wave soldering. As the solder flows on the legs, any excess can end up on those pads, not shorting out the pins of the IC.

fifi_22
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  • That's pretty crazy. I didn't know they wave soldered SMD components – DKNguyen Mar 21 '21 at 21:45
  • @DKNguyen As far as I know, they don't, but they might wave solder a board that already has SMD components reflowed onto it. – Hearth Mar 21 '21 at 22:23
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    Even that's pretty crazy! But it makes sense. – TimWescott Mar 21 '21 at 23:59
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    They do wave solder smd - pick'n place places the compoments on the glue, then the THT ones are pushed into the holes manually. And at the and everything goes through solder wave. – fifi_22 Mar 22 '21 at 04:56
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    It's not really a recommended process but for huge quantities if you can avoid a reflow is a huge saving! – Lorenzo Marcantonio Mar 22 '21 at 09:26
  • One million VCR's per week for ten years. I learned a lot about how the pros tweak a design for high volume and low cost by tearing apart dead VCRs. Many times a large, fine-pitch SMT chip is at a 45 degree angle WRT the edges of the board. This is to minimize bridging when going through the wave. – AnalogKid Mar 22 '21 at 14:56
  • @AnalogKid: Another fun thing I've observed looking at VCR internals is that surface mount resistors seemed to be preferred except when they would be wired directly in series with a jumper. At least one board I looked at included a shorted-out surface-mount resistor footprint in series with each throughhole resistor. My guess was that the design was intended to be adaptable to assembly using just jumper wires and throughhole resistors, but be manufacturable more cheaply if equipment was available to handle sequenced reels. – supercat Mar 22 '21 at 15:06
  • Might also be due to components supply shortage – Damien Mar 22 '21 at 15:24
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To add (may or may not be applicable for this example):

Some GND connected pads might be added as provision for surface-mount shielding for EMC/ EMI considerations. This is done if the component is either sensitive to interference (i.e. multi-GHz RF front-end), or generates a lot of digital noise and should be suppressed through "shielding" (i.e. for EMC compliance, or to prevent other sensitive components from picking up its noise).

enter image description here

Carla H.
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In addition to the accepted answer, I have seen test sockets with solder tags at these points. It provides mechanical support when screwing the lids down.

SiHa
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