Screaming Circuits: Power Components and Surface Tension


Power Components and Surface Tension

Another Parts Library issue

Surface tension can be quite handy. For one thing, without surface tension, water bugs wouldn't be able to skip along the surface of a pond. They'd sink like a rock straight to the bottom and frogs wouldn't be able to eat them. The frog population would plummet and we all know what that means.

Another handy effect of surface tension is that it acts to help BGAs and QFNs to center themselves Ground_pad_pulling_part_2 during reflow. This same effect can backfire on us though. Take a look at the TO-263 packaged regulator in the photo. By the way, the LM1086 really annoys me. Why in the world isn't it pin compatible with the ubiquitous 78XX regulator pinout? Yeah, I know it's a low-dropout but other low-dropout regulators use the standard pin out. What gives?

Anyway, what happened is that surface tension pulled the part up and nearly pulled the three legs off of their land pads. If not reworked, the legs won't be mechanically secure, they may not have sufficient current carrying capacity and if there are and mask imperfections, they are at risk of shorting to the copper fill area.

There are a couple of possible remedies here:

  1. Mask off the top of the thermal pad so the part can't slide up that far.
  2. If the pad needs to be fully exposed for better convection, just use a strip of soldermask as a solder dam. If you use this approach, don't forget to modify the paste layer so that no solder paste is spread above the mask dam.
  3. Extend the thermal pad down so that when the body of the part is centered, the legs will still be appropriately located on their pads.

Duane Benson
What about newts?


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