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I'm experimenting with a TI Dual Output Buck Converter, the TPS54383. In section 11.3, on page 43, the datasheet says

11.3 PowerPAD Package The PowerPAD package provides low thermal impedance for heat removal from the device. The PowerPAD derives its name and low thermal impedance from the large bonding pad on the bottom of the device. The circuit board must have an area of solder-tinned-copper underneath the package. The dimensions of this area depend on the size of the PowerPAD package. Thermal vias connect this area to internal or external copper planes and should have a drill diameter sufficiently small so that the via hole is effectively plugged when the barrel of the via is plated with copper. This plug is needed to prevent wicking the solder away from the interface between the package body and the solder-tinned area under the device during solder reflow. Drill diameters of 0.33 mm (13 mils) work well when 1-oz. copper is plated at the surface of the board while simultaneously plating the barrel of the via. If the thermal vias are not plugged when the copper plating is performed, then a solder mask material should be used to cap the vias with a diameter equal to the via diameter of 0.1 mm minimum. This capping prevents the solder from being wicked through the thermal vias and potentially creating a solder void under the package. (See the Related Documentation section.)

Also, in the pin function table for the thermal pad it says

This pad must be tied externally to a ground plane and the GND pin.

My questions 1) does this mean I need to have a solder connection between the chip and the pad? Or will having enough solder so that the pad on the IC touches the pad on the PCB? 2) If I need a connection, like for pins, how do I heat up the solder underneath enough to reflow? I only have a solder iron, no hot air gun.

Edit 1 Here is a picture of the pads on the PCB. I started to tin one corner but stopped because I wasn’t sure how to connect the thermal pad (if I need to).

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  • \$\begingroup\$ Unrelated to your question, the soldering on most of the 0603/0805 parts is very poor. You are using way too much solder, likely using too thick solder. Most of them look like cold joints too. You need to revisit all of these parts with a flux pen, then pin the component down with tweezers and touch each joint with the iron. Let the surplus solder follow the iron away from the joint. Alternatively wick braid everything away and re-do the joint. \$\endgroup\$ – Lundin Jun 18 at 6:28
  • \$\begingroup\$ This is my first attempt at SMD soldering. I only have 0.6mm solder wire. How do you tell it’s a cold solder joint? I ordered a flux pen, should be here tomorrow. \$\endgroup\$ – Sean Jun 18 at 15:35
  • \$\begingroup\$ You should use 0.25mm, anything thicker will make a mess. You can tell that it is cold if the solder hasn't wet ("floated out" all over) both surfaces. Cold joins also look matte and dull light gray. With leaded solder it should look shiny metallic, less so with RoHS solder, which also looks more grainy. \$\endgroup\$ – Lundin Jun 24 at 14:15
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Well in the past I have used a large via in the power pad as a way of getting a small iron tip into the hole and then soldered from underneath.

In my experience if you aren't pushing the part to its limits it will be ok not soldering it at all.

But if you want to solder it, buy some solder paste and stick it in the oven when the other half isn't looking.

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This imformation may allow you to further analyze your heat-removal challenge.

A via with 1:1 perimeter/height ratio, with standard 1 ounce/square_foot foil or plating (which is 1.4 mil thick,o r35 microns), has thermal resistance of 70 ° C per watt.

A via with 0.5:1 perimeter/height ratio, has thermal resistance of 140 ° C per watt.

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