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I have soldered through-hole and SMT before but this is the first time I have to do SMT (QFN) package which has no legs. I tried the traditional approach, Flux, solder, then heating the component. But here the pads are underneath that it's been a nightmare to reach em to apply heat.

I worked with somebody back in the day who soldered in a different way. He applied something on the pads(very thick paste), I'm guessing Solder/flux paste. It managed to keep the components right where there are. Then he put the board in a microwave and ran it for a few minutes. Then the components were soldered perfectly. No issues what so ever.

My question is, what's that paste? Is my guess right? What exactly is it called so that I can buy one?

If I'm wrong, then how does one normally solder these components?

My component is a micro msp430fr2433.

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    \$\begingroup\$ Toaster oven, NOT microwave oven. And don't use it for food, ever, after this, especially if you use lead-based solder. \$\endgroup\$ – Spehro Pefhany Jan 11 at 18:49
  • \$\begingroup\$ There are a bunch of similar questions on this site about soldering QFN (take a look at them for ideas), but the usual way is with reflow soldering. You can also do it with an iron, although this is more difficult. \$\endgroup\$ – user1850479 Jan 11 at 18:51
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    \$\begingroup\$ The thick substance must be solder paste. \$\endgroup\$ – mkeith Jan 11 at 18:52
  • \$\begingroup\$ Try tiny strip of foil in a microwave (not) \$\endgroup\$ – Tony Stewart EE75 Jan 11 at 18:56
  • \$\begingroup\$ I realize that. I'll not use it for food ever again. \$\endgroup\$ – varun Jan 11 at 19:10
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I certainly would not use a microwave oven here. That runs the risk of damaging your board, components, and also the oven itself.

I've had good success with a "rework reflow" process where you do the following:

  1. Apply suitable flux to the pads and to the pins on the device.
  2. Apply a small bead of low temp solder paste to the pads. It's OK if there is some between the pads.
  3. Place the part using some glue under the center if it won't stay put.
  4. Use a heat gun that produces a high enough temperature to flow your solder paste to the pads until the solder reflows.

Alternatively you can use a hot plate or a toaster oven to heat the entire board but I've had the best luck with the heat gun.

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  • \$\begingroup\$ They seem pretty pricey. I just need to solder this one component this way. I can solder other SMT packages without any issues. Unless there is no other option, I would not wanna buy a heat gun. \$\endgroup\$ – varun Jan 11 at 19:18
  • \$\begingroup\$ I got a small heat gun that is sold in craft stores. Less than US$20. \$\endgroup\$ – jwh20 Jan 11 at 19:26
  • \$\begingroup\$ But an oven will work, toaster oven, or a paint stripper heat gun from the hardware store. \$\endgroup\$ – jwh20 Jan 11 at 19:27
  • \$\begingroup\$ There are many ways to do it. You asked for how one does it. I explained how I've done it with reasonable success. The soldering iron "swipe" method also works but I've had a lot of trouble with solder bridging with that method. \$\endgroup\$ – jwh20 Jan 11 at 19:31
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    \$\begingroup\$ It takes some practice. Don't start with your good stuff! Get some "SMT practice" boards and parts and get a feel for how this works. \$\endgroup\$ – jwh20 Jan 11 at 20:21
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The usual way is solder paste (mixture of tiny balls of solder and flux) and a reflow oven. The paste can be applied using a syringe or by speading the paste over the board using a stencil.

For small numbers, I'd just use a regular soldering iron, a fine tip, and a roll of thin solder and do them by hand.


I've never heard of using a microwave oven for reflow soldering. High power microwaves would almost certainly damage the board or the components.

Hobbyists often modify toaster ovens for reflow use.

Maybe your coworker used a microwave oven with a "grill" function like a toaster oven where the microwave function isn't used.

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  • \$\begingroup\$ You could be right. It's been years. So I don't exactly remember what it was. Do you think this will work? digikey.com/en/products/detail/chip-quik-inc/TS391AX50/7802229 I'll apply this to the pads, place the components, then place the board in a toaster oven for a few minutes. \$\endgroup\$ – varun Jan 11 at 19:14
  • \$\begingroup\$ I don't do reflow. I'd just get out the soldering iron and have at it. \$\endgroup\$ – JRE Jan 11 at 19:35
  • \$\begingroup\$ @varun that solder paste is OK. it has relatively low melting point, so should be easy to use with heat gun too \$\endgroup\$ – Maple Jan 11 at 20:07
  • \$\begingroup\$ @maple Thank you for confirming. \$\endgroup\$ – varun Jan 11 at 20:18

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